Spectrophotometric determination of tungsten ( VI ) after extraction of its 6 - chloro - 3 - hydroxy - 2 - phenyl - 4 - oxo - 4 H - 1 - benzopyran complex into chloroform

6-chloro-3-hydroxy-2-phenyl-4-oxo-4H-1-benzopyran CHPB) has been used for the first time as an analytical reagent for the spectrophotometric determination of tungsten. The yellow colored W (VI) – CHPB complex has λmax at 420 nm, is extractable into chloroform from 0.04 – 0.32 M HCl medium containing 0.6 – 1.6 mL of 0.1% CHPB (in acetone). Tungsten (VI) in presence of several cations, anions and complexing agents forms a yellow 1: 2 complex with CHPB which is stable for about 20 h in organic extract. The molar absorptivity and Sandell’s sensitivity of the complex at 420 nm are calculated to be 3.125×10 dm mol cm and 0.00294 μg W cm, respectively. Beer’s law is obeyed over the concentration range 0 – 2.9 μg W(VI) mL. However, the optimum range of determination of tungsten from Ringbom’s curve at 420 nm is 0.9– 3.1 μg mL. For 2 μg W mL, the standard deviation is 0.000726 and relative standard deviation of ± 0.21% absorbance unit. The method is simple, selective, rapid and precise and has been applied to the determination of tungsten in synthetic and standard samples.