xvEPA
ted States
Environmental Protection
Agency
fice of Pesticides
and Toxic Substances
Washington, DC 20460
EPA-560/11-80-
October 1980
Toxic Substances
Support Document
Test Data Development
Standards
Physical/Chemical and
Persistence Characteristics :
Density/Relative Density
Melting Temperatures
Vapor Pressure
Octanol/Water Partition Coefficient
Soil Thin Layer Chromatography
Proposed Rule, Section 4
Toxic Substances Control Act
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EPA 560/11-80-02?
SUPPOET DOCUMENT
TEST DATA DEVELOPMENT STANDARDS
PHYSICAL/CHEMICAL & PERSISTENCE CHARACTERISTICS:
DENSITY/RELATIVE DENSITY
MELTING TEMPERATURES
VAPOR PRESSURE
OCTANOL/WATER PARTITION COEFFICIENT
SOIL THIN LAYER CHROMATOGRAPHY
PROPOSED RULE, SECTION 4
TOXIC SUBSTANCES CONTROL ACT
Exposure Evaluation Division
Office of Toxic Substances
OCTOBER 1980
U. S. Environmental Protection Agency
Office of Pesticides & Toxic Substances
Washington, D.C. 202^60
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TABLE OF CONTENTS
[Numbers Refer to Those at Top of Page]
Density/Relative Density 1
Melting Temperatures 27
Vapor Pressure 51
Octanol/Water Partition
Coefficient 79
Soil'fMn Layer
Chromatography 121
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DENSITY/RELATIVE DENSITY
STANDARD FOR TEST DATA DEVELOPMENT
PROPOSED RULE, SECTION 4, TSCA
Refers to
Part 772 -- Standards for Development of Test Data
Subpart I. Physical, Chemical and Environmental
Persistence Characteristics
Section 772.122-1 Density/Relative Density
October 1980
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CONTENTS
Paqe
1. NEED FOR THE TEST 1
2 . SCIENTIFIC ASPECTS 4
2.1. Selection of Test Conditions 4
2.1.1. Equipment 4
2.1.2. Materials 5
2.2. Selection of Test Procedures 5
2.2.1. Temperature of Test 5
2.2.2. Selection of Measurement Technique... 6
2.2.3. Calculation of Ideal Gas Density 10
2.3. Test Data Required 10
3 . ECONOMIC ASPECTS 12
4. COMPATABILITY WITH OTHER TEST STANDARDS 13
4.1. TSCA/FIFRA Compatibility 14
4.2. OECD Compatibility 14
4.3. IRLG Compatibility 14
5. RESPONSES TO COMMENTS RECEIVED ON PROPOSED
GUIDANCE FOR PREMANUFACTURE TESTING 15
5.1. General 15
5.2. Responses to Specific Comments 15
6. REFERENCES 19
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1. NEED FOR THE TEST
The purpose of this test is to develop data on density and
relative density (specific gravity) of chemical substances and
mixtures.
EPA will use the data obtained from testing of density and
relative density to evaluate the potential for human and
environmental exposure to chemicals. Data also will be used as a
screening test in order to determine what additional testing will
be required for physical and chemical characteristics and for
effects on the various portions of the living and nonliving
environment.
Density is an important factor affecting the path that a
chemical will take in moving through the environment and
affecting where the material may eventually accumulate. As
chemical substances enter the atmosphere or water, as a result of
normal practices of manufacture, transportation, use, ultimate
disposal, or accidental spill, they will tend to rise, sink, or
disperse as a function of their density. In water, for example,
low density substances (liquid or solid) will float to the
surface, dense substances will tend to sink to the bottom, and
materials with approximately the same density as the water will
tend to become dispersed. EPA, therefore will use information on
density to determine if principal exposure may be to bottom-
dwelling organisms (e.g., shellfish), to plants or animals
(e.g., waterfowl) that live at the surface, to swimmers who use
the shallow beach areas, or to swimming fish which contact the
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dispersed chemicals. The information will be used in selecting
which types of plants or animals to test for toxic effects, and
it will be used in the ultimate evaluation of risk to the human
and nonhuman environment.
Similar considerations need to be made for materials that
enter the atmosphere through any of a number of ways, intentional
or accidental. Dense gases will tend to accumulate near the
surface of the earth. An example is chlorine, which occasionally
is spilled as a result of train, barge, or truck wrecks or the
rupture of storage vessels. Because of its density, the gas
remains near the surface and causes a great risk to nearby human
and animal populations. Light gases, on the other hand, tend to
rise and diffuse in the atmosphere where they may be subject to
photochemical degradation. However, if they are not degraded
they may be transported great distances and ultimately be
redeposited through various processes of atmospheric deposition.
One way that EPA will use data from density testing is as
part of the input to several mathematical models for assessing
transport and fate of chemicals in the environment. These
models, which consider movement in the atmosphere, surface
waters, and ground water, all require information on density to
evaluate the movement of materials, their ultimate deposition,
and the biological and chemical processes of transformation that
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will affect them. Results from the modeling work are then
combined with the results of toxicity and other hazard
evaluations in order to assess the total human and environmental
risk from the chemicals.
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2. SCIENTIFIC ASPECTS
2.1. Selection of Test Conditions
2.1.1. Equipment
The use of calibrated ASTM-specification thermometers is
required. The particular type of thermometer is usually
specified in the standard technique identified in Table 1 of the
Test Standard.
Widespread laboratory experience has found that manufactured
thermometers are sometimes inaccurate by several degrees. Such
inaccuracy may result in incorrect density values that would
adversely affect EPA's ability to use the data in models. A
minimal, satisfactory, easily attainable traceability to NBS is a
comparison by the manufacturer or the analyst with a thermometer
that has been calibrated by NBS. Compliance with this condition
of the test is relatively easy, but it is an important factor
that often is overlooked.
The other apparatus required is fully described in the
methods and procedures referenced in Table 1 of the Standard.
EPA has reviewed and adopted these procedures with their
appropriate equipment requirements.
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2.1.2. Materials
The reference materials, water, mercury, cyclohexane, and
toluene have been recommended by the IUPAC Commission on
Physicochemical Measurements and Standards (1975, 1976), and
recommended density values have been given. These, or other
appropriately selected substances, are to be used in this
procedure for calibrating the apparatus as described in several
of the techniques.
The suggested immersion liquids are water and a series of
organic compounds useful in the event that the substance is
soluble in water. The liquids listed are representative and have
been recommended in one or more of the techniques requiring
immersion of the specimen. The immersion liquids include several
of high density, to be used in the sink-float technique. The
comparison gases are for use with the gas-comparator pycnometer
technique.
2.2. Selection of Test Procedures
2.2.1. Temperature of test
The IUPAC has for many years recommended (IUPAC 1972) the
reporting of physical-chemical-properties measurements at the
convenient temperature of 25°C, and this is the preferred
temperature. Testing at this temperature will provide the
reliable data that EPA requires for use in its modeling
techniques for evaluating environmental fate and exposure. A
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temperature of 25°C is slightly above customary laboratory
temperature, which makes for convenient adjustment and
maintenance of a constant temperature bath at 25°C for
measurements.
However, engineering groups, such as the American Petroleum
Institute and the American Gas Association have established a
much lower reference temperature. Their most common reference
temperature for many years had been 60°F (15.55°C). Within the
last few years, with the shift to SI units, and to harmonize
their standards with the European standards (through ISO) they
have agreed to shift to 15°C as the reference temperature. This
temperature is representative of many environmental temperatures
and EPA will be able to adjust its evaluating techniques to use
density data determined at 15°C.
The CIPAC group has adopted (Raw 1970) 20°C as the standard
reporting temperature for the properties of pesticides. A
substantial body of data on pesticides has been determined at
this temperature and the results are useable by EPA. Pesticides
are commonly used at temperatures which are somewhat warmer than
annual temperature because they are generally used during the
growing season or indoors.
2.2.2. Selection of a Measurement Technique
The procedure for testing chemicals under the Toxic
Substances Control Act (TSCA) must be broad enough to cover the
entire range of chemicals subject to the law. Measurement of
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density, in itself, is not a complicated problem for most
chemicals, however, the techniques for handling different kinds
of chemicals are quite different because of different physical
characteristics.
Many simple procedures have been developed for making
measurements on common classes of chemicals. These are based on
well known physical principles, such as Archimedes' principle
(see, for example Bauer and Lewin [1971], p.91) or on the
definition of density itself. The different techniques have
specific limitations because of the shapes of the measuring
devices, the incompatibility of certain chemicals with immersion
liquids, the viscosity and the volatility of the test substance,
or the size of the particles that may have to be tested.
EPA has reviewed a broad range of methods from the
scientific literature and has adopted the large number of
standardized protocols listed in Table 1 of the Standard. Each
method has been evaluated for suitability for use with different
types of substances as described by characteristics given in
Table 2 of the Standard. Section (e)(l)(ii) of the Standard
specifies which types of techniques EPA has judged suitable for
use with the different types of substances. The person doing the
testing then is free to choose from among the appropriate
standardized protocols listed in Table 1.
Some contingencies may not be covered by the identified
techniques in this procedure. In these cases the method to be
used will be specified in the test rule.
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An important feature of Table 1 is its application to the
choice of a technique that is suitable to the physical
characteristics of the material to be tested. For assistance in
this process, Table 2 of the Test Standard has been developed to
give the important characteristics that must be taken into
account in making the selection of a technique. If further
guidance is necessary, EPA recommends the book by Bauer and Lewin
(1971), Table 2.7, which outlines recommended techniques for
various special problems.
For gaseous substances the principal decision is whether to
calculate or measure the density. In consideration of the
relatively small differences between results produced by the two
techniques, the Agency will accept a calculation based on the
ideal gas law as adequate. This method is suitable for
assessment purposes, as indicated by a comparison of real gas and
ideal gas densities of several typical industrial gases (allene,
ammonia, 1,3-butadiene, chlorine, methylamine, helium,
fluoromethane, and nitrous oxide), at atmospheric pressure and
ambient temperatures. The real gas density is higher than the
ideal gas density in all cases but one. However, the average
relative error is only 3.0 percent, with a standard deviation in
the relative error of 4.7 percent. However, because easily used
devices are available commercially for measuring gas density,
[ANSI/ASTM D 1070] the option is given of measuring rather than
calculating the density.
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For liquids, the volatility and viscosity are factors in the
selection of a method. The sensitivity of the techniques to
these factors is not very great, however, so for certain ranges
of volatilities and viscosities, one technique or another may
work equally well. For volatilities, this range is defined by 15
kPa < P° < 80 kPa, where 1 kPa = 0.00987 atm. and P° is the
vapor pressure of the pure liquid at 25°C. Similarly there is a
range of viscosities in which one technique or another may
equally well be used. This range is defined by 15 cST < T
(258C) < 400 St where T is the viscosity. The viscosity
primarily affects the ease with which the liquid flows through
capillary tubes used in some techniques. Table 2 of the
Standards accounts for the range of overlap.
For solids, the size of particles, the interfacial tension
with water, and the solubility in water affect the choice of
method. The selection is not strongly dependent on the
characteristic, however, so the Standard leaves room for the
judgment of the analyst.
Solubility of the solid in water in amounts greater than 1
mass percent precludes use of water as an immersion liquid, and
some other liquid must be chosen. Also, immersion methods in
which the change in weight of an object is used as a measure of
its density, require pieces large enough to be handled
individually. Wettability is another important parameter,
particularly for powders, if the density is to be determined by
displacement of water.
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Table 2 lists the characteristics of interest in a
systematic way for convenience, but it gives limits only in some
cases for which quantitative criteria seem to be useful. Within
the scope of this Table and Table 1, the sponsor is expected to
determine which technique to use and to apply it appropriately in
making the test.
The accuracies obtainable by the standard techniques are
stated in the published descriptions of the techniques. In every
case they meet or exceed the requirements of the Agency.
Techniques for exposure assessment used by EPA generally accept
an accuracy within 5 percent as adequate, except in the region of
relative density from 0.95 to 1.05. Here, in order to determine
how a chemical will act in water, the relative density needs to
be known to 0.01.
2.2.3. Calibration
Calibration of laboratory equipment has long been a
standard requirement for testing or research. The step, while
not burdensome, assures that the data will not be affected by
equipment that is damaged or has drifted out of calibration.
2.2.4. Calculations of Ideal Gas Density
The density, p, is calculated from the formula
p = pM/RT
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in which p is the pressure, M the molar mass, R the thermodynamic
gas constant, and T the thermodynamic temperature. Using T =
298.15 K (25°C) and R = 8.3143 J mol^R-1, and with the mass M
expressed in grams and the pressure in kilopascals, the gas
density can be calculated in kilograms per cubic meter.
The relative density of an ideal gas with respect to air is
calculated from the ratio of the molecular weight of the gas to
the average molecular weight of air. For the average molecular
weight of air, we have taken 28.964 from Weast (1978-1979).
2.3. Data Required
The sponsor is allowed the option of reporting either the
density or the relative density because the calculation from one
to the other is simple and unambiguous. The sponsor is also
asked for a statement of the technique that was chosen and for
calibration information. Because, in some instances, the density
of the material tested may not indicate the range of acceptable
densities in a material of variable composition, a statement of
the acceptable range of densities is requested.
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3. ECONOMIC ASPECTS
A survey of commercial laboratories to estimate costs for
performing the tests outlined in the Standard found a range of
$15 to $75, with four of the five estimates having a top range of
$30, and with a "best estimate" of $28 based on the survey. A
cost estimate also was made by separating the protocol into
components and estimating the cost of each component, including
direct labor cost, overhead cost, other direct cost, general and
administrative costs, and profit or fee. The protocol best
estimate of cost was $72, with an estimated range of $36 to $109
based on ± 50 percent of the best estimate. An analysis of the
discrepancy between the protocol estimate and the survey pointed
out that the nature of the chemical to be analyzed has an impact
on the cost of analysis. The physical state of the compound
(e.g., liquids versus chunk or particulate -old'3-* determines the
method to be used. Different methods will cause variations in
cost. The number of samples for analysis may also affect the
cost per sample. Discounts are offered by some laboratories
based on the number of samples to be analyzed.
The above cost estimates were made assuming that all the
requirements of Good Laboratory Practice Standards, as specified
in section (c) of the Density Standard, are being satisfied.
Details of the cost estimates are contained in a report by Enviro
Control, Inc. (1980).
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4. COMPATABILITY WITH OTHER TEST STANDARDS
4.1. TSCA/FIFRA Compatibility
Methods for determining density given in FIFRA guidelines
(USEPA 1978) refer to techniques as follows: CIPAC MT3; ASTM D
941-55; ASTM D 1480-62; ASTM D 1481-62; and ASTM D 1217-54. No
guideline is given as to which technique should be used. All of
the above techniques are offered in the present test protocol, as
well as some additional techniques which may be needed because of
the greater variety of chemicals to which TSCA applies. In
addition instructions are given in the TSCA Standard for the
selection of an appropriate technique.
The FIFRA guidelines (USEPA 1978) specify the reporting only
of density for solids and only of relative density (specific
gravity)* for liquids. We offer a broader option of either
density or relative density for both solids and liquids. Either
type of information is equally suitable for use in the type of
exposure analysis used for TSCA chemicals.
The FIFRA guidelines (USEPA 1978) specify a reporting
temperature of 20°C. The TSCA protocol includes that temperature
*The IUPAC (McGlashan et al. 1979) has recommended the term
relative density in preference to specific gravity; preferring to
reserve the word "specific" to refer to a quantity per unit mass,
as in specific volume. The term "specific gravity," with the
same meaning as relative density is therefore obsolescent. This
practice has been adopted also by ASTM (1978) for their
standards.
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among three options, all of which are used to some extent in
nationally and internationally standardized tests. The 20°C
reference temperature is adopted by CIPAC (Raw 1970), and this
provides the justification for its use. However, as explained in
Section III, there is substantial justification for allowing
other reporting temperatures. In summary, the TSCA protocol is
compatible with the FIFRA guidelines (and with proposed
revisions) but is somewhat broader in scope and both more
specific and more flexible in its requirements.
4.2. OECD Compatibility
An OECD Test Guideline for Density (OECD Expert Group
Physical chemistry, 1978) lists many of the same standard
techniques. We believe the present document is entirely
compatible with the OECD standard, but to assure adequate
selection of applicable techniques, the TSCA Standards specify
which protocol is to be used for specific types of materials.
C. IRLG Compatibility
The IRLG has not proposed a standard for density.
We expect to submit this standard to IRLG as a proposed
standard.
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among three options, all of which are used to some extent in
nationally and internationally standardized tests. The 20°C
reference temperature is adopted by CIPAC (Raw 1970), and this
provides the justification for its use. However, as explained in
Section III, there is substantial justification for allowing
other reporting temperatures. In summary, the TSCA protocol is
compatible with the FIFRA guidelines (and with proposed
revisions) but is somewhat broader in scope and both more
specific and more flexible in its requirements.
4.2. OECD Compatibility
An OECD Test Guideline for Density (OECD Expert Group
Physical chemistry. 1978) lists many of the same standard
techniques. We believe the present document is entirely
compatible with the OECD standard, but to assure adequate
selection of applicable techniques, the TSCA Standards specify
which protocol is to be used for specific types of materials.
4.3. IRLG Compatibility
The IRLG has not proposed a standard for density.
We expect to submit this standard to IRLG as a proposed
standard.
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5. RESPONSES TO COMMENTS RECEIVED ON PROPOSED GUIDELINES
FOR PREMANUFACTURE TESTING (USEPA 1979).
5.1. General
Following publication of the Guidelines a reconsideration
of the variety of chemical materials requiring testing and of the
availability of standardized procedures for such testing brought
us to the conclusion that the techniques proposed then were
inadequate to cover the multiplicity of identifiable situations
that would arise. As a result this proposed Test Standard goes
into considerably more detail with respect to the characteristics
of materials that influence the selection of a technique, and it
specifies appropriate techniques for handling materials with
these characteristics. The requirement for the temperature at
which the test is to be carried out was also changed to allow
selection of one of three options.
5.2. Responses to specific comments
In response to the Proposed Guidelines, comments dealing
specifically with density/relative density were received from six
respondents.
5.2.1. General Approval
The largest group of responders who commented on the test
expressed general approval of the test requirement, on the basis
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that the test can be and routinely is made on materials for use
in marketing and handling, and can be done with the accuracy
required here.
5.2.2. Marginal Utility
One comment was that the test result is useful in
understanding the general nature of a specific chemical but is of
marginal utility in determining its environmental effects. The
Agency position is that the information is needed to evaluate
environmental fate, and therefore plays an important part in risk
assessment. The usefulness in determining environmental fate is
discussed in Section I of this document.
5.2.3. Limitations on Materials to Which the Test
Should Be Applied.
