Extraction and spectrophotometric determination of tungsten with thiocyanate and amides

Abstract

A spectrophotometric method for the determination of microgram amounts of tungsten(VI) is described. The method is based on the formation of trithiocyanatotungsten(V) by the reduction of the metal with tin(II) chloride in strong hydrochloric acid solution, the reaction of tungsten(V) with thiocyanate and its extraction with amide into benzene as an adduct in 4.0–8.0 M HCl. The extraction of the metal with various amides was examined and ten amides were found to be suitable extraction reagents for the spectrophotometric determination. The molar absorptivities of the adducts lie in the range 0.92 × 10⁴–1.43 × 10⁴ l mol^–1 cm^–1 at 405 nm. When the method was modified as a qualitative test for tungsten, the detection limit was 0.005 µg ml^–1. The method is simple, convenient, reproducible and applicable to alloy steels containing up to 0.05% of tungsten. The effect of diverse ions was examined and the results indicated that Fe, Co, Ni, Cu, Zn, Cd, Mn, Al, Sb, Bi, Ti, Hf, V, Cr, Mo, Re, Pd, etc., did not interfere. Niobium was found to interfere but was removed by masking with oxalate.