Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 128

TABLE 4.3
Measured Air-Water Partition Constants for Selected Compounds
in Natural Waters
Compound Matrix Composition K aw Reference

Toluene Distilled water 0.244 Yurteri et al. (1987)

Tap water 0.231

“Creek” water 0.251

Chloroform Distilled water 0.125 Nicholson et al. (1984)

Natural lake water 0.121

CH 3 CCl 3 Distilled water 0.53 Hunter-Smith et al. (1983)

Seawater 0.94

CHClBr 2 Distilled water 0.036 Nicholson et al. (1984)

Natural lake water 0.034

CHBr 3 Distilled water 0.018 Nicholson et al. (1984)

Natural lake water 0.017

Hexachlorobenzene Distilled water 0.054 Brownawell (1986)

Seawater 0.070

2,4,4 -Trichlorobiphenyl Distilled water 0.00595 Brownawell (1986)

Seawater 0.00885

effects on Henry's constants in the presence of co-solvents such as methanol and iso-
propanol up to concentrations as large as 10 g/L (i.e., co-solvent mole fraction of
0.5 10 − 3 ) . It was observed that the effects were pronounced for very hydrophobic

solutes.A compound that interacts better with the co-solvent exhibits the effect at low
co-solvent concentrations than does a compound that is more hydrophilic. In gen-
eral, for any hydrophobic solute, the presence of a co-solvent increases the aqueous
solubility and lowers K aw . These conclusions have been substantiated by others who
determined the Henry's constants of volatile solutes such as toluene and chloroform
in distilled, tap, and natural lake waters. Table 4.3 is a summary of these observations.
The effects of inorganic salts on air-water partition constants are manifest through
their effects on the infinite dilution activity coefficients that are described in Chap-
ter 3. Most hydrophobic organic compounds show the salting-out behavior, whereby
the mole fraction solubility in water is decreased (or activity coefficient is increased).
Consequently, the air-water partition constant decreases in the presence of salts. A
number of investigators have found that, for example, the partition constant for chlori-
natedVOCs such as chlorofluorocarbons (CFCs) and chlorobiphenyls in seawater are
20-80% greater than the values determined in distilled or deionized water (Table 4.3).

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E XAMPLE 4.6 K aw AND I ONIC S TRENGTH
Determine K aw for phenanthrene in an industrial wastewater with 1 M total ion
concentration at 292 K.