Extractive separation of Tungsten as phosphotungstate

Tungsten can be extracted quantitatively as phosphotungstate in micro as well as milligram concentrations by extraction with MIBK from 0.1-1M hydrochloric acid if the w/w tungsten:phosphorus ratio = 7, and separated from Fe, Ni, Co, Cr, Mn, Cu, Ca, U, Th, As, Sb, Bi and Si, after reduction of Fe(III) and cr(VI) with thiosulphate, in natural and industrial samples. Mo and V suppress the extraction of tungsten and therefore require prior separation. The method takes 15 min for a single separation and gives highly satisfactory results with an overall error of about 0.1-0.2% over the range 10-100 mg tungsten in the sample.