Activity Coefficients of the System {yNaCl+(1–y)NaH2PO4}(aq) at T = 298.15 K

Abstract

Zero-current cell-potential measurements were used to determine the solution activity coefficient in a ternary system {yNaCl + (1-y)NaH2PO4}(aq) at a temperature T = 298.15 K, in the total ionic strength range I = (0.1-1.0) molkg-1. The Na–ISENaCl(mNaCl), Na2HPO4 ( )AgAgCl, cell was used where m_NaCl and m_(NaH_2 PO_4 ) are the stoichiometric molalities of the NaCl and NaH2PO4 electrolytes in the mixture. The ionic strength fraction of NaCl in the total ionic strength of the mixed solution was y = (0.199;0.310; 0.4101; 0.5051; 0.6090; 0.7776; 0.9037). The standard electrode potential of the electrode pair is Eo = 23.22737 mV. The values of the mean ionic coefficients of NaCl in the mixed electrolyte solution, γ_(±NaCl) , were estimated using Nerst equation (1): (1) where E_m is the electrode potential of the cell, γ_(〖±NaH〗_2 PO_4 ) is mean ionic activity coefficient of NaH2PO4 in the mixture and Kpot is selectivity coefficient having values less than K^pot<10^(-4) and therefore, was neglected. The ionic strength of the solution is I =〖 m〗_NaCl+m_(NaH_2 PO_4 ) whereas the ionic strength fraction of NaCl is: y=〖 m〗_NaCl/I. To generate a set of parameters for the system {yNaCl + (1-y)NaH2PO4}(aq) that can be applied in the investigated range of mixed solution ionic strengths, Pitzer models[1] were used to fit the experimental data, represented in Figure 1. Parameter values for the pure electrolyte solutions of the second and third virial coefficients, B_NaCl 〖,B〗_(NaH_2 PO_4 ),〖 C〗_NaCl^T,〖 C〗_(NaH_2 PO_4)^T, respectively, that are mixture components, were taken from literature[2,3]. The treatment of experimental data using Pitzer’s model yielded the following values of the mixing parameters θ_(Cl,〖H_2 PO〗_4 ) = - 0.710 and Ψ_(Na,Cl,〖H_2 PO〗_4 ) = 0.1269.The experimental and calculated values of the thermodynamic properties of the studied system were in excellent agreement, with standard deviation s.d. (NaCl)= 0.0041