United States       Prevention, Pesticides     EPA712-C-96-043
          Environmental Protection    and Toxic Substances     August 1996
          Agency        (7101)
&EPA    Product Properties
          Test Guidelines
          OPPTS 830.7950
          Vapor Pressure

     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

     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 document 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,    internet:     http://
fedbbs.access.gpo.gov, or call 202-512-0132 for disks or paper copies.
This guideline is available in ASCII and PDF (portable document format)
from the EPA Public Access Gopher (gopher.epa.gov) under the heading
"Environmental Test Methods and Guidelines."

OPPTS 830.7950  Vapor pressure.
    (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 materials used in developing this har-
monized OPPTS test guideline are the OPPT  guideline under 40  CFR
796.1950 Vapor pressure, OPP guideline  63-9 Vapor pressure (Pesticide
Assessment Guidelines, Subdivision  D: Product Chemistry, EPA  Report
540/9-82-018,  October 1982) and OECD guideline 104 Vapor Pressure

    (b) Guidance information—(1) Data required. Melting point/melt-
ing range; boiling point/boiling range; molecular mass (for vapor pressure
balance method and gas saturation method).

    (2) The vapor pressure balance method and the gas  saturation method
are not routine procedures. Improvement of these methods in the future
will depend upon further exchange of experiences and results.

    (c) Method—(1) Introduction,  purpose, scope, relevance, applica-
tion,and limits of test, (i) The following account  for the environmental
relevance of vapor pressure:

    (A) Vapor pressure gives an indication of the probability of the phase
transitions liquid/gas and solid/gas.

    (B) Vapor pressure, together with the solubility in water, is the major
auxiliary variable for calculating the volatility of a substance from an aque-
ous solution.

    (C) Vapor pressure is a significant factor  for predicting atmospheric

    (D) The vapor pressure of a  substance can furthermore be useful as
a basis for deciding whether or not a photochemically induced degradation
(in the  homogeneous gas phase or in  an absorbed phase) is  necessary.

    (E) There is no single vapor pressure measurement procedure applica-
ble to the  entire range of vapor pressures.  Therefore, several methods are
recommended for the measurement of vapor pressure from <10'3 Pa to
105 Pa. The OECD  Laboratory  Intercomparison  Testing Programme
showed that the gas saturation method may allow measurements of consid-
erably lower vapor pressure (as low as approximately 10-5 Pa). Vapor pres-
sure testing is  not required for chemicals with a standard boiling point

    (F) The dynamic method, static method,  and isoteniscope method can
be applied to pure and commercial grade  substances although impurities

    will affect the results.  The vapor pressure balance method and the gas
    saturation method can only be applied to pure substances.

         (2)  Definitions and units, (i) The vapor pressure of a substance is
    defined as the saturation pressure above  a solid or liquid  substance. At
    thermodynamic equilibrium, the vapor pressure is a function of tempera-
    ture only.

         (ii)  The SI unit of pressure which should be used is the Pascal (New-
    ton/m2).  Units which have been employed historically, together with their
    conversion factors, are:

         (A) 1 Torr (mm Hg) = 1.333 x 102 Pa

         (B)  1 atmosphere (physical) = 1.013 x 105 Pa

         (C)  1 atmosphere (technical) = 9.81 x 104 Pa

         (D) 1 bar = 105 Pa

         (3)  Reference substances. The following reference compounds need
    not be employed in all  cases when investigating a new substance.  They
    are provided primarily so that calibration of the method may be performed
    from time  to time  and  to  offer the  chance to compare the results when
    another method is  applied.  The values presented in the following tables
    1. through 6.  are average and range values of vapor pressure from OECD
    and EEC-Laboratory Intercomparison Testing Programmes  (from  cal-
    culated regression curves) and not necessarily representative of the results
    which can be obtained with this test  guideline as they have been derived
    from an earlier version of the test guideline.
    Table 1.—Vapor Pressure of Toluene: Dynamic Method (OECD), three laboratories
Temperature, °C

