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§799.6786 TSCA water solubility: Generator column method.
(a) Scope -- (1) Applicability. This section is intended to meet the testing requirements of the Toxic Substances Control Act (TSCA) (15 U.S.C. 2601).
(2) Source. The source material used in developing this TSCA test guideline is the Office of Pollution Prevention, Pesticides and Toxics (OPPTS) harmonized test guideline 830.7860 (March 1998, revised final guideline). The source is available at the address in paragraph (e) of this section.
(b) Introduction -- (1) Purpose. (i) The water solubility of a chemical is defined as the equilibrium concentration of the chemical in a saturated aqueous solution at a given temperature and pressure. The aqueous phase solubility is an important factor in governing the movement, distribution, and rate of degradation of chemicals in the environment. Substances that are relatively water soluble are more likely to be widely distributed by the hydrologic cycle than those which are relatively insoluble. Furthermore, substances with higher water solubility are more likely to undergo microbial or chemical degradation in the environment because dissolution makes them "available" to interact and, therefore, react with other chemicals and microorganisms. Both the extent and rate of degradation via hydrolysis, photolysis, oxidation, reduction, and biodegradation depend on a chemical being soluble in water (i.e., homogeneous kinetics).
(ii) Water provides the medium in which many organisms live, and water is a major component of the internal environment of all living organisms (except for dormant stages of certain life forms). Even organisms which are adapted to life in a gaseous environment require water for normal functioning. Water is thus the medium through which most other chemicals are transported to and into living cells. As a result, the extent to which chemicals dissolve in water will be a major determinant for movement through the environment and entry into living systems.
(iii) The water solubility of a chemical also has an effect on its sorption into and desorption from soils and sediments, and on volatilization from aqueous media. The more soluble a chemical substance is, the less likely it is to sorb to soils and sediments and the less likely it is to volatilize from water. Finally, the design of most chemical tests and many ecological and health tests requires precise knowledge of the water solubility of the chemical to be tested.
(2) Definitions. The following definitions apply to this section.
Concentration (C) of a solution is the amount of solute in a given amount of solvent or solution and can be expressed as a weight/weight or weight/volume relationship. The conversion from a weight relationship to one of volume incorporates density as a factor. For dilute aqueous solutions, the density of the solvent is approximately equal to the density of the solution; thus, concentrations expressed in milligrams per liter (mg/L) are approximately equal to 10-3 g/103 g or parts per million (ppm); those expressed in micrograms per liter (μg/L) are approximately equal to 10-6 g/103 g or parts per billion (ppb). In addition, concentration can be expressed in terms of molarity, normality, molality, and mole fraction. For example, to convert from weight/volume to molarity molecular mass is incorporated as a factor.
Density is the mass of a unit volume of a material. It is a function of temperature, hence the temperature at which it is measured should be specified. For a solid, it is the density of the impermeable portion rather than the bulk density. For solids and liquids, suitable units of measurement are grams per cubic centimeter (g/cm3). The density of a solution is the mass of a unit volume of the solution and suitable units of measurement are g/cm3.
Extractor column is used to extract the solute from the saturated solutions produced by the generator column. After extraction onto a chromatographic support, the solute is eluted with a solvent/water mixture and subsequently analyzed by high-pressure liquid chromatography (HPLC), gas chromatography (GC), or any other suitable analytical procedure. A detailed description of the preparation of the extractor column is given in paragraph (c)(1)(i)(D) of this section.
Generator column is used to produce or generate saturated solutions of a solute in a solvent. The column, see figure 1 in paragraph (c)(1)(i)(A) of this section, is packed with a solid support coated with the solute, i.e., the organic compound whose solubility is to be determined. When water (the solvent) is pumped through the column, saturated solutions of the solute are generated. Preparation of the generator column is described in paragraph (c)(1)(i)(A) of this section.
Response factor (RF) is the solute concentration required to give a 1 unit area chromatographic peak or 1 unit output from the HPLC recording integrator at a particular recorder attenuation. The factor is required to convert from units of area to units of concentration. The determination of the RF is given in paragraph (c)(3)(ii)(B)(2) of this section.
Sample loop is a Saturated solution is a solution in which the dissolved solute is in
equilibrium with an excess of undissolved solute; or a solution in equilibrium
such that at a fixed temperature and pressure, the concentration of the solute
in the solution is at its maximum value and will not change even in the presence
of an excess of solute.
