[Code of Federal Regulations]
[Title 40, Volume 28]
[Revised as of July 1, 2002]
From the U.S. Government Printing Office via GPO Access
[CITE: 40CFR796.1950]

[Page 82-87]
 
                   TITLE 40--PROTECTION OF ENVIRONMENT
 
         CHAPTER I--ENVIRONMENTAL PROTECTION AGENCY (CONTINUED)
 
PART 796--CHEMICAL FATE TESTING GUIDELINES--Table of Contents
 
               Subpart B--Physical and Chemical Properties
 
Sec. 796.1950  Vapor pressure.

    (a) Introduction--(1) Background and purpose. (i) Volatilization, 
the evaporative loss of a chemical, depends upon the vapor pressure of 
chemical and on environmental conditions which influence diffusion from 
a surface. Volatilization is an important source of material for 
airborne transport and may lead to the distribution of a chemical over 
wide areas and into bodies of water far from the site of release. Vapor 
pressure values provide indications of the tendency of pure substances 
to vaporize in an unperturbed situation, and thus provide a method for 
ranking the relative volatilities of chemicals. Vapor pressure data 
combined with water solubility data permit the calculation of Henry's 
law constant, a parameter essential to the calculation of volatility 
from water.
    (ii) Chemicals with relatively low vapor pressures, high 
adsorptivity onto solids, or high solubility in water are less likely to 
vaporize and become airborne than chemicals with high vapor pressures or 
with low water solubility or low adsorptivity to solids and sediments. 
In addition, chemicals that are likely to be gases at ambient 
temperatures and which have low water solubility and low adsorptive 
tendencies are less likely to transport and persist in soils and water. 
Such chemicals are less likely to biodegrade or hydrolyze and are prime 
candidates for atmospheric oxidation and photolysis (e.g., smog 
formation or stratospheric alterations). On the other hand, nonvolatile 
chemicals are less frequently involved in atmosphere transport, so that 
concerns regarding them should focus on soils and water.
    (iii) Vapor pressure data are an important consideration in the 
design of other chemical fate and effects tests;

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for example, in preventing or accounting for the loss of violatile 
chemicals during the course of the test.
    (2) Definitions and units. (i) ``Desorption efficiency'' of a 
particular compound applied to a sorbent and subsequently extracted with 
a solvent is the weight of the compound which can be recovered from the 
sorbent divided by the weight of the compound originally sorbed.
    (ii) ``Pascal'' (Pa) is the standard international unit of vapor 
pressure and is defined as newtons per square meter (N/m2). A 
newton is the force necessary to give acceleration of one meter per 
second squared to one kilogram of mass.
    (iii) The ``torr'' is a unit of pressure which equals 133.3 pascals 
or 1 mm Hg at 0 [deg]C.
    (iv) ``Vapor pressure'' is the pressure at which a liquid or solid 
is in equilibrium with its vapor at a given temperature.
    (v) ``Volatilization'' is the loss of a substance to the air from a 
surface or from solution by evaporation.
    (3) Principle of the test methods. (i) The isoteniscope procedure 
uses a standardized technique [ASTM 1978] that was developed to measure 
the vapor pressure of certain liquid hydrocarbons. The sample is 
purified within the equipment by removing dissolved and entrained gases 
until the measured vapor pressure is constant, a process called 
``degassing.'' Impurities more volatile than the sample will tend to 
increase the observed vapor pressure and thus must be minimized or 
removed. Results are subject to only slight error for samples containing 
nonvolatile impurities.
    (ii) Gas saturation (or transpiration) procedures use a current of 
inert gas passed through or over the test material slowly enough to 
ensure saturation and subsequent analysis of either the loss of material 
or the amount (and sometimes kind) of vapor generated. Gas saturation 
procedures have been described by Spencer and Cliath (1969) under 
paragraph (d)(2) of this section. Results are easy to obtain and can be 
quite precise. The same procedures also can be used to study 
volatilization from laboratory scale environmental simulations. Vapor 
pressure is computed on the assumption that the total pressure of a 
mixture of gases is equal to the sum of the pressures of the separate or 
component gases and that the ideal gas law is obeyed. The partial 
pressure of the vapor under study can be calculated from the total gas 
volume and the weight of the material vaporized. If v is the volume 
which contains w grams of the vaporized material having a molecular 
weight M, and if p is the pressure of the vapor in equilibrium at 
temperature T (K), then the vapor pressure, p, of the sample is 
calculated by

p=(w/M)(RT/v),


where R is the gas constant (8.31 Pa m2 mol-1 
K-1) when the pressure is in pascals (Pa) and the volume is 
in cubic meters. As noted by Spencer and Cliath (1970) under paragraph 
(d)(3) of this section, direct vapor pressure measurements by gas 
saturation techniques are more directly related to the volatilization of 
chemicals than are other techniques.
    (iii) In an effort to improve upon the procedure described by 
Spencer and Cliath (1969) under paragraph (d)(2) of this section, and to 
determine the applicability of the gas saturation method to a wide 
variety of chemical types and structures, EPA has sponsored research and 
development work at SRI International (EPA 1982) under paragraph (d)(1) 
of this section. The procedures described in this Test Guideline are 
those developed under that contract and have been evaluated with a wide 
variety of chemicals of differing structure and vapor pressures.
    (4) Applicability and specificity. (i) A procedure for measuring the 
vapor pressure of materials released to the environment ideally would 
cover a wide range of vapor pressure values, at ambient temperatures. No 
single procedure can cover this range, so two different procedures are 
described in this section, each suited for a different part of the 
range. The isoteniscope procedure is for pure liquids with vapor 
pressures from 0.1 to 100 kPa. For vapor pressures of 10-5 to 
10 3 Pa, a gas saturation procedure is to be used.
    (ii) With respect to the isoteniscope method, if compounds that boil 
close to

