United States Prevention, Pesticides EPA712-C-98-060
Environmental Protection and Toxic Substances January 1998
Agency (7101)
&EPA Fate, Transport and
Transformation Test
Guidelines
OPPTS 835.2210
Direct Photolysis Rate in
Water By Sunlight
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INTRODUCTION
This guideline is one of a series of test guidelines that have been
developed by the Office of Prevention, Pesticides and Toxic Substances,
United States Environmental Protection Agency for use in the testing of
pesticides and toxic substances, and the development of test data that must
be submitted to the Agency for review under Federal regulations.
The Office of Prevention, Pesticides and Toxic Substances (OPPTS)
has developed this guideline through a process of harmonization that
blended the testing guidance and requirements that existed in the Office
of Pollution Prevention and Toxics (OPPT) and appeared in Title 40,
Chapter I, Subchapter R of the Code of Federal Regulations (CFR), the
Office of Pesticide Programs (OPP) which appeared in publications of the
National Technical Information Service (NTIS) and the guidelines pub-
lished by the Organization for Economic Cooperation and Development
(OECD).
The purpose of harmonizing these guidelines into a single set of
OPPTS guidelines is to minimize variations among the testing procedures
that must be performed to meet the data requirements of the U. S. Environ-
mental Protection Agency under the Toxic Substances Control Act (15
U.S.C. 2601) and the Federal Insecticide, Fungicide and Rodenticide Act
(7U.S.C. I36,etseq.).
Final Guideline Release: This guideline is available from the U.S.
Government Printing Office, Washington, DC 20402 on The Federal Bul-
letin Board. By modem dial 202-512-1387, telnet and ftp:
fedbbs.access.gpo.gov (IP 162.140.64.19), or call 202-512-0132 for disks
or paper copies. This guideline is also available electronically in ASCII
and PDF (portable document format) from EPA's World Wide Web site
(http://www.epa.gov/epahome/research.htm) under the heading "Research-
ers and Scientists/Test Methods and Guidelines/OPPTS Harmonized Test
Guidelines."
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OPPTS 835.2210 Direct photolysis rate in water by sunlight.
(a) Scope—(1) Applicability. This guideline is intended to meet test-
ing requirements of both the Federal Insecticide, Fungicide, and
Rodenticide Act (FIFRA) (7 U.S.C. 136, et seq.) and the Toxic Substances
Control Act (TSCA) (15 U.S.C. 2601).
(2) Background. The source material used in developing this har-
monized OPPTS test guideline is 40 CFR 796.3700 Photolysis in Aqueous
Solution in Sunlight.
(b) Introduction—(1) Background and purpose. Numerous chemi-
cals enter natural aquatic systems from a variety of sources. For example,
chemical wastes are discharged directly into natural water bodies, and
chemicals leach into natural water bodies from landfills. Pesticides are ap-
plied directly into water bodies, and are applied to soils and vegetation,
and subsequently leach into water bodies. Pollutants present in aqueous
media can undergo photochemical transformation in the environment (i.e.
in sunlight by direct photolysis or by sensitized photolysis). As a result,
there is considerable interest in photolysis in solution, especially the pho-
tolysis of pesticides. However, most of these studies have been qualitative
in nature and involved the identification of photolysis products. Quan-
titative data in the form of rate constants and half-lives are needed to deter-
mine the importance of photochemical transformation of pollutants in
aqueous media. This test method describes a two-tiered screening level
approach for determining direct photolysis rate constants and half-lives of
chemicals in water in sunlight.
(2) Definitions and units. The definitions in section 3 of TSCA and
in 40 CFR Part 792—Good Laboratory Practice Standards (GLP) apply
to this test guideline. The following definitions also apply to this test
guideline.
Absorbance (Ax) is the logarithm of the ratio of the initial intensity
(Io) of a beam of radiant energy to the intensity (I) of the same beam
after passage through a sample at a fixed wavelength A,. Thus, AX = log(Io/
I).
The Beer-Lambert law states that the absorbance of a solution of a
given chemical species, at a fixed wavelength, is proportional to the thick-
ness of the solution (1), or the light pathlength, and the concentration of
the absorbing species (C).
Direct photolysis is the direct absorption of light by a chemical fol-
lowed by a reaction which transforms the parent chemical into one or more
products.
A first-order reaction is a reaction in which the rate of disappearance
of a chemical is directly proportional to the concentration of the chemical
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and is not a function of the concentration of any other chemical present
in the reaction mixture.
The Grotthus-Draper law, the first law of photochemistry, states that
only light which is absorbed can be effective in producing a chemical
transformation.
The half-life (t\/2) of a chemical is the time required for the concentra-
tion of the chemical being tested to be reduced to one-half its initial value.
Molar absorptivity (ex) is the proportionality constant in the Beer-
Lambert law when the concentration is given in terms of moles per liter
(i.e. molar concentration). Thus, AX = exC/, where AX and ex represent
the absorbance and molar absorptivity at wavelength X and / and C are
defined in (3). The units of ex are molar^1 cm-1. Numerical values of
molar absorptivity depend upon the nature of the absorbing species.
Radiant energy, or radiation, is defined as the energy traveling as
a wave unaccompanied by transfer of matter. Examples include X-rays,
visible light, UV light, radio waves, etc.
The reaction quantum yield (§x) for an excited-state process is de-
fined as the fraction of absorbed light that results in photoreaction at a
fixed wavelength X. It is the ratio of the number of molecules that
photoreact to the number of quanta of light absorbed or the ratio of the
number of moles that photoreact to the number of einsteins of light ab-
sorbed at a fixed wavelength A,.
The solar irradiance in water (L^) is related to the sunlight intensity
in water and is proportional to the average light flux (in the units of 10 3
einsteins cm 2 day :) that is available to cause photoreaction in a wave-
length interval centered at A over a 24-hour day at a specific latitude and
season date.
The Stark-Einstein law, the second law of photochemistry, states that
only one molecule is activated to an excited state per photon or quantum
of light absorbed.
The sunlight direct aqueous photolysis rate constant (^PE) is the first-
order rate constant in the units of day : and is a measure of the rate of
disappearance of a chemical dissolved in a water body in sunlight.
A glossary of symbols can be found under paragraph (c)(5) of this guide-
line.
(3) Principle of the test method, (i) This test method is based on
the principles developed by Zepp and Cline under paragraph (e)(8) of this
guideline, Zepp under paragraph (e)(ll) of this guideline, Mill et al. under
paragraphs (e)(4), (e)(5), and (e)(6) of this guideline, and Dulin and Mill
under paragraph (e)(2) of this guideline.
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(ii) Zepp and Cline, under paragraph (e)(8) of this guideline published
a paper on the rates of direct photolysis in aquatic environments. The rates
of all photochemical processes in a water body are affected by solar spec-
tral irradiance at the water surface, radiative transfer from air to water,
and the transmission of sunlight in the water body. It has been shown
that for photolysis of a chemical in an optically thin aqueous solution,
the kinetics of direct photolysis can be described by the following equa-
tions:
Equation 1
In (Co/Ct) = kpEt
Equation 2
tl/2E = 0.693/kpE
Equation 3
kpE = 4»Eka
where §E is the reaction quantum yield of the chemical in dilute solution
and is independent of the wavelength, ka = Zkax, the sum of kax values
of all wavelengths of sunlight that are absorbed by the chemical (i.e. the
light absorption rate constant), t is the time, Co and Ct are the concentra-
tions of chemical at t = 0 and t, and ti/2E represents the half-life. The
term kpE represents the first-order photolysis rate constant for a water body
in sunlight in the units of reciprocal time.
(iii) Furthermore, under the same conditions cited above, the first-
order direct photolysis rate constant, kpE, is given by the following equa-
tion:
Equation 4
kpE = (fcZexLx
where ([>£• is the reaction quantum yield, e?i is the molar absorptivity in
the units molar^1 cm-1, LX is the solar irradiance in water in the units
of 10 3 einsteins cm 2 day : [Mill et al. under paragraph (e)(5) of this
guideline], and the summation is taken over the range A, = 290 to
800 nm. LX is the solar irradiance at shallow depths for a water body
under clear sky conditions and is a function of latitude and season of the
year.
(iv) The method of Zepp and Cline under paragraph (e)(8) of this
guideline and the method of Mill et al. under paragraph (e)(5) of this
guideline are applicable to sunlight incident on a water surface such as
natural water body. However, the method developed in this guideline
measures rate constants in tubes (e.g. 13 x 100 mm) and the rate is faster
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in tubes. This is discussed in more detail in paragraph (c)(2)(i)(J) of this
guideline. Thus, equations 1 and 2 have to be modified to take this into
account. For simplicity, the following nomenclature is used. For water
bodies, the rate constant is designated as kpn with the subscript E designat-
ing rates in the environment in water bodies. For tubes, the rate constant
is designated as kp. The corresponding half-lives for water bodies and
tubes are ti/zn and ti/2, respectively. Thus, for tubes, equations 1 and 2
can be written as:
Equation 5
ln(Co/Ct) = kpt
Equation 6
ti/2 = 0.693/kp
(v) A simple first-tier screening test has been developed using Equa-
tion 4 under paragraph (b)(3)(iii) of this guideline. As an approximation,
it is assumed that the reaction quantum yield ([>E is equal to one, the maxi-
mum value. As a result, the upper limit for the direct photolysis sunlight
rate constant in aqueous solution is obtained and Equation 4 under para-
graph (b)(3)(iii) of this guideline becomes
Equation 7
(kpE)max = Ze?tLx
Using equation 7 in equation 2 under paragraph (b)(3)(ii) of this guideline,
the lower limit for the half-life is then given by
Equation 8
(tl/2E)min = 0.693/(kpE)max
The molar absorptivity can be determined experimentally by the method
outlined in paragraph (c)(l) of this guideline and values of LX are given
in Tables 3 to 6 as a function of latitude and season of the year under
paragraph (c)(3) of this guideline. These data can then be used in equation
7 to calculate (kpE)max. Finally, (kpE)max can then be substituted in Equa-
tion 8 tO Calculate ti/2E)min.
