Raman Studies on Species in Single and Mixed Solutions of Molybdate and Vanadate

Raman spectroscopy was applied to investigate the speciation in both single and mixed solutions of molybdate and vanadate at pH values from 10.0 to 1.0. Evidence was obtained for the difference of existing forms between these two elements. Vanadium mainly exists as (VO₃)_n^n- while Mo is MoO₄^2- in the pH range of 9.0-7.5. This difference is the theoretical basis for many available separation process. The species in the binary system was identified by comparing the Raman spectra with that in the single systems. Molybvanadates are formed below pH=6.5, which may partly be ascribed to the replacement of V atoms by Mo atoms in some V-O-V groups. Vanadium mainly exists as the decavanadate species in the pH range of 6.0-2.0. The predominant species of Mo are heteropolyanions having structural features of heptamolybdate rather than Mo₈O₂₆^4- and Mo₃₆O₁₁₂^8- which are the predominant Mo species in single solution at pH=2.0-1.0.