[[Page 84]]

or form azeotropes with the test material are present, it is necessary 
to remove the interfering compounds and use pure test material. 
Impurities more volatile than the sample will tend to increase the 
observed vapor pressure above its true value but the purification steps 
will tend to remove these impurities. Soluble, nonvolatile impurities 
will decrease the apparent vapor pressure. However, because the 
isoteniscope procedure is a static, fixed-volume method in which an 
insignificant fraction of the liquid sample is vaporized, it is subject 
to only slight error for samples containing nonvolatile impurities. That 
is, the nonvolatile impurities will not be concentrated due to 
vaporization of the sample.
    (iii) The gas saturation method is applicable to solid or liquid 
chemicals. Since the vapor pressure measurements are made at ambient 
temperatures, the need to extrapolate data from high temperatures is not 
necessary and high temperature extrapolation, which can often cause 
serious errors, is avoided. The method is most reliable for vapor 
pressures below 10 3 Pa. Above this limit, the vapor 
pressures are generally overestimated, probably due to aerosol 
formation. Finally, the gas saturation method is applicable to the 
determination of the vapor pressure of impure materials.
    (b) Test procedures--(1) Test conditions. (i) The apparatus in the 
isoteniscope method is described in paragraph (b)(2)(i) of this section.
    (ii) The apparatus used in the gas saturation method is described in 
paragraph (b)(2)(ii) of this section.
    (2) Performance of the tests--(i) Isoteniscope Procedure. The 
isoteniscope procedure described as ANSI/ASTM Method D 2879-86 is 
applicable for the measurement of vapor pressures of liquids with vapor 
pressures of 0.1 to 100 kilopascals (kPa) (0.75 to 750 torr). ASTM D 
2879-86 is available for inspection at the Office of the Federal 
Register, 800 North Capitol Street, NW., suite 700, Washington, DC. This 
incorporation by reference was approved by the Director of the Office of 
the Federal Register. This material is incorporated as it exists on the 
date of approval and a notice of any change in this material will be 
published in the Federal Register. Copies of the incorporated material 
may be obtained from the Non-Confidential Information Center (NCIC) 
(7407), Office of Pollution Prevention and Toxics, U.S. Environmental 
Protection Agency, Room B-607 NEM, 401 M St., SW., Washington, DC 20460, 
between the hours of 12 p.m. and 4 p.m. weekdays excluding legal 
holidays, or from the American Society for Testing and Materials (ASTM), 
1916 Race Street, Philadelphia, PA 19103. The isoteniscope method 
involves placing liquid sample in a thermostated bulb (the isoteniscope) 
connected to a manometer and a vacuum pump. Dissolved and entrained 
gases are removed from the sample in the isoteniscope by degassing the 
sample at reduced presssure. The vapor pressure of the sample at 
selected temperatures is determined by balancing the pressure due to the 
vapor of the sample against a known pressure of an inert gas. The vapor 
pressure of the test compound is determined in triplicate at 
25[plusmn]0.5 [deg]C and at any other suitable temperatures 
([plusmn]0.5[deg]). It is important that additional vapor pressure 
measurements be made at other temperatures, as necessary, to assure that 
there is no need for further degassing, as described in the ASTM method.
    (ii) Gas saturation procedure. (A) The test procedures require the 
use of a constant-temperature box as depicted in the following Figure 1.

[[Page 85]]

[GRAPHIC] [TIFF OMITTED] TC01AP92.036

        Figure 1--Schematic Diagram of Vapor Saturation Apparatus

The insulated box, containing sample holders, may be of any suitable 
size and shape. The sketch in Figure 1 shows a box containing three 
solid sample holders and three liquid sample holders, which allows for 
the triplicate analysis of either a solid or liquid sample. The 
temperature within the box is controlled to [plusmn]0.5[deg] or better. 
Nitrogen gas, split into six streams and controlled by fine needle 
valves (approximately 0.79 mm orifice), flows into the box via 3.8 mm 
(0.125 in.) i.d. copper tubing. After temperature equilibration, the gas 
flows through the sample and the sorbent trap and exits from the box. 
The flow rate of the effluent carrier gas is measured at room 
temperature with a bubble flow meter or other suitable device. The flow 
rate is checked frequently during the experiment to assure that there is 
an accurate value for the total volume of carrier gas. The flow rate is 
used to calculate the total volume (at room temperature) of gas that has 
passed through the sample and sorbent [(vol/time) x time = volume]. The 
vapor pressure of the test substance can be calculated from the total 
gas volume and the mass of sample vaporized. If v is the volume of gas 
that transported mass w of the vaporized test material having a 
molecular weight M, and if p is the equilibrium vapor pressure of the 
sample at temperature T, then p is calculated by the equation

    p=(w/M)(RT/v).