(vi) In a second-tier test method, an aqueous photolysis screening test
has been developed to determine rate constants and half-lives in the pres-
ence of sunlight using Equations 1, 2, 4, 5, and 6 (Mill et al. under para-
graphs (e)(4), (e)(5) and (e)(6) of this guideline, and Dulin and Mill under
paragraph (e)(2) of this guideline). The second-tier test method is divided
into two phases. In phase one, the test chemical is photolyzed in sunlight
in order to obtain an approximate rate constant, kcp. This method only
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gives an approximate rate constant since it fails to measure sunlight inten-
sities incident on the sample during photolysis.
(vii) In phase two, a standard /7-nitroacetophenone-pyridine actinom-
eter (PNAP/PYR) is used to measure sunlight intensities incident on the
sample during photolysis (Mill et al. under paragraph (e)(6) of this guide-
line and Dulin and Mill under paragraph (e)(2) of this guideline). The
rate constant for this actinometer, kap, can be adjusted to match the approx-
imate rate constant of the test chemical by adjusting the concentration of
pyridine. Since the rate constant is a function of the reaction quantum
yield of the actinometer, the rate constant can be adjusted according to
the equation
Equation 9
= 0.0169[PYR]
where [P YR] is the molar concentration of pyridine for a /7-nitroacetophe-
none (PNAP) concentration of 1.00 x 10 5 M. The reaction quantum yield
for the test chemical, 4>CE, is given by
Equation 10
The reaction quantum yield of the test chemical, (|)CE, can be determined
in the following way. By measuring the concentration of test chemical
and actinometer (PNAP) as a function of time t in sunlight, the ratio of
rate constants, (kcp/kap), can be determined using Equation 5 under para-
graph (b)(2)(i)(H) of this guideline. The reaction quantum yield <^E can
be determined from Equation 9 at the molar concentration of pyridine used
in the standard actinometer. The term ZeaxLx for the actinometer has been
tabulated as a function of latitude and season of the year in Table 2 under
paragraph (c)(3) of this guideline. The term ZecxLx for the test chemical
can be obtained from the experimentally measured molar absorptivities
under paragraph (b)(l) of this guideline and the values of LX listed in
Tables 3 to 6, as a function of latitude and season of the year under para-
graph (c)(3) of this guideline.
(viii) With the values of 4>CE, £cx, and the appropriate LX values, kpE
for the test chemical can be calculated as a function of latitude and season
of the year in the United States using Equation 4 under paragraph
(b)(3)(iii) of this guideline. The corresponding half-life can be calculated
using kpE in Equation 2 under paragraph (b)(3)(ii) of this guideline.
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(4) Applicability and specificity, (i) This test method is applicable
to all chemicals which have UV/visible absorption maxima in the range
of 290 to 800 nm. Some chemicals have absorption maxima significantly
below 290 nm and consequently cannot undergo direct photolysis in sun-
light (e.g. chemicals such as alkanes, alkenes, alkynes, saturated alcohols,
and saturated acids). This is a direct consequence of the Grotthus-Draper
law. Some chemicals have absorption maxima significantly below 290 nm
but have measurable absorption tails above the baseline in their absorption
spectrum at wavelengths greater than 290 nm. Photolysis experiments
should be carried out for these chemicals.
(ii) These test methods are only applicable to pure chemicals and not
to the technical grade.
(iii) The first-tier screening test can be employed to estimate
(kpE)max and (ti/2E)min. If these data indicate that aqueous photolysis is
an important process relative to other transformation processes (e.g. bio-
degradation, hydrolysis, oxidation, etc.), then it is recommended that the
second-tier photolysis tests be carried out to determine environmentally
relevant rate constants and half-lives in sunlight. The data obtained from
this test can be used to determine kpn for the test chemical as a function
of latitude and season of the year anywhere in the United States. These
rate constants are in a form suitable for preliminary mathematical modeling
for environmental fate of a test chemical.
(iv) The second-tier screening test is applicable to the direct photoly-
sis of chemicals in a homogeneous dilute solution with absorbance less
than 0.05 in the reaction cell at all wavelengths greater than 290 nm and
at shallow depths (less than 0.5 m). These results are applicable to direct
sunlight photolysis for water bodies and clear sky conditions. In addition,
these experiments are limited to the direct photolysis of chemicals in air-
saturated pure water.
(v) This screening test has been designed to determine the molar ab-
sorptivity of a test chemical, 8xc, and its reaction quantum yield, (|)CE. These
parameters can be used to determine environmentally relevant rate con-
stants at low absorbance and shallow depths in pure water as a function
of latitude and season of the year. Tables of solar irradiance (Tables 3
to 6) under paragraph (c)(3) of this guideline have been included in this
test method to carry out all the calculations. However, the method is really
very general and can be extended to determine the rates of photolysis over
a range of other environmental conditions using a computer program. Zepp
and Cline under paragraph (e)(8) of this guideline have written a computer
program to calculate the rates of photolysis as a function of depth in water,
as a function of the attenuation coefficient of the water (ax) for natural
water bodies, the average ozone layer thickness that pertains to the seasons
and location of interest, and as a function of latitude and season of the
year. This program has been recently updated with the best available solar
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irradiance data and is called the GC SOLAR program. The GC SOLAR
computer program is available on request as referenced under paragraph
(e)(10) of this guideline.
(c) Test procedures—(1) Tier 1 Test: UV/visible absorption
spectra-estimation of aqueous photolysis maximum rate constant and
minimum half-life in sunlight. The UV/visible absorption spectra in
aqueous solution can be determined by the methods described in OPPTS
830.7050. It is recommended that the following additional procedures be
followed:
(i) For chemicals which ionize or protonate (e.g. carboxylic acids,
phenols, amines), carry out UV/visible absorption studies at pHs at least
two orders or magnitude above the pKa and at least two orders of mag-
nitude below the pKa. Prepare buffer solutions at 25 °C using reagent
grade chemicals and distilled water as follows: pHs in the range 3-6—
NaH2PO4/HCl; pHs in the range 6-8—KH2PO4 /NaOH; pHs in the range
>8—prepare buffers as described in the Handbook of Chemistry and Phys-
ics.In the case of pHs 3-6 and 6-8, use the minimum concentration of
buffers to attain the desired pH. Check the pH of all the buffer solutions
with a pH meter at 25 °C and adjust to the proper pH, if necessary. These
buffer solutions can then be added to the test chemical solution until the
desired pH is obtained. If these buffers are inadequate, then adjust the
pH of the test chemical solution with 1 M HC1 or NaOH at 25 °C.
(ii) (A) Measure the absorbance, AX, as a function of wavelength in
the range of 290 to 800 nm in duplicate. If applicable, measure AX at
each experimental pH. Record, in duplicate, the baseline when both the
sample and reference cells are filled with blank solutions. These data will
be used to calculate the molar absorptivities for the appropriate wavelength
intervals and wavelength centers in Table 1 under paragraph (c)(3) of this
guideline, where the test chemical absorbs light. The wavelength center
is defined as the midpoint of the interval range.
(B) It must be emphasized that the molar absorptivities of the test
chemical must be carefully determined, especially in the tails of the ab-
sorption bands at X > 290 nm. Large errors will be encountered in calculat-
ing photolysis rate constants and half-lives if these measurements are not
carefully carried out.
(2) Tier 2 Test: Aqueous Photolysis in Sunlight—(i) Test condi-
tions—(A) Special laboratory equipment. It is recommended that quartz
tubes be used for the photolysis of chemicals with appreciable absorption
at wavelengths below 340 nm. Chemicals that absorb appreciably at wave-
lengths greater than 340 nm may be tested in borosilicate tubes. Thin-
walled borosilicate or quartz tubes are recommended. Disposable culture
tubes (13 x 100 mm) with Teflon-lined screw caps or quartz tubes with
quartz or borosilicate stoppers, Teflon-lined, may be used as reaction ves-
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sels. Tubes of 11 mm i.d. are recommended. For some chemicals, it may
be difficult to determine the concentration of the test chemical in reaction
tubes of small volume. For these chemicals, larger volume reaction vessels
are recommended provided that the cell walls are thin and the pathlength
of radiation through the vessel is less than 0.5 meter.
(B) Purity of water. Reagent grade water, e.g. water meeting ASTM
Type II A standards, or an equivalent grade, is recommended to minimize
biodegradation. ASTM Type II A water is described in ASTM D 1193-
77—Standard Specification for Reagent Water. Air-saturated water can be
easily prepared by allowing the water to equilibrate in a vessel plugged
with sterile cotton. Copies may be obtained from the American Society
for Testing and Materials (ASTM), 1916 Race Street, Philadelphia, PA
19103.
(C) Sterilization. It is extremely important to sterilize all glassware
and to use aseptic conditions in the preparation of all solutions and in
carrying out all photolysis experiments to eliminate or minimize biodeg-
radation. Glassware can be sterilized in an autoclave or by any other suit-
able nonchemical method.
(D) pH effects. It is recommended that all photolysis experiments
be carried out at pHs at least two orders of magnitude above the pKa
and at least two orders of magnitude below the pKa for any chemical
which ionizes or protonates (e.g. carboxylic acids, phenols, and amines).
Buffers described in paragraph (b)(2)(ii)(B) of this guideline should be
used.
(E) Volatile chemical substances. Special care should be taken when
testing a volatile chemical so that the chemical substance is not lost due
to volatilization during the course of the photolysis experiment. Thus, it
is important to effectively seal the reaction vessels. Disposable culture
tubes with Teflon-lined screw caps or quartz tubes with quartz or
borosilicate stoppers, Teflon-lined, are recommended. Volatile compounds
can be conveniently studied in culture tubes equipped with Mininert®
valves. Samples can be introduced into or removed from the tubes through
the septum in these valves with no loss of substrate. As an alternative,
the tubes can be sealed with a torch. In addition, the reaction vessels
should be as completely filled as is possible to prevent volatilization to
any air space.
(F) Control solution. It is extremely important to take certain pre-
cautions to prevent loss of chemical from the reaction vessels by processes
other than photolysis. For example, biodegradation and volatilization can
be eliminated or minimized by use of sterile conditions and minimal air-
space in sealed vessels. Hydrolysis is a process which cannot be minimized
by such techniques. Thus, control vessels containing test substances which
are not exposed to sunlight are required. In this way, the loss of test chemi-
8
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cal for processes other than photolysis may be determined and eliminated.
