Precision Measurements of Benzoic Acid Dissociation Constants by Concentration Spectrophotometry

The pK^T value of benzoic acid in a binary solution of benzoic acid–water at 25 ± 0.5°C without any additions of titrants, buffer solutions, and supporting salts was obtained by concentration UV–Vis spectrophotometry, and found to be 4.294 ± 0.002. The obtained value differs from the literature data. A new model is proposed that includes the effects of symmetric and asymmetric electrolytes on the acid-base equilibria. The dependences of the observed dissociation constants of benzoic acid in aqueous salts at their concentrations from 10^–3 to 10^–1 mol dm^–3 is nonlinear. It is shown that at decreased concentrations of a strong electrolyte, the observed dissociation constants of benzoic acid tend to the value obtained in a pure solvent, i.e., to the thermodynamic constant.