Journal of Solution Chemistry, Vol. 28, No. 6, 1999 Activity coefficients for sodium chloride in the NaCl + Na2SO4 +H2O ternary system were determined from emf measurements of the cell Na-glass|NaCl(mA), Na2SO4(mB), AgCl(s)|Ag at 15, 25, 35, and 45°C and at total ionic strengths from 0.1 to 6 mol-kg-1. These activity coefficient were analyzed by using the Harned rule and the treatments of Scatchard-Rush-Johnson and Pitzer. The analysis of activity coefficients at "trace" concentration was also achieved and qualitatively interpreted by consider- ing the ionic interactions that can take place in the mixed system. Finally, the excess Gibbs energy of the mixtures was calculated and qualitatively analyzed against the composition of the system. 1. INTRODUCTION In this work, the ternary system NaCl + Na2SO4 +H2O has been investigated, since the electrolytes involved are present in many interesting 1 Depattamento de Ingenieria Quimica, Universidad de Antofagasta, Casilla 170, Antofa- gasta, Chile. 2Departamento de Quimica Fisica, Universidad de La Laguna, 38204 La Laguna, Tenerife, Spain. Email: mesteso@ull.es. +We dedicate this paper to the memory of Kenneth S. Pitzer in recognition of his many invaluable contributions to solution chemistry. Thermodynamic Study of the NaCl + Na2SO4 + H2O System by Emf Measurements at Four Temperatures + H. R. Galleguillos-Castro,1 F. Hernandez-Luis, 2 L. Fernandez-Merida,2 and M.A. Esteso2, * Received November 26, 1998; revised January 29, 1999 KEY WORDS: Activity coefficients; emf measurements; ternary systems; NaCl + Na2SO4. 791 0095-9782/99/0600-791$ 16.00/0 © 1999 Plenum Publishing Corporation