The dissolution of vanadium pentoxide in aqueous solutions of oxalic and mineral acids

The dissolution of vanadium pentoxide in perchloric and oxalic acids has be en studied at 25.0 degrees C and ionic strength 0.50 mol dm(-3) (NaClO4). T he results are described in terms of an extended surface complexation appro ach. The rate data in HClO4 are used to derive an estimated value of the pH of the point of zero charge, pH(o) similar to 3. The whole set of rate dat a is accounted for by surface protolytic and complexation reactions, follow ed by the first order transfer of surface species to solution. The transfer rates are fast but are controlled by surface reactions; the first order ra te constants are of the order of 1 s(-1). Protonated, mono- and dioxalato s urface complexes are involved. Because of the fast transfer rates, it is po ssible that the surface concentrations are steady state rather than equilib rium values. Thus, the idea of surface complexes as transient species is pu t forward. Because of the pH dependence of surface protonation and complexa tion by oxalate, this scheme leads to dissolution rate enhancements by oxal ic acid (with respect to dissolution in mineral acid) that increase with pH . (C) 1999 Academic Press.