For simplicity, if the loss of chemical in the control is small (i.e. approxi-
mately 10 percent or less), one can calculate a first-order loss, kioss, and
subtract it from (kp)0bs to give the corrected direct photolysis rate constant
kp. If hydrolysis is found to be significant (i.e. greater than 10 percent),
hydrolysis studies should be carried out first under OPPTS 835.2110.
(G) Absorption spectrum as a criterion for performing the aque-
ous photolysis test. This aqueous photolysis screening test is applicable
to all chemicals which have UV/visible absorption maxima in the range
of 290 to 800 nm. Some chemicals have absorption maxima significantly
below 290 nm but have measurable absorption tails above the baseline
in their absorption spectrum at wavelengths greater than 290 nm. Photoly-
sis experiments should be carried out for these chemicals. The absorption
spectrum of the chemical in aqueous solution can be measured by OPPTS
830.7050.
(H) Sunlight actinometer. (7) In order to quantify the rate of pho-
tolysis more precisely, it is necessary to measure the sunlight intensity
incident on the sample during photolysis. A standard/7-nitroacetophenone-
pyridine actinometer (PNAP/PYR) has been developed (Mill et al. under
paragraphs (e)(4) and (e)(6) of this guideline and Dulin and Mill under
paragraph (e)(2) of this guideline) to measure the sunlight intensity inci-
dent on the sample during photolysis and this actinometer has been incor-
porated in this section. According to Equation 4 under paragraph (b)(3)(iii)
of this guideline, the rate constant is a function of the reaction quantum
yield. Furthermore, the reaction quantum yield can be adjusted by varying
the molar concentration of the pyridine according to Equation 9 under
paragraph (b)(3)(vii) of this guideline. Hence, by varying the pyridine con-
centration, the actinometer photolysis rate constant can be adjusted so that
the half-life can range from several hours to several weeks. The initial
concentration of PNAP is set at 1.00 x 10 5 M.
(2) Using the test chemical photolysis rate constant, kpc, determined
in Tier 2, Phase 1, and the variable kaa ( = ZeaxLx), listed in Table 2
under paragraph (c)(3) of this guideline the molar concentration needed
to adjust the rate of disappearance of PNAP in PNAP/PYR to match the
rate of disappearance of the test chemical is given by
Equation 11
[PYR] = 26.9 (kpc/ka*)
(3) Experiments are carried out by simultaneously photolyzing the
test chemical and actinometer solutions. The concentrations of test chemi-
cal and actinometer are measured periodically as a function of time. These
data are then used to determine the ratio of the rate constants, kpc/kpa,
using linear regression analysis on the following equation:
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Equation 12
ln(C0/Ct)c = (kpckpa) ln(C0/Ct)a
with In (C0/Ct)a as the independent variable and ln(C0/Ct)c as the depend-
ent variable. The slope of the best straight line is the ratio of the rate
constants, kpc/kpa.
(I) Solar irradiance data. In order to calculate the reaction quantum
yield of the test chemical, ([>EC, and then calculate kpEc and ti/2E, it is ti/2E
necessary to use the solar irradiance parameter L^-Lx values are propor-
tional to the average light flux that is available to cause photolysis in a
wavelength interval centered at X over a 24-hour day at a specific latitude
and season date. The LX values are defined by the angle of declination
of the sun at -20° for winter, -10° for fall, +10° for spring, and +20°
for summer. The actual dates for 1982 that correspond to these angles
of declination are January 21, April 16, July 24, and October 20, for win-
ter, spring, summer, and fall, respectively (AA (1982) under paragraph
(e)(l) of this guideline). The LX values for these season dates are listed
in Tables 3 to 6 under paragraph (c)(3) of this guideline as a function
of latitude and are applicable to clear sky conditions, water bodies, shallow
depths, and for chemicals whose absorbance is less than 0.05 in pure water
(Mill et al. under paragraph (e)(7) of this guideline).
(J) Geometry of the reaction vessel. The method of Zepp and Cline
under paragraph (e)(8) of this guideline and the method of Mill et al.,
under paragraph (e)(5) of this guideline are applicable to sunlight incident
on a water surface such as a natural water body while the method devel-
oped in this test method measures rate constants (kp) in tubes (e.g.
13 x 100 mm). However, rates in tubes are faster than in water bodies and
it has been experimentally observed (Mill et al. under paragraph (e)(6)
of this guideline) that
Equation 13
kp = 2.2kpE
Because tubes are the simplest and easiest reaction vessels to use, this
test method recommends the use of tubes as reaction vessels and the meth-
od has been modified to take into account the increased rate in tubes (equa-
tion 13).
(K) Chemical analysis of solution. (7) In determining the concentra-
tion of the chemical in solution, an analytical method should be selected
which is most applicable to the analysis of the specific chemical substance.
Chromatographic methods are generally recommended because of their
chemical specificity in analyzing the parent chemical substance without
interference from impurities. Whenever practicable the chosen analytical
method should have a precision of + 5 percent or better.
10
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(2) The /7-nitroacetophenone in the chemical actinometer solution is
conveniently analyzed by high-pressure liquid chromatography using a
30 cm Cig reverse-phase column and a UV detector set at 280 nm. The
mobile phase in volume percent is 2.5 percent acetic acid, 50 percent ace-
tonitrile, and 47.5 percent water which is passed through the column at
a flow rate of 2 mL/min.
(ii) Preparations—(A) Preparation of test chemical solution. Pre-
pare homogeneous solutions with the chemical at less than one-half of
its solubility in water and at a concentration such that the absorbance is
less than 0.05 in the photolysis reaction vessel at wavelengths greater than
290 nm. For very hydrophobic chemicals, it is difficult and time consum-
ing to prepare aqueous solutions. To facilitate the preparation of aqueous
solutions containing very hydrophobic chemicals and to allow for easier
analytical measurement procedures, the following procedure may be used
to aid in the dissolution of the chemical. Dissolve the pure chemical in
reagent grade acetonitrile. Add pure water as described under Test Condi-
tions, in paragraph (b)(2)(i)(B) of this guideline, or buffer solution as de-
scribed under Preparations, in paragraph (b)(2)(ii)(B) of this guideline, for
chemical substances which ionize or protonate, to an aliquot of the acetoni-
trile solution. Do not exceed one volume-percent of acetonitrile in the final
solution. Place the reaction solution in the appropriate photolysis reaction
tubes as described in paragraph (b)(2)(i)(A) of this guideline.
(B) Preparation of buffer solutions. Prepare buffer solutions accord-
ing to the procedures outlined in paragraph (b)(l)(i) of this guideline using
reagent grade chemicals and pure water as described under Test Condi-
tions, in paragraph (b)(2)(i)(B) of this guideline.
(C) Preparation of actinometer solution. (7) Using the test chemical
photolysis rate constant, kpc, determined in Tier 2, Phase 1, and the vari-
able kaa listed in Table 2 under paragraph (c)(3) of this guideline, the
molar concentration of pyridine needed to adjust the rate of disappearance
of/7-nitroacetophenone (PNAP) to match the rate of disappearance of the
test chemical can be obtained from equation 11 under paragraph
(b)(2)(i)(H)(2) of this guideline. The variable kaa ( = ZexaLx) is equal
to the day-average rate constant for sunlight absorption by PNAP which
changes with season and latitude. The value of kaa is selected from Table
2 under paragraph (c)(3) of this guideline for the season nearest the mid-
experiment date of the Tier 2, Phase 1, studies and the decadic latitude
nearest the latitude of the experimental site.
(2) Once the molar concentration of pyridine [PYR] has been deter-
mined, an actinometer solution can be prepared as follows. Dissolve 0.165
gm. of PNAP in 100 mL of acetonitrile (0.01 M). Add 1 mL of this solu-
tion to a 1-L volumetric flask. Add to the volumetric flask the mass in
grams, or the volume (V) of pyridine at 20° C, obtained from the equations
11
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Equation 14
mass(grams) = 79.1 [PYR]
V(mL) = 80.6 [PYR]
Fill the volumetric flask with pure water as described in paragraph
(b)(2)(i)(B) of this guideline to give 1 L of solution and shake vigorously
to make sure that the solution is homogeneous. The PNAP/PYR solution
should be wrapped with aluminum foil and kept from bright light.
(iii) Performance of the tests—(A) Phase 1 experiments. (7) For
all experiments, prepare an aqueous solution of the chemical substance,
as described in paragraph (b)(2)(ii)(A) of this guideline, and a sufficient
number of samples in quartz or borosilicate glass tubes to perform all the
required tests. Fill the tubes as completely as possible and seal them. Pre-
pare two control samples in the absence of UV light and totally exclude
light by wrapping the tubes with aluminum foil or by other suitable meth-
ods. These samples are analyzed for the chemical substance immediately
after completion of the experiment to measure the loss of chemical in the
absence of light. Place the samples, including the controls, outdoors in
an area free of shade and reflections of sunlight from windows and build-
ings. Place the samples on a black, nonreflective background and inclined
at approximately 30° from the horizontal with upper end pointing due
north (in the northern hemisphere). Conduct the photolysis experiments
during a frost-free time of year (e.g. May, June, July, August, or Septem-
ber in the northern hemisphere—weather permitting) and start the experi-
ments initially at noon (1200 hours). Record the date and time the experi-
ment was begun, the date and time completed, the time of sunrise and
sunset on all days when photolysis experiments were performed, the times
exposure was stopped and restarted for intermittent exposure, the weather
conditions during the period, and the latitude of the site. For chemical
substances that ionize or protonate, carry out photolysis experiments at
the required pHs as described under Test Conditions under paragraph
(b)(2)(i)(D) of this guideline.
(2) If a significant loss of test chemical has occurred in the control
samples, determine the cause and eliminate or minimize the loss. If hydrol-
ysis is found to be significant, hydrolysis studies should be carried out
first under paragraph (b)(2)(i)(F) of this guideline.
(3) Use one of the following procedures, depending on how fast the
chemical substance photolyzes.
(/) Procedure 1. If the chemical substance transforms 50 to 80 per-
cent within 28 days, measure the concentration of the chemical substance,
in duplicate, at time t = 0 and periodically (at least four data points at
approximately equal time intervals) at noon (1200 hours) until at least 50
percent of the substance has been consumed. As a simplification, the sam-
12
-------
pling times can be estimated as the photolysis experiments progress. Deter-
mine the concentration of test chemical from two, freshly opened, reaction
tubes for each time point. Determine the concentration in each of the two
control solutions as soon as the photolysis experiments are completed.
