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 ΘH,Gd = 0.07 ± 0.03 andΨH,Gd,Cl = 0.14 ± 0.03. These values were determined using literature values ofβ(0), β(1), and Cψ for GdCl3 at 25°C and estimates of the effect of temperaturefrom 5 to 55°C using enthalpy and heat capacity data.