Determination of stoichiometric dissociation constants of benzoic acid in aqueous sodium or potassium chloride solutions at 25°C

Abstract

Equations were determined for the calculation of the stoichiometric (molality scale) dissociation constant K m of benzoic acid in dilute aqueous NaCl and KCl solutions at 25°C from the thermodynamic dissociation constant K a of this acid and from the ionic strength I m of the solution. The salt alone determines mostly the ionic strength of the solutions considered in this study and the equations for K m were based on the single-ion activity coefficient equations of the Hiickel type. The existing literature data obtained by conductance measurements and by electromotive force (EMF) measurements on Harned cells were first used to revise the thermodynamic value of the dissociation constant of benzoic acid. A value of K a = (6.326 ± 0.005) x 10 -5 was obtained from the most precise conductivity set [Brockman and Kilpatrick] and this value is supported within their precisions by the less precise conductivity set of Dippy and Williams and by the EMF data set measured by Jones and Parton with quinhydrone electrodes. The new data measured by potentiometric titrations in a glass electrode cell were then used for the estimation of the parameters of the Huckel equations of benzoate ions. The resulting parameters were also tested with the existing literature data measured by cells with and without a liquid junction. The Huckel parameters suggested here are close to those determined previously for anions resulting from aromatic and aliphatic carboxylic acids. By means of the calculation method based on the Huckel equations, K m can be obtained almost within experimental error at least up to I m of about 0.5 mol-kg -1 for benzoic acid in NaCl and KCl solutions.