Thermodynamics of aqueous solutions of 1, 2 .. electrolytes at very low concentrations

The solubility of very slightly soluble salts in water, CaSO4 and PbCl2, were analyzed in the presence of another, soluble salt. In one case, CaCl2 addition, no interaction between the salt (CaSO4) and the third component was found. In all the others cases new interaction parameters were needed to describe the increased solubility. These interaction parameters are different from those obtained from the salt-salt phase diagrams. When the both salts are present in a very dilute state, the ions can form complexes which increase the solubility of the slightly soluble salts. This effect is only observable with very slightly soluble salts. In highly soluble salts at the solubility limit the relative concentration of ions is negligible compared to the undissociated salt.