Average values all labs in Pa
1 7 x 1 03
3 0 x 1 03
4.9x 103
7.8x 103

Range values in Pa
(1 7 to 1 8) x 1 03
(29 to 3 1) x 103
(4.8 to 5.0) x 103
(7.7 to 8.9) x 103

  Table 2.—Vapor Pressure of Hexachlorobenzene: Gas saturation method1, (OECD), two
Temperature, °C
A n

Average values all labs in Pa
8. Ox 10-4
2.6 x 10-3
8.1 x 10-3
2*5 ^, *\r\ ~>

6-1 . . -| n 9
.1 X 1U"2
Range values in Pa
3.7 x 10-4to 2.5 x 10-3
1.6x 10-3 to 5.7 x 10-3
5.9 x 10-3 to 1.2 x 10-2
2-1 , , -i r\ 9 j.~ <•> c , , -i r\ 9

.y X ILr2 TO D./ X ILr2
1 test temperature 10-50 °C

    Table 3.—Vapor Pressure of Hexachlorobenzene: Vapor pressure balance1, (EEC), two
Temperature, °C
A n


Vapor pressure, average values all
labs in Pa
20 xx -i n 4

3Q ,, A n -3

30 xx -i n ?
.y x u -L
Range values in Pa

4 1 x 1 0-4 to 1 5 x 1 0-3
0~7 +r\ £ A\ xx -1 n ^

6 4 x 1 0-3 to 1 8 x 1 0-2
/O O +r\ £ Q\ xx -1 n 9
\£..£. TO O.O) X U z
 1 test temperature 30-150 °C

Table 4.—Vapor Pressure of Dibutylphthalate: Vapor pressure balance, (EEC), two laboratories
Temperature, °C
-1 n


Average values all labs in Pa
6c , , -i n 4.

2.3x 1C-3
70 ^/ -i n -i

2.4 x 1C-2
7n , , A n 9
.U X ID'2
Range values in Pa
(A A *r\ o ^ \ xx ^ n 4

(1.7 to 3.1 )x 1C-3
/c ^ +/-\ o 7\ xx ^ n ^

(2.0 to 2.8) x 1C-2
/c o +/-\ 7 Q\ xx ^ n 9
(D.z TO / .0) X ILr2
  Table 5.—Vapor Pressure of Benzoic acid: Gas saturation method (OECD), two laboratories;
                     vapor pressure balance (OECD), two laboratories
Temperature, °C

Average values all labs in Pa
gas saturation
vp balance
Range values in
0.012 to 0.03
0.04 to 0.07
0.1 5 to 0.26
0.48 to 0.89
1.24 to 2. 9
Table 6.—Vapor Pressure of Di(2-ethylhexyl)phthalate: Gas saturation1, (OECD), one laboratory;
                     vapor pressure balance2, (OECD), one laboratory

                                         Regression curve values in Pa


A n

1.1 x 10-6

2.8x 10-5
-1 <•> , , -I n 4

4.9x 10-4
gas saturation
3.2 x 10-6
10 xx ^ n s

4.7 x 10-5
1c xx ^ n 4

5.0 x 10-4
 1 test temperature 10-50 °C
 2 test temperature 80-120 °C

           (4)  Principle of the test methods.  Five methods are proposed for
      determining  the vapor pressure  in different vapor pressure ranges. The
      measured results are plotted in a  log p versus 1/T graph and yield rectilin-
      ear curves for limited temperature ranges.

     (i) Dynamic method. (A) In the dynamic method, the boiling tem-
perature which pertains to a specified pressure is measured.

     (B) Recommended range is 103 Pa up to 105 Pa, between 20 °C and
100 °C.

     (C) This method has also been recommended for the determination
of boiling points, and is useful for that purpose up  to 350 °C.

     (ii) Static method. (A) In the static process, at thermodynamic equi-
librium, the vapor pressure established in a closed system is determined
at a specified temperature. This method is suitable for one-component and
multicomponent solids and liquids.

     (B) Recommended range  is 10 Pa up to 105 Pa, between 0 °C and
100 °C.