One respondent stated that testing would be unneccessary
if the relative density had already been determined to be less
than 0.97 or more than 1.03 in the temperature range between 10
and 30°C, or if the solubility in water is 10 percent or more.
The criteria as to whether testing is to be required under a
Section 4 Test Rule are somewhat different from the criteria for
testing under Section 5 Premanufacture Test Guidelines. It might
indeed be decided that for a particular chemical which available
information put within the bounds suggested by the respondent, no
test requirement would be made. On the other hand, for other
material testing may be required. An example might be a dense
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material that may be expected to get into flowing water where it
would be transported in turbulent suspension, as sand is
transported. For such material, knowledge of density is required
to apply equations of turbulent transport to estimate where and
how the material will move in the environment.
5.2.4. Limitations on Accuracy and Care Required
One responder suggested that special care cited in the
Proposed Guideline is not required if the accuracy specified for
the test is only ± 0.1 g cm"3. This is an editorial comment and
an appropriate change has been made.
5.2.5. Additional Test Procedures Suggested
Individual respondents suggested as alternatives: (1) a
microsyringe method developed by Burroughs and Goodrich (1974);
(2) Volumetric flask (Class A) (NBS 1959); (3) Simpler
techniques approved by the Association of Official Analytical
Chemists (AOAC 1975).
The first response to these suggestions is that the number
of specified acceptable test procedures in this Test Standard is
substantially larger than the number in the Guidelines (USEPA
1979), but that EPA still has not attempted to include all of the
available tests that may be acceptable. While some techniques
that might be acceptable are not included here, EPA feels that it
has included a broad range of tests that are widely accepted and
in common use. Many laboratories use the techniques, and their
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applicability, limitations, and special precautions necessary
will not present a hardship or increase the cost of testing. The
exclusions of other methods are considered to be of minor impact
in view of the large number of acceptable techniques and the long
period of time over which they have been developed.
The microsyringe technique (Burroughs and Goodrich 1974) is
not included, principally because no evidence was found that it
has been found generally useful enough to be widely adopted. It
seems most useful for rare or research chemicals that are
available only in very small amounts. This criterion does not
apply to chemicals subject to control under TSCA. The use of a
calibrated volumetric flask is included in the specified array of
acceptable techniques for the present Test Standard. The
standards of AOAC (1975) included several techniques, all of
which are also found in the specified array of techniques, though
not specifically identified as AOAC standards.
5.2.6. Temperature of the test
One respondent suggested that the temperature range
(20 ± 3)°C proposed by EPA (1979) be modified to (25 ± 3)°C in
order to include most common laboratory temperatures without the
need for a thermostated water bath or laboratory air-
conditioning. The present test standard specifies one of three
temperatures, 15°C, 20°C, or 25°C, discussed and justified in
section II.B.(l) of this document.
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6.
REFERENCES
ASTM. 1978 . American Society for Testing and Materials.
Annual book of ASTM Standards, books 1-47. Philadelphia, PA:
American Society for Testing and Materials.
AOAC. 1975. Association of Official Analytical Chemists.
Official methods of analysis of the association of official
analytical chemists, 12th ed. Washington, DC: Association of
Official Analytical Chemists.
Bauer N, Lewin SZ. 1971. The determination of density. In:
Weissberger A, Rossiter BW, eds. Techniques of chemistry. New
York: Wiley-Interscience, Vol. I, Part IV, Chapter 2.
Burroughs VE, Goodrich CP- 1974. Rapid method for determining
densities of liquids using microsyringes. Anal. Chem. 46:
1614.
Enviro Control, Inc. 1980. Cost analysis methodology and
protocol estimates: environmental standards. Rockville, MD:
Enviro Control, Inc., Borriston Laboratories, Inc.
IUPA. 1975. International Union of Pure and Applied
Chemistry. Commission on Physicochemical Measurements and
Standards. Herrington EFG, ed. Recommended reference materials
for the realization of physicochemical properties. Pure Appl.
Chem. 40:393-472.
IUPAC. 1976. International Union of Pure and Applied Chemistry,
Commission on Physicochemical Measurements and Standards.
(a)Herrington EFG, ed. Recommended reference materials for the
realization of physicochemical properties. Brown J, Lane JE,
collators. Section: Density. Pure Appl. Chem. 45:1-9.
(b) Physicochemical measurements: Catalogue of reference
materials from national laboratories. Pure Appl. Chem. 48ROS-
SIS.
IUPAC. 1972. International Union of Pure and Applied Chemistry,
Commission on Thermodynamics and Thermochemistry. A guide to
procedures for the publication of thermodynamic data. Pure Appl.
Chem. 29:397-407.
*As a matter of good practice, the most recent available edition
that contains the desired Standard should be used.
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McGlashan ML, Paul MA, Whiffen DH, eds. 1979. Manual of Units,
2nd revised 1973 edition. New York: Pergamon Press. See
also: Pure Appl. Chem. 51:1-41.
NBS. 1959. National Bureau of Standards. Capacity tolerances
for volumetric flasks. National Bureau Standards Circular 602.
Washington, DC: U.S. Government Printing Office.
OECD. 1978. Organization for Economic Cooperation and
Development Expert Group Physical Chemistry. Test guidelines for
density. Draft October 11, Berlin, FRG: Umweltbundesamt.
Raw, GR, ed. 1970. CIPAC handbook, Ashworth RdeB, Henriet J,
Lovett JF, compilers. Volume I. Analysis of technical and
formulated pesticides. Herpender, Herts. UK. Collaborative
International Pesticides Council, Ltd. (Obtain from National
Agricultural Chemicals Association, Washington DC). Method MT-
3. pp. 830-839.
USEPA. 1978. U.S. Environmental Protection Agency. Proposed
guidelines for registering pesticides in the United States. Fed.
Regist., July 10, 1978, 43 29695-29741.
USEPA. 1979. U.S. Environmental Protection Agency. Toxic
substances control. Discussion of premanufacture testing Policy
and technical issues; request for comment, Fed. Regist. March 16,
1979, 44 16240-16292.
Weast RF, ed. 1978-79. Handbook of chemistry and physics, 59th
edition. Cincinnati, OH: CRC Publishing Co.
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MELTING TEMPERATURES
STANDARD FOR TEST DATA DEVELOPMENT
PROPOSED RULE, SECTION 4, TSCA
Refers to
Part 772 — Standards for Development of Test Data
Subpart 1. Physical, Chemical and Environmental
Persistence Characteristics
Section 772.122-2 Melting Temperatures
October 1980
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CONTENTS
Page
INTRODUCTION AND SUMMARY OF THE STANDARD 1
1. NEED FOR THE TEST 3
2. SCIENTIFIC ASPECTS 4
2.1. Melting Behavior of Materials 4
2.2. Test Material 5
2.3. Good Laboratory Practices 6
i
2.4. Test Conditions 7
2.4.1. Thermometers 7
2.4.2. Apparatus 8
2.4.3. Reference Materials 8
2.5. Test Procedures 9
2.5.1. Visual Technique 9
2.5.2. Preliminary Examination 9
2.5.3. Selection of a Standard Technique 10
2.5.4. Precise Study 12
2.6. Test Data Required 12
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CONTENTS (continued)
3 . ECONOMIC ASPECTS 14
4. TSCA/FIFRA COMPATIBILITY 15
5. DISCUSSION OF COMMENTS SUBMITTED IN RESPONSE TO
PROPOSED GUIDELINES FOR PREMANUFACTURE TESTING 17
5.1 General 17
5. 2 Specific Comments 17
6 . REFERENCES 19
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INTRODUCTION AND SUMMARY OF THE STANDARD
The Support Document to this test summarizes the scientific
basis of the melting temperature test and its applicability in
the assessments of chemicals.
The proposed test standard, when applied through a testing
rule will require sponsors to determine melting temperature of
the test chemical, or the range of temperature over which it
melts, within the range from -30°C to +250°C. The standard is
written in a general way to permit selection of a technique
appropriate to the particular chemical from a variety of
available techniques. The techniques proposed are limited to
visual techniques in which the actual melting process is
observed.
A preliminary examination of the chemical is required in
order to provide information for the selection of an appropriate
technique. Criteria include: (1) decomposition on melting;
(2) volatility; (3) reactivity with air or with possible
apparatus material; (4) lack of sharpness of melting; and
(5) melting outside the range specified.
A variety of standardized techniques are stipulated for the
test, but at the option of the sponsor techniques of greater
accuracy than is required by the Agency are allowed. Under this
standard, persons subject to TSCA physical properties test rules
would develop and submit to the Agency the following kinds of
information.
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(1) Information and results obtained from the test,
including a numerical value of the melting temperature;
range of melting temperatures, or pour point; and a
statement of special characteristics of the substances
that influenced the selection of technique such as
volatility, chemical decomposition, or range of
softening temperature;
(2) Information relating to the technique used;
justification for the technique selection, if a
technique was used that is not specified in the
standard; and
(3) Information relating to good laboratory practices, such
as nature and purity of the test substance,
qualifications of personnel conducting the measurement
and of the laboratory in which the test was made, and
quality control practice in the laboratory.
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1. NEED FOR THE TEST
The normal melting temperature describes quantitatively the
conditions under which a chemical substance or mixture will be
solid or fluid under ambient pressure. It therefore provides
information on the physical form in which the chemical will be
found when in the free state.
Information on the physical form of a chemical in the
environment, as solid or liquid, is important in order to assess
the movement and fate of the chemical and the human and
environmental exposure to it. Physical form determines whether
the chemical will stand in place or flow away from spills. When
combined with density information and solubility information it
determines how the chemical will move in a waterway. When
combined with other information such as volatility and particle
size, it determines the distribution resulting from dispersal of
the material. All of this information, when assessed together,
is used to determine what human populations, classes of
organisms, or sectors of the environment will be affected.
Knowledge of physical form also is of value in assessing the
mode of interaction and speed of interaction at interfaces, which
is strongly dependent on whether a substance is solid or
liquid. A liquid, for instance may wet the skin and cause an
inflammation or absorption much more rapidly than a solid.
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2. SCIENTIFIC ASPECTS
2.1. Melting Behavior of Materials
Melting temperature is the temperature at which the change
from solid to liquid takes place. The term "melting point" is
often used instead.
The melting temperature of a material is affected by its
composition. Impurities added to a pure substance cause a
lowering of the melting temperature if the impurities are soluble
in the liquid and if the mixed substances segregate on
solidifying. If the mixed substances form a liquid solution and
do not segregate on solidifying, the melting temperature will
usually be a weighted average of the melting temperatures of the
substances in the mixture.
Other materials that form glasses have special melting
behavior because of complex intermolecular forces, and the
distinction between solid and liquid is not clear cut.
Thus, in practice, many materials, particularly those that
are mixtures or form glasses, do not melt sharply at a well
defined temperature, but rather soften and liquefy over a range
of temperatures. Under some circumstances the melting
temperature is lowered many kelvins (degrees Celsius) by
impurities. For these materials other terms are used, including
pour point, congealing temperature, drop melting temperature,
initial and final melting temeprature, or melting range. Such
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parameters are included in this test standard and are defined in
the standard.
Definitions related to normal melting temperature are given
to allow for more precision of statement in describing the
results of tests on impure materials and materials that do hot
have sharp melting temperatures. They also are useful in
describing tests that inherently do not strictly determine the
true melting temperature, but only an approximation to it. These
terms are in common practice in test laboratories and test
standards.
The term "normal" is often used to specify that the phase
change occurs at 1 atm pressure. Because the effect of pressure
changes is rather small, we do not emphasize the use of this
term. The term "melting temperature" is used instead of "melting
point" to specify that it is the temperature rather than pressure
or volume that is of interest, and to emphasize the fact that
"point" refers to a "point" on a phase diagram (i.e., an
invariant), while the melting temperature is a variable depending
on pressure and composition.
2.2. Test Material
The type or grade of material to be tested is not specified
in this Standard, but will be specified in Test Rules that
incorporate the Standard. The following discussion relates to
the choice of material to be tested.
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The normal melting temperature of a pure material is one
means of characterizing the substance and usually is considered a
worthwhile standard reference datum.
The technical grade of substance is often a mixture of the
substance with other substances that are not easily separated and
do not interfere with the technological applications of the
material. Because of the phase behavior of materials briefly
outlined in earlier parts of this Standard, the technical grade
material will usually have a melting temperature different from
that of the pure substance and melt over a range of temperature,
rather than sharply at a single temperature. These effects may
amount to several kelvins, and may not be the same from one batch
of technical material to the next. The technical grade is the
material that will normally be found in commerce, and the
temperature differences that will be encountered are within the
range of fluctuations that occur periodically in the
environment.
2.3. Good Laboratory Practices
Laboratories that do not have proper quality control
sometimes produce data that are less than adequate for supporting
sound analyses and decisions. Good Laboratory Practice
Standards, cited in section (c) of the test standard, are
designed to ensure that all data generated in response to Test
Rules will be valid. For further discussion of the details
contained in Good Laboratory Practice Standards see the Preamble
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for Good Laboratory Practice Standards for Physical, Chemical,
Persistence, and Ecological Effects Testing (Section 772.110-2)
proposed in the same issue of the Federal Register in which this
proposed Standard for density is proposed.
2.4. Test Conditions
2.4.1. Thermometers
A good thermometer is the most important item of equipment
needed for measuring melting temperature. Specifications for all
the thermometers needed for the techniques listed are given by
ASTM.
Thermometers frequently are inaccurate because of errors in
manufacture or accidents which damage them, such as by separation
of the mercury or fluid column. They, therefore, invariably need
to be calibrated if they are to be accurate indicators of
temperature.
The U.S. temperature scale is maintained at the National
Bureau of Standards, and measurement accuracy is propagated
throughout the nation by calibrations of thermometers originating
at the National Bureau of Standards. Traceability to the
National Bureau of Standards may be by comparison of thermometers
of which one was calibrated at the National Bureau of Standards,
or by measurement of the melting temperature of a substance
certified for melting temperature at the National Bureau of
Standards, or by a longer sequence of comparisions. The
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comparison can be done by the manufacturer or by the laboratory
doing the testing. If this traceability is not available for the
thermometer being used, there is no way to verify readings that
are taken during the course of testing.
2.4.2. Apparatus
See the discussion of test procedures (section 2.5. of this
document) for a description of how specific test procedures were
selected and adopted from among standardized methods. Published
protocols for the standardized methods generally include
descriptions of specific pieces of equipment or apparatus and
accessories required for the test. The equipment, as specified
in the protocol, is considered to be an integral part of the
standardized test, and therefore is required by EPA as one of the
conditions of this Standard.
2.4.3. Reference Materials
See the discussion of the use of reference materials for the
calibration of thermometers in a previous paragraph in this same
section (2.4.) of this support document. In addition, the
reference materials should be available and should be used for
the calibration of the total apparatus and procedure used. Some
of the standardized protocols adopted in this standard
specifically require a calibration step with a standard reference
material. Several recommended materials are listed in the
standard, and should be selected by the analyst according to the
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39
method used and the type of material being tested. The standard
also allows for the selection of other materials as the analyst
may judge necessary.
2.5. Test Procedures
2.5.1. Visual Technique^
The criterion of visual evidence of melting is necessary to
establish that the transition reported is actually that between
solid and liquid. This is because of the pronounced difference
in the behavior of solids and liquids in dispersion, in rate of
reaction, and in kind of surface interaction with other
materials.
The list of acceptable techniques could be expanded by
including nonvisual techniques such as differential scanning
calorimetry (DSC). This would be permissible only after evidence
was obtained in preliminary visual examination that the melting
occurred in a general temperature range within which a single
DSC-observed transition could be indentified. This seemed to
constitute a more complex test than those adopted and was thus
rejected.
2.5.2. Preliminary Examination
The purpose of the preliminary examination required by this
Test Standard (Section (e)(2)) is to identify special features of
the test material that would invalidate a test of melting
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temperatures and to guide the selection of a suitable
technique. The following section describes how the information
from the preliminary examination is applied in the selection of
one of the standard techniques referenced in the Test Standard.
2.5.3. Selection of a Standard Technique
Section (e)(3) and Table 1 of the Test Standard require the
use of certain standardized protocols to be selected according to
the information determined from the preliminary examination.
In selecting the many standardized techniques listed in
Table 1, EPA reviewed many published protocols, and determined
that those selected were capable of producing good and reliable
data for the broad range of substances that are expected to be
tested under the provisions of TSCA. These protocols are
believed to be widely used for the routine determination of
melting temperature, and therefore provide economical and
suitable means of producing the data that EPA requires for
evaluation of environmental exposure.
Paragraphs (e)(3)(i)-(iv) of the Standard specify which type
of technique(s) (e.g., Thiele tube, cooling curve, pour point,
etc.) is(are) to be used according to the type of melting
characteristics which different types of materials have. These
specifications were based on a detailed review of the protocols
and their stated or EPA's judgment of their applicability. For
material for which the standard gives several types of suitable
techniques, the analyst is free to select the most applicable
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type, and likewise, where Table 1 lists several suitable
protocols for a type of test, the analyst may select the most
convenient or suitable standard method to follow.
The following several paragraphs give additional detail as
to how the stipulated techniques in sections (e)(3)(i)-(iv)
relate to the melting characteristics of the test materials.
Softening temperatures are best determined by techniques not
suitable for materials with sharp melting temperatures. If
decomposition or other chemical reaction occurs on melting, the
temperature observed depends strongly on the rate of heating.
This is because the properties of products of the reaction affect
the results. Therefore, the technique must take account of this
possibility- Vaporization might cause disappearance of the
sample and excessively volatile substances should be enclosed for
the test.
Techniques designed for substances that melt sharply are
unsuitable for substances that have an extended softening
interval. This is because the techniques will show a broad range
of melting temperature or will give results which have
considerable uncertainty. Techniques that rely on physical
appearance or movement under rigorously standardized conditions
narrow the range of uncertainty for such substances.
For materials that decompose on melting, a technique needs
to be used in which decomposition does not influence the
results. For example, the Kofler hot bench technique effectively
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divides a lot of material into small sublets, each of which is
exposed to a different temperature and is separately
observable. Thus, the decomposition at one temperature does not
unduly influence the state of an adjacent substance which is at a
slightly lower temperature. Volatilization, reaction with air,
or other reactions tend to invalidate the test; hence, this
limits the kind of measurement technique that can be used.
2.5.4. Precise Study
A well established research and development facility or a
quality control laboratory may very accurately determine the
melting temperature using procedures referenced in section (e)(4)
of the Standard. The results of these more accurate tests
certainly satisfy the Agency's needs, and therefore are
acceptable in lieu of the standardized tests specified in section
2.6. Test Data Required
No melting temperature measurement is required for
substances that sublime or decompose at normal pressure because
the melting process does not occur under real world conditions.
