Activity coefficients for ternary systems: VI. The system HCl + MgCl₂ + H₂O at different temperatures; application of Pitzer's equations

Electromotive-force measurements have been made on HCl-MgCl₂-H₂O mixtures at 5, 15, 25, 35 and 45°C at eleven different ionic strengths from 0.1-5.0 mol-kg^-1. The results are interpreted in terms of the simple Harned's equations, as well as the more complicated Pitzer ion-component treatment of multicomponent electrolyte mixtures. Activity coefficients for HCl in the salt mixtures obey Harned's rule up to and including I=5.0. For the salt in the acid mixtures, Harned's rule holds true up to and including I=0.5. The contribution of higher-order electrostatic terms (E_θ and E'_θ) in the Pitzer equations is important for accurate evaluations of unlike cation-cation interactions (θ_H,Mg), and cation-anion-cation interactions (Ψ_H,Mg,Cl). The values of^Sθ_H,Mg and Ψ_H,Mg,Cl (determined with E_θ and E'_θ), θ_H,Mg and Ψ_H,Mg,Cl (determined without E_θ and E'_θ), as well as the trace activity coefficients of HCl, γ_tr^A, in solutions of MgCl₂ (where ionic strength fraction of the salt, y_B = 1) at all the experimental temperatures and ionic strengths, are reported. Results of this study are compared with those for similar systems. At I=0.1 and 25°C, the results of the Brönsted-Guggenheim specific interaction theory are discussed briefly.