Activity Coefficients of HCl + GdCl3 + H2O System from 5 to 55°C. Application of Pitzer Formalism

Abstract

The emf of the cell

$${\text{Pt, H}}_{\text{2}} ({\text{g, 1 atm}})|{\text{HCI(}}m_{\text{A}} {\text{),GdCl}}_{\text{3}} (m_B )|{\text{AgCl, Ag}}$$

without a liquid junction was used to investigate the HCl + GdCl₃ + H₂O mixedelectrolyte system. The emf of the cell was measured for HCl + GdCl₃ + H₂Osolutions at ionic strengths of 0.025, 0.05, 0.1, 0.5, 1.0, 1.5, and 2.0 mol-kg⁻¹and at eleven temperatures ranging from 5 to 55°C at 5°C intervals. The meanactivity coefficients for HCl in the mixtures were determined using the Nernstequation. About 793 experimental emf data points were treated by the Harnedequations. Results show that hydrochloric acid follows Harned's rule at all ionicstrengths, but the quadratic term is needed for I = 1.5 mol-kg⁻¹. Theion-interaction treatment of Pitzer was used to evaluate the results. The binary andternary mixing parameters at 25°C were found to be Θ_H,Gd = 0.07 ± 0.03 andΨ_H,Gd,Cl = 0.14 ± 0.03. These values were determined using literature values ofβ⁽⁰⁾, β⁽¹⁾, and C ^ψ for GdCl₃ at 25°C and estimates of the effect of temperaturefrom 5 to 55°C using enthalpy and heat capacity data.