Information about any nonspecified test procedure is needed
to determine that acceptable data have been generated and that
the testing laboratory had good reason for using a nonstandard
method .
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The level of accuracy required in reporting is 1 to 2°C. In
general this level of accuracy is inherent in the technique, is
easy to determine, and is approximately the accuracy with which
ambient temperatures are known. This point is discussed further
in section 5.2.(2).
Sections (f)(4)-(ll) specify the observable end points which
are to be reported for the different types of testing
techniques. These reportable end points have been specified in
the standard to assure that data from the different techniques
are reported in similar manners that are suitable for use in
EPA's data storage systems, are suitable for comparison from
chemical to chemical, and are correctly reported for checking the
performance of the method when used with standard reference
materials.
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3. ECONOMIC ASPECTS
A survey of three commercial laboratories to estimate costs
for performing the tests outlined in this Standard found a range
of $25 to $75, with a "best estimate" of $50. A cost estimate
also was made by separating the protocol into components and
estimating the cost of each component, including direct labor
cost, overhead cost, other direct costs, general and
administrative costs, and profit or fee. The protocol best
estimate of cost was $83, with an estimated range of $42 to $125
based on ± 50 percent of the best estimate. An analysis of the
discrepancy between the protocol estimate and the survey pointed
out that the nature of the chemical to be analyzed has an impact
on the cost of analysis. The actual melting temperature of the
compound has an influence on cost. For example, a compound
melting at 250°C will take longer to analyze than one with a
lower melting point. Separate methods also are required for
compounds that decompose or melt, compounds that show extended
softening, and compounds that react or volatilize. Differences
in the methods will cause variations in the cost.
The above cost estimates were made assuming that all the
requirements of Good Laboratory Practice Standards, as specified
in section (c) of the melting temperature standard, are being
satisfied. Details of the cost estimate are in a report by
Enviro Control, Inc. (1980).
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4. TSCA/FIFRA/IRLG/OECD COMPATIBILITY
In Pesticides Guidelines (USEPA 1978) Section 163.61 8(c)(3)
a melting temperature or melting range in °C is required for a
solid technical chemical unless the technical chemical sublimes
or decomposes. This criterion is consistent with the present
Test Standard.
In Pesticides Guidelines (USEPA 1978) Section 163.61,
Appendix to Product Chemistry calls for CIPAC test MT 2. This is
one of the standard tests using the Thiele tube to which we
refer. Any data acceptable under Pesticide Guildlines would
therefore be acceptable under this Test Standard, provided that
the test substance was appropriate and did not require use of one
of the specialized methods specified in Section (e)(3)(i), (ii),
or (iii) of the Standard.
There is a difference in the scope of requirements for FIFRA
and TSCA in that the FIFRA applies solely to pesticides, which
are a more uniform chemical group than those to which TSCA
applies. The CIPAC test is an adequate test for many pesticides;
it is not adequate for the whole range of chemicals to which TSCA
applies. Therefore, this Test Standard appropriately includes a
broader range of tests than Pesticide Guidelines because the
greater range of chemical types requires a greater range of
techniques.
The OECD (1979) has issued a Draft Test Guideline for
Melting Point/Melting Range. This OECD draft guideline includes
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a variety of techniques, allowing for making the determination on
materials with various melting characteristics, and in this way
it is comparable to the proposed TSCA standard. The OECD draft
is considerably longer and gives many experimental details and
other parameters that are included in the proposed TSCA standard
only by reference to ASTM or other standards. A technique-tay-
technique comparison shows that the OECD guideline and the EPA
proposed standard have scopes which seem to differ principally in
the inclusion in the EPA standard of provision for materials that
soften and melt only over an extended range, and giving a drop
melting temperature or pour point. The proposed TSCA Standard
does not discuss automated methods, whereas the OECD standard
discusses the applicability of automation to several, and
includes one (photocell detection) that is only offered as an
automated technique. From EPA's point of view, automation is a
peripheral factor because TSCA tests fundamentally are not
production-line processes. The EPA standard outlines to a much
greater degree how the testing of a particular substance is to be
approached but avoids many of the details found in the OECD
standard.
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5. DISCUSSION OF COMMENTS SUBMITTED IN RESPONSE TO
PROPOSED GUIDELINES FOR PREMANUFACTURE TESTING
(USEPA 1979).
5.1. General
Following publication of EPA's Proposed Guidelines for
Premanufacture Testing, the Agency reconsidered the variety of
chemical materials requiring testing under Section 4 of TSCA and
the availability of standardized procedures for determining
melting temperature. EPA concluded that the techniques
originally proposed were inadequate to cover the multiplicity of
identifiable situations that would arise. As a result, this
proposed Test Standard gives considerably more detail with
respect to the special characteristics of materials that
influence the selection of a technique, and it specifies
appropriate techniques for handling materials with these
characteristics.
5.2. Specific Comments
In response to the Proposed Guidelines (USEPA 1979) comments
referring specifically to melting temperature were received from
10 responders. Their comments may be grouped in five categories:
(1) General concurrence. Three responders expressed
general approval of the tests and the reference to
standardized procedures such as those of ASTM.
(2) Precise measurements are not needed. We concur.
However, we do not believe the requirement of accuracy
can be relaxed so far as to allow a range of 12°C as
suggested by one responder. While environmental
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temperatures sometimes fluctuate daily by this amount
and seasonal changes are even larger, the physical
state of the chemical will also fluctuate if it
happens to have a melting temperature within the daily
or seasonal fluctuation range. Thus, for a reasonably
good assessment of the state and manner of movement of
the chemical in a -particular environment the
temperature of melting should be known with
approximately the same certainty as the temperature of
the environment is known, 1 to 2°C.
(3) Measurement is redundant and should be omitted. Three
responders made this argument on the basis that
melting and boiling temperature were lumped together
for discussion in the Proposed Guidelines. They
argued that vapor pressure measurement gives the
essential information contained in a boiling
temperature measurement. An examination of the
argument reveals that it must have been directed
toward the boiling temperature alone and is invalid
when applied to the melting temperature. The boiling
temperature is a rapidly varying function of pressure,
while melting temperature is a slowly varying function
and futhermore, melting temperature cannot be derived
from the vapor pressure alone.
(4) Measurement as described is inapplicable to
polymers. We concur with the two responders making
the point that the Proposed Guideline (USEPA 1979) was
inadequate in this respect. We believe the present
proposed Test Standard with additional protocols is
versatile enough to provide for the necessary
softening or melting temperatures that are useful for
characterizing polymers.
(5) Refer to specific test techniques. One responder
indicated that their laboratory regularly used
differential thermal analysis (d.t.a.) for this test
and that a test costs $2500. The d.t.a. method was in
the Proposed Guidelines (USEPA 1979), but d.t.a. has
been omitted from the current Test Standard because of
excessive cost. The fact that the responder quotes a
cost of $2500, compared to costs for traditional
melting temperature tests in the range of less than
$125 (Section III) supports our contention that the
d.t.a./d.s.c. technique is more elaborate and costly
than the techniques specified. At the sponsor's
option, however, the d.t.a./d.s.c. techniques still
could be used under provisions of section (e)(4).
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6. REFERENCES
Enviro Control, Inc. 1980. Cost analysis methodology and
protocol estimates: environmental standards. Rockville, MD.
Enviro Control, Inc., Borriston Laboratories, Inc. April 28.
OECD. 1979. Organization for Economic Cooperation and
Development. Draft test guideline for melting point/melting
range. Berlin: Umweltbundesamt. June 20.
USEPA. 1979. U.S. Environmental Protection Agency. 1979. Toxic
substances control. Discussion of premanufacture testing policy
and technical issues; request for comment. Fed. Regist., March
16, 1979, 44 16240-16292.
USEPA. 1978. U.S. Environmental Protection Agency. Proposed
guidelines for registering pesticides in the United States. Fed.
Regist., 1978, 43 29696 - 29713.
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VAPOR PRESSURE
STANDARD FOR TEST DATA DEVELOPMENT
PROPOSED RULE, SECTION 4, TSCA
Refers to
Part 772 — Standards for Development of Test Data
Siibpart L. Physical, Chemical and Environmental
Persistence Characteristics
Section 772.122-3 Vapor Pressure
October 1980
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CONTENTS
PAGE
1. Need for the Test 1
2. Scientific Aspects 5
2.1. Test methods 5
2.2. Test procedures 9
2.2.1. Temperature of the test 9
2.2.2. Sorbent for gas saturation procedure 10
2.2.3. Gas flow rates in gas saturation procedure 11
2.2.4. Calculations of vapor pressure in gas
saturation procedure 11
2.3. Test data required 12
3. Economic Aspects 14
4. Replies to comments on March 16, 1979 document 16
5. TSCA/FIFRA Compatibility 20
6. TSCA/OECD Compatibility 21
7. References 22
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1. NEED FOR THE TEST
The vapor pressure of a chemical is an important parameter
in determining the environmental fate of the chemical.
The atmosphere is a major route for the widespread distribu-
tion of chemicals. There are several ways by which chemicals may
become airborne and subsequently be transported by wind currents.
Airborne solids and foamy emulsions are commonly observed, but
these are not believed to be major factors in atmospheric trans-
port because they involve particulate matter which settles out in
a relatively short time (Seiber et al. 1975). Aerosols, from
spray applications, manufacturing and formulation sites, and
aerated waste treatment systems may constitute more important
sources of chemical for air transport since very small droplets
(5 micrometers or less in diameter) may be formed and carried
considerable distances (Edwards 1973). However, it appears that
volatilization from land and water surfaces is the most important
source of material for airborne transport (Hartley 1969, Lich-
tenstein 1971, MacKay and Wolkoff 1973, Seiber et al. 1975).
Volatilization is the evaporative loss of a chemical com-
pound. Volatilization rates are dependent on the vapor pressure
of the chemical and the environmental factors which influence
diffusion from the evaporative surface. Harper et al. (1976, p.
236) noted that "volatilization is probably the single largest
means by which pesticides are lost and transported over wide
areas and into bodies of water far from the application
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location." The airborne vapors of a hazardous chemical may
present a threat to plant and animal life exposed to those
vapors, not only in the area of chemical release but also at
sites remote from the volatilization site. This occurs when
vapors are removed from the air, primarily by precipitation with
rain or snow.
Volatilization rates are related to vapor pressure, which
varies with temperature. However, volatilization from soil or
water is influenced by other environmental conditions and the
effective vapor pressure may be considerably lower than the
potential vapor pressure. Nevertheless, vapor pressure is the
one common factor governing the tendency of a compound to
volatilize.
According to kinetic theory there is a continuous flight of
molecules from the surface of a liquid or solid into the free
space above it. At the same time vapor molecules return to the
surface at a rate depending on the concentration of the vapor.
If there is no removal of vapor from the surface (for example, by
air currents), equilibrium will be established where the rate of
vaporization is exactly equal to the rate of condensation. The
pressure exerted by the equilibrium vapor is known as the vapor
pressure (Daniels and Alberty 1955) and is dependent upon
temperature.
Knowledge of the vapor pressure of a compound allows the
ranking of a chemical as relatively nonvolatile, highly volatile,
or of some intermediate volatility- When vapor pressure data are
combined with solubility data to calculate Henry's Law constants,
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as described by MacKay and Leinonen (1975) and Dilling (1977),
rates of the evaporation of dissolved chemicals from water can be
estimated.
Evaporation from an exposed surface will depend upon other
factors such as wind speed (which reduces the vapor density above
the surface) and adsorption (which may act to hold the substance
on the surface). Volatilization from aqueous systems depends
also on the solubility of the compound and its movement to the
water surface. In soils the rate of volatilization of a chemical
will depend upon such factors as adsorption on soil, solubility
in soil water, and on the amount of soil water and its rate of
evaporation. Volatilization from soils can become a diffusion
controlled process as mass transfer to the soil surface is
reduced by low water evaporation due to high humidity or to the
lack of soil water in a dry soil (Bailey et al. 1974).
Chemicals that have relatively low vapor pressures and that
sorb readily to solids or dissolve readily in water are not
likely to vaporize significantly at ambient temperatures. For
that reason, airborne transport is not a major transport mecha-
nism for these chemicals and assessment of them should be focused
on their chemical fate and environmental effects in soils,
sediments, and water. However, chemicals with high vapor pres-
sures or with relatively low water solubility and low adsorptiv-
ity to solids are less likely to reside only in soils, sediments
or water, since volatilization can be a potentially significant
factor in their environmental transport. Chemicals that are
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58
gases at ambient temperatures and that have low water solubility
and low adsorptive tendencies will be transported to a
significant degree in the atmosphere and are prime candidates for
photolysis and for involvement in adverse atmospheric effects
such as smog formation or stratospheric alterations. Further-
more, effects testing of those chemicals should also focus on
inhalation and surface contact as potential routes for direct
exposure.
An understanding of how a chemical is likely to partition
among the various environmental media (air, water, soil, and
sediment) is needed in judging whether or not a chemical will be
subject to various transformation possibilities, such as
oxidation by hydroxyl ion or ozone in the atmosphere. Vapor
pressure data can influence decisions on whether or not it is
appropriate to conduct photolysis, adsorption/desorption,
partition coefficient, and certain biodegradation tests. Vapor
pressure data are an important consideration in the design of
other fate and effects tests, for example in preventing or
accounting for the loss of volatile materials during the course
of the test. Clearly, a knowledge of vapor pressure combined
with information on water solubility and adsorptive tendencies is
necessary in predicting environmental transport and in providing
guidance as to which persistence and effects tests need to be
considered and how those tests should be designed.
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2. SCIENTIFIC ASPECTS
2.1. -Test Methods
A procedure for measuring the vapor pressure of materials
released to the natural environment ideally would cover a range
of vapor pressure values, at ambient temperatures, of about 10~5
Pa to 10 Pa (approximately 10~7 to 760 Torr). Because no single
procedure can cover this range, two different procedures are
described, each suited for a different part of the range. The
isoteniscope procedure (ASTM 1978) is for pure liquids with vapor
pressures from 0.1 to 100k Pa. For vapor pressures of 10 to
103 Pa, a gas saturation procedure is to be used. The Knudsen
effusion procedure (Thomson and Douslin 1971) may be used for low
vapor pressure values.
Each of the tests must be performed under conditions of
normal laboratory room temperatures in order to allow for careful
control of the temperatures in thermostated baths or chambers
containing the test apparatus.
The isoteniscope procedure uses a standardized technique
that was developed to measure the vapor pressure of certain
liquid hydrocarbons. It is applicable to pure liquids with vapor
pressures of 0.1 kPa (0.75 Torr) or more at ambient temper-
atures. The sample is purified within the equipment by removing
dissolved and adsorbed gases until the measured vapor pressure is
constant. This process is called "degassing." The procedures do
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not remove higher boiling impurities, decomposition products, or
compounds that boil close to or form azeotropes with the material
under test. If compounds that boil close to or form azeotropes
with the test material are present, it is necessary to remove the
interfering compounds and use pure test material. Impurities
more volatile than the sample will tend to increase the observed
vapor pressure above its true value but the purification steps
will tend to remove these impurities. Soluble, nonvolatile
impurities will decrease the apparent vapor pressure. However,
because the isoteniscope procedure is a static, fixed-volume
method in which an insignificant fraction of the liquid sample is
vaporized, it is subject to only slight error for samples
containing nonvolatile impurities. That is, the nonvolatile
impurities will not be concentrated due to vaporization of the
sample.
Gas saturation (or transpiration) procedures use a current
of inert gas passed through or over the test material slowly
enough to ensure saturation and subsequent analysis of either the
loss of material or the amount (and sometimes kind) of vapor
generated (Bellar and Lichtenberg 1974, Thomson and Douslin
1971).
The gas saturation procedures have been described by Spencer
and Cliath (1969). Results are easy to obtain and can.be quite
precise. The same procedures also can be used to study
volatilization from laboratory scale environmental simulations.
Vapor pressure is computed on the assumption that the total
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pressure of a mixture of gases is equal to the sum of the
pressures of the separate or component gases. The partial
pressure of the vapor under study can be calculated from the
total gas volume and the weight of the material vaporized. If v
is the volume which contains w grams of the vaporized material
having a molecular weight M, and if p is the pressure of the
vapor in equilibrium at temperature T (K), then the vapor
pressure, p, of the sample is calculated by
p = (w/M)(RT/v)
where R is the gas constant (8.31 Pa m mol K ) when the
pressure is in pascals (Pa) and the volume in cubic meters. As
noted by Spencer and Cliath (1970), direct vapor pressure
measurements by gas saturation techniques are more directly
related to the volatilization of chemicals than are other
techniques.
In an effort to improve upon the procedures described by
Spencer and Cliath and to determine the applicability of the gas
saturation method to a wide variety of chemical types and
structures, EPA is sponsoring research and development work at
SRI International (EPA Contract No. 68-01-5117). The procedures
described in Appendix A are those developed under that contract
and currently being evaluated with a wide variety of chemicals of
differing structure and vapor pressures.
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In addition to the above methods, other procedures have been
described for the measurement of vapor pressure (Daniels et al.
1956, Glasstone 1946, Nesmeyanou 1963, Thomson and Douslin
1971). These include boiling point procedures, effusion
techniques, and many highly specialized techniques that are
restricted to the determination of very precise vapor pressure
values or to the measurement of vapor pressures of specific kinds
of materials. These highly specialized methods were rejected by
EPA because they do not have general applicability to either a
wide variety of chemicals or a relatively broad range of vapor
pressure values at ambient temperatures.
EPA rejected boiling point procedures, such as that using
Ramsey and Young apparatus, because the accuracy is very poor
below 10 Pa (Thomson and Douslin 1971) and because boiling point
methods provide inaccurate estimates of the vapor pressures at
ambient temperatures if there is a change of state or a
transition temperature between the boiling temperature and
ambient temperatures.
Effusion techniques, particularly those employing the
Knudsen effusion apparatus, are used to measure vapor pressures
from about 10~5 to 1 Pa and have provided some good data (Hamaker
and Kerlinger 1969). Those procedures were rejected because they
require working with systems under vacuum and because it is
necessary to saturate the capsule space with vapor during the
measurement periods. The lack of equilibrium saturation has been
postulated as a reason for inaccurate published vapor pressures
data (Spencer and Cliath 1970). However, it must be recognized
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that there are laboratories which have employed Knudsen effusion
techniques successfully and which have considerable experience
with the method, especially for determining very low vapor
pressure values, such as 10~5 to 10~3 Pa. For such laboratories,
the Knudsen effusion methods are a satisfactory alternative to
the gas saturation method in the determination of low vapor
pressure values. However, it seems reasonable to require that
the laboratory using effusion methods supply documentation to
substantiate successful utilization of the effusion procedures
with other compounds.
