Common ion equilibria of hydrochloride salts and the Setschenow equation.

A simple equation was derived to describe the relationship between the aqueous solubility of sparingly soluble salts (S0) and the empirical Setschenow salting-out constant (k): k = 0.217/S0. This relationship and the Setschenow equation were found to be valid only at low concentrations of added salt. This equation agreed with recently published data when compared for the effect of the chloride ion on the solubility of a series of drug hydrochloride salts. The theoretical treatment also predicts the curvature which has been reported in literature Setschenow plots at higher salt concentrations. As the concentration of added salt increases, the apparent k value is not constant but is dependent on solubility and the rate of change of solubility with added salt concentration. It was concluded that the Setschenow treatment is generally inappropriate for description and analysis of common ion equilibria.