CCA-2635 Original Scientific Paper Extraction and Characterization of Niobium(V) Thiocyanate Complexes

Extraction of niobium(V) from sulphuric and hydrochloric acid so-lutions containing an excess of thiocyanate ions with tetraphen-ylphosphonium (TPP) and tetraphenylarsonium (TPA) chloride into chloroform was investigated. The maximum extraction of nio-bium from sulphuric acid solutions was about 91 % and from hydro-chloric acid solutions about 94%. Chloride ions produced a moder-ate synergistic effect. Niobium(V) was quantitatively extracted (over 99%) from sulphuric and hydrochloric solutions only if chlo-ride 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 thiocya-nate and more than 4 M chloride in the aqueous phase, and TPP (TPA) concentration in the organic phase higher than 3 10–3 M. The extracted complexes in chloroform had an absorption maxi-mum at 390 nm or at 390 and 320 nm, depending on the composi-tion 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 spectropho-tometric determination of niobium by measuring the absorbances of the organic phase at 390 nm were 1–2 M HCl, 0.1–0.5 M thiocy-anate and TPP (TPA) chloride in chloroform of at least 5 10–3 M. A total chloride concentration lower than 2.2 M had no effect on ab-sorbance. The mechanism of extraction based on the formation of ion-associated compounds between the onium cation and the thio-cyanatoniobate(V) or mixed ligand chlorothiocyanatoniobate(V) an-ion is discussed. CROATICA CHEMICA ACTA CCACAA 73 (1) 57¿68 (2000