Extraction and Characterization of Niobium(V) Thiocyanate Complexes

Extraction of niobium(V) from sulphuric and hydrochloric acid Solutions containing an excess of thiocyanate ions with tetraphenylphosphonium (TPP) and tetraphenylarsonium (TPA) chloride into chloroform was investigated. The maximum extraction of niobium from sulphuric acid Solutions was about 91% and from hydrochloric acid Solutions about 94%. Chloride ions produced a moderate synergistic effect. Niobium(V) was quantitatively extracted (over 99%) from sulphuric and hydrochloric solutions only if chloride ions were also in excess and a lower excess of thiocyanate and extractant was required than in extraction without addition of chloride. The optimal conditions for the quantitative extraction of niobium were 0.2-0.5 M H2SO4 or 0.2-4 M HCl, 0.2-0.7 M thiocyanate and more than 4 M chloride in the aqueous phase, and TPP (TPA) concentration in the organic phase higher than 3 x 10-3 M. The extracted complexes in chloroform had an absorption maximum at 390 nm or at 390 and 320 nm, depending on the composition of the aqueous phase. Absorption spectra were studied as a function of H2SO4, HCl, thiocyanate and chloride concentrations in the aqueous phase. The optimal conditions for the spectrophotometric determination of niobium by measuring the absorbances of the organic phase at 390 nm were 1-2 M HCl, 0.1-0.5 M thiocyanate and TPP (TPA) chloride in chloroform of at least 5 x 10-3 M. A total chloride concentration lower than 2.2 M had no effect on absorbance. The mechanism of extraction based on the formation of ion-associated compounds between the onium cation and the thiocyanatoniobate(V) or mixed ligand chlorothiocyanatoniobate(V) anion is discussed