Extraction and spectrophotometric determination of niobium by tetraphenylarsonium and phosphonium chloride from a hydrochloric acid solution

Abstract

The extraction of niobium(V) in the form of a chloro complex has been studied. Radiometrical and spectrophotometrical studies show that niobium(V) is extracted practically completely from a solution containing more than 9 mol dm −3 chloride in the range of 2–5 M hydrogen ion concentration by chloroform solutions of tetraphenylarsonium (TPA) and tetraphenylphosphonium (TPP) chloride and that niobium is not extracted at chloride concentrations less than 6 mol dm −3. The mechanism of extraction is based on the formation of the ion-associated compounds that form between the onium cation and the oxotetra-chloroniobate(V) anion. The extracted complexes in chloroform have a maximum absorbance at 282 nm (TPA) and 285 nm (TPP); they obey Beer's law in the range of 1–10 μg Nb ml −1, and are stable for at least 24 hr. The molar absorptivity of the method is 1.33 × 10 4 dm 3 mol −1cm −1. The composition of the extracted species [(C 6H 5) 4 X] [NbOCl 4] where X = As or P was determined spectrophotometrically, radiometrically, and by characterization of the crystalline compounds isolated.