2.2. Test Procedures
2.2.1. Temperature
The test procedures generally require a thermostated bath or
test chamber temperature of 25 ± 0.5° C. Laboratories should be
able to carry out vapor pressure measurements without the need
for elaborate temperature control devices. Control of the bath
or chamber to ±0.5°C will permit substantial confidence in the
data without requiring unnecessarily costly apparatus.
The International Union of Pure and Applied Chemistry has
for many years (IUPAC 1972) recommended the reporting of
physical-chemical properties measurements at the temperature of
25°C. A temperature of 25°C is slightly above most laboratory
room temperatures and this allows for convenient adjustment and
maintenance of constant temperature baths and enclosures.
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Because of the nature of the isoteniscope procedure, it is
necessary in that test to conduct some measurements at
temperatures above and/or below 25°C in order to determine
whether the sample needs further degassing. Also, for some
chemicals, it may be necessary to require vapor pressure data at
temperatures other than 25°C and, in such cases, this requirement
will be noted in Test Rules for specific chemicals. Examples of
when this requirement may be applicable include situations where
there is evidence that the vapor pressure may change signi-
ficantly with relatively small changes in ambient temperature or
when the boiling temperature for a chemical is at an ambient
temperature below 25°C.
2.2.2. Sorbent for Gas Saturation Procedure
The choice of sorbent and desorption solvent are dictated by
the nature of the compound being evaluated. Charcoal sorbent is
inexpensive and may be desorbed with carbon disulfide, a conven-
ient solvent for use with flame ionization detector. Many com-
pounds, however, do not desorb efficiently from charcoal and more
expensive sorbents, such as Tenax GC and XAD-2, must be used.
The desorption efficiency of a particular compound from a sorbent
with a solvent is defined as the weight of the compound which can
be recovered from the sorbent divided by the weight of the com-
pound originally adsorbed. It must be measured for every combi-
nation of sample, sorbent, and solvent used. Desorption
efficiency may vary with concentration, so it is important to
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65
measure it at or near the concentration of the actual sample. It
is sometimes necessary to interpolate between two measured
efficiencies.
2-2.3. Gas Flow Rates in Gas Saturation Procedure
Accurate control of gas flow rates is essential to assure
that a known volume of carrier gas is passed through the system.
Very long sampling times are required for compounds with low
vapor pressures, and it is difficult to control very low flow
rates for very long times. It is necessary to use fine needle
valves to control the flow rates and to measure the flow rates
frequently during the test period in order to make corrections
for variation which can occur, e.g. due to changes in atmospheric
pressure.
2.2.4. Calculations of Vapor Pressure in the Gas Saturation
Procedure
The calculation of vapor pressure is straightforward. The
weight of the sample desorbed from a sorbent section is divided
by the desorption efficiency to give the weight of the sample
collected by the sorbent trap. With the volume of carrier gas
calculated from the flow rate, the ideal gas law is used to
calculate the vapor pressure of the sample. To assure that the
carrier gas is indeed saturated with the compound vapor, each
compound is sampled at three different gas flow rates. If the
vapor pressure calculated shows no dependence on flow rate, then
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the gas is assumed to be saturated. The method also assumes that
there are no interactions between vaporized sample and the
carrier gas and that the molecular weight of the vaporized sample
is the same as for the sample liquid or solid. If there are any
indications that these may not be valid assumptions, the vapor
should be analyzed both qualitatively and quantitatively using
such techniques as gas chromatography combined with mass
spectrometry (Heller et al. 1975).
2.3. Test Data Required
The average calculated vapor pressure for the test material
at each required test temperature must be reported. The reported
data must also include the individual values from triplicate
determinations and the calculated standard deviation for each
average calculated vapor pressure. It might be preferable for
assessment purposes to require that each vapor pressure
determination be made in sufficient replication to provide a
given degree of reproducibility. However, the precision
attainable will vary not only with the number of replications but
also with the procedure employed and the test chemical. For a
given chemical, the only way to determine how many replications
of a given procedure are necessary to provide vapor pressure data
with some specified precision is to repeat the procedure until
the data provide that precision. This may take a few or many
replications and a requirement for numerous replications is not
justified unless the specified precision is needed for assessment
purposes with an individual chemical. The minimum requirement
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EPA would impose would be a statistical analysis of vapor
pressure data to provide standard deviation calculations based on
triplicate determinations.
For the isoteniscope method, the vapor pressure data
generated during the degassing operation, including a plot of log
p vs 1/T, must be included to provide evidence of successful
degassing. For the gas saturation method, the data showing that
vapor pressure does not vary with flow rate must be included to
provide evidence of saturation of the carrier gas with the sample
vapor. The data also must include a complete description of all
analytical techniques and results, a description of the sorbents
and desorption solvents used and the desorption efficiency
calculations.
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3. ECONOMIC ASPECTS
A survey of commercial laboratories to estimate costs for
performing the tests outlined in this Standard predicted a range
of $64 to $193 per substance for the isoteniscope method and $185
to $554 for the gas saturation method. The "best estimate"
costs, based on the survey, were $129 for the isoteniscope method
and $369 for the gas saturation method. The cost estimates were
made assuming that all the requirements of Good Laboratory
Practice Standards (GLPs), as specified in section (c) of the
Test Standard, are being satisfied. Details of the cost
estimates are contained in a report by Enviro Control, Inc.
(1980).
The cost estimates in the Enviro Control report do not
include the costs associated with specialized analytical
methodology or for analytical method development that may be
required for vapor pressure analysis by the gas saturation
method. It is not possible to estimate the cost of such special
analytical requirements since the costs are related to the nature
of the chemical to be analyzed and because such cost may be
prorated among the analytical needs related to other Test
Standards. Also, the nature of the chemical to be tested for
vapor pressure may cause drastic variations in price when, for
example, very long sampling times are required in the gas
saturation method for substances with very low vapor pressures.
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In reviewing the state of technology concerning this area of
testing, the Agency has not identified any adequately developed
tests that would be significantly less costly while providing
comparable information.
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4, REPLIES TO FEDERAL REGISTER OF MARCH 16
The Fedegal- Register of March 16, 1979, (EPA 1979) presented
a discussion of policy issues, alternative approaches, and test
methods under consideration as guidance for premanufacture
testing. Included in the test methods was a section on vapor
pressure (pp. 16256-16257) which discussed the isoteniscope
procedure and the gas saturation procedure as potential methods
for use in developing information on new chemicals. Among the
comments received in response to the discussion were several
comments related to the vapor pressure methods.
Several commentors contended that a requirement for vapor
pressure measurements at each of three ambient temperatures (10,
20, and 30°C) would be unnecessarily time consuming and too
costly for the needs of a chemical fate assessment. There were
two reasons for obtaining vapor pressure data at each of three
temperatures. First, it seemed desirable to obtain vapor
pressure data at representative environmental temperatures to
determine if vapor pressure changed significantly over that range
of temperatures. Second, the determination of vapor pressure at
each of three temperatures provides a check on the validity of
the data. If the vapor pressure data are determined correctly,
then a plot of log p vs 1/T (where p = vapor pressure as Pa and T
= test temperature as K) will produce a straight line. However,
for most substances it will not be necessary to have knowledge of
the vapor pressure at each of three rather closely spaced
temperatures and a requirement for the determination of vapor
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71
pressure at each of three temperatures solely as a check on the
validity of the method would be unduly time consuming.
Therefore, the requirement has been changed to a determination of
vapor pressure at one temperature, 25°C, unless otherwise noted
in a specific Test Rule.
For some chemicals there may be indications that the vapor
pressure changes significantly over the range of environmental
temperatures or there may be evidence that there is a change of
state at an environmental temperature. Either of these
circumstances may justify a requirement for vapor pressure
measurements at temperatures in addition to 25°C. Note, also,
that the isoteniscope method requires a determination at 25°C
plus a sufficient number of other temperatures to ascertain
whether or not further degassing is needed. Although the vapor
pressure determinations at temperatures other than 25°C need not
be made in triplicate (unless they are temperatures specified in
a Test Rule), the data can serve to confirm the validity of the
vapor pressure results.
Some commentors suggested that the investigator should be
able to use any technique, even employing high temperatures for
the vapor pressure measurement and an Antoine equation nomograph
to determine the vapor pressure at ambient temperatures. The
problem with this appraoch is that there may be a change of state
or a transition temperature between the test temperature and
ambient temperatures. If this is the case, extrapolation will
yield incorrect and misleading results. The use of differential
thermal analysis and thermal evolution analysis, suggested by one
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commentor, involves variations on the melting point/boiling point
procedures for estimating vapor pressures and are subject to the
same errors described above. It may be appropriate to use such
estimation techniques to provide guidance in performing the vapor
pressures measurements but those estimated values for vapor
pressure would not be sufficiently reliable to be used in
estimating volatility using water solubility and calculating the
Henry's law constant. Such predictions of volatility based on
Henry's law constant will be an important consideration in
evaluating the environmental fate of a chemical.
It was commented that ionizable and highly soluble compounds
should be exempt from the requirements for vapor pressure data.
This would be a reasonable suggestion if it could be demonstrated
that the chemical in question would reside exclusively in the
aqueous environment upon release to the environment. If the ,
chemical can occur in non-aqueous environments, such as soils or
on solid surfaces, its vapor pressure is needed regardless of
whether or not it is ionizable or highly soluble in water.
It was stated that there should be no need to determine
vapor pressure as low as 10"^ Pa. Unfortunately, experience has
shown that compounds with very low vapor pressures can volatilize
to a significant extent when low vapor pressure is combined with
low water solubility. Examples include DDT and the
polychlorinated biphenyls (PCBs). When dealing with the wide
variety of chemical compounds which exist in the commercial
chemical inventory, it is necessary to conclude a concern for
those which have low vapor pressures and low water solubilities
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73
and which may, as a result of these factors, evaporate from water
bodies and become airborne contaminants.
One commentor suggested simply determining the weight loss
/
at 1 Torr (about 102 Pa) and 30°C in place of a determination of
vapor pressure. This simplistic sort of procedure can provide
some information on the tendency of a compound to evaporate from
a non-adsorptive surface, provided the vapor pressure is
sufficiently high to observe results in a relatively short time,
but no quantitative information is obtained which is of value in
assessing environmental fate.
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5. TSCA/FIFRA COMPATIBILITY
In the 1978 Proposed Pesticide Guidelines (EPA 1978) it is
suggested that vapor pressure be determined by ASTM Method D-
3074-72. This method measures the pressure within an aerosol can
and is related to packaging. It is not applicable to environ-
mental fate considerations. In the 1975 Proposed Guidelines (EPA
1975), it was suggested that vapor pressure should be determined
by the Knudsen effusion or the gas saturation (transpiration)
procedures, both of which are discussed in this document. The
FIFRA proposed guidelines have not contained any reference to a
vapor pressure prcedure which would be applicable to compounds
with relatively high vapor pressures, in excess, say, of 1 kPa at
environmental temperatures. This may be because the FIFRA
concern is primarily with compounds of low vapor pressure,
although there are some pesticides which do not fit that picture,
such as fumigants. Since TSCA must deal with the entire range of
chemicals, the Test Standards available must include methodology
suitable for chemicals with high vapor pressures, such as the
isoteniscope procedure.
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6. TSCA/OECD COMPATIBILITY
The Organization for Economic Cooperation and Development
(OECD) has been developing a testing plan for toxic chemicals
which will aid in assessing environmental fate and effects. The
plan, which has not yet been published, will include recommended
methodology for vapor pressure. The approach being considered by
OECD is similar to that proposed by TSCA in the Fede-r-al- Reqi-ster
discussion document of March 16, 1979, (EPA 1979). The isoteni-
scope method and the gas saturation method are recommended for
the determination of vapor pressure at 30°C. OECD also
recommends a vapor pressure balance method which seems to be
similar in principle to the Knudsen effusion method and which
measures the pulse generated on a sensitive balance by a vapor
jet in a high vacuum. This appears to be a rather complicated
procedure employing delicate apparatus. There are no literature
references to its use in the OECD draft reports. The equipment
and expertise may be readily available in Europe but it does not
seem to be a procedure in common use in the United States. For
laboratories which have the equipment and successful application
experience, the Vapor Pressure Balance method may be an
acceptable procedure for determining relatively low vapor
pressure values, but EPA would need to have more information on
the method and its use before acceptance. Finally, the OECD
suggests the use of a boiling point method which EPA believes
would not provide acceptable data, because of poor repro-
ducibility and the possibilty of gross inaccuracies, as noted
earlier.
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7. REFERENCES
ASTM. 1978. American Society for Testing and Materials Annual
book of standards, part 24. Philadelphia, PA. pp. 740-745.
Bailey GW, Swank RR, Jr., and Nicholson HP. 1974. Predicting
pesticide runoff of agricultural land: a conceptual model. J.
Environ. Qual. 3:95-102.
Balson EW. 1947. Studies in vapor pressure measurement, Part
III—An effusion manometer sensitive to 5 x 10 mm Hg: Vapor
pressure of DDT and other slightly volatile substances. Trans.
Fara. Soc. 43:54-60.
Bellar TA and Lichtenberg JJ. 1974. Determining volatile
organics at microgram-per-litre levels by gas chromatography. J.
Am. Water Works Assn., 66:739-744.
Daniels F and Alberty RA. 1955. Physical Chemistry, New York:
John Wiley and Sons, p. 157.
Daniels F, Mathews JH, Williams JW, Bender P, and Alberty RA.
1956. Experimental physical chemistry. New York: McGraw-Hill
Book Co., pp. 47-511, 370-373.
Enviro Control Inc. 1980. Cost analysis methodology and
protocol estimates: environmental standards. Rockville, MD:
Enviro Control, Inc., Borriston Laboratories, Inc.
Dilling WL. 1977. Interphase transfer processes. I.
Evaporation rates of chloromethanes, ethanes, ethylenes, pro-
panes, and propylenes from dilute aqueous solutions. Comparisons
with theoretical predictions. Env. Sci. Tech. 11:405-409.
Edwards CA. 1973. Persistent pesticides in the environment.
Cleveland: CRC Press, p. 21.
USEPA. 1975. U.S. Environmental Protection Agency, Office of
Pesticide Programs. Proposed guidelines for registering
pesticides in the United States. Fed. Regist., June 25, 1975,
40 p. 26889.
USEPA. 1978. U.S. Environmental Protection Agency, Office of
Pesticide Programs. Proposed guidelines for registering
pesticides in the United States. Fed. Regist., July 10, 1978 43
p. 29711- 29712.
USEPA. 1979. U.S. Environmental Protection Agency, Office of
Toxic Substances. Discussion of premanufacture testing policy
and technical issues, Fed. Regist. March 16, 1979 44 p. 16240-
16292.
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Glasstone S. 1946. Textbook of physical chemistry, 2nd ed. New
York: D. Van Nostrand Co., p. 446-449.
Hamaker JW and Kerlinger WO. 1969. Vapor pressure of
pesticides. Adv. Chem. Series 86:39-54.
Harper LA, White AW, Jr., Bruce RR, Thomas AW, and Leonard RA.
1976. Soil and micro climate effects of trifluralin
volatilization. J. Environ. Qual. 5:236-242.
Hartley GS. 1969. Evaporation of pesticides. Adv. Chem. Series
86:115-134.
Heller SR, McGuire JM and Budde WL. 1975. Trace organics by
GC/MS. Environ. Sci. Tech. 9:210-213.
International Union of Pure and Applied Chemistry. IUPAC,
1972. Commission on Thermodynamics and Thermochemistry- 1972. A
guide to procedures for the publication of thermodynamic data.
Pure and Appl. Chem. 29:397-407-
Lichtenstein EP. 1971. Environmental factors affecting fate of
pesticides in Degradation of Synthetic Organic Molecules on the
Biosphere. Washington, DC: National Academy of Sciences, p.192.
MacKay D and Leinonen PJ. 1975, Rate of evaporation of low-
solubility contaminants from water bodies to atmosphere.
Environ. Sci. Technol. 9:1178-1180.
MacKay D and Wolkoff AW. 1973. Rate of evaporation of low-
solubility contaminants from water bodies to atmosphere.
Environ. Sci. Technol. 7:611-614.
Seiber JM, Shafik TM and Enoa HF. 1975. Determination of
pesticides and their transformation products in water. In Haque
R and Fried VH eds. Environmental dynamics of pesticides, New
York: Plenum Press, p.18.
Spencer WF and Cliath MM. 1969. Vapor density of dieldrin J.
Agric. Food Chem. 3:664-670.
Spencer WF and Cliath MM. 1970. Vapor density and apparent
vapor pressure of lindane. J. Agric. Food Chem. 18:529-530.
Thomson GW and Douslin DR. 1971. Vapor pressure. In: _Physical
methods of chemistry. Vol. I. Part V., Weissberger A and Rossiter
BW, eds. New York: Wiley-Interscience, p. 46-89.
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OCTANOL/WATER PARTITION COEFFICIENT
STANDARD FOR TEST DATA DETOOIMENT
PROPOSED RULE, SECTION 4, TSCA
Refers to
Part 772 — Standards for Development of Test Data
Subpart L. Physical, Chemical and EnyirpnmBntal
Persistence Characteristics
Section 772.122-4 Octanol/Water Partition Coefficient
October 1980
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TABLE OF CONTENTS
1. NEED FOR THE TEST 1
2 . SCIENTIFIC ASPECTS 3
2.1. Rationale for the Use of the Octanol/Water Partition
Coefficient to Estimate Bioconcentration Potential 3
2.2. Rationale for the Selection of the Test Method 4
(2.2.1.) The Conventional Method of Determining the
Octanol/Water Partition Coefficient KOW 4
(2.2.2.) Other Experimental Methods of Determining KQW.... 5
(1) Reverse-Phase High-Pressure Liquid Chro-
matography as a Method of estimating KQW.... 5
(2) Thin-Layer Chromatography as a Method of
Estimating KQW < 7
(3) Estimation of K_,T from Water Solubility
Data ?V 7
2.3. Rationale for the Selection of the Test Conditions 9
(2.3.1.) Theory of the Distribution Law and the
Octanol/Water Partition Coefficient 9
(2.3.2.) Factors which Affect the Value of KQW 12
(1) Effect of Temperature 12
(2 ) Purity of the Solvents 13
(3) Concentration of Solute 13
(4) Equilibration Time 14
(5) Octanol/Water Volume Ratio » ... 14
(6) Chemical Analysis of the Octanol and
Water Phases 17
(7) Emulsification and Ultracentrifugation 18
(8) Equilibration Vessel 18
(9) Speciation Effects 19
(10) Presaturation of the Solvents * 20
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PAGE
2.4. Reference Compounds 20
2.5. Test Data Required 21
2.6, Statistical Analysis of the Data 23
3 . ECONOMIC ASPECTS 25
4i REPLIES TO THE COMMENTS ON THE PUBLISHED SECTION 5
TESTING GUIDELINES » . 27
5 . TSCA/FIFRA COMPARABILITY 32
6. TSCA/OECD COMPARABILITY 33
7. REFERENCES 35
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1. NEED FOR THE TEST
Bioconcentration, the accumulation of a substance in living
tissues or other organic matter as a result of net chemical
uptake from the medium (e.g., water), is a factor in determining
the movement of a chemical in the environment and the potential
effects of the chemical on biota. Hydrophobia chemicals which -
are present in the aqueous environment at subtoxic concentrations
may accumulate to toxic levels once inside organisms, presumably
through diffusion into nonpolar cell components where they
accumulate because of their greater solubility. Further movement
of the substance in living tissues may occur as a result of
ingestion of lower trophic level organisms, i.e., food chain
effects.
