Thermodynamic studies of mixed aqueous solutions of binary electrolytes of Na 2 SO 4 and K 2 SO 4 Rodica Sirbu Department of Chemistry, Ovidius University of Constantza, Blvd. Mamaia, No. 124, Constantza, 8700, Romania Received 2 June 1999; received in revised form 12 October 1999; accepted 8 November 1999 Abstract The thermodynamic study of Na 2 SO 4 and K 2 SO 4 binary and ternary aqueous solutions is materialized in obtaining both average ionic activity coefficients and excess Gibbs energies, from cryometric data. Differential cryometric method utilized for obtaining experimental data has permitted the measurement of cryometric effect with a precision of 210 4 K. The experimental data process has been obtained using a specific calculation program. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Excess Gibbs energy; Ionic activity coefficient; NaSO 4 /K 2 SO 4 binary aqueous solution; Ternary aqueous solution 1. Introduction Many systems, biological or geological as well as chemical, involve the interest of mixed aqueous elec- trolytes. The representation of excess Gibbs energy and activity coefficients for aqueous electrolyte sys- tems is a fundamental problem in the design and operation of many industrial processes. The thermodynamic properties of aqueous electro- lytes have been extensively investigated both experi- mentally and theoretically in works by authors like Harned and Owen [1], Robinson and Stokes [2], and others [3–5]. The numerous works which study the binary solu- tions represent a rich source of data for the calculation of the thermodynamic activities of the components of various systems. The differential cryoscopy, considered a very accu- rate method, was extended to the investigation of the multicomponent systems [6], representing a way to make evident the behaviour of mixed electrolytes in aqueous solutions at temperatures close to the freezing point of the pure solvent. 2. General equations from cryometric data The Harned law [7] and the Bro¨nsted–Guggenheim equations [8] allow to express the activity coefficient of a solute from a ternary mixture with total constant molality, depending on the other solute, in dilute solutions. We could thus correlate thermodynamic magni- tudes specific to the components of ternary systems. The established equations are based on numerous experimental data, elaborated especially by mea- surements of electromotive forces and solubility measurements. We tried to study by colligative method the interionic actions of electrolytes in mixture. Thermochimica Acta 352–353 (2000) 1–10 E-mail address: rsirbu@gmb.ro (R. Sirbu) 0040-6031/00/$ – see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0040-6031(99)00429-3