Enthalpy of Ionization of Water from Electro-motive Force Measurements

Abstract

New e.m.f. data have been obtained for the cell: Pt, H2|NaOH, NaCl|AgCl|Ag, over the temperature range 5–85°C, from which values of the ionic product (Kw) and the enthalpy of ionization of water (ΔH°) can be calculated.

A general expression is given for the standard error in the enthalpy of ionization (ΔH°) in terms of the errors of the parameters of the general polynomial-logarithmic equations used to represent the variation of ionization constant (K) with temperature. The standard error in ΔH° has only been calculated previously for certain polynomial expansions, such as the Clarke and Glew equations. Selected data for the ionization constant of water derived from e.m.f. measurements have been analysed with various pK =f(T) expressions to establish the standard error in ΔH° from each, and to determine whether the reported discrepancy of 200 cal mol–1 between ΔH° derived calorimetrically and from e.m.f. measurements can arise from the method of analysing the pK data. It is concluded that only systematic errors and not statistical errors in the e.m.f. data can account for the discrepancy. The extrapolation of the e.m.f. results to zero ionic strength to obtain Kw is suggested as the most likely cause of the discrepancy between the e.m.f.-derived and calorimetric ΔH° values