The octanol/water partition coefficient KQW has been shown
to be a good predictor of the tendency of chemicals to
bioconcentrate in fish (Neely et al. 1974). Since 1974, KQW has
been used as a measure of bioconcentration potential in fatty
tissues in aquatic and other living organisms. The numerical
value of the octanol/water partition coefficient is one factor to
be considered in determining whether to conduct fish biocon-
centration studies. Other factors must also be taken into
account. For example, transformation rates (e.g., rates of
biodegradation, hydrolysis, photolysis, and oxidation) must also
be considered. If a chemical transforms readily by one of these
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84
processes, the potential for bioconcentration will be reduced
significantly and fish bioconcentration studies may not be
needed.
The octanol/water partition coefficient has been introduced
by Hansch to correlate biological activity and chemical structure
(Hansch 1969; Hansch and Fujita 1964). Numerous papers have been
published by Hansch and his coworkers on this subject in the
ensuing years. A monograph has been published on the Hansch
approach (Gould 1972).
A recent publication has indicated that the sorption of
several hydrophobic pollutants on natural sediments can be
related to the octanol/water partition coefficient. Karickhoff
et al. (1979) showed that a reasonable estimate (within a factor
of two) of the sorption behavior of hydrophobic pollutants can be
made from knowledge of the particle size distribution and
associated organic content of the sediment and the octanol/water
partition coefficient.
A recent publication has described a novel method for esti-
mating the distribution of a chemical in the environment (Mackay
1979). KOW is used in this partitioning analysis. This parti-
tioning analysis will be used as a guide to ecological and health
effects testing.
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85
2. Scientific Aspects
2.1. Rationale for the Use of the Octanol/Water Partition
Coefficient to Estimate Bioconcentration Potential
Intuitively, the absorption and fat storage of xenobiotic
chemicals in living organisms seem to be related to lipophilicity
or preferential solubility in fats as compared to water. By
definition, the octanol/water partition coefficient KQW expresses
the equilibrium concentration ratio of an organic chemical
partitioned between octanol and water in dilute solution. If one
assumes that octanol simulates fats in its solubilizing effect on
organic chemicals, then KOW should be a potential measure of the
ease of storage of organic chemicals in fats. For example, a
large value of KQW indicates that an organic chemical is not very
soluble in water but soluble in octanol. Hence, this would
suggest the potential for a large storage of the organic chemical
in fats. Davies et. al. (1975) reported human pesticide
poisoning by a fat-soluble organophosphate, dichlofenthion. The
octanol/water partition coefficient KQW was found to be very high
(1.37x10^) which correlated with the high fat storage of this
chemical.
Neely et al. (1974) found a pronounced correlation between
K and the bioconcentration in trout muscle. Specifically,
^J\w
these researchers obtained a linear correlation between the log
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86
of bioconcentration and the log of calculated K for a series of
organic chemicals. Since that time, KQW has been used by
researchers as an index of bioconcentration potential in living
organisms. The Office of Pesticide Programs (OPP) [EPA 1975,
1978] has proposed, and the Organization for Economic Cooperation
and Development (OECD 1978) is considering the use of Kow as a
measure of bioconcentration potential in aquatic organisms.
2.2. Rationale for the Selection of the Test Method
2.2.1. The Conventional Method of Determining the
Octanol/Water Partition Coefficient Kow
The conventional method for determining a distribution
coefficient is carried out by distributing a chemical between two
immiscible liquids in a vessel and measuring the concentration of
the chemical in the two liquid phases after equilibration
(Glasstone 1946; Leo et al. 1971). This method can be applied to
the determination of the octanol/water partition coefficient
KQW. Numerous researchers use the conventional method of
determining KQW and have published papers using this method
(e.g., Fujita et al. 1964; Hansch and Anderson 1967; Leo et al.
1971; Chiou et al. 1977). OPP (EPA 1975, 1978) has proposed, and
OECD (1978) is considering the conventional method of determining
K_,7. Most chemical companies that determine octanol/water
Ovv
partitioning use the conventional method of determining K .
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87
Hence, the proposed test standard utilizes the conventional
method of determining the octanol/water partition coefficient
KQW. it should be noted that there is no validated standard test
method for determining KQW (e.g., an ASTM method). The proposed
method in this standard was developed from a thorough review of
the research literature on the experimental determination of Kow
and by talking to researchers who have considerable experience in
carrying out these experiments.
2.2.2. Other Experimental Methods of Determining KQW
(1) Reverse-Phase High-Pressure Liquid Chromatography
as a Method of Estimating KOW. A rapid method
based on reverse-phase high-pressure liquid Chromatography has
been developed by Veith (Veith and Morris 1978; Veith et al.
1979) to estimate the octanol/water partition coefficient of
organic chemicals. Using the solvent mixture water/methanol
(15/85 v/v) as the elutant, the log of the retention time [log
(tR)] of organic chemicals on a permanently bonded (C-18)
reverse-phase high-pressure liquid chromatographic system has
been found to be linearly related to log KQW. This relationship
has been expressed by the equation
1Q9 Kow = A J-ogftR) - B' (1)
where A and B are constants determined from the experimental data
for some organic chemicals. Using a mixture of the chemicals
benzene, bromobenzene, biphenyl, p,p -DDE [2,2-bis(p-
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88
chlorophenyl)-l,l-dichloroethylene] and 2,4,5,2',5*-
pentachlorobiphenyl, A and B were found to be 5.106 and 1.258,
respectively, with a correlation coefficient of 0.975. It must
be emphasized that this correlation is limited with respect to
being representative of the organic chemicals encountered. This
calibration mixture was selected largely on the basis of the log
KQW values reported in the literature, and the correlation is
linear over five orders of magnitude of Kow- To determine the
accuracy of this method of estimating log KQW by comparison with
data reported in the literature, Veith and coworkers measured the
retention time of 18 chemicals, and the standards and log KQW
values were calculated from the regression equation (1)- The
results indicated that log KQW can be estimated to within (22.8
± 20.0) percent when compared with the values reported in the
literature from measurements using other methods. The percent
error was calculated assuming the literature value is the correct
log KQW/ an assumption these researchers had some reservations
about. It should be noted that some of the greatest relative
errors were observed with polar chemicals which dissociate in
water (e.g., m-chlorobenzoic acid, 2,4,5-trichlorophenol, and
diphenylamine). This method has a definite advantage since the
estimation of KQW can be made rapidly and relatively easily in
comparison to the determination of K by the conventional
method. Futhermore, KQW can be estimated for individual
chemicals in complex mixtures (e.g., solid wastes) without
knowing the specific chemical structure of each chemical.
Further work is being carried out by Veith (at the Duluth EPA
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89
Laboratory), and others, to develop this method so that it may be
applicable to a large number of organic chemicals with a wide
range of different organic structures. If this research is
successful, this high-pressure liquid chromatographic method of
estimating KQW will be proposed as a test standard.
Other researchers have developed high-pressure liquid
chromatographic methods to determine KQW (Mirrless et al. 1976;
Yamana et al. 1977; Carlson et al. 1975; Hulshoff and Perrin
1976; McCall 1975). However, these methods are based on a very
limited number of experiments and considerably more work is
needed to develop them.
(2) Thin-Layer Chromatography as a Method of
Estimating KOW. It has been reported that thin-
layer chromatography can be used to estimate KQW (Mirrless et
al., 1976; Hulshoff and Perrin 1976). However, high-pressure
liquid chromatography (HPLC) is far superior to thin-layer
chromatography (TLC) because of its accuracy (i.e., definition of
the peak), reproducibility. ease of detection in many cases, and
above all the range of applicability (HPLC is applicable over 5
order of magnitude of KQW while TLC is only applicable over 1.5
orders of magnitude of KQW\ ( Mirrless et al. 1976).
(3) Estimation of K^.lT from Water Solubility Data.
The octanol/water partition coefficient is defined as the ratio
of the equilibrium molar concentation of the chemical in octanol
and water. Thus, low molecular mass (i.e., molecular weight)
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90
organic chemicals with a low water solubility should have a high
value of Kow (e.g., hydrophobic organic chemicals). Therefore,
there should be a correlation between KQW and water solubility.
Chiou et al. (1977) studied the relationship between KQW and the
water solubility, S, and found that for 34 organic chemicals, an
excellent linear correlation was observed between log Kow and log
S which extended to more than eight orders of magnitude in water
solubility (10~3 to 104 ppm), and six orders of magnitude in Kow
(10 to 107). Chiou et al. (1977) found the following regression
equation
log KQW = 5.00 - 0.670 log S, (2)
where Kow is the octanol/water partition coefficient, S is the
water solubility in ymol/L, and the correlation coefficient (r )
was 0.970 for these 34 chemicals. Thus, KQW can be estimated
from the experimental value of the water solubility of an organic
chemical. This method would have a definite advantage in that
KQW could be estimated directly from water solubility data
without having to experimentally measure K . Thus, the
octanol/water test standard could eventually be eliminated,
thereby reducing the cost of testing. However, considerably more
experimental work is necessary to extend the correlation to a
large number of organic chemicals with different structures
before it can be used as a test standard. EPA will sponsor
research work to develop this method.
8
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91
2.3. Rationale for the Selection of Test Conditions
A detailed study of the theory of the distribution law, the
partition coefficient, and the published literature on the
conventional determination of KQW indicates that it is extremely
important that numerous factors (or test conditions) be
standardized. In order to establish these factors clearly, the
theory of the distribution law and its relation to these factors
are discussed in detail in the following sections.
2.3.1. Theory of the Distribution Law and the Octanol/Water
Partition Coefficient
The distribution coefficient or partition coefficient can be
derived using thermodynamic theory (Glasstone 1946). Consider a
mixture of two immiscible liquids which is shaken with a solute
(organic chemical). The solute distributes itself between the
two liquids in such a way that at equilibrium, in dilute
solution, the ratio of the concentrations of the solute in the
two layers is a constant at a given temperature. The tendency of
a chemical to distribute itself between two immiscible liquids
with a constant concentration ratio, in dilute solution, is a
direct consequence of the thermodynamic requirements for
equilibrium. To illustrate this, consider a pair of immiscible
liquids A and B in contact with each other containing the same
solute in solution. The chemical potential of a solute in
solvent A is given by
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92
y» = y° + RT in a. , (3)
A A A
where y is the chemical potential of the solute in solvent
A
A, y° is the standard chemical potential of the solute in the
same solvent (i.e., the value of y at a,. = 1), while aA, the
activity of the solute in the solvent A is the effective
concentration taking into account intermolecular interactions of
the solute in the solvent. R is the gas constant and is equal to
8.314 joules/°K/mol, while T is the absolute temperature in °K.
Similarly, for solvent B
UB = y°B + RT in aB, (4)
where all the quantities have the same significance as in equa-
tion (3). At equilibrium between the layers Ay = 0; hence
Ay = y — y = O,
and
UR - WA. (5)
D f\
Using equations (3) and (4) in (5) yields
y°B + RT In aB = y°A + RT In aft (6)
in
ft RT
However, at a given temperature, u°B and u°A are constants for a
given solute in a particular solvent; hence
ln
10
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93
and,
!* = K. (7)
aA
Equation (7) is the mathematical statement of the distribution
law which states that a substance will distribute itself between
two solvents until at equilibrium and the ratio of the activities
of a chemical in the two layers is a constant at a fixed
temperature, irrespective of the absolute values of aA or aB.
The activity aA can be written as
aA = YA CA , (8)
where YJ> is the activity coefficient and takes into account the
interaction between molecules A in solution, and CA is the molar
concentration. In dilute solution as
CA * o ,
hence,
YA * 1 ; (9)
limit
CA * ° (aA> = CA*
The same argument follows for the solute in solvent B and
limit
C* O (a_) = C_. Using these results in equation (7), the
B B is
distribution coefficient K, in dilute solution, becomes
0 limit Cn
K° = C > O ( -?£- } . (11)
For the specific case for the octanol/water partition
coefficient, B is the solvent n-octanol, A is the solvent water,
and K° = KQW. Thus, equation (11) becomes
11
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94
= . (12)
ow C ^
water
According to Nernst, the distribution law applies only to
individual molecular species in solution. If a molecule
dissociates or associates in octanol and water, then equation
(12) must be modified. In general, if a represents the fraction
of the total solute that is dissociated or associated, assuming
that either association or dissociation occurs in each solvent,
then
(1 — n \ f
K - Oct.' UOCt.
K
OW l _ c
water water
since (1-a) gives the fraction of unchanged molecules in each
phase. For the special case where no association takes place in
octanol, equation (13) reduces to
°» - War
2.3.2. Factors Which Affect the Value of Kow
(1) Effect of Temperature. From the theory of the
distribution law as outlined in section 2.3.1., the distribution
coefficient K is a function of the temperature (equation (6)),
and is a constant at a fixed temperature (equation (7)). Since
KQW is a distribution coefficient, it should also vary with
temperature and is a constant at a fixed temperature. Hence, in
carrying out octanol/water partition coefficient experiments by
12
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95
the conventional method, the temperature should be controlled.
However, variations due to temperature are small compared to
those inherent in the errors in the other measurements, e.g., the
errors in measuring the concentration of solute in octanol and
water. Therefore, for reasonably accurate determinations of KQW,
it is sufficient to control the temperature to ± 1°C. Since most
physical properties of chemicals are reported at 25°C, the
proposed standard requires that KQW be determined at this
temperature.
(2) Purity of the Solvents. Trace amounts of
impurities present in n-octanol tend to produce emulsions and
must be removed (Fujita et al. 1964; Hansch and Anderson 1967;
Chiou et al. 1977). Emulsions give poor phase separation and
result in a wide scatter in the value of KQW. in addition, im-
purities in octanol may affect the analysis for the solute.
Hence, the proposed test standard required that the octanol be
_>99.9% pure. Distilled or reagent grade water (ASTM Type II)
should be used.
(3) Concentration of Solute. From the theory of the
distribution law, as outlined in section 2.3.1., equations (12),
(13), and (14) only apply in dilute solution. Hence the proposed
test standard requires that all experiments be carried out at
molar concentration C < 0.01M in octanol and water.
13
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96
(4) Equilibration Time. For many chemicals, 5 minutes
of gentle agitation of the two phase system established
equilibrium and produced consistent results (Leo et al. 1971).
Studies by Craig and Craig (1950) indicated that when the phases
were of about equal volume, equilibrium was rapidly attained.
When high ratios of water to octanol (>100:1) were used, longer
shaking was necessary to establish equilibrium. High ratios of
water to octanol are used to determine KQW for very hydrophobia
organic chemicals (sections 2.3.2.(5) and (8)) as described
below. Therefore, for most chemicals, gentle agitation for 1
hour should be adequate to reach equilibrium. For surfactants,
at least 16 hours of agitaton is necessary to reach
equilibrium. This is an empirical observation obtained by
researchers who have carried out experiments with surfactants.
It is undoubtedly due to the nature of surfactant chemicals.
(5) Octanol/Water Volume Ratio. Depending upon the
solubility of the solute in octanol and water, the ratio of the
volume of octanol to water should be adjusted. For hydrophobic
solutes, which are very insoluble in water, considerably more
water than octanol should be used. Adjustment of solvent volumes
can decrease the effect of analytical errors and consequently
decrease the error in determining KQW (Leo et.al. 1971). The
following example will illustrate this point. Consider a
chemical with a molecular mass (i.e., molecular weight) MW
(mg/mmol). KQW = 200. Twenty mg of chemical are dissolved in 100
mL of octanol, and 100 mL of water are added to this system.
14
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97
After equilibration, the mass of chemical in each phase can be
calculated as follows. The chemical will partition with x mg in
the water phase and (20-x) mg in the octanol phase.
(20-x)mg
MW (mg/mmol) _ (20-x) mmol _ (20-x) Molar
oct. 100 mL ~ 100 MW mL 100 MW
x mg
MW(mg/mmol) _ x mmol _ x
Cwater = 100 mL = 100 MW mL 100 MW
Since
K
Cwater
(20-x)
100 MW (20-x).
200 = Z =
x
100 MW
20
Then x = = 0.0996 = 0.10 mg = mass of chemical in the
201
water phase; and (20-x) = 19.9s: 20 mg = mass of chemical in the
octanol phase.
Consider an analytical error of +_ 0.05 mg in the aqueous phase
(i.e., 0.10 - 0.05 = 0.05 and 0.10 + 0.05 = 0.15).
20
MW
100 20
K0w = ^05" = oTbT = 40°
MW
100
20
MW
100 20
0.15 - 0.15
MW
100
15
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98
Therefore, an analytical error of ± 0.05 mg in the aqueous phase
means that KQW can range from 400 to 133, and a very large error in
KQW occurs.
Now consider the same example as above but now the solvents are
adjusted to 200 mL of water and 5 mL of octanol. After equilbration,
the mass of chemical in each phase is now:
(20-x)
MW
oct.
X
MW
vow
200 =
200
coct.
cwater
(20-x)
5 MW
x
200 MW
(20-x)
5 MW
200 MW
Molar
Molar
(20-x)
(40)
20
Then x = — = 3.33 mg = mass of chemical in the water phase; and
6
(20-x) = 16.7 mg = mass of chemical in the octanol phase.
Now consider the same analytical error of ^KD.05 mg in the aqueous
phase (i.e., 3.33 - 0.05 = 3.28 and 3.33 + 0.05 = 3.38)
16.7
MW
K
OW
3.28
MW
200
16.7
MW
16.7
3.38 (40) = 203
K,
ow
3.28
MW
200
16.7
3.28 .(40) _ 197.
16
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99
Now the analytical error of ± 0.05 mg in the aqueous phase means that
Kow can range from 197 to 203 and the error in Kow has been reduced
dramatically.
(6) Chemical Analysis of the Octanol
and Water Phases. Consider a partitioning experiment
in which a chemical is dissolved in octanol at a low concentration
(less than 0.01 molar). The conventional partitioning experiment is
carried out and only one phase is analyzed for the molar concentration
of the solute. Using a mass balance, the molar concentration of the
solute in the other phase is obtained by difference. However, if
there is a loss of chemical by adsorption to the surface of the glass
walls, a serious error will occur at this low concentration. This is
especially true for very hydrophobic chemicals (Chiou et al. 1977) and
for ionic solutes (Leo et al. 1971). Therefore, the proposed test
standard requires that both the octanol and water phases be analyzed.