In this equation, R is the gas constant (8.31 Pa m\3\mol-1 
K-1). The pressure is expressed in pascals (Pa), the volume 
in cubic meters (m\3\), mass in grams and T in kelvins (K). T=273.15+t, 
if t is measured in degrees Celsius ([deg]C).
    (B) Solid samples are loaded into 5 mm i.d. glass tubing between 
glass wool plugs. The following Figure 2 depicts a drawing of a sample 
holder and absorber system.

[[Page 86]]

[GRAPHIC] [TIFF OMITTED] TC01AP92.037

                Figure 2--Solid Compound Sampling System
    (C) Liquid samples are contained in a holder as shown in the 
following Figure 3.
[GRAPHIC] [TIFF OMITTED] TC01AP92.038

                Figure 3--Liquid Compound Sampling System

The most reproducible method for measuring the vapor pressure of liquids 
is to coat the liquid on glass beads and to pack the holder in the 
designated place with these beads.
    (D) At very low vapor pressures and sorbent loadings, adsorption of 
the chemical on the glass wool separating the sample and the sorbent and 
on the glass surfaces may be a serious problem. Therefore, very low 
loadings should be avoided whenever possible. Incoming nitrogen gas 
(containing no interfering impurities) passes through a coarse frit and 
bubbles through a 38 cm column of liquid sample. The stream passes 
through a glass wool column to trap aerosols and then through a sorbent 
tube, as described above. The pressure drop across the glass wool column 
and the sorbent tube are negligible.
    (E) With both solid and liquid samples, at the end of the sampling 
time, the front and backup sorbent sections are analyzed separately. The 
compound on each section is desorbed by adding the sorbent from that 
section to 1.0 ml of desorption solvent in a small vial and allowing the 
mixture to stand at a suitable temperature until no more test compound 
desorbs. It is extremely important that the desorption solvent contain 
no impurities which would interfere with the analytical method of 
choice. The resulting solutions are analyzed quantitatively by a 
suitable analytical method to determine the weight of sample desorbed 
from each section. The choice of the analytical method, sorbent, and 
desorption solvent is dictated by the nature of the test material. 
Commonly used sorbents include charcoal, Tenax GC, and XAD-2. Describe 
in detail the sorbent, desorption solvent, and analytical methods 
employed.
    (F) Measure the desorption efficiency for every combination of 
sample, sorbent, and solvent used. The desorption efficiency is 
determined by injecting a known mass of sample onto a sorbent and later 
desorbing it and analyzing for the mass recovered. For each combination 
of sample, sorbent, and solvent used, carry out the determination in 
triplicate at each of three concentrations. Desorption efficiency may 
vary with the concentration of the actual sample and it is important to 
measure the efficiency at or near the concentration of sample under gas 
saturation test procedure conditions.

[[Page 87]]

    (G) To assure that the gas is indeed saturated with test compound 
vapor, sample each compound at three differing gas flow rates. 
Appropriate flow rates will depend on the test compound and test 
temperature. If the calculated vapor pressure shows no dependence on 
flow rate, then the gas is assumed to be saturated.
    (c) Data and reporting. (1) Report the triplicate calculated vapor 
pressures for the test material at each temperature, the average 
calculated vapor pressure at each temperature, and the standard 
deviation.
    (2) Provide a description of analytical methods used to analyze for 
the test material and all analytical results.
    (3) For the isoteniscope procedure, include the plot of p vs. the 
reciprocal of the temperature in K, developed during the degasing step 
and showing linearity in the region of 298.15 K (25[deg]C) 
and any other required test temperatures.
    (4) For the gas saturation procedure, include the data on the 
calculation of vapor pressure at three or more gas flow rates at each 
test temperature, showing no dependence on flow rate. Include a 
description of sorbents and solvents employed and the desorption 
efficiency calculations.
    (5) Provide a description of any difficulties experienced or any 
other pertinent information.
    (d) References. For additional background information on this test 
guideline the following references should be consulted:
    (1) U.S. Environmental Protection Agency. Evaluation of Gas 
Saturation Methods to Measure Vapor Pressures: Final Report, EPA 
Contract No. 68-01-5117 with SRI International, Menlo Park, California 
(1982).
    (2) Spencer, W.F. and Cliath, M.M. ``Vapor Density of Dieldrin,'' 
Journal of Agricultural and Food Chemistry, 3:664-670 (1969).
    (3) Spencer, W.F. and Cliath, M.M. ``Vapor Density and Apparent 
Vapor Pressure of Lindane,'' Journal of Agricultural and Food Chemistry, 
18:529-530 (1970).

[50 FR 39252, Sept. 27, 1985, as amended at 53 FR 12525, Apr. 15, 1988; 
53 FR 21641, June 9, 1988; 60 FR 34466, July 3, 1995]