Spectrophotometric study and extraction of niobium(V) complexes with catechol and its derivatives as ion-pairs with quaternary ammonium ions: Application to the separation of niobium from other refractory metal ions

The reactions of niobium(V) with catechol and several substituted derivatives were studied. Optimal conditions for colour development in the niobium(V)-catechol system are: pH 6.7–8.0, and heating at 85° for at least 6 min; the wavelength of maximal absorption is 365 nm, and the molar absorptivity is 1.02·10⁴. Catechol derivatives require lower temperatures and develop colours with niobium(V) in neutral or slightly acidic solutions. The best of the derivatives studied were 3-methyl-catechol and 4-chlorocatechol. Extraction of the ion-pairs formed between niobium-(V)-catechol and different quaternary ammonium ions, was studied with representative solvents. Similar studies were made on other refractory metal ions, i.e., Ti(IV), V(V), Mo(VI) and W(VI), under the optimal conditions for the reaction with niobium(V). Separations of binary (1:1) mixtures of niobium(V) with Ti(IV), V(V), Mo(VI) or W(VI) at the 5·10^-5M level were shown to be possible under the predicted conditions.