Solution is a homogeneous mixture of two or more substances
constituting a single phase.
(3) Principle of the test method. (i) This test method is based on the
dynamic coupled column liquid chromatographic (DCCLC) technique for determining
the aqueous solubility of organic compounds that was initially developed by May
et al. (as described in the references listed in paragraphs (e)(5) and (e)(6) of
this section), modified by DeVoe et al. (as described in the reference listed in
paragraph (e)(1) of this section), and finalized by Wasik et al. (as described
in the reference listed in paragraph (e)(11) of this section). The DCCLC
technique utilizes a generator column, extractor column and HPLC coupled or
interconnected to provide a continuous closed flow system. Saturated aqueous
solutions of the test compound are produced by pumping water through the
generator column that is packed with a solid support coated with the compound.
The compound is extracted from the saturated solution onto an extractor column,
then eluted from the extractor column with a solvent/water mixture and
subsequently analyzed by HPLC using a variable wavelength ultraviolet (UV)
detector operating at a suitable wavelength. Chromatogram peaks are recorded and
integrated using a recording integrator. The concentration of the compound in
the effluent from the generator column, i.e., the water solubility of the
compound, is determined from the mass of the compound (solute) extracted from a
measured volume of water (solvent).
(ii) Since the HPLC method is only applicable to compounds that absorb in the
UV, an alternate GC method, or any other reliable procedure (which must be
approved by OPPTS), can be used for those compounds that do not absorb in the
UV. In the GC method the saturated solutions produced in the generator column
are extracted using an appropriate organic solvent that is subsequently injected
into the GC, or any other suitable analytical device, for analysis of the test
compound.
(4) Reference chemicals. Table 1 of this section lists the water
solubilities at 25 °C for a number of reference chemicals as obtained from the
scientific literature. The data from Wasik et al. (as described in the reference
listed in paragraph (e)(11) of this section), Miller et al. and Tewari et al.
(as described in the references listed in paragraphs (e)(7) and (e)(10) of this
section, respectively) were obtained from the generator column method. The water
solubilities data were also obtained from Mackay et al. and Yalkowski et al. (as
described in the references listed in paragraphs (e)(4) and (e)(12) of this
section, respectively) and other scientists by the conventional shake flask
method. These data have been provided primarily so that the generator column
method can be calibrated from time to time and to allow the chemical testing
laboratory an opportunity to compare its results with those listed in table 1 of
this section. The water solubility values at 25 °C reported by Yalkowski et al.
are their preferred values and, in general, represent the best available water
solubility data at 25 °C. The testing laboratory has the option of choosing its
own reference chemicals, but references must be given to establish the validity
of the measured values of the water solubility. (5) Applicability and specificity. (i) Procedures are described in
this section to determine the water solubility for liquid or solid compounds.
The water solubility can be determined in very pure water, buffer solution for
compounds that reversibly ionize or protonate, or in artificial seawater as a
function of temperature (i.e., in the range of temperatures of environmental
concern). This section is not applicable to the water solubility of gases.
(ii) This section is designed to determine the water solubility of a solid or
liquid test chemical in the range of 1 ppb to 5,000 ppm. For chemicals whose
solubility is below 1 ppb, the water solubility should be characterized as "less
than 1 ppb" with no further quantification. For solubilities greater than 5,000
ppm, the shake flask method should be used, see paragraph (e)(15) of this
section.
(c) Test procedure -- (1) Test conditions -- (i) Special
laboratory equipment -- (A) Generator column. (1) Either of
two different designs shall be used depending on whether the eluted aqueous
phase is analyzed by HPLC in paragraph (c)(3)(ii) of this section or by solvent
extraction followed by GC (or any other reliable quantitative) analysis of
solvent extract in paragraph (c)(3)(iv) of this section. The design of the
generator column is shown in the following figure 1:
(2) The column consists of a 6 mm (
(B) Constant temperature bath with circulation pump-bath and capable of
controlling temperature to ± 0.05 °C, see paragraph (c)(3) of this section.
(C) HPLC equipped with a variable wavelenth UV absorption detector operating
at a suitable wavelength and a recording integrator in paragraph (c)(3)(ii) of
this section.
(D) Extractor column -- 6.6 × 0.6 cm stainless steel tube with end fittings
containing 5 μm frits filled with a superficially porous phase packing
(Bondapack C (E) Two 6-port high-pressure rotary switching valves in paragraph (c)(3)(ii)
of this section.
