«The "Salt Effect".»—In the second place,[204] it seems possible that the presence of strong electrolytes in a solution may modify the ‹ionizing power of the solvent› in such a way as to increase it, and to increase it the more, the more concentrated the ions are in the solution.[205] The ionizing power of solvents, as has been explained, is intimately connected with their dielectric properties. [p110]
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]