(if) Procedure 2. If the chemical substance transforms in the range
of 20 to 50 percent in 28 days, determine the concentration of the chemical
substance, in duplicate, at time t = 0. Determine the concentration of the
chemical in the two separate reaction tubes and the two control tubes after
28 days of photolysis.
(fff) Procedure 3. For chemical substances that transform in sunlight
50 to 80 percent within one or two days, place the samples outside at
noon (1200 hours) and analyze two samples for the concentration of the
chemical substance at t = 0, and in two, freshly opened, reaction tubes
at noon (1200 hours) the next day, and again, in two, freshly opened,
reaction tubes at noon (1200 hours) the second day. Determine the con-
centration of the test chemical in each of the two control solutions after
the first day of photolysis and as soon as the photolysis experiments have
been completed on the second day.
(fv) Analytical methodology. Select an analytical method which is
most applicable to the analysis of the specific chemical being tested under
paragraph (b)(2)(i)(K) of this guideline.
(B) Phase 2 experiments. (7) Using the test chemical photolysis rate
constant, kcp, determined in Tier 2, Phase 1, prepare an actinometer solu-
tion, as described in paragraph (b)(2)(ii)(C) of this guideline and a suffi-
cient number of samples in quartz tubes to perform all the required tests.
Fill all the tubes as completely as possible, seal them, and cover them
with aluminum foil as soon as possible after preparation. Prepare an aque-
ous solution of test chemical, as described in paragraph (b)(2)(ii)(A) of
this guideline, and a sufficient number of samples in quartz or borosilicate
tubes to perform all the required tests. Fill these tubes as completely as
possible, seal them, and cover them with aluminum foil as soon as possible
after preparation. Place all the samples outdoors in an area free of shade
and reflections of sunlight from windows and buildings. Place the samples
on a black, nonreflective background and inclined at approximately 30°
from the horizontal with the upper end pointing due north (in the northern
hemisphere). Remove the foil from all samples except for the test chemical
control solutions and the actinometer control solutions at noon (1200
hours). Based on the results of the Phase 1 experiments, determine the
concentration of test chemical and actinometer (PNAP), in triplicate, at
time t = 0 and periodically (at least five data points at approximately equal
time intervals). Determine the concentration of PNAP in the three actinom-
eter control solutions and the concentration of test chemical in the three
control solutions for each time point.
13
-------
(2) Select an analytical method which is most applicable to the analy-
sis of the specific chemical tested, in paragraph (b)(2)(i)(K) of this guide-
line and follow the procedure given in paragraph (b)(2)(i)(K) of this guide-
line for the analysis of PNAP.
(d) Data and reporting—(1) Tier 1 Test: UV/visible Absorption
Spectra—Estimation of Aqueous Photolysis Maximum Rate Constant
and Minimum Half-Life in Sunlight—(i) Treatment of results. (A) The
molar absorptivity can be determined from the absorption spectra using
the expression.
Equation 15
exc = Ax/Cl
where AX is the absorbance at wavelength A,, C is the molar concentration
of test chemical, and 1 is the cell pathlength in centimeters. The molar
absorptivity of the chemical should be determined for the wavelengths list-
ed in Table 1 under paragraph (d)(3) of this guideline for a solution of
concentration C and in a cell with pathlength. /. If the absorption curve
is flat within the interval around the wavelength ^center, ex may be deter-
mined from the absorbance AX at ^center using equation 15. If a large
change in absorbance occurs within this interval, obtain an average
absorbance AX at ^center based on the absorbances at the two boundaries
of the interval. Calculate an average ex using the average value of AX
in equation 15. Determine the molar absorptivity for each replicate and
calculate a mean value.
(B) Using the molar absorptivities obtained from the spectra and the
values of the LX from Tables 3 to 6 under paragraph (d)(3) of this guide-
line, the maximum rate constant (kpE)max can be calculated at a specific
latitude and season of the year using equation 7 under paragraph (b)(3)(v)
of this guideline. The minimum half-life, (ti/2E)min can then be calculated
using this (kpE)max in equation 8 under paragraph (b)(3)(v) of this guide-
line.
(C) Two hypothetical examples are presented in paragraph (d)(4)(i)
of this guideline to illustrate how the test data obtained in the first-tier
screening test can be used.
(ii) Test data report. (A) Submit the original chart, or photocopy,
containing a plot of absorbance of test chemical vs. wavelength plus the
baseline. Spectra should include a readable wavelength scale, preferably
marked at 10 nm intervals. Each spectrum should be clearly marked with
the test conditions.
(B) Report the concentration of the test chemical solution, the type
of absorption cell used (quartz or borosilicate glass) and the pathlength.
14
-------
(C) Report AX and 8x at Center for each replicate and the mean value.
(D) Report (kpE)max and (ti/2E)min for the summer and winter solstices
using the appropriate LX values from Tables 3-6 closest to the latitude
of the chemical manufacturing site.
(E) Report the identity and compositon of the solvent used in the
spectral absorption study.
(F) For ionizable chemicals, report its pKa. Report the type and con-
centration of the buffers employed for each pH. Report the pHs in which
the photolysis experiments were carried out.
(G) Describe the method used in determining the concentration of
the test chemical.
(H) Report the name, structure, and purity of the test chemical.
(I) Submit a recent test spectrum on appropriate reference chemicals
for photometric and wavelength accuracy.
(J) Report the name and model of the spectrophotometer used.
(K) Report the various control settings employed with the
spectrophotometer. These might include scan speed, slit width, gain, etc.
(2) Tier 2 Test: Aqueous photolysis in sunlight—(i) Phase 1 ex-
periments—(A) Treatment of results. (7) If a small loss of test substance
in the control tubes has occurred, use this data to make corrections to
the measured photolysis rate in paragraph (b)(2)(i)(F) of this guideline.
Note the site of photolysis and its latitude and the weather conditions.
For Procedures 1 and 2 note the dates and times of actual exposure includ-
ing times of sunrise and sunset and, in case the cells are moved to prevent
freezing or for other reasons, make sure that these times are recorded and
that the cells are kept in a dark place when exposure is not in progress.
(/) For chemical substances which transform 50 to 80 percent within
28 days, use a concentration Ct, which corresponds to less than 50 percent
of the initial concentration of chemical substance remaining, and the cor-
responding time t, in days, along with the initial molar concentration Co,
in Equation 5 to calculate kp in days-1. From the analysis of the two sam-
ples at time t = 0 and t, calculate a mean value of Co and Ct, respectively,
and a value of kp. If a slight loss of chemical has been detected in the
controls, then calculate a rate constant as follows: Calculate an average
concentration Ct, based on the duplicate measurements of concentration
in the controls. Use this concentration along with the average initial con-
centration in Equation 5 and calculate a rate constant kioss. Using this rate
constant along with the observed rate constant, the corrected rate constant
is then
15
-------
Equation 16
Calculate the half-life, ti/2 using the corrected kp value in Equation 6 under
paragraph (b)(3)(iv) of this guideline.
(//) For chemical substances which transform 20 to 50 percent in 28
days, use the mean concentration Ct remaining at t = 28 days along with
the mean value of Co to calculate kp. Use the same procedure as described
above to calculate the value of kp and ti/2. If less than 20 percent of the
chemical substance degrades in 28 days, report the mean concentration
of Ct and C0. In this case the apparent half-life is reported as greater than
3 months.
(///) For chemical substances which transform 50 percent or more in
the first day, as described in Procedure 3, calculate a full day kp value
using the mean concentration Ct of chemical substance remaining at noon
(1200 hours) after the first day along with the mean value of Co using
Equation 5 under paragraph (b)(3)(iv) of this guideline. For chemical sub-
stances which degrade less than 50 percent at noon (1200 hours) after
the first day but 50 percent or more at noon (1200 hours) the second day,
calculate kp using the mean concentration of chemical substances remain-
ing at noon (1200 hours) the second day. Calculate the half-life, ti/2, using
the mean value of kp in Equation 6 under paragraph (b)(3)(iv) of this
guideline. If a small loss of test substance in the control tubes has oc-
curred, use this data to make corrections to the measured photolysis rate
as described. Note the dates of photolysis, the latitude, and the site.
(2) A hypothetical example is presented in paragraph (d)(4)(ii) of this
guideline, to illustrate how the test data obtained in the Tier 2, Phase 1,
test method can be used.
(B) Specific analytical and recovery procedures. (7) Provide a de-
tailed description or reference for the analytical procedures used, including
the calibration data and precision.
(2) If extraction methods were used to separate the solute from the
aqueous solution, provide a description of the extraction method as well
as the recovery data.
(C) Other test conditions. (7) Report the size, approximate cell wall
thickness, and type of glass used for the reaction tubes.
(2) Report the initial pH of all test solutions, if appropriate.
(3) For all procedures, report the dates of photolysis, the time of sun-
rise and sunset on each photolysis day, the site of photolysis and its lati-
tude, and the weather conditions. For Procedures 1 and 2 submit the dates
and times of actual exposure, and the duration of exposure, and, for inter-
16
-------
mittent exposure, the fraction of each day during which photolysis oc-
curred.
(4) If acetonitrile was used to solubilize the test substance, report the
percent, by volume.
(5) If a significant loss of test chemical occurred in the control solu-
tion, indicate the causes and how they were eliminated or minimized.
(D) Test data report. (7) For each photolysis experiment, report:
(/) The initial molar concentration of test chemical (Co) of each rep-
licate and the mean value.
(//) The molar concentration of test chemical for each replicate and
the mean value for each time point t.
(///) The molar concentration of each replicate control sample and
the mean value after completion of the photolysis experiments.
(2) For Procedure 1, 2, or 3, report the value of kp. If small losses
of chemical are observed, report (kp)0bs, kioss and kp. Report the half-life
(ti/2) calculated using the value of kp.
