TERNARY NaCl+LiCl+H₂O MIXED ELECTROLYTE SYSTEM: DETERMINATION OF ACTIVITY COEFFICIENTS BASED ON POTENTIOMETRIC METHOD

Abstract

The mean activity coefficients for NaCl in a ternary electrolyte system were determined by the potentiometric method, at 25°C, using a solvent polymeric (PVC) sodium-selective membrane electrode (Na⁺ ISE), containing N,N′-dibenzyl-N,N′-diphenyl-1,2-phenylenedioxydiacetamide as ionophore, and combined with an Ag/AgCl electrode. The potentiometric measurements were performed at the same ionic strengths in different series of mixed salt solutions, each characterized by a fixed salt molal ratio r (where r = m₁/m₂ = 1, 10, 50, 100). The nonideal behavior of the ternary NaCl(m₁) + LiCl(m₂) + H₂O electrolyte system was described based on the Pitzer ion-interaction model for mixed salts over the ionic strength ranging from 0.01 up to about 4 mol/kg. Two- and three-particle Pitzer interaction parameters for a mixed electrolyte system were determined based on potentiometric data, and the critical role of potentiometric selectivity coefficient (K ₁₂) of ISE as limiting factor in the potentiometric measurements was analyzed.

Keywords:

• Activity coefficient
• Lithium chloride
• Pitzer equations
• Sodium chloride
• Sodium ion-selective electrode

Notes

^a , where n is the number of experimental potentiometric cell data used; σ was obtained by a minimization-iteration procedure from which the resulting Pitzer parameters (β⁽⁰, β⁽¹⁾, and C ^φ) for NaCl in H₂O were determined.