[Code of Federal Regulations]
[Title 40, Volume 31]
[Revised as of July 1, 2006]
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
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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; 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/m\2\). 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),
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where R is the gas constant (8.31 Pa m\2\ 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 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 National Archives and Records
Administration (NARA). For information on the availability of this
material at NARA, call 202-741-6030, or go to: http://www.archives.gov/
federal--register/code--of--federal--regulations/ibr--locations.html.
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.
[[Page 85]]
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 0.5 [deg]C and at
any other suitable temperatures (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.
[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 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
[[Page 86]]
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.
[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,
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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.
(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; 69 FR 18803, Apr.
9, 2004]