Activity coefficients of HCl + GdCl₃ + H₂O system from 5 to 55°C. Application of Pitzer formalism

The emf of the cell Pt, H₂ (g, 1 atm) | HCl (m_A), GdCl₃ (m_B) | AgCl, Ag without a liquid junction was used to investigate the HCl + GdCl₃ + H₂O mixed electrolyte system. The emf of the cell was measured for HCl + GdCl₃ + H₂O solutions at ionic strengths of 0.025, 0.05, 0.1, 0.5, 1.0, 1.5, and 2.0 mol-kg^-1 and at eleven temperatures ranging from 5 to 55°C at 5°C intervals. The mean activity coefficients for HCl in the mixtures were determined using the Nernst equation. About 793 experimental emf data points were treated by the Harned equations. Results show that hydrochloric acid follows Harned's rule at all ionic strengths, but the quadratic term is needed for I = 1.5 mol-kg^-1. The ion-interaction treatment of Pitzer was used to evaluate the results. The binary and ternary mixing parameters at 25°C were found to be Θ_H,Gd = 0.07 ± 0.03 and Ψ_H,Gd,Cl = 0.14 ± 0.03. These values were determined using literature values of β⁽⁰⁾, β⁽¹⁾, and C^φ for GdCl₃ at 25°C and estimates of the effect of temperature from 5 to 55°C using enthalpy and heat capacity data.