Thermodynamic study of aqueous rubidium and cobalt selenate system at the temperature 298.15 K

Abstract

The isopiestic method has been used to determine the osmotic coefficients of the binary solutions Rb 2SeO 4(aq) at the temperature T=298.15 K from (1.072 to 5.200) mol·kg −1. The molalities m of ( m 1Rb 2SeO 4+ m 2CoSeO 4)(aq) have been investigated by the physicochemical analysis method. The crystallization of a new double salt Rb 2SeO 4·CoSeO 4·6H 2O has been established. This double salt crystallizes in a monoclinic crystal system, space group P2 1/ a, a=0.9281(2) nm , b=1.2422(2) nm , c=0.6276(9) nm , β=105.70 (2)° , and V=0.6966(2) nm 3 . The double salt is isostructural with Rb 2SeO 4·ZnSeO 4·6H 2O. T.g. and d.t.a measurements indicate that the double salt loses the crystallization water in two steps in the temperature interval from 393.15 K to 523.15 K. The Pitzer ion-interaction model has been used in the thermodynamic analysis of the experimental osmotic and solubility data obtained. The thermodynamic parameters needed (binary and ternary parameters of ionic interaction, thermodynamic solubility products) have been calculated and the theoretical solubility isotherm has been plotted. The experimentally obtained and the calculated solubilities are in very good agreement. The standard molar Gibbs energy of the synthesis reaction Δ r G 0 m of the double salt Rb 2SeO 4·CoSeO 4·6H 2O from the corresponding simple salts Rb 2SeO 4 and CoSeO 4·6H 2O, as well as the standard molar Gibbs energy of formation Δ f G 0 m have been determined.