     (iii) Isoteniscope.  (A) This standardized method is also a static meth-
od, but is usually not suitable for multicomponent systems. Additional in-
formation is available in ASTM method D-2879-75 under paragraph (f)(4)
of this guideline.

     (B) Recommended range  is from  100  Pa to 105 Pa, between 0 °C
and 100 °C.

     (iv) Vapor pressure balance. (A) The quantity of substance leaving
a cell per unit time through an  aperture of known  size is determined under
vacuum in such a way that return of the substance  into the cell is negligible
(e.g.,  by measuring the pulse generated on a sensitive balance by a vapor
jet or by measuring the  weight loss).

     (B) Recommended range  is 10-3  Pa to 1  Pa,  between  0  °C and
100 °C.

     (v) Gas saturation method.  (A) A stream of inert carrier gas is
passed over the substance in such a way that it becomes saturated with
its vapor and the vapor is collected in a suitable trap.  Measurement of
the amount  of material transported by a known amount of carrier gas is
used to calculate the vapor pressure at a given temperature.

     (B) Recommended range is up to 1  Pa.

     (vi) Effusion  method:  loss of weight. (A) The  method is based on
the estimation of the mass  of test substance flowing out per unit of time
of a Knudsen cell (see paragraph (f)(9) of this guideline) in the form of
a vapor, through a micro-orifice under ultra-vacuum conditions. The mass
of effused vapor can be obtained either by determining  the loss of mass
of the cell or by condensing the vapor at low temperature and determining
the amount of volatilized substance using chromatography. The vapor pres-

sure is calculated by applying the Hertz-Knudsen relation with correction
factors that depend on parameters of the apparatus.
     (B) Recommended range is 10-3 to 1 Pa.

     (vii) Spinning rotor method. (A) This method uses a spinning rotor
viscosity gauge, in which the measuring  element is a small steel  ball
which, suspended  in a magnetic field, is made to spin by rotating fields
(see paragraph (f)(9)  of this guideline. Pick-up  coils allow its spinning
rate to be measured. When the ball has reached a given rotational speed,
usually about 400 revolutions per second, energizing is stopped and decel-
eration, due to gas friction, takes place. The  drop of rotational speed is
measured  as a function of time. The  vapor pressure is deduced from the
presssure-dependent slow-down of the steel ball.

     (B) The recommended range is 10-4 to 0.5 Pa.

     (5) Quality criteria. The various methods of determining the vapor
pressure  are  compared  as to  application,  repeatability,  reproducibility,
measuring range, and ability to standardize in the following table 7:
                        Table 7.—Quality criteria
Dynamic meth-

Static method

Effusion meth-
od: Vapor
pressure bal-
Gas saturation
Effusion meth-
od: Loss of
Spinning rotor












up to 25%





up to 25%

up to 50%

up to 50%


Recommended range

103 Pa to 2 x 103 Pa
2x 103 Pa to 105 Pa
10 Pa to 105 Pa
102 Pa to 105 Pa

10-3 Pa to 1 Pa

<10-3 Pa to 1 Pa

10-3 Pa to 1 Pa

10-4 Pa to 0.5 Pa
Ability to






     (d) Description of the test procedures—(1) Dynamic measurement
procedure—(i) Apparatus. (A) The measuring apparatus typically con-
sists of glass, metal,  and glass-metal connections and is composed of a
boiling  vessel with attached cooler  (see figure 1  under this paragraph),
equipment for regulating  and measuring the temperature and equipment
for regulating and measuring the pressure.

  Figure 1.—Apparatus for Determining the Vapour Pressure Curve
                 According to the Dynamic Method
                                     42V-1 SOW
1 = thermocouple

2 = vacuum buffer volume

3 = pressure gauge

4 = Cottrell pump

5 = measuring point

6 = heating element 42 voltage DC, 150 W.

    (B) A typical measuring apparatus shown in the drawing is made from
heat resistant  glass  and is composed of five parts: The large, partially dou-
ble-walled tube consists of a ground jacket joint, a cooler, a cooling vessel,
and an inlet.