An analytical method should be selected that is the most
applicable to the analysis of the specific chemical. However, large
errors can occur as a result of traces of more-water-soluble
contaminants that are not analytically distinguishable from the parent
chemical. This error is very significant when the analytical method
is ultraviolet absorption spectroscopy or radiometry, since these
methods can be nonspecific for many solutes. Therefore,
chromatographic methods are preferable because of their compound
specificity in analyzing the parent chemical without interference from
17
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100
impurities (Karickhoff and Brown 1979). Whenever practicable, the
chosen analytical method should have a precision within ± 5 percent.
(7) Emulsification and Ultracentrifugation. Many chemicals
can cause troublesome emulsions to form between octanol and water and
emulsification can result in large errors in KQW (Leo et al. 1971;
Chiou et al. 1977). This is especially true for hydrophobia
chemicals. Therefore, gentle shaking shall be used to minimize the
formation of emulsions. In addition, incomplete separation of the two
phases is one of the most serious sources of error. To break any
emulsion formed and to completely separate the octanol and water
phases, the proposed test standard requires that the two phase system
be ultracentrifuged at 25°C for 20 minutes. The acceleration G value
required to break an emulsion and to completely separate the octanol
and water phases must be determined by trial-and-error
experimentation. Since the clarity of the two phases is not a
dependable criterion of the absence of an emulsion and complete
separation of the two-phase system, the proposed test standard
requires that a turbidimeter be used to make sure that the emulsion is
broken and the octanol and water phases have been completely
separated.
(8) Equilibration Vessel. To simplify the experimental
procedure, equilibration should be carried out in a centrifuge tube
(special glass tubes can be used up to approximately 12,000 G and
stainless steel centrifuge tubes can be used at higher G values) with
a scalable cap. This will avoid a transfer step and volatile
18
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101
chemicals can be handled easily. The centrifuge tube shall be almost
completely filled with the two phase mixture to minimize partitioning
with air. This is especially important when determining KQW for
volatile chemicals (Hansch and Anderson 1967).
Very hydrophobic chemicals, with KQW in the order of 104 to 106,
require relatively large volumes of the aqueous phase (section 2.3.2.
(5)). Hence, for these chemicals, the proposed test standard requires
that equilibration be carried out in a large ground-glass stoppered
flask.
(9) Speciation Effects. The details of speciation have
been discussed in the theory of the distribution law and the
octanol/water partition coefficient, section 2.3.1.
If the chemical does not associate or dissociate in octanol and
water, then the proposed test standard requires that equation (12) be
used and Kow be determined at concentrations C < 0.01M and C^ =
0.1C. Under these experimental conditions, if KQW is constant, then
association or dissociation has been minimized or eliminated.
If the chemical associates in octanol or water or in both
liquids, then the proposed test standard requires that equation (13)
be used and KQW be determined at concentations C < 0.01M, C^ = 0.1C,
C2 = 0.01C, C3 = 0.001C, When KQW is constant at two
concentrations differing by a factor of 10, then the effect of
association has been minimized or eliminated.
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102
For chemicals which reversibly ionize or protonate (e.g.,
carboxylic acids, phenols or anilines), the proposed test standard
requires that equation (14) be used with water buffered at pH 5.0,
7.0, and 9.0, the pHs of environmental concern.
(10) Presaturation of the Solvents. Presaturation of
octanol with water and water with octanol is required for this test
standard. The preparation of these saturated solutions is very simple
to carry out. This requirement is extremely important when
determining KQW for very hydrophobic chemicals since the ratio of
water to octanol will be very large. In this case, if the experiment
is carried out without presaturation of the water with octanol, then
all the octanol will dissolve in the aqueous phase and KQW cannot be
determined.
2.4. Reference Compounds
It would be very desirable to have reference compounds which
cover a KQW range of 10 to 106. These reference compounds would
provide the experimenter with comparative reference values to
determine how well the test has been conducted. Unfortunately, these
reference compounds are not available. The EPA is funding work at the
National Bureau of Standards to develop such reference compounds.
When this work is completed, these reference compounds will be
recommended for use in this test standard. In the interim, it is
recommended that the book by Hansch and Leo (1979) be used for the
selection of potential reference compounds.
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103
2.5. Test Data Required
The tendency of an organic chemical to partition out of water
into other environmental compartments containing hydrophobia
constituents (e.g., aquatic organisms) can be inferred from the values
of the octanol/water partition coefficient Kow. Chiou et al. (1977)
developed regression equations relating log KQW with water solubility
S (in ymol/L) and bioconcentration in rainbow trout (BF). Assuming
log KQW is between 1 and 6, S and BF can be calculated; these results
are summarized in Table 1 (note that S has been converted to mol/L).
Furthermore, assuming that the average molecular mass (i.e. molecular
weight) of an organic chemical is 300 gm/mol, the water solubility can
be converted to ppm; these results are also summarized in Table 1. It
is apparent that for log Kow =6 (i.e., KOW = 106), the water
solubility will be extremely low (9.7 x 10~^ ppm or 9.7 ppb) and
the predicted BF is 1.48 x 10 . Hence, the data indicate that
the chemical will partition out of the water phase and into the
fat of the fish (i.e., the hydrophobic phase). For log Kow = 1
(i.e., KQW =10), the water solubility will be very high (2.80 x
105 mg/L or 280 gm/L) and the predicted BF is 2.4. Hence, these
data indicate that the chemical will remain in the water phase
and will not partition significantly into the fat of the fish
(i.e., the hydrophobic phase)- Therefore, the proposed test
standard is designed to determine the value of KOW in the range
10 to 106. Low molecular mass organic chemicals with a KQW value
less than 10 will not partition significantly into, or tend to
21
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104
Table 1. Summary of Calculated Values of Water Solubility and
Bioconcentration in Rainbow Trout as a Function of Log Kowa
KQW S(mol/L)b S(mg/L or ppm)c BFd
6 3.24 x 10~8 9.7 x 10~3 1.48 x 104
5 1 x 10~6 0.30 2.57 x 103
4 3.09 x 10~5 9.3 4.47 x 102
3 9.77 x 10~4 2.93 x 102 7.76 x 101
2 3.02 x 10~2 9.06 x 103 1.35 x 101
1 9.33 x 10"1 2.80 x 105 2.4
a Regression equations taken from Chiou et al. (1977).
" Water solubility
c Water solubility in ppm assuming a molecular mass of an
organic chemical is 300 gm/mol.
Bioconcentration in rainbow trout
22
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105
accumulate in, any hydrophobia environmental compartments. Low
molecular mass organic chemicals with KQW in excess of 106 will
tend to accumulate into all hydrophobia enviromental
compartments. For low molecular mass organic chemicals outside
the range 10 to 10 , the proposed test standard requires that
KQW be characterized as < 10 or > 106 with no further
quantification.
Specific analytical and recovery procedures are needed to
determine whether acceptable data have been generated.
2.6. Statistical Analysis of the Data
Numerous researchers have published data on the determina-
tion of the octanol/water partition coefficient by the conven-
tional method (e.g., Chiou et al. 1977; Davies et al. 1975;
Fujita et al. 1964; Hansch and Anderson 1967; Leo et al. 1971).
However, none of these researchers has analyzed the data
statistically and the precision of KQW as determined by the
conventional method has not be.en clearly established. The pre-
cision is, in part, a function of the nature of the specific
chemical. As the hydrophobicity of the chemical increases, KQW
increases and the precision of Kow decreases. Furthermore, the
precision is also a function of the analytical procedure used.
In general, the lower the concentration to be measured, the
poorer is the precision of the analytical procedure. Therefore,
no reliable precision can be stated at this time for determining
KQW. Obviously, the precision can be improved by making numerous
23
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106
replicate determinations. However, in order to minimize cost, it
has been decided to determine KOW with three replicates. The
submitter of the test results must analyze the data statis-
tically. When a large number of chemicals have been determined
by the proposed method, the data will be analyzed statistically
and the level of precision can be defined for various ranges of
Kow'
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107
3. ECONOMIC ASPECTS
A survey of three commercial laboratories to estimate costs
for performing the test outlined in this Standard found a range
of $100 to $1,400 and with a "best estimate" of $553. A cost
estimate was also made by separating the protocol into components
and estimating the cost of each component including direct labor
cost, overhead cost, other direct costs, general and
administrative costs, and profit or fee. The estimated range for
this test ranged from $229 to $688 with a "best estimate" of
$458. The cost estimates were made assuming that all the
requirements of Good Laboratory Practice Standards, as specified
in section (c) of this Test Standard, are being satisfied.
Details of the cost estimates are contained in a report by Enviro
Control, Inc. (1980).
The cost estimates in the Enviro Control report do not
include the costs for specialized analytical methodology or for
analytical method development that is required to determine the
concentration of the chemical in octanol and water at
equilibrium. It is not possible to estimate the cost of such
special analytical methodology since the costs are related to the
nature of the chemical to be analyzed and related because such
method development costs may be prorated among the analytical
needs of other test standards.
In reviewing the state of technology concerning this area of
testing, the Agency has not identified any adequately developed
25
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108
tests that would be significantly less costly while providing
comparable information.
26
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109
4. REPLIES TO THE COMMENTS ON THE PUBLISHED SECTION 5
TESTING GUIDELINES
The Federal Register of March 16, 1979 (EPA 1979) presented
a discussion of the policy issues, alternative approaches, and
test methods under consideration as guidance for premanufacture
testing. Included in the test methods was a section on the
octanol/water partition coefficient, KQW (pages 16254-16256),
which discussed a method for determining this parameter for use
in developing information on new chemicals. Among the comments
received in response to the discussion were several comments
related to the determination of K_,..
^y Vr
One commentor made the following statement: "The rationale
for including this test in the proposed premanufacture testing
program is based upon published data indicating that there is a
correlation of KQW with bioconcentration or biomagnification
potential of chemical substances in aquatic and other living
organisms. However, researchers (Chiou et al. 1977; Lu and
Metcalf 1975; Metcalf et al. 1975) have shown that there is a
correlation between bioconcentration and aqueous solubility
suggesting that the assessment for bioconcentration potential can
be obtained from water solubility data. Under these circum-
stances, the determination of KQW would be redundant." However,
K has other uses in addition to predicting bioconcentration
potential. KQW is related to partitioning into all hydrophobic
environmental compartments (e.g., sediments). In addition, K
is an indicator of biological activity. Therefore, KQW is a very
27
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110
important and useful physical/chemical property- The research
work by Chiou is significant because KOW can be estimated from
water solubility data and this work has been thoroughly discussed
in section 2.2.2. (3). Research work will be sponsored by EPA to
develop this method.
Another commentor indicated that HPLC has been overlooked as
an alternative method for determining KOW- However, this method
has been addressed in the TSCA Federal Register guidelines and
has been thoroughly discussed in section 2.2.2. (1). HPLC is
much simpler to run than the conventional method. Hence,
research will be sponsored by EPA to develop this method.
Two commentors indicated that the range of K should be
limited. One commentor suggested that the range should be 10 to
104 while another commentor indicated that there is very little
value in pushing the method to 10^ or to very low values. This
subject has been thoroughly discussed in section 2.5. The data
presented in this section indicated that the most useful range of
KQW is 10 to 10^ and for values outside this range, KQW should be
characterized by <10 or >106 without further quantification. The
value of KQW proposed by one commentor (i.e., 104) is too low
since a significant amount of a chemical would still be in the
water phase, bioconcentration is moderate (447), and partitioning
is not. complete. It is only when KQW = 106 that the water solu-
bility is very low (9.7 ppb), bioconcentration is high (1.48 x
104) and partitioning into a hydrophobic environmental compart-
ment is essentially complete.
28
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Ill
There were two comments related to equilibration time. One
commentor indicated that the equilibrium time of 1 hour is
entirely too short since it can take several weeks or months to
establish equilibrium in a water solublity experiment for very
hydrophobia chemicals. However, in determining water solubility,
the equilibration is between the molecules in the solid state and
in solution and this process is very slow for very hydrophobia
chemicals. On the other hand, in determining Kow, the chemical
is dissolved in octanol, water is added, and equilibrium is
involved with molecules in solution. Under these conditions,
equilibration is rapid (5-15 minutes). Equilibrium has been
discussed in section 2.3.2. (4).
Another commentor indicated that in order to ensure that
equilibrium has been established, several measurements of Kow are
necessary at different times, e.g., 1, 6, 16 hours. However,
this procedure would be too costly since several determinations
must be made. As indicated in section 2.3.2. (4), equilibrium is
attained rapidly for most chemicals (5-15 minutes) and 1 hour is
sufficient to reach equilibrium. For surfactants, 16 hours is
sufficient to reach equilibrium.
Three commentors discussed the determination of KQW at pH 5,
7, and 9. One commentor indicated that the measurement Of KQW at
different pHs is inappropriate since the objective of this
measurement is to simulate bioaccumulation in fatty tissues under
physiological and surface water conditions (pH 7). This state-
ment is not true. In general, the pH of surface waters can vary
29
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112
from 5 to 9 and is not necessarily at 7- Hence for chemicals
which reversibly ionize or protonate, the pH of the surface water
will determine the type of molecular species present and this
molecular species is the one which will be bioconcentrated. In
addition, this test will give information on sorption into any
hydrophobic compartment (e.g., sediments) which is a function of
the nature of these molecular species and, therefore, will have a
direct effect on partitioning in the environment. Another
commentor suggested that it would be more than sufficient to
measure the partition coefficient of the chemical at the pH (5 or
9) that will give the lowest solubility since this will most
likely yield the highest partition coefficient. However, Kow
will be used in a partitioning analysis and it is necessary to
know KQW at pH 5, 7, or 9, the pHs of environmental concern. The
third commentor suggested that the same buffers described in the
hydrolysis protocol (p. 16268) be used. The buffers listed in
the hydrolysis procedure were designed to minimize buffer
effects. Hence, these buffers have been recommended for use in
the proposed test standard.
Two commentors discussed the use of glass-stoppered centri-
fuge tubes at 37,000 G. They indicated that currently there are
no glass-stoppered centrifuge tubes that can be used at 37,000
G. This suggestion has been incorporated in the proposed test
standard. Special glass centrifuge tubes are available
commercially which can be used to approximately 12,000 G and
stainless steel centrifuge tubes can be used at higher G values.
30
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113
One commentor discussed the inadequacy of the sampling
procedure. The commentor indicated that since the octanol layer
is on top of the water layer, it would be extremely difficult to
sample the aqueous phase without contaminating the sampling
device (the pipet) with the concentrated octanol phase. The test
standard has incorporated a method which will avoid this
problem. The test standard requires that the octanol phase be
sampled first with a pipet. Then the remainder of the octanol
phase and the interfacial layer can be removed and discarded.
Finally, a new pipet can be used to sample the aqueous layer.
One commentor discussed the problem of emulsification. It
was indicated that even gentle shaking for 1 hour could lead the
same horrendous emulsions for certain chemicals. This problem
has been recognized and thus ultracentrifugation is required to
break any emulsion and to separate the octanol and water phases.
31
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114
5. TSCA/FIFRA COMPARABILITY
The methods for determining the octanol/water partition
coefficient described in the FIFRA Guidelines (EPA 1975, 1978)
and this Test Standard are similar. However, the FIFRA Guide-
lines only give general guidance for determining K_t, Because
tJW •
the determination of KQW is a difficult procedure, as explained
above, a number of variables must be controlled in order to
obtain a reliable result. Therefore, detailed procedures for
controlling these variables have been included in the TSCA Test
Standard. Furthermore, adherence to these detailed procedure is
required to insure that a uniform method is used by all
submitters, thereby allowing the data to be compared.
32
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115
6. TSCA/OECD COMPARABILITY
The Organization for Economic Cooperation and Development
issued a draft of a test protocol for determining KQW (OECD,
1978). This draft is old but is the only one available for com-
parison with the TSCA proposed test standard. In general, the
OECD test protocol and the TSCA proposed standard are very simi-
lar. However, there are a few differences which are discussed
below. OECD requires that KQW be determined quantitatively
regardless of the value for a specific chemical. The TSCA test
standard specifies that Kow only be determined quantitatively in
the range 10 to 10^. The justification for this range is out-
lined in section 2.5.
OECD indicates that KQW is "ideally" dependent on tempera-
ture. Kow is, indeed, a function of temperature as discussed in
section 2.3.2. However, OECD indicates that the effect of
temperature on KOW is small (i.e., approximately 0.01 logarithmic
units per degree Celsius and can be positive as well as
negative). Hence, OECD does not require temperature control and
their test method specifies that KQW be measured at room
temperature. However, no experimental data are given to support
*
this finding. Since KQW is an equilibrium constant and is
dependent on temperature (section 2.3.2.), the TSCA proposed test
standard requires that KQW be determined at 25 +_ 1°C. Precise
temperature control is not required as indicated by the range of
temperature control of + 1°C.
33
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116
OECD does not require that K be determined as a function
of pH for chemicals that reversibly ionize or protonate. The
TSCA proposed test standard requires that chemicals that
reversibly ionize or protonate should be tested at pHs 5, 7, and
9, the pHs of environmental concern. The effect of pH is
discussed in section 2.3.2. (!).
OECD does not indicate how to handle volatile chemicals or
very hydrophobia chemicals while the TSCA test standard gives
detailed procedures for handling these types of chemicals. The
reasons for giving precise directions on handling these types of
chemicals are discussed in section 2.3.2. (8).
OECD requires an equilibration time of 5 minutes while the
TSCA proposed standard requires an equilibrium time of one hour
for most chemicals and 16 hours for surfactants. Equilibration
time is discussed in section 2.3.2. and also has been discussed
in section 4.
OECD indicates that the reproducibility is in general
+_ 3 percent. However, no data are given to substantiate this
finding. A thorough literature search has been made and no data
have been published which have established the precision of Kow
for a large number of chemicals. Hence, no precision is
specified in the TSCA proposed test standard. The statistical
analysis of the data is discussed in section 2.5.
34
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117
8. REFERENCES
Carlson RM, Carlson RE, Kopperman HL. 1975. Determination of
partition coefficients by liquid chromatography. J.
Chromatogr. 107:219.
Chiou CT, Freed VH, Schmedding DWf Kohnert RL. 1977. Partition
coefficient and bioaccumulation of selected organic chemicals.
J. Environ. Sci. Tech. 11:475.
Craig LC , Craig D. 1950. In: Technique of organic chemistry,
Vol. Ill, pt. I, Chapter 4. New York: Interscience Publishers,
Inc.