(ii) Phase 2 experiments—(A) Treatment of results. (7) The objec-
tives of this set of experiments is to determine the sunlight reaction quan-
tum yield, 4>CE, for a specific test chemical. 4>CE can be calculated using
Equation 10 under paragraph (b)(3)(vii) of this guideline,
(|)CE = (|)aE
by the following steps:
(/) Determine the ratio of the rate constants, kap/kcp, as described in
paragraph (b)(2)(i)(H) of this guideline using Equation 12. If a slight loss
of test chemical or actinometer (PNAP) was detected in the controls at
any time t, then employ the following procedure. Consider, as an example,
the loss of test chemical in the control at time t. Using the average con-
centration of the test chemical in the controls from the replicates at time
t and the average initial concentration, calculate ln(Co/Ct)c ioss. Using the
average concentration of test chemical from the replicates after photolysis
time t, calculate ln(Co/Ct)c0bs. The corrected term is then
Equation 17
17
-------
The same procedure can be applied to obtain a corrected term from the
actinometer (PNAP). Using the corrected terms for test chemical and/or
actinometer in Equation 12 under paragraph (b)(2)(i)(H)(3) of this guide-
line, determine the ratio of the rate constants (kap/kcp) as described in para-
graph (b)(2)(i)(H) of this guideline.
(//) Determine the quantum yield of the actinometer, (f)^, using Equa-
tion 9 and the molar concentration of pyridine [PYR] present in the acti-
nometer.
(///) Determine the value of ZecxLx for the test chemical as follows:
the molar absorptivities, ecx, have been determined by the procedure given
in paragraph (b)(l) of this guideline and the results have been tabulated
according to paragraph (d)(l)(ii) of this guideline. Choose the appropriate
LX values (Tables 3 to 6 under paragraph (d)(3) of this guideline) that
correspond to the season closest to the season in which the Phase 2 experi-
ments were performed and to the latitude nearest the latitude of the experi-
mental site. Calculate the product of ecx and LX for each wavelength inter-
val where ex has a nonzero value. Sum the products of eaxLx over all
wavelength intervals.
(iv) Determine the value of ZexaLx for the actinometer, as follows:
These values have been calculated and are given in Table 2 under para-
graph (d)(3) of this guideline. Choose the appropriate value that cor-
responds to the season closest to the season in which the Phase 2 experi-
ments were performed and to the latitude nearest the latitude of the experi-
mental site.
(v) Substitute the values of kap/kcp, ^"E, Z£axLx, and Z£cxLx in Equa-
tion 10 under paragraph (b)(3)(iii) of this guideline and calculate 4>CE, the
quantum yield of the test chemical in the environment (i.e. in sunlight).
(2) Once (|)CE has been determined, equation 4 under paragraph
(b)(3)(iii) of this guideline can be used to calculate kpn at any season of
the year and latitude using the measured values of the molar absorptivities,
ecx, and the appropriate LX values (Tables 3 to 6 under paragraph (d)(3)
of this guideline). The half-life can then be calculated using kpE in Equa-
tion 2 under paragraph (b)(3)(ii) of this guideline.
A hypothetical example is presented in paragraph (d)(4)(iii) of
this guideline, to illustrate how the test data obtained in the Tier 2, Phase
1, test method can be used.
(B) Other test conditions. (7) Report the size, approximate cell wall
thickness, and type of glass used for tubes to hold the test chemical and
actinometer solutions.
(2) Report the initial pH of all test chemical solutions, if appropriate,
and the type and concentration of the buffers employed for each pH.
18
-------
(3) If acetonitrile was used to solubilize the test chemical, report the
percent, by volume, of the acetonitrile, which was used.
(4) If significant loss of test chemical occurred in the control solution,
indicate the causes and how they were eliminated or minimized.
(C) Test data report. (7) Report the initial molar concentration of
chemical (Co) of each replicate and the mean value.
(2) Report the initial molar concentration of PNAP and the molar
concentration of pyridine used in the actinometer.
(3) Report the time and date the sunlight photolysis experiments were
started, the time and date the experiments were completed, and the elapsed
photolysis time in days.
(4) For each time point, report the three separate values for the molar
concentration of test chemical and PNAP and the mean values.
(5) For each time point, report the three separate values of the molar
concentration of test chemical and PNAP for the controls and the mean
values.
(6) Tabulate and report the following data: t, ln(Co/Ct)c, and ln(Co/
Ct)a. From the linear regression analysis, report the ratio of the rate con-
stants, kpc/kpa, and the correlation coefficient.
(T) If loss of test chemical and/or actinometer was observed during
photolysis, then report the data ln(Co/Ct)Corr, ln(Co/Ct)0bs, ln(Co/Ct)ioss for
the test chemical and/or actinometer at each time t. From the linear regres-
sion analysis of ln(Co/Ct)cCorr and ln(Co/Ct)aCorr, report the ratio of the rate
constants, kpc/kpa and the correlation coefficient.
($) Report the reaction quantum yield of the actinometer (4>aE).
(9) Report the value of kaa for the actinometer corresponding to the
season closest to the season in which the photolysis experiments were car-
ried out and to the latitude nearest the latitude of the experimental site.
(Iff) Tabulate the values of ^center, 8xc, LX, and 8xcLx for the test
chemical corresponding to the season closest to the season in which the
photolysis experiments were carried out and to the latitude nearest the lati-
tude of the experimental site.
(11) Report the value £ecxLx for the test chemical from step 10.
(72) Report the reaction quantum yield of the test chemical.
(13) Report kpE and ti/2E for the summer and winter seasons using
the appropriate LX values from Tables 3-6 under paragraph (d)(3) of this
guideline closest to the latitude of the chemical manufacturing site.
19
-------
(14) For chemicals that ionize, report the data for steps 1-13 for the
experiments at the required pHs.
(3) Tables of solar irradiance and related tables.
Table 1.—Wavelength Center and Intervals for LA,
?i center (nm)
297.5
300.0
302.5
305.0
307.5
310.0
312.5
315.0
317.5
320.0
323.1
330.0
340.0
350
360
370
380
390
400
410
420
430
440
450
460
470
480
490
500
525
550
575
600
625
650
675
700
750
800
Interval from (nm)
296.2
298.7
301.2
303.7
306.2
308.7
311.2
313.7
316.2
318.7
321.2
325.0
335.0
345.0
355.0
365.0
375.0
385.0
395.0
405.0
415.0
425.0
435.0
445.0
455.0
465.0
475.0
485.0
495.0
512.5
537.5
562.5
587.5
612.5
637.5
662.5
687.5
725.0
775.0
Range to (nm)
298.7
301.2
303.7
306.2
308.7
311.2
313.7
316.2
318.7
321.2
325.0
335.0
345.0
355.0
365.0
375.0
385.0
395.0
405.0
415.0
425.0
435.0
445.0
455.0
465.0
475.0
485.0
495.0
505.0
537.5
562.5
587.5
612.5
637.5
662.5
687.5
712.5
775.0
825.0
>A,(nm)
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
3.8
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
10.0
25
25
25
25
25
25
25
25
50
50
20
-------
Table 2—Day Averaged Rate Constant (kaa)1 for Sunlight Absorption by PNAP as
a Function of Season and Decadic Latitude
Latitude (de-
grees north)
20
30
40
50
Season
Spring
515
483
431
362
Summer
551
551
532
496
Fall
409
333
245
154
Winter
327
233
139
64
1kaa =
in day-1.
Table 3— U Values for Latitude 20° N.1 23
A center
(nm)
297.5
300.0
302.5
305.0
307.5
310.0
312.5
315.0
317.5
320.0
323.1
330.0
340.0
350.0
360.0
370.0
380.0
390.0
400.0
410.0
420.0
430.0
440.0
450.0
460.0
470.0
480.0
490.0
500.0
525.0
550.0
575.0
600.0
625.0
650.0
Spring
1 1Q(— 4)
4.06(-4)
1 . 1 0(— 3)
2.37(-3)
4.24(-3)
6.65(-3)
9.42(-3)
1.24(-2)
1.54(-2)
1.82(-2)
3.23(-2)
1.1 0(— 1)
1.37(— 1)
1.52(— 1)
1.67(— 1)
1.78(— 1)
1.89(-1)
1 79(— 1 )
2:57(-1)
3.38(-1)
3.47(-1)
3.35(-1)
3.95(-1)
4.45(-1)
4.50(-1)
4.65(-1)
4.76(-1)
4.50(-1)
4.59(-1)
1.21
1.26
1.27
1.29
1.29
1.30
Summer
1.52(-4)
5.26(-4)
1.35(-3)
2.79(-3)
4.86(-3)
7.45(-3)
1.04(-2)
1.35(-2)
1 .66(-2)
1.96(-2)
3.45(-2)
1 . 17(— 1)
1 .45(— 1)
1 .60(— 1)
1 70(_1)
1 88(— 1)
2!00(-1)
1 .89(— 1)
2.71(-1)
3.57 (-1)
3.67 (-1)
3.54(-1)
4.18(— 1)
4.70(-1)
4.75(-1)
4.91 (-1)
5.03(-1)
4.76(-1)
4.85(-1)
1.28
1.33
1.35
1.36
1.37
1.38
Fall
7.77(-5)
2.96(-4)
8.21 (-4)
1.79(-3)
3.24(-3)
5.13(-3)
7.33(-3)
9.68(-3)
1.21 (-2)
1 .44(-2)
2.55(-2)
8.75(-2)
1 . 10(— 1)
1 .22(— 1)
1 .35(— 1)
1 .45(— 1)
1.55(-1)
1 46(— 1 )
2!09(-1)
2.76(-1)
2.84(-1)
2.74(-1)
3.25(-1)
3.65(-1)
3.70(-1)
3.83(-1)
3.92(-1)
3.72(-1)
3.80(-1)
1.00
1.05
1.06
1.07
1.08
1.09
Winter
3. 71 (-5)
1.62(-4)
4.99HO
1.1 7(— 3)
2.25(-3)
3.72(-3)
5.47(-3)
7.40(-3)
9.38(-3)
1 13(— 2)
2.04(-2)
7.08(-2)
9.02(-2)
1.01(— 1)
1.1 2(— 1)
1.21(— 1)
1.30(-1)
1 .22(— 1 )
1.75(-1)
2.31 (-1)
2.38(-1)
2.30(-1)
2.72(-1)
3.07(-1)
3.1 1(— 1)
3.22(-1)
3.31 (-1)
3.13(— 1)
3.20(-1)
8.48(-1)
8.83(-1)
8.92(-1)
9.05(-1)
g 1 5(— 1 )
9!24(-1)
21
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Table 3—U Values for Latitude 20° N.123—Continued
A, center
(nm)
675.0
700.0
750.0
800.0
Spring
1.30
1.29
2.48
2.38
Summer
1.38
1.36
2.62
2.51
Fall
1.09
1.08
2.08
2.00
Winter
9.27(-1)
9.21 (-1)
1.78
1.71
1 Units of LA, are 10 3 einsteins cm 2 day -1. Multiplication of LA, by EA in units of
molar -1 cm-1 gives rate constants in units of day -1
2The second number in the columns in parenthesis is the power of ten by which the
first number is multiplied.