    (C) The glass cylinder with a Cottrell pump is  mounted in the boiling
section of the tube  and has  a rough surface of crushed glass for avoiding
"bumping" in the boiling process.

    (D) The  temperature is measured by using a thermocouple or resist-
ance thermometer which is  immersed in a small quantity  of oil. It is in-
serted into the charging tube which has  a male ground joint and is sealed
on the bottom.

    (E) The  cross-piece makes the necessary connections  to the pressure
regulation and measurement equipment.

     (F) The bulb, which acts as a buffer volume, is connected with the
measuring apparatus by means of a capillary tube.

     (G) A cartridge heater, which is inserted into the glass apparatus ex-
ternally from below,  is used for heating the boiling vessel. The  desired
heating current is set by means  of a voltage-regulating transformer and
is monitored by means of an amperometer.

     (H)  An oil pump is used  for  setting the  desired vacuum between
102 and roughly 105 Pa.

     (I) A nitrogen cylinder is used for setting  a  desired pressure and  is
connected via a valve which is  also used for ventilating the apparatus.

     (J) A precision pressure gauge which is  connected to the cross-piece
is used for pressure measurement.

     (ii) Measurement procedure.  (A) The vapor pressure is measured
by determining the boiling point of the sample  at various  specified  pres-
sures between roughly 103 and  105 Pa. The temperature constancy  (at a
constant pressure) indicates that the boiling point (boiling equilibrium in
the case of a mixture) has been reached.

     (B) All glass parts are first thoroughly cleaned and dried and evacu-
ated under a gas ballast. The substance is introduced  into the apparatus.
If solids are not in a powdered form, problems may occur during the filling
process, but they can be circumvented by heating the cooling water jacket.
Frothing  substances cannot be measured using this method. After filling,
the apparatus is flanged together and the substance degassed.  The lowest
desired pressure is set, and the  heating system is switched on. Simulta-
neously, the thermocouple or the resistance thermometer is connected to
a recorder.  Equilibrium is reached when a constant boiling temperature
can be read at a constant pressure. After recording this equilibrium point,
a higher pressure is  set.  The process is  continued  in this manner  until
105 Pa has been reached (approximately 5 to 10 points in all). As a control,
equilibrium  points must be repeated at decreased pressures.

     (2) Static  measurement procedure—(i) Apparatus. (A) A typical
measuring apparatus (see figure  2 under this  paragraph) consists of glass,
metal and glass-metal connections. The measuring apparatus also includes
a heating  and a  cooling system for bringing the sample  to  a regulated
temperature and for measuring the temperature, as well as equipment for
setting and measuring the pressure.

  Figure 2.—Apparatus for Determining the Vapour Pressure Curve
                   According to the Static Method
                                          1 = Pressure gauge
                                          2 = Vacuum
                                          3 = Sample
    (B) The sample chamber is terminated on one side in a high-vacuum
valve  made of stainless steel and on the other side by a U-tube containing
a suitable manometric  fluid. The other end of the U-tube  terminates in
a cross-piece,  one branch of which leads to the vacuum pump, another
to the nitrogen cylinder, and the third to the pressure gauge.

    (C) For bringing the substance to a regulated temperature, the entire
sample chamber,  including the valve  vertical support  and  a sufficiently
large  section of the U-tube  (for practical purposes, up to  the height of
the valve vertical support), is placed in an  appropriate constant temperature
bath. The temperature is measured using a thermocouple or resistance ther-
mometer very  close to the outside  of the  sample chamber and can be re-

    (D)  Liquid nitrogen or a Dry Ice-ethyl alcohol mixture is suitable
for  supercooling the sample. An ultra-cryomat  is used for  measuring at
low temperatures.

