The dependence of the oxidation state of vanadium on the oxygen pressure in melts of VOxNa2O-VOx and CaO-S1O2-VOx

The oxidation state of vanadium has been determined as a function of oxygen pressure in pure VOx melts at 808 degrees C to 1000 degrees C, in Ha2O-VOx melts with the initial molar rations Na2O/V2O5 = 0.2, 0.5, and 1.0 at 1000 degrees C, and in CaO-SiO2-VOx melts with the molar ratios CaO/SiO2 = 0.71, 1.00, and 1.36 at 1600 degrees C. In the VOx melts, x is close to 2.5 in the range of oxygen pressure from pO2 = 10-3 to 0.94 atm. The deviation, δ from stoichiometric V2O5 (δ = 2.5 - x) varies approximately proportionally to pO2-1/4, indicting an equilibrium between V4+ and V5+ ions. In the Na2O-VOx and CaO-SiO2-VOx melts, x varies with log pO2 according to an S-shaped function, with x approaching 1.5 at low and 2.5 at high oxygen pressures. At given oxygen pressures, x increases with Na2O or CaO content, respectively. Hence, these oxides stabilize the higher valent vanadium ions. For the CaO-SiO2-VOx system, the determined x-pO2 dependence can be interpreted quantitatively in terms of V4+/V5+ and V3+/V4+ equilibria