THE ACTIVITY COEFFICIENT OF NON-ELECTROLYTES IN AQUEOUS SALT SOLUTIONS FROM SOLUBILITY MEASUREMENTS. THE SALTING-OUT ORDER OF THE IONS MERLE RANDBLL
AND
CRAWFORD FAIRBANKS FAILEY
UniversitzJ of California, Berkeley, California
In the preceding paper we studied the activity coefficient of gases dissolved in aqueous salt solutions. It was found that the quotient of the logarithm of the activity coefficient of the dissolved gas by the ionic strength, p , of the salt was approximately a constant. If the activity of the non-electrolyte is determined by having a solid phase in equilibrium, rather than by the pressure of a gas, the accuracy of the results should be greater. In the present paper we study the activity coefficient of iodine, phenylthiourea, and o-nitrobenealdehyde. The solubility of these substances in water is small and we may take the activity, u, of the non-electrolyte in its saturated aqueous solution equal to its molality, m", and its activity coefficient, yu, is, therefore, unity. In the presence of the salt, since the solid phase is present, a2 = my = constant. Hence, the activity coefficient in any salt solution is given by the expression, y u = mo/m, where m is the molality of the non-electrolyte in the salt solution. Linderstrom-Lang (1) in a recent paper has investigated the effect of a large number of salts on the solubilities of hydroquinone, quinone, succinic acid, and boric acid. He calculated the quantity (log(S"/S))/c where So is the solubility in pure water of the substance investigated, expressed as mols per liter, S its solubility in the salt solution, and c the equivalent concentration of the salt. This quantity (log(S"/S))/c was found to be approximately constant for a given saturating substance and a 285
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MERLE RANDALL AND CRAWFORD FAIRBANKS FAILEY
single salt at different concentrations, increasing with concentrations of salt in some cases, in others decreasing. We do not have the necessary density determinations to enable us to calculate (log y , ) / p . From approximate densities we estimate that this quantity will also be approximately constant, its variation being in some cases greater, in some less, than LinderstromLang's expression. TABLE 1
Activily coeficient of iodine2 in aqueous salt solations' Solubilit in water = 0.001321 M at 25"-0.001815 M at 35"
-
SALT
-
P
P
-NazSO4, 25'
1.267 1.376 0.1094 1.997 1,6530.1093 NaN03, 25" 4.095 2.784 0.1086 5.166 3.628 0.1083
Na2SOd, 35"
0.224 0.499 1.095 2.122 3.666 5.646 7.155 9.843 0.45
NaN08, 25"
1
1.0280.0535 1.1160.0955 1.2980 .lo34 Xai\'08, 35" 1.6190.0986 2 ,459 0.1066 3.897 0.1046 5.657 0.1052 0.72 0,1047 2.67 0.1056
0.736 1.072 0.0410 NaH2PO+ 25" 1.469 1.178 0.0484 2,120 1.2870.0517 4.063 1.6410.0529 4.991 1.904 0.0560
yu
log Y, -
-6.455 2.204 0.0532 7.787 2.6130.0536 9.302 3.161 0.0537 I