    (E) A suitable pump is used to evacuate the apparatus to the required

    (F) The vapor pressure  of a substance is usually measured indirectly
via  a  zero indicator. The  zero indicator is usually a liquid, but membrane


capacity manometers, for example, also exist. In a temperature-controlled
bath, the vapor pressure moves the liquid in the U-tube out of equilibrium.
Nitrogen is allowed into the apparatus from a connected nitrogen cylinder
via a valve to compensate the  effect of the vapor pressure and to bring
the pressure gauge fluid back to zero.  The nitrogen pressure required for
this is read off at a  pressure gauge which is at ambient temperature and
corresponds to the vapor pressure of the substance at a corresponding tem-
perature constancy. There  are various precision pressure gauges for the
pressure range from 10 Pa up to 105 Pa.

     (G) There are various liquids available,  according  to pressure range
and the chemical behaviour of a substance, which can be used as U-tube
liquids  for zero  balancing  at the temperature of the substance: Mercury,
silicone oils,  and phthalates. Mercury may be used  from 102 Pa  up to
105 Pa, silicone oils and phthalates  from below  102  Pa down to  10 Pa;
the membrane capacity manometer can be applied even below 10-1  Pa.

     (ii) Measurement procedure. (A) Before  measurement, all parts of
the apparatus shown in figure 2  are thoroughly  cleaned with solvents and
dried in a vacuum. The U-tube  is filled with the desired pressure  gauge
fluid, which should  be degassed at elevated  temperature prior to  filling.

     (B) After having  been filled with the  substance   the apparatus is
flanged together and the sample chamber sufficiently supercooled. The en-
closed air is  pumped  out of the apparatus for several  minutes with the
valve open above the sample chamber.  The valve above the  substance is
closed, the sample is brought to the selected temperature, and the resulting
displacement of the columns is observed and compensated to the zero posi-
tion with nitrogen, if necessary, until a constant temperature is reached.
The sample chamber is again supercooled. If residual pressure is observed
in the supercooled condition, it is due either to air contained in the sample
which is released during the heating process and which can be drawn off,
or to the  cooling temperature's not being low  enough. Liquid nitrogen
must be used as a coolant.

     (C) After the sample has been sufficiently degassed, the temperature
dependency of the vapor pressure is determined at sufficiently small tem-
perature intervals.  The  vapor pressure  values are given  in Pa in a table.
In addition, a semilogarithmic graph is prepared in which log p is plotted
as a function of 1/T.

     (3) Isoteniscope.  (i) For a complete description of this method see
ASTM-D 2879-75 under paragraph (f)(4) of this guideline. For the prin-
ciple of the measuring device see the following figure 3:

       Figure 3.—Isoteniscope (according to ASTM D 2879-75)
1 = pressure control measurement system

2 = tube, o.d. 8 mm

3 = dry nitrogen in pressure system

4 = sample vapor

5 = small tip

6 = liquid sample

     (A)  Similarly to the static method described under paragraph (d)(2),
the isoteniscope is appropriate for the investigation of solids and liquids.
In the case  of liquids, the substance itself serves as the packing fluid in
the auxiliary manometer. In the case of solids, depending on the pressure
and temperature range, the manometer liquids listed in the description of
the static method under paragraph (d)(2)(ii)(G) of this guideline are used.
The  sphere  of the isoteniscope for liquids  is filled with the  substance to
be investigated, which is degassed at elevated temperature during boiling.

     (B)  Simultaneously,  a part of the liquid is distilled out of the sphere
and is condensed  in the upper cooled sphere and returned to the U-tube.
When the latter is  sufficiently filled with degassed liquid, the lower sphere
with the  U-tube in a thermostat-controlled bath is brought up to  the chosen
temperature, and the resulting vapor pressure is indirectly measured as de-
scribed under paragraph (d)(2)(ii) of this guideline.


     (C)  In the case of solids, the degassed packing fluid is filled into
the bulge on the long arm of the isoteniscope. The solid to be  investigated
is  filled  into the  lower sphere and is  degassed at  elevated temperature.
The  isoteniscope is tilted  so that the manometer liquid can flow into the
U-tube. The measurement of vapor pressure as a function of temperature
is  done as in the static method  under paragraph (d)(2) of this guideline.

