INTERACTION OF OXYGEN WITH MOLTEN ALKALI CHLORIDES

It was shown by acidometric and iodometric titration of the reduction products of oxygen on a platinum cathode that the oxygen electrode exchanges not only oxide (O2-) but also peroxide (O-2(2-)) and superoxide (O2-) ions with molten NaCl, KCl and CsCl. The emf of the cells with oxygen and chlorine electrodes: O2 (g), Pt\M2O + M2O + MO2 + MCl(melt)\MCl(melt)\C, Cl2(g), where M is Na, K or Cs, was measured as a function of temperature, gas pressure on its electrodes and the total concentration of M2O + M2O2 + MO2. It was established that the coefficient B in empirical equations epsilon = Const - B ln P(O2) is a constant equal to 0.028 +/- 003 V. It is independent of temperature and electrolyte composition, which shows that n in the theoretical expression B = RT/nF does not depend on temperature (n < 4). An original technique of determining mole fraction of M2O, M2O2 and MO2 in the products of cathodic reduction of oxygen was developed. It enabled the expressions of temperature dependences to be derived from experimental observations for conditional standard potentials of the oxygen electrode: E(O2/O2-)*, E(O2/O22-)* and E(O2/O2)*, as well as for conditional formal redox potentials: E(O2-/O22)* E(O2/O2-)*2 and E(O2-/O2-)* in molten NaCl, KCl and CsCl. Conditional equilibrium constants, enthalpies and conditional standard entropies were calculated from the experimental data for the reactions: O2(g) + 4MCl(melt) half arrow right over half arrow left 2M2O(melt) + 2Cl2(g) O2(g) + 2MCl(melt) half arrow right over half arrow left M2O2(melt) + Cl2(g) O2(g) + MCl(melt) half arrow right over half arrow left MO2(melt) + 1/2 Cl2(g). The activity coefficients of M2O, M2O2 and MO2 in their dilute solutions in molten alkali chlorides were estimated on the basis of the literature and experimental thermodynamic data. The first two of them show negative deviations and the last one shows positive deviation from ideal mixtures. The possibility is indicated for oxygen transfer through molten alkali chlorides by peroxide and superoxide ions: 2M2O2(melt) half arrow right over half arrow left 2M2O(melt) + O2(g) and 4MO2(melt) half arrow right over half arrow left 2M2O(melt) + 3O2(g).