3 Based on the GC SOLAR program.
Table 4—LA Values for Latitude 30° N.123
^center
(nm)
297.5
300.0
302.5
305.0
307.5
310.0
312.5
315.0
317.5
320.0
323.1
330.0
340.0
350.0
360.0
370.0
380.0
390.0
400.0
410.0
420.0
430.0
440.0
450.0
460.0
470.0
480.0
490.0
500.0
525.0
550.0
575.0
600.0
Spring
5.73(-5)
2.50(-4)
7.65(-4)
1.79(-3)
3.43(-3)
5.64(-3)
8.27(-3)
1.1 2(— 2)
1.41 (-2)
1.70(-2)
3.04(-2)
1.05(— 1)
1.33(— 1)
1.47(— 1)
1.62(— 1)
1.73(— 1)
1.84(— 1)
1.74(— 1)
2.50(-1)
3.29(-1)
3.38(-1)
3.26(-1)
3.86(-1)
4.34(-1)
4.39(-1)
4.54(-1)
4.65(-1)
4.40(-1)
4.49(-1)
1.18
1.23
1.24
1.25
Summer
1.09(-4)
4. 11 (-4)
1 14(— 3)
2.46(-3)
4.45(-3)
7.02(-3)
1.00(-2)
1.32(-2)
1.64(-2)
1.95(-2)
3.46(-2)
1 . 18(— 1)
1 .48(— 1)
1 .63(— 1)
1 .80(— 1)
1 .91 (— 1)
2.04(-1)
1 .93(— 1)
2.77(-1)
3.64(-1)
3.74(-1)
3.61 (-1)
4.26(-1)
4.79(-1)
4.85(-1)
5.01 (-1)
5.13(— 1)
4.85(-1)
4.95(-1)
1.31
1.36
1.37
1.38
Fall
3.18(-5)
1 .46(-4)
4.64(-4)
1 12(— 3)
2.19(-3)
3.67(-3)
5.46(-3)
7.43(-3)
9.48(-3)
1 . 15(— 2)
2.07(-2)
7.23(-2)
9.23(-2)
1 .03(— 1)
1 . 15(— 1)
1 .24(— 1)
1 .33(— 1)
1 .25(— 1)
1 .79(— 1)
2.36(-1)
2.43(-1)
2.35(-1)
2.79(-1)
3.14(— 1)
3.19(— 1)
3.30(-1)
3.38(-1)
3.20(-1)
3.27(-1)
8.67(-1)
9.03(-1)
9.1 1(— 1)
9.24(-1)
Winter
6.78(-6)
4.23(-5)
1.71 (-4)
4.95(^)
1.11 (-3)
2.04(-3)
3.26(-3)
4.69(-3)
6. 21 (-3)
7.76(-3)
1.43(-2)
5.17(-2)
6.75(-2)
7.65(-2)
8.60(-2)
9.31 (-2)
1.01(— 1)
9.39(-2)
1.35(— 1)
1.79(— 1)
1.84(— 1)
1.78(— 1)
2.12(— 1)
2.39(-1)
2.42(-1)
2.51 (-1)
2.58(-1)
2.44(-1)
2.50(-1)
6.61 (-1)
6.87(-1)
6.93(-1)
7.04(-1)
22
-------
Table 4—U Values for Latitude 30° N.123—Continued
^center
(nm)
625.0
650.0
675.0
700.0
750.0
800.0
Spring
1.26
1.27
1.28
1.27
2.44
2.34
Summer
1.39
1.40
1.40
1.39
2.67
2.57
Fall
9.34(-1)
9.45(-1)
9.48(-1)
9.42(-1)
1.82
1.75
Winter
7.15(-1)
7.27(-1)
7.32(-1)
7.31 (-1)
1.41
1.37
1 Units of LA, are 10 3 einsteins cm 2 day-1. Multiplication of LA, by ex in units of
molar ! cm-1 gives rate constants in units of day-1.
2The second number in the columns in parenthesis is the power of ten by which the
first number is multiplied.
3 Based on the GC SOLAR program.
Table 5—U Values for Latitude 40° N.123
^center
(nm)
297.5
300.0
302.5
305.0
307.5
310.0
312.5
315.0
317.5
320.0
323.1
330.0
340.0
350.0
360.0
370.0
380.0
390.0
400.0
410.0
420.0
430.0
440.0
450.0
460.0
470.0
480.0
490.0
500.0
525.0
550.0
Spring
1.85(-5)
1.06(-4)
3.99(-4)
1.09(-3)
2.34(-3)
4.17(-3)
6. 51 (-3)
9.18(-2)
1.20(-2)
1.48(-2)
2. 71 (-2)
9.59(-2)
1.23(-1)
1.37(-1)
1.52(-1)
1.63(-1)
1.74(-1)
1.64(-1)
2.36(-1)
3.10(-1)
3.19(-1)
3.08(-1)
3.65(-1)
4.11(-1)
4.16(-1)
4.30(-1)
4.40(-1)
4.16(-1)
4.25(-1)
1.12
1.16
Summer
6.17(-5)
2.70(-4)
8.30(-4)
1.95(-3)
3.74(-3)
6.17(-3)
9. 07 (-3)
1 .22(-2)
1.55(-2)
1.87 (-2)
3.35(-2)
1.16(-1)
1.46(-1)
1.62(-1)
1.79(-1)
1.91(-1)
2.04(-1)
1.93(-1)
2.76(-1)
3.64(-1)
3.74(-1)
3.61 (-1)
4.26(-1)
4.80(-1)
4.85(-1)
5.02(-1)
5.14(-1)
4.86(-1)
4.96(-1)
1.31
1.36
Fall
7.83(-6)
4.76(-5)
1.89(-4)
5.40(-4)
1.19(-3)
2.19(-3)
3.47(-3)
4.97(-3)
6.57(-3)
8.18(-3)
1.51 (-2)
5.44(-2)
7.09(-2)
8.04(-2)
9.02(-2)
9.77(-2)
1.05(-1)
9.86(-2)
1.42(-1)
1.87(-1)
1.93(-1)
1.87(-1)
2.22(-1)
2.51(-1)
2.54(-1)
2.63(-1)
2.70(-1)
2.56(-1)
2.62(-1)
6.93(-1)
7.21(-1)
Winter
5.49(-7)
5.13(-6)
3.02(-5)
1.19(-4)
3.38HO
7.53(-4)
1.39(-3)
2.22(-3)
3.19(-3)
4.23(-3)
8.25(-3)
3.16(-2)
4.31 (-2)
4.98(-2)
5.68(-2)
6.22(-2)
6.78(-2)
6.33(-2)
9. 11 (-2)
1.20(-1)
1.24(-1)
1.20(-1)
1.43(-1)
1.61(-1)
1.64(-1)
1.69(-1)
1.74(-1)
1.65(-1)
1.68(-1)
4.45(-1)
4.61 (-1)
23
-------
Table 5—U Values for Latitude 40° N.123—Continued
^center
(nm)
575.0
600.0
625.0
650.0
675.0
700.0
750.0
800.0
Spring
1.17
1.18
1.20
1.21
1.22
1.21
2.33
2.25
Summer
1.37
1.38
1.40
1.41
1.41
1.40
2.69
2.59
Fall
7.22(-1)
7.39(-1)
7.50(-1)
7.62(-1)
7.68(-1)
7.66(-1)
1.48
1.43
Winter
4.61 (-1)
4.69(-1)
4.82(-1)
4.95(-1)
5.03(-1)
5.05(-1)
9.84(-1)
9.56(-1)
1 Units of U are 10 3 einsteins cm 2 day-1. Multiplication of LA, £A in the units of
molar ! cm-1 gives the rate constant in units of day-1.
2The second number in the columns in parenthesis is the power of ten by which the
first number is multiplied.
3 Based on the GC SOLAR program.
Table 6—LA Values for Latitude 50° N.1 23
^center
(nm)
297.5
300.0
302.5
305.0
307.0
310.0
312.5
315.0
317.5
320.0
323.1
330.0
340.0
350.0
360.0
370.0
380.0
390.0
400.0
410.0
420.0
430.0
440.0
450.0
460.0
470.0
480.0
490.0
500.0
Spring
3.61 (-6)
3.05(-5)
1.54(-4)
5.24(-4)
1.32(-3)
2.66(-3)
4.53(-3)
6.82(-3)
9.34(-3)
1.19(-2)
2.25(-2)
8.26(-2)
1.09(-1)
1.22(-2)
1.36(-1)
1.47(-1)
1.57(-1)
1.48(-1)
2.12(-1)
2.80(-1)
2.89(-1)
2.79(-1)
3.31 (-1)
3.73(-1)
3.78(-1)
3.90(-1)
4.00(-1)
3.78(-1)
3.86(-1)
Summer
2.86(-5)
1.50(-4)
5.33(-4)
1.39(-3)
2.89(-3)
5.05(-3)
7.75(-3)
1.08(-2)
1 .40(-2)
1.71 (-2)
3.12(-2)
1.10(-1)
1.40(-1)
1.57(-1)
1.74(-1)
1.86(-1)
1.99(-1)
1.87(-1)
2.69(-1)
3.55(-1)
3.65(-1)
3.52(-1)
4.17(-1)
4.69(-1)
4.75(-1)
4.91 (-1)
5.03(-1)
4.76(-1)
4.85(-1)
Fall
9.58(-7)
8.27(-6)
4.47(-5)
1 .63(-4)
4.39(-4)
9.32(-4)
1 .66(-3)
2.58(-3)
3.64(-3)
4.76(-3)
9.19(-3)
3.48(-2)
4. 71 (-2)
5.43(-2)
6.18(-2)
6.76(-2)
7.37(-2)
6.89(-2)
9.90(-2)
1.31(-1)
1.35(-1)
1.31(-1)
1.55(-1)
1.75(-1)
1.78(-1)
1.84(-1)
1.89(-1)
1.79(-1)
1.83(-1)
Winter
5.47(-8)
4.17(-7)
2.62(-6)
1.34(-5)
5.14(-5)
1.49(-4)
3.43HO
6.52(-4)
1.07(-3)
1.57(-3)
3.39(-3)
1.45(-2)
2.12(-2)
2.53(-2)
2.96(-2)
3.30(-2)
3.65(-2)
3.49(-2)
4.98(-2)
6.54(-2)
6. 71 (-2)
6.47(-2)
7.66(-2)
8.62(-2)
8.74(-2)
8.95(-2)
9.15(-2)
8.62(-2)
8.77(-2)
24
-------
Table 6—U Values for Latitude 50° N.123—Continued
^center
(nm)
525.0
550.0
575.0
600.0
625.0
650.0
675.0
700.0
750.0
800.0
Spring
1.01
1.05
1.05
1.06
1.08
1.10
1.11
1.11
2.15
2.08
Summer
1.28
1.33
1.34
1.35
1.37
1.38
1.39
1.38
2.66
2.57
Fall
4.84(-1)
5.03(-1)
5.04(-1)
5.13(-1)
5.26(-1)
5.39(-1)
5.47(-1)
5.49(-1)
2.07
1.04
Winter
2.28(-1)
2.32(-1)
2.28(-1)
2.32(-1)
2.42(-1)
2.53(-1)
2.61 (-1)
2.66(-1)
5.22(-1)
5.11(-1)
1 Units of LA, are 10 3 einsteins cm 2 day-1. Multiplication of LA, by EA in the units of
molar ! cm-1 gives the rate constant in units of day-1.