     (4) Effusion method: vapor pressure balance—(i)  Apparatus. (A)
There are several different designs presented under  paragraphs (f)(2) and
(f)(3) of this guideline. The one described here is illustrative  of the prin-
ciples involved. A number of parts can  be  seen in the following figure

  Figure 4.—Apparatus for Determining the Vapour Pressure Curve
                According to the Vapour Jet Method
1 = base plate

2 = moving coil instrument

3 = bell jar

4 = balance with scalepan

5 = vacuum measuring device

6 = refrigeration box and cooling bar

7 = evaporator furnace

8 = Dewar flask with liquid nitrogen

9 = shield

     (B) These are base plate and bell jar, a pump with a vacuum measur-
ing device and equipment for measuring the vapor pressure with a visual
display  of the pointer deflection.  The  following built-in  equipment is
mounted on the base plate:

     (7) An evaporator furnace with flange and rotary feed-through. The
evaporator furnace  is a flat, cylindrical  copper vessel.  (The furnace  can
also be made of glass surrounded by a copper wall.) It is placed in a
copper retainer which is screwed onto a piece of stainless steel by its lower
protruding edge. The piece  of stainless steel,  in turn,  is mounted on the
base plate by means  of a flange so  that it can be rotated  about the axis
of the furnace. Heating is  provided by a heater coil inside the piece of
stainless steel, closed off from the vacuum chamber.

     (2) The furnace lid is made of copper and has three evaporation open-
ings of various diameters which are located  at 90° to  one another. By
rotating the  furnace, the desired opening or an intermediate position  can
be placed under the  slot in the cooler which  is positioned eccentrically
to the furnace, aiming the molecular beam at the balance pan or diverting
it. A thermocouple or resistance thermometer  is mounted  in the furnace
wall for temperature measurement.

     (3) The  balance is a moving-coil instrument. The  pointer is replaced
by  a small tube on which  are mounted the balance beam and  counter-
weight.  The  balance  beam  has  a replaceable pan made of a  thin piece
of gold-plated aluminium.  A 0.1 mm thick constantan wire onto which
calibration weights can be  set is attached to the  approximate  center of
the balance beam.  The vapor pressure  can be recorded using a photo-
electric null-point recording method.

     (4) A cylindrical brass pot  surrounds the balance pan on all sides
with the exception of the two slots for the movement of the  balance beam
and  a narrow opening for the entrance  of the molecular beam. Heat dis-
sipation to the outside  is provided by a copper bar on top of the brass
pot.lt is routed via  a  stainless steel tube through the base plate and ther-
mally insulated from it. The bar is immersed in a Dewar flask  containing
liquid nitrogen under the base plate.

     (ii) Measurement procedure. (A) The copper furnace is filled with
the substance, the  lid closed, and the plate orifice,  shield,and cooler  slid
over the furnace. The  bell is mounted, and the vacuum pumps are switched
on. The final pressure before beginning measurement is roughly 10-4 Pa.
The cooling  of the refrigeration box is begun from 10~2 Pa downwards.
After a period of time, the  balance  will have  attained a sufficiently low
temperature to allow the escaping vapor jet to condense on  the scale pan.


This condensation produces a signal on the connected recorder. This signal
can be used in two ways. For the particular apparatus  described here, the
vapor pressure is determined directly from the pressure  on the scale pan
(the molecular mass is not required). At the same time the mass condensed
is determined, and  the evaporation rate can therefore  be calculated from
the time of deposition. This latter property applies to more general appara-

     (B) The vapor pressure may also be  calculated from the  evaporation
rate and molecular mass using the Herz relationship,

                  p = G V 2 RT/Mr


     G = evaporation rate

     Mr = relative molecular mass

     T = temperature in K

     R = universal molar gas constant

     P = vapor pressure

     (C) After the  necessary vacuum is reached,  the  series of measure-
ments  commences at  the lowest desired measuring temperature.  The nec-
essary orifice is opened, and the vapor jet passes the shield directly mount-
ed above the cover  and strikes  the cooled scale pan. The size  of the scale
pan is chosen so that the entire jet is collected in its cosinal distribution.
The momentum of the vapor jet results in  a force onto the scale pan where
the molecules condense on its cooled surface. Due to the force of the vapor
jet, the scale  beam will be deflected from the equilibrium state. At the
end of the scale  beam is a small tab which is registered  optically via a
prism system and two photodiodes. A connected control circuit momentar-
ily regulates  and resets the debalanced scale beam  back to the balanced
state. The required  torque is recorded and corresponds, after a calibration
with weights, to the vapor pressure of the substance.

