Activity coefficients of (hydrogen chloride + europium chloride) (aq) using Harned's rule and the Pitzer formalism

Activity coefficients of HCl in (hydrogen chloride + europium chloride) have been determined at temperatures ranging from 278.15 K to 328.15 K at 5 K intervals and at eight constant total ionic strengths of (0.025, 0.05, 0.1, 0.25, 0.5, 1.0, 1.5, and 2.0) mol kg^-1. Experimental cell potentials were obtained using a cell without liquid junction of the type Pt|H₂(g, p = 101.325 kPa)|HCl (m_A), EuCl₃ (m_B)|AgCl(s)|Ag(s). The treatment of Harned has been used to describe the mean molal activity coefficient of hydrochloric acid as a function of ionic strength in acid salt mixtures. Results indicate that the quadratic form is sufficient for the full range of Y_B (the ionic strength fraction of the salt) to 0.9 at all the experimental ionic strengths. The ion-interaction model of Pitzer has also been used to calculate Pitzer's mixing parameters Θ_H,Eu and ψ_H,Cl,Eu for the system H⁺ + Eu³⁺ + Cl^- + H₂O. The values found, Θ_H,Eu = 0.2 ± 0.03 and ψ_H,Cl,Eu = 0, were similar to those for other trivalent ions studied previously. The parameters used in this study reproduce the activity coefficients of HCl in the mixtures to within 0.016 over the entire range of ionic strengths and within 0.009 for I > 0.05 mol kg^-1 over the entire temperature range.