2The second number in the columns in parenthesis is the power of ten by which the
first number is multiplied.
3 Based on the GC SOLAR program.
(4) Examples of application of methodology—(i) Tier 1 Test: UV/
visible absorption spectra—estimation of aqueous photolysis rate con-
stant and minimum half-life in sunlight—(A) illustrative example 1.
A neutral organic chemical A was dissolved in water at a concentration
of 1.00 x 10 3 M. UV/visible absorption spectra were obtained in a 10.0
cm quartz absorption cell and no absorbance was detected above the base-
line in the region 290 nm and greater (i.e. AX = 0) for A, > 290 nm).
Since AX = 0, then 8xc = 0 (Equation 15). Using this result in Equation
7, it is found that (kpE)max = 0, indicating that no direct photolysis can
take place in sunlight at any latitude or season of the year. This example
illustrates the principle of the Grotthus-Draper law. That is, in order for
direct photolysis to take place in sunlight, the chemical must absorb sun-
light in the region A> 290 nm.
(B) Illustrative example 2. (7) Consider a plant located in Columbus,
Georgia on the Chattahoochee River which produces an organic chemical
B which is not an acid or a base. The waste effluent passes through a
primary and secondary treatment plant and is then discharged directly into
the river. The plant produces chemical B continuously every day of the
year. The plant is located at 32.5° north latitude. Estimate the maximum
sunlight direct photolysis rate constant and the corresponding minimum
half-life for this chemical in the river for the winter and summer seasons
under clear skies.
(2) Laboratory experiments, data, and calculations: (/) The water solu-
bility of chemical B is 1.00 x 10 3 M at 25 °C. Chemical B was dissolved
directly in water and a 1.00 x 10 4 molar solution was prepared at 25
°C. The UV/visible absorption spectra were obtained according to the Tier
1 procedure in a 10.0 cm quartz absorption cell in duplicate. Using the
25
-------
wavelength interval range (from Table 1 under paragraph (d)(3) of this
guideline), the average absorbance of the duplicate runs at ^center was
obtained and the results are summarized in the following Table 7:
Table 7—Summary of Photolysis Data for Chemical B
Spectral Data
?i center (nm)
2975
300.0
302.5
3050
3075
3100
312.5
315.0
3175
3200
323 1
330.0
Ax
1 684
1.434
1.221
0919
0742
0208
0.138
0.094
0057
0009
0002
0.000
ecx (IVH cm-1)
1684
1434
1221
919
742
208
138
94
57
9
2
0
Photolysis Data
A, center (nm)
297.5
300.0
302.5
305.0
307.5
310.0
312.5
315.0
317.5
320.0
323.1
330.0
Summer
Lx1
1.09(-4)
4. 11 (-4)
1.14(-3)
2.46(-3)
4.45(-3)
7.02(-3)
1.00(-2)
1.32(-2)
1.64(-2)
1.95(-2)
3.46(-2)
1.18(-1)
excLx (day -i)
0.18
0.59
1.39
2.26
3.30
1.46
1.38
1.24
0.94
0.18
0.07
0.00
12^99
Winter
Lx1
6.78(-6)
4.23(-5)
1.71 (-4)
4.95(-4)
1.11 (-3)
2.04(-3)
3.26(-3)
4.69(-3)
6.21 (-3)
7.76(-3)
1.43(-2)
5.17(-2)
ecxLx (day _i)
0.01
0.06
0.21
0.45
0.82
0.42
0.45
0.44
0.35
0.07
0.03
0.00
Vsi
1The units of l_x are in 10 3 einsteins cm 2 day -1. The second number in the col-
umns in parenthesis is the power of ten by which the first number is multiplied.
(//) From the above data and Equation 15 under paragraph (d)(l)(i)(A)
of this guideline, the average molar absorptivity is
Equation 18
ex = 1,000 Ax
26
-------
From the average AX value at X center, the average molar absorptivity
can be obtained from Equation 18 and the results are summarized in Table
7 under paragraph (d)(4)(i)(B)(2)(/) of this guideline. Since the plant is
located at 32.5° north latitude, the closest LX values are at 30° north lati-
tude. These values are obtained from Table 4 under paragraph (d)(3) of
this guideline and are summarized in Table 8 under paragraph
(d)(4)(i)(B)(2)(/) of this guideline for the summer and winter seasons.
Using the data from Table 7 under paragraph (d)(4)(B)(2)(/) of this guide-
line and Equations 7 and 8 under paragraph (b)(3)(v) of this guideline,
the following results are obtained.
Summer
(kpE)max = ZexcLx =13.0 day !
(ti/2E)min = 0.053 day
Winter
(kpE)max = ZecLx = 3.31 day*
(ti/2E)min= 0.21 day
Since the chemical transforms rapidly for the summer and winter seasons,
it is necessary to carry out Tier 2 experiments to more accurately define
direct photolysis rates in aqueous media as a function of the season of
the year.
(ii) Tier 2 Phase 1: Aqueous photolysis in sunlight—illustrative
example 3. (A) Consider the same scenario as described in illustrative
example 2, under paragraph (d)(4)(i)(B) of this guideline. Using the Tier
2, Phase 1 Procedure, carry out experiments to estimate the rate of direct
photolysis and half-life in aqueous solution in the spring for water bodies.
(B) Photolysis experiments and calculations: Since chemical B ab-
sorbs appreciably below 340 nm, 11 mm i.d. quartz tubes were used (note:
this tube has an approximate pathlength of 1 cm). Chemical B was dis-
solved directly in pure water and a 1.00 x 10 5 molar solution was pre-
pared at 25°C. Since the water solubility is 1.00 x 10 3 M at 25°C, this
sample solution was well below one-half its water solubility. The UV spec-
trum of this solution in a 1 cm absorption cell indicated that AX was less
than 0.05 at 290 nm. Hence, under these conditions, first-order kinetics
are applicable.
(C) A series of quartz tubes were filled with this aqueous solution,
sealed, and photolysis experiments were carried out in sunlight according
to the appropriate procedure described in paragraph (b)(2)(iii)(A) of this
guideline. The experiments were started at noon (1200 hours) on May 8,
1982. The weather conditions are summarized for this period of time and
27
-------
the concentration data given represent the mean of duplicate determina-
tions.
(7) May 2, 1982: at t = 0 (noon—1200 hours) C0 = 1.00 x 10 5
M.
(2) May 2, 1982: Noon to sunset—clear and sunny.
(3) May 3, 1982: Noon (1200 hours), Ct = 0.840 x 10 5 M.
(4) May 3, 1982: at 1400 hours the weather conditions were cloudy
with rain. The rain and cloudy weather continued until 2200 hours.
(5) From sunrise, May 4, 1982 through 1200 hours May 8, 1982,
the weather was clear and sunny. At 1200 hours, May 8, 1982, analysis
of the samples gave an average concentration of Ct = 0.400 x 10 5 M.
Since 60 percent of chemical B transformed, the photolysis experiments
were terminated and the control samples were analyzed. The average con-
centration of the control samples was 0.997 x 10 5 M which was essen-
tially the same as Co. Hence, no adventitious processes occurred and the
loss of chemical was only due to sunlight photolysis.
(D) Listed in the following Table 8 are the times of sunrise and sunset
for the dates sunlight photolysis experiments were carried out along with
the total number of hours of sunlight.
28
-------
Table 8—Summary of Times for Sunrise and Sunset for the Period May 2-8,
1982
Date (1982)
May 2
May 3
May 4
May 5
May 6
May 7
May 8
Sunrise (a.m.)
0600
0559
0558
0557
0556
0555
0554
Sunset (p.m.)
2010
2011
2012
2013
2014
2015
2016
Total sunlight hours
14.2
14.2
14.2
14.3
14.3
14.3
14.4
(E) The following data summarizes the dates photolyzed, the times
exposed to sunlight, the total sunlight photolysis time for each date in
days, the total number of days of sunlight photolysis, and the calculation
of kcP andti/2.
Date
May 2
May 3
May 3
May 4
May 5
May 6
May 7
Mav 8
Times photolyzed
1200 to 2010 hours (8 2/14 2)
0559 to 1200 hours (6 0/14 2)
1200 to 1400 hours (2 0/14 2)
0558 to 2012 hours
0557 to 2013 hours
0556 to 2014 hours
0555 to 2015 hours
0554 to 1200 hours (6.1/14.4)
Sunlight photolysis time
for each (days)
058
042
0 14
1 00
1 00
1 00
1 00
0.42
t = 5.6 days; C0 = 1.00 x 10 5; Ct = 0.400 x 10 5
1 Total hours.
ln(Co/Ct) =
kcP = (1/t) ln(Co/Ct) = (1/5.6) ln(1.00 x 10 5/0.400 x 10 5)
kcP = 0.16 days !
ti/2 = 0.693/0.16 days-1 = 4.3 days
Therefore, the rate constant for direct photolysis of chemical B in tubes
in pure water is 0.16 days * and the corresponding half-life is 4.3 days
for the period of photolysis May 2-8, 1982, at 32.5° north latitude. Using
equation 13, under paragraph (b)(2)(i)(J) of this guideline, the direct pho-
tolysis rate constant (kpn) for water bodies is 0.073 days * and the cor-
responding half-life (ti/zn) is 9.5 days.
