The Effect of Acid-base Properties on the Redox Equilibrium of Vanadium in Molten V₂O₅-M₂O and V₂O₅-MPO₃ (M: alkali metal)

Abstract

To elucidate the relation between the redox equilibrium of transition metals and acidbase properties in molten oxysalts, the variation of V (IV)-V (V) equilibria with the composition of melts was investigated on V₂O₅-M₂O and V₂O₅-MPO₃ systems (M: alkali metal). The structure of quenched sample was examined by means of X-ray diffraction, IR spectroscopy and paper chromatography. The summary of results is as follows:
(1) In molten V₂O₅-M₂O, percent reduction of vanadium, V(IV)×100/[V(IV)+V(V)] decreases with increasing basicity and becoms nearly zero at the composition of meta vanadate (Fig. 3). Bronze-like structure, V(IV)-O-V(V) network, is also found to be less stable for oxidation in molten state than in the solid one.
(2) In molten V₂O₅-KPO₃, KPO₃ is more acidic than V₂O₅ and acid-base reactions represented by eqs. (8)-(14) are in equilibrium between both components. Equilibrium ratio of V(IV)/V(V) increases with increasing KPO₃ content, that is, decreasig the basicity of the melt, up to 90mol%, and decreases again at 99mol% (Figs. 5 and 6). Paper chromatographic analysis of phosphate ions suggests that, when KPO₃ content is higher than 70mol%, vanadium component exists in the forms of VO₂⁺, VO³⁺ and VO²⁺. In this range of high concentration of KPO₃, the dependence of redox equilibrium of vanadium on the concentration of free oxygen ions can be explained with the following equations,
VO₂⁺_??_VO²⁺+1/2O^2-+1/4O₂…(a)
VO³⁺+1/2O^2-_??_VO²⁺+1/4O₂…(b)
V(IV)/V(V)=K_aPo₂^-1/4/{(O^2-)^1/2+K_a/K_b(O^2-)^-1/2}…(c)
(3) The shift of redox equilibrium of vanadium towards the more oxidized state in K-salt than in Na-salt, Fig. 7, means that K⁺ ions keep the melt more basic than Na ions do. This difference may result from the difference in the polarizing power between Na⁺ and K⁺ ions.
(4) The ion fraction of free oxygen ions calculated by applying Masson's approach to silicate melts does not agree with one estimated from eq. (c) (Figs. 13 and 14). This suggests that the effects of cation on the basicity of melt must be taken account as well as the anionic distributions.