Ionic product and enthalpy of ionization of water from electromotive force measurements

Abstract

New e.m.f. data have been obtained for the cell: Pt, H₂|MOH,NX|AgX|Ag at 25°C where M, N = Li, Na, K, Cs and X = Cl, Br, from which the ionic product (K_w) of water can be obtained (pK_w= 14.004). Application of a theory of mixed electrolyte solutions to the results enables the variation of the apparent ionic product (K^′_w) with ionic strength and cation to be expressed in terms of the properties of the pure alkali metal halide and hydroxide solutions at the same total ionic strength. The conditions M = Li, N = K, X = Cl were selected for a study over the temperature range 0–60°C and from these measurements the enthalpy of ionization of water was calculated (ΔH°= 13430 ± 5 cal mol^–1). The use of mixed cations in the cell minimises the variation of apparent ionic product with ionic strength, thereby reducing the uncertainty in the extrapolation to zero ionic strength felt previously to be the principal contributing factor leading to the discrepancy between calorimetric and e.m.f. derived values for the ionization enthalpy of water.