THERMODYNAMIC AND VOLTAMMETRIC STUDY OF OXYGEN SYSTEMS IN MOLTEN NA2CO3-K2CO3 (56-44 MOL-PERCENT) EUTECTIC AT 750-DEGREES-C

The acidity-potential diagrams of molten Na2CO3-K2CO3 have been calculated from both thermochemical tables and experimental data (literature) at different temperatures between 727 and 927-degrees-C. They allow one to define the stability domains of the various redox species of oxygen (O2, O2- and O22-) and to estimate the solubility of peroxide and superoxide species in this medium. Thermodynamic predictions are then compared with results obtained from voltammetric studies at gold electrodes. A first reduction step that should be attributed to superoxide ions was observed in moderately acidic and in moderately basic media. A second reduction step characteristic of peroxide ions was observed in moderately basic and in basic media; oxide ions are responsible of the associated oxidation current. The system O22-/O2- is a two-electron reversible process. It is shown that the CO2 does not participate in the rate-determining reduction steps. The solubility of Na2O in molten Na2CO3-K2CO3 has been estimated at 750-degrees-C.