THERMODYNAMIC STUDY OF MIXED ANIONIC SOLID-SOLUTIONS USING GRADIENT SOLID ELECTROLYTES - SYSTEM K2CO3 - K2SO4

The thermodynamic properties of K2CO3 -KSO, solid solutions with hexag onal structure have been measured using a solid-state cell, incorporat ing a composite solid electrolyte with step-changes in composition. Th e cell with the configuration Pt, CO2' + O2' \\ K2CO3 \ K2(CO3)x(SO4)1 -x \\ CO2'' + O2'' + Pt X =1 X=X was investigated in the temperature r ange of 925 to 1165 K. The composite gradient solid electrolyte consis ted of pure K2CO3 at one extremity and the solid solution under study at the other. The Nernstian response of the cell to changes in partial pressures of CO2 and O2 at the electrodes and temperature was demonst rated. The activity of K2CO3 in the solid solution was measured by thr ee techniques. All three methods gave identical results, indicating un it transport number for K+ ions and negligible diffusion potential due to concentration gradients of carbonate and sulfate ions. The activit y of K2CO3 exhibits positive deviation from Raoult's law. The excess G ibbs energy of mixing of the solid solution can be represented using a subregular solution model DELTAG(E) = X(1 - X)[5030X + 4715(1 - X)] J mol-1 By combining this information with the phase diagram, mixing pr operties of the liquid phase were obtained.