Davies JE, Barquet A, Freed V, Haque R, Morgade C, Sonneborn RE,
Vaclavek C. 1975. Human poisonings by a fat-soluble organo-
phosphate insecticide. Arch. Environ. Health 30:608.
Enviro Control, Inc. 1980. Cost analysis methodology and
protocol estimates; Environmental Review Division standards.
Rockville, Md., Enviro Control, Inc., Borriston Laboratories,
Inc.
USEPA. 1975. U.S. Environmental Protection Agency. Office of
Pesticide Programs. Proposed guidelines for registering
pesticides in the United States. Fed. Regist. 1975 40, 26802.
USEPA. 1978. U.S. Environmental Protection Agency, Office of
Pesticide Programs. Proposed guidelines for registering
pesticides in the( United States. Fed. Regist. 1978 43, 29696.
Fujita T, Iwasa J, Hansch C. 1964. A new substituent constant,
derived from partition coefficients. J. Am. Chem. Soc. 86:5175.
Glasstone S. 1946. Textbook of physical chemistry. New York:
Van Nostrand Co.
Gould RF ed. 1972. Biological correlations—the Hansch
approach. Adv. Chem. Ser. No. 114. Washington, D.C.: American
Chemical Society.
Hansch C. 1969. A quantitative approach to biomedical
structure-activity relationships. Ace. Chem. Res. 2:232.
Hansch C, Anderson SM. 1967. The effect of intramolecular
hydrophobia bonding on partition coefficients. J. Org. Chem.
23:2583.
35
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118
Hansch C, Fujita T. 1964. p-ot-ir anaysis. A method for the
correlation of biological activity and chemical structure. J.
Am. Chem. Soc. 86:1616.
Hansch C, Leo A. 1979. Substitueht constants for correlation
analysis in chemistry and biology. New York: J. Wiley & Sons.
Hulshoff A, Perrin JH. 1976. A comparision of the determination
of partition coefficients of 1,4-benzodiazepines by high-
performance liquid chromatography and thin-layer
chromatography. J. Chromatogr. 129:263.
Karickhoff SW, Brown DS. 1979. Determination of octanol/water
distribution coefficients, water solubilities, and
sediment/water partition coefficients for hydrophobic organic
pollutants. EPA-600/4-79-032.
Karickhoff SW, Brown DS, Scott TA. 1979. Sorption of
hydrophobic pollutants on natural sediments. Water Res. 13:241.
Leo A, Hansch C, Elkins D. 1971. Partition coefficients and
their uses. Chem. Rev. 71:525.
Lu PY, Metcalf RL. 1975. Environmental fate and
biodegradability of benzene derivatives as studied in a model
aquatic ecosystem. Environ. Health Perspect. 10:269.
Mackay D. 1979. Finding fugacity feasible. Environ. Sci. Tech.
13:1218.
Metcalf RL, Sanborn JR, Lu PY, Nye D. 1975. Laboratory model
ecosystem studies of the degradation and fate of radiolabelled
tri-, tetra-, and pentachlorobiphenyl compared with DDE. Arch.
Environ. Cont. 3:151.
McCall JM. 1975. Liquid-liquid partition coefficients by high-
pressure liquid chromatography. J. Med. Chem. 18:549.
Mirrless MS, Moulton SJ, Murphy CT, Taylor PJ. 1976. Direct
measurement of octanol-water partition coefficients by high-
pressure liquid chromatography. J. Med. Chem. 19:615.
Neely WB, Branson DR, Blau GE. 1974. Partition coefficient to
measure bioconcentration potential of organic chemicals in
fish. Environ. Sci. Tech. 8:113.
OECD-Chemicals Testing Program. 1978. Draft test protocol for
the determination of the partition coefficient of solid and
liquid substances in the system water/n-octanol.
Veith GD, Morris RT. 1978. A rapid method for estimating log P
for organic chemicals. EPA-600/3-78-049.
36
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119
Veith GD, Austin NM, Morris RT. 1979. A rapid method for
estimating log P for organic chemicals. Water Res. 13:43.
Yamana T, Tsuja A, Miyamoto E, Kubo O. 1977. Novel method for
determination of partition coefficients of penicillins and
cephalosporins by high-pressure liquid chromatography. J. Pharm.
Sci. 66:747-
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121
SOIL THIN LAYER CHROMATOGRAPHY
STANDARD FOR TEST DATA DEVELOPMENT
PROPOSED RULE, SECTION $, TSCA
Refers to
Part 772 — Standards for Development of Test Data
Subpart L. Physical, Chemical and Environmental
Persistence Characteristics
Section 772.122-5 Soil Thin Layer Chromatography
October 1980
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123
CONTENTS
Page
1. NEED FOR THE TEST 1
2. SCIENTIFIC ASPECTS OF SOIL LEACHING 2
2.1. Introduction 2
2.2 Basic Processes Affecting Soil Leaching 2
2.3 Chemical Properties Affecting Leaching 4
2.4 Soil Properties Affecting Leaching 4
2 . 5 Types of Adsorptive Forces 7
2.6 Surface Transformations 8
3. ECONOMIC ASPECTS 11
4. SCIENTIFIC ASPECTS OF THE TEST 12
4.1 Development of Soil Thin Layer
Chromatography (TLC) 12
4.2 Rationale for the Selection of Soil TLC 15
4.3 Rationale for Selection of Experimental
Conditions and Procedures 17
4.4 Replies to Federal Register 20
5 . TSCA/FIFRA/OECD COMPARABILITY 23
6. REFERENCES 24
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125
1. NEED FOR THE TEST
Leaching of chemicals through soil is an important process
which affects a chemical's distribution in the environment. If a
chemical is tightly adsorbed to soil particles, it will not leach
through the soil profile but will remain on the soil surface. If
a chemical is weakly adsorbed, it will leach through the soil
profile and may reach ground waters and then surface waters.
Knowledge of the leaching potential is essential under certain
circumstances for the assessment of the fate of chemicals in the
environment.
Chemical leaching also affects the assessment of ecological
and human health effects of chemicals. If a chemical reaches
ground water, deleterious human health effects may arise due to
the contamination of drinking water. If a chemical remains at
the soil surface, deleterious environmental and human health
effects may arise due to an increased concentration of the
chemical in the zone of plant growth, possibly resulting in
contamination of human food supplies.
Soil thin layer chromatography (TLC) is a qualitative
screening tool suitable for obtaining an estimate of chemical's
leaching potential. This test is the first of several tests
which will be used in obtaining a rough estimation of a
chemical's leaching potential and is the test most amenable to
standardization at the present time.
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126
2. SCIENTIFIC ASPECTS OF SOIL LEACHING
2.1. Introduction
Since chemical leaching in soils is affected by a large
number of interacting processes, this section of the support
document will discuss these processes as they relate to this
phenomenon .
2.2. Basic Processes Affecting Soil Leaching
As it occurs in the real world, leaching through soil is a
complex phenomenon consisting of several major processes (Hamaker
1975). These processes are operative in the soil TLC method and
will be discussed in detail below.
The general equation (Guenzi 1974) for chemical movement
through porous media under steady state soil-water flow
conditions is :
--
where B = soil bulk density (g/cm^)
0 = volumetric water content (cm 'cm '
S = amount of chemical adsorbed at the
soil/water interface (g/g soil).
t =• time ( sec. )
C'= solution concentration of chemical
D'= dispersion coefficient (cm /sec)
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127
V = average pore-water velocity (cm/sec)
X = space coordinate measured normal to the
section
Most mass transport equations represent simplifications of
"real world" conditions. Equation 1 and similar mathematical
expressions try to describe the chromatographic distribution of
the chemical in the soil profile and are gross simplifications of
a phenomenon affected by a number of complex interacting
processes including but not limited to precipitation,
evaporation, evapotranspiration and hydrodynamic dispersion.
In general, chemical leaching is dependent upon three major
processes: the mass transport of water (the direction and rate,
of water flow), diffusion, and the adsorption characteristics of
the chemical in soil (Guenzi 1974). Diffusion is the transport
of matter resulting from random molecular motion caused by
molecular thermal energy. This random motion will lead to the
uniform distribution of molecules in a closed system since there
is net movement from regions of higher to lower concentrations.
In this document, adsorption refers to the equilibrium
distribution of a molecule between a solid phase and a solution
phase. As the degree of adsorption increases the concentration
of the chemical in the soil water and the soil air decreases.
This equilibrium process is governed by two opposing rate
processes. The adsorption rate is the rate to which molecules
from the liquid phase transfer into the adsorbed state in the
solid phase. The desorption rate is the opposite process, i.e..
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128
the rate at which molecules transfer from the adsorbed state in
the solid phase into the liquid phase. In general, the mass
transport, diffusion, and adsorption processes produce the
observed leaching pattern of a chemical in soil.
2.3. Chemical Properties Affecting Leaching
The main process of the three processes discussed above
which determines a chemical's leaching potential (as described
mathematically in equation 1) is adsorption. Adsorption is
governed by the properties of both the adsorbent and the
adsorbate. The important properties of the absorbate affecting
adsorption by soil colloids (Bailey and White 1970) are: (1)
chemical structure and conformation (2) acidity or basicity of
the molecule (pka or pkb), (3) water solubility. (4) permanent
charge, (5) polarity. (6) molecular size, and (7) polariz-
ability. There are many ways in which each of these adsorbate
properties interact and are manifested in the overall adsorption
reaction (Bailey and White 1970).
2.4. Soil Properties Affecting Leaching
Soil is the unconsolidated organic and mineral material on
the immediate surface of the earth which serves as a natural
medium for the growth of plants. The combined actions of
climate, microorganisms and macroorganisms over long periods of
time on different parent geologic and bio£ic materials form soils
that differ widely in their physical, chemical, and morphological
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129
characteristics. The wide variations in the amounts and types of
clay and organic matter, soil pH, primary and secondary minerals,
structure, texture, and exchange capacity create soils of
substantial heterogeneity within the United States. There are
currently 10 Soil Orders, at least 43 Suborders, over 200 Great
Groups and over 7,000 soil series recognized in the United States
(Buckman and Brady 1969). The test rule will specify the minimum
number of soils depending on the number of sites of manufacture,
distribution, use, and disposal of the test chemical. The soil
physical/chemical property ranges for pH, organic matter and
cation exchange capacity will be specified so that soils used in
the TLC method will be representative of U.S. humid and semiarid
region mineral soils.
The soil properties affecting the adsorption and desorption
of organics include organic matter content, type and amount of
clay, exchange capacity, and surface acidity (Adams 1973; Bailey
and White 1970; and Helling 1970). Soil organic matter is a
primary soil parameter responsible for the adsorption of many
pesticides. Helling (1970) lists many examples where the organic
matter primarily influenced the adsorption of pesticides.
Although organic matter and clay are the soil components most
often implicated in pesticide adsorption, the individual effects
of either organic matter or clay are not easily ascertained.
Since the organic matter in most soil is intimately bound to the
clay as a clay-metal-organic complex (Stevenson 1973), two major
types of adsorbing surfaces are normally available to the
chemical, namely, clay-organic and clay alone. Clay and organic
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130
matter function more as a unit than as separate entities and the
relative contribution of organic and inorganic surfaces to
adsorption will depend on the extent to which the clay is coated
with organic substances. Comparative studies between known clay
minerals and organic soils suggest that most, but not all,
pesticides have a greater affinity for organic surfaces than for
mineral surfaces (Stevenson 1973). Since typical soil studies
compare soils in which both clay and organic matter increase and
do not utilize multiple regression analyses to isolate the
governing parameter (Helling 1970), only generalizations
concerning the relative importance of clay and organic matter can
be made.
The activity of protons in the bulk suspension (i.e., as
measured by pH) and the activity of protons at or in close
proximity to the colloidal surface (i.e., the acidity in the
interfacial region) may differ significantly. The term "surface
acidity" as applied to soil systems is the acidity at or in close
proximity to the colloidal surface and reflects the ability of
the system to act as a Lewis acid. Surface acidity is a
composite term which reflects both the total number of acidic
sites and their relative degree of acidity. Surface acidity is
probably the most important property of the soil or colloidal
system in determining the extent and nature of adsorption of
basic organic chemicals as well as determining if acid-catalyzed
chemical transformation occurs (Bailey and White 1970). There is
overwhelming evidence, mainly from infrared studies, pointing to
the fact that there is protonation of basic chemicals by clays
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131
having hydrogen and aluminum as the predominant exchangeable
cation and by clays saturated with alkali, alkaline earth, and
transition metal cations. A summary of recent investigations
indicates that the protonation of chemicals in the interfacial
region of clays is a function of the basicity of the molecule,
the nature of the exchangeable cation on the clay, water content
of the clay system, and the origin of negative charge in the
aluminosilicate clay (Bailey and White 1970).
In summary, the chemical properties discussed in (c) and the
soil properties discussed in (d) both govern the extent of
adsorption in soils.
2.5 Types of Adsorptive Forces
The specific type of interaction of organic molecules with
soil will depend on the specific chemical properties of the
organic molecule and the type of soil. These specific
interactions or adsorptive forces are usually classified as: van
der Waals forces, charge transfer, ion exchange, and hydrophobic
bonding (Adams 1975, Goring and Hamaker 1972).
The van der Waals forces arise from the fluctuations in a
molecule's electron distribution as the electrons circulate in
their orbitals. These fluctuations produce instantaneous dipoles
which cause that molecule's attraction to other atoms and
molecules. Charge transfer involves the formation of a donor-
acceptor complex between an electron donor molecule and an
electron acceptor molecule with partial overlap of their
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132
respective molecular orbitals and a partial exchange of electron
density. Hydrogen bonding is an example of a particular type of
charge transfer. Ion exchange refers to the exchange between
counterions balancing the surface charge on the soil colloid and
the ions in the soil solution. The driving force for this
interaction is the requirement for electroneutrality: the
surface electric charge must be balanced by an equal quantity of
oppositely charged counterions. In general, ion exchange is
reversible, diffusion controlled, stoichiometric and, in most
cases, exhibits some selectivity or preferential adsorption for
one ion over another competing ion. Hydrophobia bonding refers
to the preference of an organic molecule for a hydrocarbon
solvent or hydrophobia region of a colloid over a hydrophilic
solvent. This preference is due to the fact that hydrocarbon
regions of a molecule have greater solubility in liquid
hydrocarbons (or most organic solvents) than in water. In
general, one or more of these specific interactions or adsorptive
forces may occur at the same time depending on the presence and
magnitude of the chemical and soil properties discussed above.
2.6. Surface Trans formations
A special type of interaction between organic molecules and
soils deals with the transformation of organic chemicals into new
compounds containing different chemical structures through the
catalytic activity of the soil colloid surfaces. Although several
theories exist to account for the mechanism of these
transformations, no scheme predicting the occurrence of such
8
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133
surface reactions presently exists. Therefore, parent compound
mass balances shall be performed and reported as part of the
requirements of this soil TLC Test Standard in order to ascertain
the extent of such transformations during soil leaching experi-
ments. Also, the leaching pattern (a diagram or photograph of
the TLC plate showing the position of the chemical) can give a
qualitative indication of the extent of such transformations and
shall be reported. The scientific literature shows that a number
of chemicals and chemical classes undergo colloid surface induced
chemical transformations. Poly-(dimethylsiloxane) fluids in
intimate contact with many soils undergo siloxane bond redistri-
bution and hydrolysis, resulting in the formation of low mole-
cular weight cyclic and linear oligomers (Buch and Ingebrightson
1979)- S-triazines (White 1976) and organophosphorus pesticides
(Yaron 1978, and Mingelgrin et al. 1977) undergo clay colloid
induced hydrolysis. Benzene and phenol polymerize into high
molecular weight species by adsorption and reaction at the
surface of smectite saturated with transition metal cations
(Mortland and Halloran 1976). Gallic acid, pyrogallol,
protocatechuic acid, caffeic acid, orcinol, ferulic acid, p-
coumaric acid, syringic acid, vanillic acid and p-hydroxybenzoic
acid undergo oxidative polymerization in the presence of various
clay minerals (Wang and Li 1977 and Wang et al. 1978). In
general, testing methods that do not take into account surface
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134
transformations should not be used in determining the leaching
potential of chemicals.
In summary, the interfacial region is important in
determining the adsorption mechanism, the energy by which the
adsorbate is held, and in determining if the adsorbed chemical is
transformed. This information is important in determining the
persistence and ultimate toxicity of the molecule since the
transformation product(s) (1) may be more or less toxic than the
original compound, (2) may be more or less tightly bound than the
original compound, and (3) may have a water solubility either
greater than or less than the original compound, thereby
affecting its leaching and movement into the groundwater.
10
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135
3. ECONOMIC ASPECTS
A survey of commercial laboratories to estimate costs for
performing the test outlined in this standard found a range of
$390-$850 with a "best estimate" of $620. A cost estimate also
was made by separating the standard into components and
estimating the cost of each component including direct labor
cost, overhead cost, other direct costs general and admini-
strative costs and profit or fee. The best estimate of cost was
$344 with an estimated range of $172 to $516. An analysis of the
T
discrepancy between the price estimate and survey pointed out
that the nature of the method of analysis, after solvent
migration, has an impact on the cost of analysis. Different
methods will cause variations in cost. Also, the price estimate
does not include costs for any analytical work which may be
necessary before the test can be performed. In practice, testing
laboratories receive chemicals in a pure form, and most identi-
fication and analytical methodology has already been developed.
Extraction, GC, MS, etc. methodology may be necessary, however,
and it is estimated that this would cost less than $1000 for 90
percent of the chemicals tested.
The above cost estimates were made assuming that all the
requirements of Good Laboratory Practice Standards (GLPs) are
being satisfied. Details of the cost estimate are contained in a
report by Enviro Control, Inc. (1980).
11
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136
4. SCIENTIFIC ASPECTS OF THE TEST
4.1. Development of Soil Thin Layer Chromatography (TLC)
Before 1968, methods of investigating the mobility of
nonvolatile organic chemicals within soils were based on the use
of field analysis, soil adsorption isotherms, and soil columns.
In 1968, Helling and Turner introduced soil thin layer
Chromatography (soil TLC) as an alternate procedure. It is
analogous to conventional TLC, with the use of soil instead of
silica gels, oxides, etc. as the adsorbent phase.
In their initial report. Helling and Turner used Lakeland
sandy loam, Chillum silt loam, and Hagerstown silty clay loam.
Medium sand (>250 pm dia.) was removed from Chillum and
Hagerstown soils and coarse sand (>500 um ) from Lakeland soil
by dry-sieving. Aqueous slurries were prepared and 500 um (silt
loam, silty clay loam) or 750 um (sandy loam) thick layers were
spread on TLC plates using conventional TLC apparatus. After
drying, six or seven radiolabelled pesticides were applied near
the base of a 20 x 20 cm plate and developed ten cm with water by
ascending Chromatography. Pesticide movement was visualized by
autoradiography. Movement was expressed by the conventional R
designation, although this referred to the front of pesticide
movement rather than its maximum concentration. The soil TLC
data are most appropriately compared with other mobility data
which indicate the depth to which an organic chemical may be
12
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137
leached. The ranking of pesticides in order of mobility is in
good agreement with general trends previously reported.
