Re-determination of mean ionic activity coefficients for the system HCl+KCl+water at 298.15 K and correlations between Harned and Pitzer equations

Abstract

A re-examination of the activity coefficients for HCl and KCl in their aqueous mixtures at 298.15 K has been made via electromotive-force measurements of the cell, Pt, H₂|HCl(m_A), KCl(m_B)|AgCl|Ag. Contrary to previous reports, a quadratic term in the Harned equation for the mean ionic activity coefficient of HCl is necessary to give significantly better fits of precise experimental results at total ionic strengths of the mixtures > 1 mol kg^–1. A method based on correlations between Harned's equations and slight modifications of Pitzer's equations is proposed and used to calculate the Harned parameters for activity coefficients of the added salt from those experimentally determined for HCl in their mixtures.

The quadratic term parameters are equal for the two electrolytes within the experimental precision; this is interpreted, in terms of an extension of Pitzer's treatment, as implying negligible ion–triplet interactions among ions of like charge at ionic strengths up to 4 mol kg^–1.