Niobium oxide solution chemistry

Abstract The solution chemistry of niobium oxide in alkaline and acidic oxalate solutions was investigated by Raman spectroscopy in order to determine the molecular structures of the various aqueous niobium oxide species. The aqueous potassium niobate species were studied as a function of pH from 14.5 to 0.55. Different types of niobium oxide ionic species such as H x Nb 6 O (x = 0–3), Nb 12 O and Nb 2 O 5 · n H 2 O exist in aqueous solutions, and both the solution pH and niobium oxide concentration determine the different niobium oxide ionic species. The aqueous niobium oxide ionic species are unstable at pH < 6.5, and hydrolyze and polymerize to form Nb 2 O 5 · n H 2 O precipitate. In acidic solutions containing oxalic acid two niobium oxalate ionic species are present, [NbO(C 2 O 4 ) 2 (H 2 O) 2 ] and [NbO(C 2 O 4 ) 3 ], and the equilibrium between these species is dependent on the solution pH, oxalic acid concentration, niobium concentration and H + concentration. The niobium oxalate ionic species are stable in the pH range 0.5‐3.0, and also from the Nb 2 O 5 · n H 2 O precipitate at pH > 3.0. This study demonstrates the ability of Raman spectroscopy to determine directly the solution chemistry and molecular structures of niobium oxide species in aqueous solutions.