     (D) For further measurements, the temperature  is  increased by small
steps until the maximum desired temperature value is reached.  The sample
is cooled again, and a second curve of the  vapor pressure may be recorded.
The  two series will only be reproducible if the sample being measured
is sufficiently pure. If the third run fails to confirm the results of the sec-
ond run, it is possible that the substance may be decomposing in the tem-
perature range being measured.

     (5) Gas saturation  method—(i) Apparatus. (A) A  typical apparatus
used is designed essentially according to paragraph (f)(5)  of this guideline.
It consists  of a number  of components shown in  the following figure 5:


  Figure 5.—An Example of a Flow System for the Determination of
  Vapour Pressure By the Gas Saturation Method Under Paragraph
                      (e)(6) of this Guideline

1 = flow regulator

2 = heat exchanges

3 = needle valves

4 = sensor (% relative humidity)

5 = saturation columns

6 = PTFE joints

7 = flow meter

8 = trap (absorber)

9 = oil trap

10 = fritted bubbler

     (B) Components. (7) Inert gas. The carrier gas must not react chemi-
cally with the test substance. Nitrogen is usually sufficient for this pupose,
but occasionally other gases may be required. The gas employed must be

     (2) Flow control. A suitable gas control system is required to ensure
a constant and selectable flow through the saturator column.

     (3) Traps to collect vapor. These are dependent on the particular sam-
ple characteristics  and the  method of analysis chosen. The vapor should
be trapped quantitatively and in a form which permits subsequent analysis.
For some test substances, traps containing liquids  such as hexane or ethyl-
ene glycol will be suitable.  For others, solid absorbants may be applicable.

     (4) Heat-exchanger. For measurements at  different  temperatures it
may be necessary to include a heat-exchanger in the assembly.

     (5) Saturator  column: The  test substance is  a  solution coated onto
a suitable inert support. The coated support is packed into  the saturator
column, the dimensions and the  flow rate of which should  be  such that
complete saturation of  the carrier gas is ensured. The saturator column
must be thermostated. For measurements at temperatures above  20  °C, the
region between the saturator column and the traps  should be  heated to
prevent condensation of the test substance.

     (ii) Test conditions. Determinations with the sample  should be made
in triplicate, preferably  at each of three temperatures:  10, 20, and 30 °C.
For some  substances, it may be necessary to carry out the procedures at
elevated temperatures (<100 °C) and extrapolate to these temperatures.

     (iii) Performance  of  test—(A) Preparation of the  saturator  col-
umn. A solution of the  test substance in a highly volatile solvent is added
to a  suitable amount of support.  Sufficient test substance should be added
to maintain  saturation for the duration of the test. The solvent is totally
evaporated in air or in a rotary evaporator, and the thoroughly mixed mate-
rial is  added to the saturator column. After thermostating  the sample, dry
nitrogen is passed through the apparatus.

     (B) Measurement  procedure. (7) The traps are connected  to the col-
umn effluent line and the time recorded. The flow rate is checked at the
beginning and at regular intervals during the experiment, using a bubble
meter (or continuously, with a mass flow-meter). The pressure at the outlet
to the  saturator must be measured. This may be done either  by including
a pressure gauge between the saturator and traps (this because of increased
dead  space  and adsorptive surface), by determining the pressure drops
across the particular trapping system used as a function  of flow  rate in
a separate experiment (may be not very satisfactory for liquid traps).


