ELECTROLYTIC REDUCTION AND ELLINGHAM DIAGRAMS FOR OXY-ANION SYSTEMS

Abstract

Standard reversible decomposition voltages, E0, for various decomposition reactions of K2CO3, Na2CO3, Li2CO3, BaCO3, CaCO3, MgCO3, Na2SO4, NaNO3, and KNO3 are calculated from thermal data and the values obtained plotted against temperature. That reaction which has the lowest values of E0 is considered to be the most likely to occur when the particular salt is electrolyzed. The results obtained indicate that in the case of the carbonates metal deposition is possible only for K2CO2 and Na2CO3 above 870 °K. In all other cases metal oxide and carbon (or CO at higher temperatures) are the favored cathode products. For Na2SO4, Na2S and Na2O are the favored cathode products and for the nitrates, nitrite and oxide or nitrogen evolution and oxide formation appear equally feasible thermodynamically.Once the likely electrolytic decomposition products are known, a method is presented whereby the products formed by reduction of the oxy-anions by metals (and in the case of the carbonates, by carbon) may be predicted. This method involves drawing an appropriate redox line on an Ellingham diagram. Finally, a comparison is made between the thermodynamic predictions and the few experimental results available.