Heats of mixing of aqueous solutions of alkali metal salts of substituted benzenesulfonic acids

The enthalpy change on mixing aqueous solutions of substituted benzene sulf onic acids and their salts, with salts having a common cation or anion, wer e measured at constant total ionic strength and at 25 degrees C. The result s are qualitatively interpreted in terms of solute-water structural propert ies and the ion size effect. The heat effects of mixing solutions having co mmon anions obey the sign rule of Young and Smith. In anion-common cation m ixings at concentration of 0.5 mol-kg(-1) only exothermic heat effects were observed, whose magnitude increase with the increasing difference in size of the mixed anions. The magnitude of the mixing effect increased with the salt concentration in cation-common anion mixing processes. In anion-common cation mixings the enthalpy of mixing changes sign from negative to positi ve, indicating a predominantly endothermic effect as concentration increase s.