A method for the determination of vanadium and iron oxidation states in naturally occurring oxides and silicates

A valence-specific analytical method for determining V(3+) in ore minerals has been developed that involves two steps: dissolution of a mineral sample without disturbing the V(3+)/V(tot) ratio, followed by determination of V(3+) in the presence of V(4+). The samples are dissolved in a mixture of hydrofluoric and sulphuric acids at 100 degrees in Teflon-lined reaction vessels. Tervalent vanadium is then determined colorimetrically by formation of a V(3+)-thiocyanate complex in aqueous-acetone medium. Fe(3+) is measured semi-quantitatively in the same solution. The method has been tested with two naturally occurring samples containing vanadium and iron. The results obtained were supported by those obtained by other methods, including electron spin resonance spectroscopy, thermogravimetric analysis, and Mössbauer spectroscopy.