29
-------
(iii) Tier 2, Phase 2: Aqueous photolysis in sunlight—illustrative
example 4. (A) Consider the same scenario as described in illustrative
examples 2 and 3. Using the Tier 2, Phase 2, procedure, carry out experi-
ments to determine the sunlight reaction quantum yield and estimate the
rate constant for direct photolysis in aqueous solution and the half-life
for water bodies and clear sky conditions for the summer and winter sea-
sons.
(B) Photolysis experiments and calculations: The sunlight photolysis
experiments were carried out in the beginning of May 1982, at 32.5° north
latitude.
(C) Preparation of the actinometer solution: (7) The results from the
Tier 2, Phase 1, experiments indicated that Kcp for the test chemical was
0.16 days-1. Since the experiments were carried out in early May at 32.5°
north latitude, the value of Kaa was chosen from Table 2 which cor-
responds to the spring season and at 30° north latitude; and the value is
483 days-1. Using Equation 11 under paragraph (b)(2)(i) (H) of this guide-
line, the molar concentration of pyridine required to adjust the actinometer
rate to match the rate of disappearance of the test chemical is
[PYR] = 26 .9 (0.16/483) = 8.91 x 10 3 molar
Using this concentration of pyridine, an actinometer solution was (ii)(C)
prepared according to the procedure described in paragraph (b)(2) of this
guideline. The quantum yield for this actinometer is calculated using equa-
tion 9 under paragraph (b)(3)(vii) of this guideline.
4>aE = 0.0169[PYR] = 0.0169(8.91 x 10 3) = 1.51 x 10 4
(2) Procedures for Tier 2, Phase 2 experiments (under paragraph
(b)(2)(iii) of this guideline) were followed and sunlight experiments were
initiated at 1200 hours on May 9, 1982. The mean initial concentration
of test chemical was 1.00 x 10 5 molar and the mean initial concentration
of PNAP was 1.00 x 10 5 molar. Samples of the chemical and actinometer
and the controls were analyzed in triplicate periodically at 1200 hours on
May 10, 11, 13, 15, and 16. On May 16, the photolysis experiments were
terminated. The mean concentrations of all samples are summarized as
follows:
30
-------
Date
May 9
May 10
May 11
May 13
May 15
Mav 16
Concentration
of chemical
(M)
1.00 x 10-5
0.820x 10-5
0.654 x 10-5
0.440 x 10-5
0.299 x 10-5
0.233 x 10-5
Concentration
of actinometer
(M)
1.00 x 10-5
0.855x 10-5
0.71 Ox 10-5
0.51 5 x 10-5
0.383x 10-5
0.304x 10-5
Concentration
of chemical
control (M)
1.00 x 10-5
0.997 x 10-5
1.00 x 10-5
0.996x 10-5
0.999 x 10-5
0.997x 10-5
Concentration
of actinometer
control (M)
1.00 x 10-5
1.00 x 10-5
0.997 x 10-5
0.999 x 10-5
0.998 x 10-5
0.996x 1C-5
Since no significant loss of PNAP or test chemical was observed in the
control samples, no adventitious processes occurred and the loss of test
chemical and PNAP was only due to sunlight photolysis.
(3) Using the above data, In (Co/Ct) for the test chemical and actinom-
eter can be calculated and the results are summarized as follows:
t
(days)
0
1
2
4
6
7
Chemical
Ctx 105(M)
1.00
0.820
0.654
0.440
0.299
0.233
In (Co/Ct)-
0.000
0.199
0.425
0.821
1.21
1.46
Actinometer
Ctx 105(M)
1.00
0.855
0.710
0.515
0.383
0.304
In (Co/Ct)-
0.000
0.157
0.343
0.664
0.960
1.19
(4) The ratio of the rate constants, kcp/kap, is defined by equation
12 under paragraph (b)(2)(i)(H)(3) of this guideline.
Equation 12
ln(C0/Ct)c =
ln(C0/Ct)a
(5) Using all the data (including the time point t = 0) and linear re-
gression analysis, the slope is found to be 1.237 with a correlation coeffi-
cient of 0.9998. Therefore,
(kcp/k*p) = 1.24
(6) Using the molar absorptivities obtained in example 2 under para-
graph (d)(4)(i)(B) of this guideline and the LX values for spring at 30°
north latitude in Table 4 under paragraph (d)(3) of this guideline, the value
can be calculated as follows:
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A, center (nm)
297.5
300.0
302.5
305.0
307.5
310.0
312.5
315.0
317.5
320.0
323.1
330.0
e^lVHcm-1)
1684
1434
1221
919
742
208
138
94
57
9
2
0
L ^
5.73 (-5)
2.50 (-4)
7.65 (-4)
1.79 (-3)
3.43 (-3)
5.64 (-3)
8.27 (-3)
1.12 (-2)
1.41 (-2)
1.70 (-2)
3.04 (-2)
1.05 (-1)
ecxU (days-1)
0.10
0.36
0.93
1.65
2.55
1.17
1.14
1.05
0.80
0.15
0.06
0.00
are 10 3 einsteins cm 2 day-1.
= 9.96 days-1-
1 The units of
For this experiment, kaa(X = eaxLx) is 483 days : (Table 2 under paragraph
(d)(3) of this guideline). All the pertinent data are summarized as follows:
kcp/kap= 1.24
= 9.96 days-1
= 483 days -1
4>aE= 1.5 Ix 10 4
Substituting these results into Equation 10 under paragraph (b)(3)(vii) of
this guideline yields
4>CE = (1.24)(483/9.96)(1.51 x 10 4)
4>cE = 9.08x 10 -3
(7) The rate constants for direct photolysis of test chemical in aqueous
media and the half-life for water bodies and clear sky conditions for the
winter and summer seasons can be calculated as follows: The values of
ZexcLx have been calculated from example under paragraph (d)(3)(i)(A)
of this guideline. For summer Z£X°LX = 13.0 days-1; for winter £ex°Lx
= 3.31 days-1. The reaction quantum yield for the chemical is 9.08 x 10 3.
Using these data in equation 4 under paragraph (b)(3)(iii) of this guideline
yields
Summer
kpE = 9.08 x 10-3 (13 o) = 0.118 days-1
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Winter
kpE = 9.08 x 10-3 (331) = 0.0301 days-1
These values can be substituted into Equation 2 under paragraph (b)(3)(ii)
of this guideline to obtain the half-lives for these two seasons.
Summer:
ti/2E = (0.693/0.118) = 5.9 days
Winter:
ti/2E = (0.693/0.0301) = 23 days
(5) Glossary of symbols
PYR = Pyridine.
PNAP =/>Nitroacetophenone.
A, = Wavelength A.
AX = Absorbance at wavelength X.
a = Actinometer (composed of PNAP/PYR).
ecx = Molar absorptivity of a chemical C.
8ax = Molar absorptivity of the actinometer.
1 = light pathlength; the distance traveled by a beam of light passing
through the system.
4>CE = Sunlight reaction quantum yield of chemical c in water.
4>aE = Sunlight reaction quantum yield of the actinometer in water. Since
the reaction quantum yield is independent of A, (f)aE = 4>a (i.e. the reaction
quantum yield of the actinometer measured in the laboratory).
[C] = Molar concentration of chemical c.
[PYR] = Molar concentration of pyridine.
-d[C]/dt = Direct photolysis rate of chemical c.
kpE = Direct photolysis sunlight rate constant in water bodies in the envi-
ronment.
(kpE)max = Maximum direct photolysis sunlight rate constant in water bod-
ies in the environment.
kcp = Direct photolysis sunlight rate constant of chemical c in water in
tubes.
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kap = Direct photolysis sunlight rate constant of the actinometer in water
in tubes.
kax = Specific light absorption of a photoreactive chemical at a low con-
centration and at wavelength A,.
ka = Specific light absorption rate constant integrated over all wavelengths
absorbed by the chemical.
kaa = Specific light absorption rate constant integrated over all wavelengths
absorbed by the actinometer.
ti/2 = Sunlight half-life of a chemical in water in tubes.
(ti/2E)min = The minimum sunlight half-life of a chemical in water bodies
in the environment.
I = The numbers of photons of light of wavelength A in the system per
cm2 per second.
LX = Solar irradiance in water in the units 10 3 einsteins cm 2 day *.
y = The geometry factor which represents the ratio of the rate constants
in tubes (kp) to the rate constant in water bodies in the environment
(e) References. For additional background information on this test
guideline the following references should be consulted:
(I) Astronomical Almanac (1982).
(2) Dulin, D. and Mill, T. Development and application of solar
actinometers. Environmental Science and Technology 16:815 (1982).
(3) Environmental Protection Agency. Mill, T. et al. Toxic substances
process data generation and protocol development, Draft final report, EPA
Contract No. 68-03-2981 with EPA Athens Research Laboratory, Office
of Research and Development (1984).
(4) Handbook of Chemistry and Physics. (Chemical Rubber Company,
Cleveland, OH)
(5) Mill, T. et al. Evaluation and Optimization of Photolysis Screen-
ing Protocols. EPA Report No. 560/5-81-003 (1981).
(6) Mill, T. et al. Laboratory Protocols for Evaluating the Fate of
Organic Chemicals in Air and Water. EPA Report No. 600/3-82-022
(1982).
(7) Mill, T. et al. Design and Validation of Screening and Detailed
Methods for Environmental Processes.
34
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(8) Zepp, R.G. and Cline, D.M. Rates of direct photolysis in aquatic
environment. Environmental Science and Technology 11:359 (1977).
(9) Zepp, R.G.Quantum yields for reaction of pollutants in dilute
aqueous solution. Environmental Science and Technology, 12:327 (1978).
(10) Zepp, R.G. Environmental Research Laboratory, U.S. Environ-
mental Protection Agency, College Station Road, Athens, Georgia 30601.
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