Absolute movement on soil TLC plates cannot be transposed
directly to field or soil column experiments. Since soil
structure in the TLC system is considerably more homogeneous that
in most other systems, band spreading will be somewhat less than
in field or column regimes. Flow rates are also higher than
those occurring naturally. For example, infiltration into
Hagerstown silty clay loam was equivalent to rainfall of about
1.2 cm/hr (Helling 1970). High flow rates are usually associated
with increased mobility, as later correlations (Helling 1968)
bore out. In spite of these problems, monitoring data utilizing
certain reference chemicals has provided the necessary infor-
mation to relate soil TLC data to column and field data. In
general, Helling and Turner (1968) indicated that soil TLC
offered a rapid, simple, and inexpensive procedure for
establishing a general mobility classification of pesticides and
organic chemicals.
Simple chromatographic theory can be used to correlate
adsorption coefficients with soil TLC R£ values. If
chromatographic movement through a soil column is treated
according to the distillation theoretical plate theory (Block et
al. 1958, Martin and Synge 1941), a formula for Rf is obtained in
terms of the relative cross-sectional areas of the liquid and
solid phases and partition of a chemical between solid and liquid
phases (Hamaker 1975):
13
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138
Rf = AL/(AL + AS) = i/[(i+ a (AL + AS)] (2)
where As and AL are cross-sectional areas of solid and liquid
phases and a is the ratio of volume concentration in the solid
phase to that in the liquid phase . For saturated conditions which
will be assumed for a soil plate, AL + As = A (cross-sectional
area), this can be written:
Rf = i/[(i + a (A/[AL - i])] (3)
When reexpressed in terms of the pore fraction of the
soil 6, density of soil solids (ds), and a soil adsorption
coefficient K, this equation becomes:
Rf =f(i + K(ds)(i/e-i)]- (4)
This ratio, A/AL, is set equal to 1/6 ' by analogy to the
treatment of soil diffusion by Millington and Quirk (1961) where
it serves to correct for the tortuosity of flow through the
porous medium. In this case, it serves to relate the pore volume
to the cross sectional area of the liquid phase in a saturated
soil. In general, equation 4 has shown that an inverse
relationship exists between the soil adsorption coefficient K and
Rf (Hamaker 1975) .
14
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139
Riley (1976) presented a general relationship between the
soil/solution distribution coefficient K and the depth of
pesticide leaching. Relating the data of Riley (1976) with the
Rf values of Helling (1968, 1971a, 1971b, 1971c) and the average
K values of Goring and Hamaker (1972) for selected pesticides,
the general relationship shown in Table 1 was developed between
the soil/solution partition coefficient, Rf, and soil mobility.
4.2 Rationale for Selection of Soil TLC
A number of laboratory tests, the soil thin layer
chromatography, soil adsorption isotherm, and soil columns have
been developed to obtain an estimate of a chemical's leaching
potential (Hamaker 1975). Soil TLC is the first of several tests
with will be recommended for use. It is the least expensive of
the available tests which measures leaching potential,is widely
used, and is the test most amenable to standardization at the
present time.
The soil TLC offers many desirable features. First,
mobility results are reproducible. Mass transfer and diffusion
components are distinguishable. The method has relatively modest
requirements for chemicals, soils, laboratory space, and
equipment. It yields data that are amenable to statistical
analyses. A chemical extraction-mass balance procedure to elicit
information on degradation and chemical transformations occurring
at colloid interfaces can be incorporated into this test. The
ease with which the Rf and mass balance are performed will depend
15
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Table 1. The General Relationship Between the Soil/Solution
Partition Coefficient K, R and Soil Mobility
K Rf Mobility class
0.1 0.95 Very Mobile
1 0.60
10 0.25 Mobile
10^.. 0.10 Low mobility
102-5 0.00
103 0.00
104 0.00 Immobile
Distance surface applied chemical may leach
Much of chemical leaches through top 20 cm soil
into subsoil
. . Much of chemical leached Into soil but peak
concentration in top 20 cm soil.
. . Only small amount of leaching and peak
concentration normally In top 5 cm soil
No siqnificant leachinq.
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141
upon the physical/chemical properties of the test chemical and
the availability of suitable analytical techniques for measuring
the chemical.
4.3. Rationale for Selection of Experimental Conditions
and Procedures
The papers by Helling (1968, 1971a, 1971b, 1971c) and
Helling and Turner (1968) were the basis of this test standard.
The soil and colloid chemistry literature and the analytical
chemistry literature substantiates the experimental conditions
specified in the suggested standard as accepted, standard
procedures. A few of these conditions will be discussed in
greater detail below.
Soil TLC can be used to determine the soil mobility of
sparingly water soluble to infinitely soluble chemicals. In
general, a chemical having a water solubility of less than 0.5
ppm need not be tested since the literature indicates that these
chemicals are, in general, immobile (Goring and Hamaker, 1972).
However, this does not preclude future soil adsorption/transfor-
mation testing of these chemicals if more refined data are needed
for the assessment process.
Soil TLC may be used to test the mobility of volatile
chemicals by placing a clean plate over the spotted soil TLC
plate and then placing both plates in a closed chromatographic
chamber.
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Soil TLC was originally designed for use with soils. The
literature shows no published use of this method with sediments
as the adsorbent phase, probably due to the fact that sediment
surface properties change significantly during air drying. The
TLC plate with the adsorbent must be air dried before leaching
studies can be undertaken.
The Test Rule will specify the minimum number of soils to be
tested. This number will depend upon the number of sites of
manufacture, distribution, use, and disposal of the test
chemical. Since there are currently 10 recognized soil orders in
the United States containing soils with wide variations in the
amounts and types of clay, organic matter, pH, primary and
secondary minerals, structure, texture, and exchange capacity the
inclusion of a requirement to test soils representing each major
order for each chemical would make this standard prohibitively
expensive.
Distilled-deionized I^O is required in order to minimize
competition effects for soil exchange sites by cationic and
anionic species normally present in tap and distilled H^O.
The purity of the test chemical will be specified in the
test rule. In general, however, the test chemical should be of
the purest grade available. Impurities may produce migration
patterns on the TLC plate independent of the parent chemical and
may be misinterpreted as transformation products. Transformation
product identification is an expensive analytical procedure that
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may be unnecessarily required as a result of the presence of
impurities.
The sieving of soils will remove the coarse (500-2,000 ym )
and medium (250-500 ym ) sand fractions. Published"testing
results showed that removal of a portion of sand had no affect on
the mobility of a test compound but aided in achieving a more
cohesive uniform soil layer and more reproducible results
(Helling 1971a).
Gentle crushing and grinding must be used to reduce soil
aggregate size. Fine particles (silt and clay) in excess of the
amount originally present may be created if excessive pressure is
exerted on the aggregates.
Application of the soil slurry to clean glass plates must be
done quickly to prevent particle size segregration. A specific
method of soil slurry application was not identified since a
number of methods which produce the acceptable layer thickness
are in use today.
Replication of the basic experimental unit was necessary in
order to estimate the standard deviation of the treatment mean.
Three replicates are considered to be the minimum number of
replicates for a statistically acceptable estimation. The soils
literature indicates that, in general, the standard deviation
should be less than 0.01 Rf units for soil TLC.
Since the available literature indicated that pesticide
mobility on soil TLC plates did not significatly change when
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temperature varied from 2° to 25°C (Helling 197la), only a room
temperature range was suggested.
The Soil Order, Series, and general clay fraction mineralogy
data may be found in Soil Survey Reports published after
approximately 1970. Pre-1970 reports may not contain mineralogy
data. Soil Survey Reports have been issued for most U.S.
counties and may be obtained from County Extension Offices; the
State Office of the U.S. Department of Agriculture Soil
Conservation Service; or the USDA-Soil Conservation Service,
Publications and Information Division, P.O. Box 2890, Washington,
DC 20013. If mineralogy data are not printed in a report, the
State Office of the U.S. Department of Agriculture Soil
Conservation Service may be contacted for assistance in obtaining
general clay mineral data of a particular soil. The test
standard does not require test soil mineral analysis since
general clay mineralogy data may already exist for the test soil.
4.4. Replies to Federal Register of March 16, 1979
The Federal Register of March 16, 1979 (EPA 1979) presented
a discussion of policy issues, alternative approaches, and test
methods under consideration for premanufacture testing. Included
in the test methods was a soil thin layer chromatography test
method (pgs. 16257-16264). Several comments received on this
method will be discussed in this section.
One commentor suggested that it is not possible to determine
the adsorption coefficients of organic compounds, having
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145
solubilities less than 0.05 ppm, with any degree of accuracy.
Therefore, a 0.5 ppm solubility limit has been adopted in the
soil TLC test standard.
Several commentors stated that the 95% extraction efficiency
in the mass balance section of the March 16, 1979, standard is
too stringent and too costly to implement. An 80 percent
extraction efficiency has been adopted in response to this
comment and is believed to be a more reasonable indicator of a
chemicals propensity to transform.
One commentor suggested that the high pressure liquid
chromatography technique HPLC be adopted instead of the soil TLC
for soil mobility testing. Sections 2.3., 2.4., and 2.5. of this
support document indicate that hydrophobia bonding is an
important effect governing soil mobility, but is not the only
effect that must be considered for all chemicals. HPLC measures
only the hydrophobia bonding potential of a neutral organic
chemical. Also section 2.6. indicates that surface
transformations may occur but no scheme to predict the occurence
of these reactions presently exists. HPLC does not measure this
effect. For these reasons, the suggestion was not adopted.
Several commentors stated that the use of soil as the
adsorption medium is impractical. A substitute adsorbent such as
carbon, clay, humic substances, oxides, and silica and/or alumina
plates could be used with several solvents of different
polarity. This suggestion was not adopted for several reasons.
First, water is the liquid environmental transport media of
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146
concern. Use of different solvents would give little insight
into the mobility of chemicals in soils. Second, section 2.4.,
2.5., and 2.6. of this document indicate that soil is a
heterogeneous mixture of numerous inorganic and organic phases.
The published literature does not substantiate the use of one
substitute adsorbent as an adequate replacement for soils during
mobility tests. Third, the literature does not indicate how data
derived from these adsorbents can be extrapolated to soils.
22
4 U. S. GOVERNMENT PRINTING OFFICE : 1880 341-085/3932
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147
5. TSCA/FIFRA/OECD COMPARABILITY
Leaching data are required to support the FIFRA registration
of all formulated products intended for terrestrial noncrop, tree
fruit/crop, and field/vegetable crop uses, and for
terrestrial/aquatic (forest) uses. One preferred technique to
support registration is soil TLC. Data derived for the Office of
Pesticide Programs will be accepted under TSCA provided chemical
mass balance data is submitted and soil physical/chemical
property guidance as discussed in the test rule have been met.
The Organization for Economic Cooperation and Development
recognizes the importance of determining the mobility of a
chemical in soils to assess the potential risk to man and the
environment due to the manufacture, distribution, use, and
disposal of chemical products. The soil adsorption isotherm was
identified by OECD as a potential method for determining the
leaching potential of chemicals in soils. This test will be
proposed as a standard method in the future.
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148
6. REFERENCES
Adams Jr. RS. 1973. Factors influencing soil adsorption and
bioactivity of pesticides. Residue Rev. 47:1-54.
Bailey GW, White JL. 1970. Factors influencing the adsorption,
desorption and movement of pesticides in soil. Residue Rev.
32:29-92.
Block RJ, Durrum EL, Zweig G. 1958. A manual of paper
chromatography and paper electrophoresis. Second Edition.
Academic Press, N.Y.
Buch RR, Ingebrigtson DN. 1979. Rearrangement of poly-
(dimethyl/siloxane) fluids on soil. Environ. Sci and Technology
13:676-679.
Buckman HO, Brady NC. 1969. The nature and properties of
soils. London: The Macmillan Company
Enviro Control, Inc. 1980. Cost analysis methodology and
protocol estimates: environmental standards. Submitted to the
Office of Regulatory Analysis, U.S. Environmental Protection
Agency, Washington, DC
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Goring CAI, Hamaker JW eds. 1972. Organic chemicals in the soil
environment. Vol. I & II. New York: Marcel Dekker, Inc.
Guenzi WD ed. 1974. Pesticides in soil and water. Madison, WI:
Soil Science Society of America, Inc.
Hamaker JW 1975. The interpretation of soil leaching
experiments. In Haque R and Freed VH eds. Environmental science
research Vol. 6: Environmental dynamics of pesticides.
Helling CS 1968. Pesticide mobility investigations using soil
thin-layer chromatography. Amer. Soc. Agron. Abstracts p. 89.
Helling CS, Turner BC. 1968. Pesticide mobility: Determination
by soil thin layer chromatography. Science 162:562.
Helling CS. 1970. Movement of s-triazine herbicides in soils.
Residue Review 32:175-210.
Helling CS. 1971a. Pesticide mobility in soils I. Parameters
of soil thin layer chromatography. Soil Sci. Soc. Amer. Proc.
35:732-737-
Helling CS. 1971b. Pesticide mobility in soils II.
Applications of soil thin layer chromatography. Soil Sci. Soc.
Amer. Proc. 35:737-743.
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Helling CS. 1971c. Pesticide mobility in soils III. Influence
of soil properties. Soil. Sci. Soc. Amer. Proc. 35:743-748.
Martin AJP , Synge RLM. 1941. A new form of chromatogram
employing two liquid phases. Biochem. J. 35:1358.
Millington RJ and Quirk JP 1961. Permeability of porous solids.
Trans. Faraday Soc. 57:1200.
Mingelgrin U, Saltzman S, Yaron B. 1977. A possible model for
the surface induced hydrolysis of organophosphorus pesticides on
kaolinite clays. Soil Sci. Soc. Amer. Jour. 41:519-523.
Mortland MM, Halloran LJ. 1976. Polymerization of aromatic
molecules on smectite. Soil Sci. Soc. Amer. Jour. 40:367-370.
Riley D. 1976. Physical loss and redistribution of pesticides
in the liquid phase. In: British Crop Protection Council
Symposium Proceedings, p. 109-115.
Shearer RC, Letey J, Farmer WJ, Klute A. 1973. Lindane
diffusion in soil. Soil Sci. Soc. Amer. Proc. 37:189-193.
Stevenson FJ. 1973. Organic matter reactions involving
pesticides in soil. In Bound and conjugated pesticide
residues. ACS Symposium Series Monograph 29/1976.
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Wang TDC, Li,SW. 1977. Clay minerals as heterogeneous catalysts
in preparation of model humic substances. Z. Pflanzenernaehr.
Bodenkd. 140:669-676.
Wang TSC, Li SW, Ferng YL. 1978. Catalytic polymerization of
phenolic compounds by clay minerals. Soil Sci. 126:15-21.
White JL. 1976. Determination of susceptibility of s-triazine
herbicides to protonation and hydrolysis by mineral surfaces.
Arch. Environ. Contain. Toxicol. 3:461-469.
Yaron B. 1978. Some aspects of surface interactions of clays
with organophosphorus pesticides. Soil. Sci. 125:210-216.
27
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S0272-101
REPORT DOCUMENTATION
PAGE
153
1. REPORT NO.
EPA 560/11-80-02?
3. Recipient's Accession No.
Title and Subtitle
Support Document, Test Data Development Standards, Physical/
Chemical & Persistence Characteristics: Density/Relative Density
Melting Temperature, Vapor Pressure. Octanol/Water Partition
5. Report Date
October 1980
(6.
7. Authors Coefficient, Soil Thin Layer Chromatography.
Geo. T. Armstrong. Robert H. Brink,
8. Performing Organization Rept. No.
Asa Lei'er
9. Performing Organization Name and Address Jam6S
U. S. Environmental Protection Agency
Office of Pesticides & Toxic Substances,
Office of Toxic Substances
Exposure Evaluation Division
401 M Street. S.W.. Washington. D.C., 20460
10. Project/Task/Work Unit No.
11. Contract(C) or Grant(Q) No.
(0
(G)
12. Sponsoring Organization Name and Address
U. S. Environmental Protection Agency
401 M Street, S.W.,
Washington, D.C., 20/4-60
13. Type of Report & Period Covered
14.
15. Supplementary Notes
This is a Support Document to a Proposed Rule for Environmental Test Standards
under the Toxic Substances Control Act (TSCA), published in the Federal Register October
16. Abstract (Limit: 200 words)
This technical Support Document provides the rationale for the development of test
standards to develop data on density/relative density, melting temperature, vapor pressure
octanol/water partition coefficient, and soil thin layer chromatography of chemical sub-
stances. EPA will use the data on these physical/chemical characteristics to evaluate
the manner and extent of environmental transport, fate and places of deposit as *>n aid
in assessing health and environmental effects of chemicals under TSCA. For density/
relative testing, an analysis is given of available methods of determining this pro-
perty of particular classes of materials with different physical characteristics. For
melting temperature testing, available methods are analyzed in terms of materials with
different physical characteristics. For vapor pressure, two procedures are given, the
isoteniscope procedure for pressures of 0.1 to 100 kPa and a gas saturation (transpiration
procedure for pressures of 10"^ to 10* Pa. The Rnudsen effusion procedures are also
given. How to determine the numerical values of the octanol/water partition coefficient
are given. Soil thin layer chromatography, an experimental method for determining the
relative mobility of ofcgsriie.'lehemicals in soils, is discussed, including scientific
- aspects of soil leaching, economic aspects of the method, and scientific history and
the rationale for selection of experimental conditions for this method.
17. Document Analysis a. Descriptors
Environmental Transport
Environmental Exposure Pathway
Adsorption
Leaching
Soil ChemiBtry
Chemical Tests
Chemical Analysis
Octanol/Water Partition Coefficient
ta. Identifiers/Open-Ended Term*
Density, Specific Gravity; Melting Point, Temperature; Gas Saturation Procedure,
Isoteniscope Procedures, Gas Transpiration Procedures; Bioaccumulation, Ecological
Concentration, Partitioning Measurement; Soil Mobility, Soil Thin Layer Chromatography,
Soil Tests.
e. COSATI Field/croup Q^Q chemistry, Physical Properties
18. Availability Statement
This document is available for general public
release.
19. Security Class (This Report)
none
20. Security Class (This Page)
21. No. of Pages
ca. 200 pp
22. Price
(See ANSI-Z39.18)
See Instructions on Reverse
OPTIONAL FORM 272 (4-77)
(Formerly NTIS-35)
Department of Commerce
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