(F) Collection vessel -- 8 × (G) GC, or any other reliable analytical equipment, which has a detector
sensitive to the solute of interest in paragraph (c)(3)(iii) of this section.
(ii) Purity of water. Water meeting appropriate American Society for
Testing and Materials (ASTM) Type II standards, or an equivalent grade, are
recommended to minimize the effects of dissolved salts and other impurities on
water solubility. ASTM Type II water is presented in the reference listed in
paragraph (e)(13) of this section.
(iii) Purity of solvents. All solvents used in this method must be
reagent or HPLC grade. Solvents must contain no impurities which could interfere
with the determination of the test compound.
(iv) Seawater. When the water solubility in seawater is desired, the
artificial seawater described in paragraph (c)(2)(ii) of this section must be
used.
(v) Effect of pH on solubility. For chemicals that reversibly ionize
or protonate with a pK (2) Preparation of reagents and solutions -- (i) Buffer
solutions. Prepare buffer solutions as follows:
(A) pH 3.0 -- to 250 mL of 0.10M potassium hydrogen phosphate add 111 mL of
0.10 M hydrochloric acid; adjust the final volume to 500 mL with reagent grade
water.
(B) pH 5.0 -- to 250 mL of 0.1M potassium hydrogen phthalate add 113 mL of
0.1M sodium hydroxide; adjust the final volume to 500 mL with reagent grade
water.
(C) pH 7.0 -- to 250 mL of 0.1M potassium dihydrogen phosphate add 145 mL of
0.1M sodium hydroxide; adjust the final volume to 500 mL with reagent grade
water.
(D) pH 9.0 -- to 250 mL of 0.075M borax add 69 mL of 0.1M HCl; adjust the
final volume to 500 mL with reagent grade water.
(E) pH 11.0 -- to 250 mL of 0.05 M sodium bicarbonate add 3 mL of 0.10 M
sodium hydroxide; adjust the final volume to 500 mL with reagent grade water.
(ii) Check the pH of each buffer solution with a pH meter at 25 °C and adjust
to pH 5.0, 7.0, or 9.0, if necessary. If the pH of the solution has changed by
±0.2 pH units or more after the addition of the test compound, then a more
concentrated buffer is required for that pH determination. The sponsor should
then choose a more suitable buffer.
(iii) Artificial seawater. Add the reagent-grade chemicals listed in
table 2 of this section in the specified amounts and order to 890 mL of
reagent-grade water. Each chemical shall be dissolved before another one is
added. (3) Performance of the test. Using either the procedures in paragraph
(c)(3)(ii) or (c)(3)(iii) of this section, determine the water solubility of the
test compound at 25 °C in reagent-grade water or buffer solution, as
appropriate. Under certain circumstances, it may be necessary to determine the
water solubility of a test compound at 25 °C in artificial seawater. The water
solubility can also be determined at other temperatures of environmental concern
by adjusting the temperature of the water bath to the appropriate temperature.
(i) Prior to the determination of the water solubility of the test chemical,
two procedures shall be followed.
(A) The saturated aqueous solution leaving the generator column must be
tested for the presence of an emulsion, using a Tyndall procedure. If colloids
are present, they must be eliminated prior to the injection into the extractor
column. This may be achieved by lowering the flow rate of the water.
(B) The efficiency of the removal of the solute (i.e. test chemical) by the
solvent extraction from the extraction column must be determined and used in the
determination of the water solubility of the test chemical.
(ii) Procedure A -- HPLC method -- (A) Scope. (1)
Procedure A covers the determination of the aqueous solubility of compounds
which absorb in the UV.
(i) The HPLC analytical system is shown schematically in the following
figure 3:
(ii) Two reciprocating piston pumps deliver the mobile phase (water or
solvent/water mixture) through two 6-port high-pressure rotary valves and a 30 ×
0.6 cm C (2) The general procedure for analyzing the aqueous phase is as
follows (a detailed procedure is given in paragraph (c)(3)(ii)(B)(4) of
this section).
(i) Direct the aqueous solution to "Waste," see figure 3 in paragraph
(c)(3)(ii)(A)(1)(i) of this section, with the switching valve in
the inject position in order to equilibrate internal surfaces with the solution,
thus ensuring that the analyzed sample would not be depleted by solute
adsorption on surfaces upstream from the valve.
(ii) At the same time, water is pumped from the HPLC pumps in order to
displace the solvent from the extractor column.
