The "Salt Effect".
Now, solid salts have higher dielectric constants than has[206] solid water, and the dielectric constant of a compound is usually much higher in the liquid than in the solid form.[207] It is possible, therefore, that the presence of salts in the solution increases the dielectric or, at any rate, the ionizing power of the solvent and there are many facts which would be explained by such a behavior. Unfortunately for any decision of the question, determinations of the dielectric constants of salt solutions have given contradictory results; the more recent, and possibly more reliable results of Drude[208] indicate that salt solutions show approximately the same dielectric behavior as water itself. Smale, in Nernst's laboratory, on the other hand, obtained results indicating that salt solutions have decidedly higher dielectric constants than pure water.[209] A final decision in the matter would be of great importance.[210] But, as explained before (p. [64]), there are other properties of a solvent which seem to be intimately related to its ionizing power and which may be modified by the presence of salts, i.e. of strong electrolytes. The value obtained for the proportion [Me+] × [X−] / [MeX] grows rapidly with increasing concentration, indicating a disproportionately large ionization in the more concentrated solutions—which is what one would expect, if electrolytes or their ions in some way increased the ionizing power of the medium. In agreement with such a conclusion, Arrhenius found[211] that the ionization of weak acids, like acetic acid, is increased by the presence of foreign neutral salts, such as sodium chloride. This means, of course, that the strength of acetic acid, as an acid, is increased likewise.
Exp. 0.5 c.c. of 0.1 molar acetic acid is added to 100 c.c. of a dilute solution of methyl orange in each of three test glasses. When some solid sodium chloride (3 grams, and then 3 grams more—altogether 0.1 mole) is added to the one solution, a plain increase in the intensity of the acid tint is observed.[212] The addition of cane sugar to a second solution has no such effect. The addition of sodium chloride to a fourth portion of the methyl orange, to which no acetic acid has been added, shows that its color remains unchanged: the effect on the indicator in the first case, then, is the result of the action of the salt on the acetic acid.[213] [p111]
The so-called "salt-effect" on the ionization of ammonium hydroxide may be illustrated in a similar way.
Exp. 0.5 cc of 0.1 molar ammonium hydroxide is added to each of two portions (100 c.c.) of a dilute solution of phenolphthaleïn. When some sodium chloride solution is then added to one of the two portions, the basicity of the solution is distinctly increased. The addition of sodium chloride to a third portion of the phenolphthaleïn solution shows that its own reaction to the indicator is neutral.
If the ionizing power of a solvent is changed by the presence of an electrolyte, then the law of chemical equilibrium, in its simple form, would not apply to the ionization of such electrolytes in varying concentrations—as little as we should expect to obtain the same constant for acetic acid in aqueous solution and in alcoholic solution, the ionizing power of alcohol being much smaller than that of water. The deviation from the law, naturally, would be most marked in the case of those electrolytes which ionize so easily as readily to produce high concentrations of ions.
It may be said that laws based on the electrical properties of salt solutions seem to be the predominating laws governing the ionization of electrolytes and modifying, in certain cases, the chemical laws based on the study of non-electrolytes.[214]
For the purposes of qualitative analysis, it will suffice to bear in mind the fact that the ionization[215] of salts, strong acids and strong bases does not conform to the laws of mass action, and the fact that practically all salts (with the notable exceptions, among common salts, given on p. [107]) are very readily ionized in aqueous solution, namely to the extent of 40 to 85% in solutions of such moderate concentration as 0.1 molar.[216]
Some Applications of the Law of Chemical Equilibrium.
[CH3CO2−] × [H+] / [CH3CO2H] = Kionization.