The ionic product of water in highly concentrated aqueous electrolyte solutions

Abstract

The ionic product of water, {Mathematical expression}, has been determined in aqueous NaCl (0.5-5.0 M), KCl (3.0 M), NaNO3 (3.0 and 5.0 M), and KNO3 (2.5 M) at 25 °C from high-precision potentiometric titrations carried out in cells with liquid junction using either glass or hydrogen electrodes. Measurements of Kw provide a set of self-consistent data that can be used in the estimation of activity coefficient changes and liquid junction potentials in the study of extremely concentrated electrolyte solutions. Where comparison is possible, results obtained by hydrogen electrode measurements are in excellent agreement (ca ± 0.005 in pKw) with other reliable experimental values and the predictions of the Pitzer activity-coefficient model. The glass electrode results are, as expected, routinely lower (by 0.03-0.05 pKw units), owing to interference by Na+ ions. This effect virtually disappears in solutions of potassium salts. Comparison of the experimental results with the Pitzer predictions shows that knowledge of the ternary interaction parameters is essential to account for specific ionic effects in the concentration dependence of pKw.