(iii) The switching valve is next changed to the load position to
divert a sample of the solution through the extractor column, and the liquid
leaving this column is collected in a weighing bottle. During this extraction
step, the mobile phase is changed to a solvent/water mixture to condition the
analytical column.
(iv) After the desired volume of sample is extracted, the switching
valve is returned to the inject position for elution and analysis. Assuming that
there is no breakthrough of solute from the extractor column during the
extraction step, the chromatographic peak represents all of the solute in the
sample, provided that the extraction efficiency is 100%. If the extraction
efficiency is less than 100%, then the extraction efficiency shall be used to
determine the actual weight of the solute extracted.
(v) The solute concentration in the aqueous phase is calculated from
the peak area and the weight of the extracted liquid collected in the weighing
bottle.
(B) Determinations -- (1) Sample-loop volume. Accurate
measurement of the sample loop may be accomplished by using the
spectrophotometric method of Devoe et al. under paragraph (e)(1) of this
section. For this method measure absorbance, A
Table 1_Water Solubilities at 25 °C of Some Reference Chemicals
----------------------------------------------------------------------------------------------------------------
Water solubility (ppm at 25 °C)
--------------------------------------------------------
Reference chemical Wasik (generator Other literature
column method) Yalkowski\1\ \5\ references
----------------------------------------------------------------------------------------------------------------
2-Heptanone............................................ \2\4080 4300 \5\4330
1-Chlorobutane......................................... \2\873 872.9 \7\666
Ethylbenzene........................................... \2\187 208 \7\162
1,2,3-Trimethylbenzene................................. \2\65.5 75.2 \7\48.2
Biphenyl............................................... \3\ \10\6.71 7.48 \8\6.62
Phenanthrene........................................... \4\1.002 1.212 _
2,4,6-Trichlorobiphenyl................................ \3\ \10\0.226 0.225 \8\0.119
2,3,4,5-Tetrachlorobiphenyl............................ \3\ \10\0.0209 0.01396 \8\0.0192
Hexachlorobenzene...................................... _ 0.004669 \9\0.00996
2,3,4,5,6-Pentachlorobiphenyl.......................... \3\ \10\0.00548 0.004016 \8\0.0068
----------------------------------------------------------------------------------------------------------------
\1\ Preferred water solubility at 25 °C by Yalkowski et al. (1990) in paragraph (e)(12) of this section
based on a critical review of all the experimental water solubility data published.
\2\ Tewari et al. (1982) in paragraph (e)(10) of this section.
\3\ Leifer et al. (1983) in paragraph (e)(3) of this section.
\4\ May, Wasik, and Freeman (1978, 1978a) in paragraphs (e)(5) and (6) of this section.
\5\ Yalkowski et al. (1990) in paragraph (e)(12) of this section.
\6\ Hansch et al. (1968) in paragraph (e)(2) of this section.
\7\ Sutton and Calder (1975) in paragraph (e)(9) of this section.
\8\ Mackay et al. (1980) in paragraph (e)(4) of this section.
\9\ The elution chromatographic method from Organization for Economic Cooperation and Development (OECD) (1981)
in paragraph (e)(8) of this section.
\10\ Miller et al. (1984) in paragraph (e)(7) of this section.
Table 2_Constituents of Artificial Seawater\1\
------------------------------------------------------------------------
Chemical Amount
------------------------------------------------------------------------
NaF.................................................. 3 mg
SrCl[INF]2[/INF].6H[INF]2[/INF]O..................... 20 mg
H[INF]3[/INF]BO[INF]3[/INF].......................... 30 mg
KBr.................................................. 100 mg
KCl.................................................. 700 mg
CaCl[INF]2[/INF].2H[INF]2[/INF]O..................... 1.47 gram (g)
Na[INF]2[/INF]SO[INF]4[/INF]......................... 4.00 g
MgCl[INF]2[/INF].6H[INF]2[/INF]O..................... 10.78 g
NaCl................................................. 23.50 g
Na[INF]2[/INF]SiO[INF]3[/INF].9H[INF]2[/INF]O........ 20 mg
NaHCO[INF]3[/INF].................................... 200 mg
------------------------------------------------------------------------
\1\ If the resulting solution is diluted to 1 L, the salinity should be
34±0.5 g/kilogram (kg) and the pH 8.0±0.2. The desired test
salinity is attained by dilution at time of use.