     (2) The time required  for collecting the quantity  of test substance
that  is necessary for  the different methods of analysis is determined in
preliminary  runs  or by estimates. Before calculating the vapor pressure
at a given temperature, preliminary runs are to be carried out to determine
the maximum flow rate that will  completely  saturate the carrier gas with
substance vapor.  This is guaranteed if the carrier gas is passed through
the  saturator slowly  enough that  a still lower rate gives no greater cal-
culated vapor pressure.
        The  specific analytical method will be determined by the nature
of the substance  being  tested (e.g.,  gas chromatography  or gravimetry).

     (4) The quantity of substance transported by a known volume of car-
rier gas is determined.

     (C)  Calculation  of vapor pressure. Vapor pressure  is calculated
from the vapor density, W/V, by means of the equation:

                  p = W/V x PT/Mr


p = vapor pressure in Pa

W = mass of adsorbed test substance in grams

V = volume of saturated gas in cubic meters

R = universal molar gas constant

T = temperature in K

Mr = relative molecular mass

Measured volume must be corrected  for pressure and temperature dif-
ferences between the flow meter and the thermostated saturator. If the flow
meter is located downstream from the vapor trap, corrections may be nec-
essary to  account  for any vaporized-trap ingredients, as discussed under
paragraph (f)(6) of this guideline.

     (e) Data and reporting — (1) Treatment of results. The vapor pres-
sure from any of the preceeding methods should be determined for at least
three temperatures in the range 0-50 °C. If the chosen method has required
measurement at temperatures above  this range, the vapor pressure curve
(log p versus 1/T)  should be extrapolated to these temperatures. Care must
be taken when extrapolating over large temperature ranges.

     (2) Test report. (A) For all methods, the vapor pressure  at  20  or
25 °C should be reported. This value should preferably be an experimental
one, but may be interpolated or extrapolated if necessary. Date  need not
be reported if vapor pressure is less than 10-5 PA (10-7  TORR).


    (B)  In addition, the test report should include the minimum three
vapor pressure and temperature values used to determine the above value.
It should also include all of the raw data and the log p versus 1/T curve
used to determine these values.

    (C) The report should also include a description of the apparatus and
method employed if they deviate from those described in this test guide-
line. Difficulties encountered and any other pertinent information should
be reported. If transitions (change of state,  decomposition) were  encoun-
tered, the following information should be noted:

    (7) Nature of the change.

    (2) Temperature at which the change occurs at atmospheric pressure.

    (3) Vapor pressure at 10 and 20  °C above and below the transition
or change of state temperature (unless the transition is from solid to gas).

    (f) References. The following references should be consulted for ad-
ditional background material on this test guideline.

    (1)  Report  by D. I. Mennicken in  AP of the  Bayer AG, 5090
Leverkusen, Federal Republic of Germany, December 17, 1968.

    (2) Report by the Leybold Company,  Bonner Str. 504,  5000 Koln-
Bayenthal, Federal  Republic of Germany, 1951.

    (3) Herlet,  A.  and G. Reich. Zeitschrift fur angewandte Physik 9:14-
23 (1957).

    (4) ASTM D  2879-75, Standard Test Method for Vapor Pressure-
Temperature Relationship and Initial Decomposition Temperature of Liq-
uids by Isoteniscope.

    (5) Spencer, W.F. and M. M. Cliath. Vapor Density of Dieldrin, Envi-
ronmental Science and Technology 3:670-74 (1969).

    (6) Thompson, G.W. and D. R. Douslin.  "Vapor Pressure" in Phys-
ical Methods of Chemistry, Arnold Weissberger and W. B. Rossiter (eds.),
Vol. 1, Part 5, pp. 47-89, Wiley-Interscience New York, (1971).

    (7) Friedrich,  K.  and  K.  Stammbach. Journal of Chromatography
16:22-28 (1964).

    (8) OECD Guideline for The Testing  of Chemicals,  Guideline 104,
Vapor Pressure (1995).

    (9) Organization for Economic Cooperation and Development, Guide-
lines for The Testing of Chemicals, OECD 104, Vapor Pressure Curve,
OECD. Paris, France.