Amidines as extractants: spectrophotometric determination of rhenium in ores as its ternary thiocyanato complex

A sensitive method for the spectrophotometric determination of ReIV is described. The method is based on the reduction of ReVII to ReIV by tin(II) chloride in H2SO4 and the extraction of the thiocyanato complex of the metal with amidine into chloroform over an acidity range of 3.5–6.5 M H2SO4. The complexes formed with 11 different amidines have molar absorptivities in the range 1.46 × 104–2.35 × 104 l mol–1 cm–1 with absorption maxima at ca. 435 nm in chloroform. Of the amidines investigated, N-(4-methylphenyl)-N′-phenylbenzamidine gave the highest sensitivity and was therefore selected for detailed studies. The detection limit of the method is 8 p.p.b. of Re. The effect of foreign ions on the extraction of Re was examined. The method is highly reproducible; it was applied to the determination of Re in an ore sample containing 40 p.p.m. of Re, with a relative standard deviation of 1.1%.