Combined HPLC-ICP-AES technique as an informative tool for the study of heteropolyniobates

Abstract

The combined HPLC-ICP-AES technique was applied to study formation of tungstoniobates and phosphomolybdoniobates. Reaction of potassium hexaniobate with tungstic acid results in the formation of mixed Nb/W Lindqvist hexametalates. The metal ratio depends on the relative amount of H₂WO₄ and reaction time. The most favored species are [Nb₄W₂O₁₉]⁶⁻ and [Nb₃W₃O₁₉]⁵⁻ which equilibrate with hexaniobate. Formation of [Nb₂W₄O₁₉]⁴⁻ was also observed; its share gradually increases with increasing amount of tungstic acid and decreasing pH. Reaction of tungstic acid with telluropentaniobate [TeNb₅O₁₉]⁷⁻ results only in substitution of W for Te producing the same mixed Nb/W Lindqvist anions, but with different distribution of the individual species. The reaction of phosphomolybdic acid (H₃PMo₁₂O₄₀) with a niobium oxalate complex, [NbO(C₂O₄)₂(H₂O)₂]⁻, leads to the formation of mixed P/Nb/Mo Keggin anions, unambiguously detected for the first time. Only [PMo₁₁NbO₄₀]⁴⁻ and [PMo₁₀Nb₂O₄₀]⁵⁻ were observed over the whole concentration range. No trisubstituted [PMo₉Nb₃O₄₀]⁶⁻ was detected in the substitution reaction, but is accessible from the self-assembly reaction of the niobium oxalate complex, phosphoric acid, and molybdate.