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Table 6.3 Effects of salts on the solubility of selected organic compounds expressed as
Setschnow constant (L/mol). Reprinted from Xie WH, Shiu WY, Mackay DA ( 1997 ) review of
the effect of salts on the solubility of organic compounds in seawater. Marine Envir Res
44:429-444. Copyright 1997 with permission of Elsevier
Salt
Benzene
Toluene
o-
Xylene
m-
Xylene
p-
Xylene
Naphthalene
Salting out
NaCl
0.195
0.225
0.227
0.248
0.251
0.260
KCl
0.166
0.206
0.205
0.222
0.217
0.204
NaNO 3
0.119
0.144
0.141
0.165
0.146
0.131
MgSO 4
0.488
0.457
0.491
0.531
0.491
0.516
BaCl 2
0.344
0.376
0.393
0.412
0.407
0.401
CH 3 COONa
0.165
0.209
0.206
0.237
0.208
0.21
Salting in
(C 2 H 5 ) 4 NBr
-0.56
-0.323
-0.414
-0.402
-0.413
-0.11
(CH 3 ) 4 NBr
-0.24
-0.163
-0.198
-0.178
-0.2
-0.5
salts. Salting-out constants for selected organic compounds and aqueous electro-
lyte solutions at 25 C are presented in Table 6.3 .
Dror et al. ( 2000a , 2000b ) report an experiment dealing with the effect of type
and concentration of electrolytes, in an artificial soil aqueous solution, on the
solubility of kerosene, a petroleum product containing more than 100 hydrocar-
bons. At increasing concentrations in water of NaCl or CaCl 2 , from 0.2 to 2.0 M, a
linear decrease in kerosene solubility occurred. It was also observed that the
decrease in kerosene dissolution in the artificial soil solution is controlled by the
electrolyte concentration but is not influenced by the sodium adsorption ratio
(SAR; defined in Eq. 2.46 ). As noted previous, the SAR is an important factor in
the relationship between soil solutions and soils. The salting-out effect is consis-
tent with published data (Xie et al. 1997 ; Schwarzenbach et al. 2003 ).
It is interesting to note that smaller ions (e.g., Na + ,Mg 2+ ,Ca 2+ ,Cl - ) form
hydration shells larger than bigger ions, which tend to bind water molecules only
very weakly. In a simple way, the salting out of nonpolar and weakly polar
compounds was explained by Schwarzenbach et al. ( 2003 ) by imagining that the
dissolved ions compete successfully with the organic compound for solvent
molecules. The freedom of some water molecules to solvate an organic molecule
depends on the type and concentration of salts.
Under natural conditions, we usually deal with the solubility of nonelectrolytes
in mixed electrolyte solutions. This aspect has not been thoroughly studied to date.
Early studies of Randall and Failey ( 1927 ) used ionic strength fractions for
modeling salting coefficients in mixed electrolyte solutions. No changes were
observed in the Setschenow constant values when the electrolyte concentration
was held constant, but NaCl ions were replaced with CaCl 2 , showing that the ionic
composition did not affect the dissolution in water of NAPL compounds. The
solubility of naphthalene in some electrolyte mixtures was determined by Gordon
and Thorne ( 1967a , b ) using the simple mixing rule
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