THE IONIC PRODUCT OF WATER IN HIGHLY CONCENTRATED AQUEOUS-ELECTROLYTE SOLUTIONS

The ionic product of water, K-w = [H+][OH-] = 10(-pKw), 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 degrees C from high-precision potentiometric titrations carried out in cells with liquid junction using either glass or hydrogen electrodes. Measurements of K-w 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 pK(w)) with other reliable experimental Values and the predictions of the Fitter activity-coefficient model. The glass electrode results are, as expected, routinely lower (by 0.03-0.05 pK(w) units), owing to interference by Na+ ions. This effect virtually disappears in solutions of potassium salts. Comparison of the experimental results with the Fitter predictions shows that knowledge of the ternary interaction parameters is essential to account for specific ionic effects in the concentration dependence of pK(w).