Spectrophotometric determination of tungsten (VI) with Bromopyrogallol Red and zephiramine

Abstract

Tungsten (VI) forms a blue complex with Bromopyrogallol Red (BPR) in the presence of excess zephiramine in an acidic solution containing hydrochloric acid. The absorption maximum of the tungsten-BPR-zephiramine complex was observed at around 621 nm, and the absorbance was strongest in (1.01.3) N hydrochloric acid solution. The calibration curve showed that Beer's law held over the range (235)μg tungsten (VI) /25 ml. The coefficient of variation of the absorbance for 30μg of tungsten (VI) was 0.4%. The molar absorptivity was 6.5×104 at 621 nm. The molar ratio of tungsten to BPR in the complex was estimated to be 1 : 1 by the continuous variation method. The sensitivity of this method is superior to that reported by Dhupar, et al. using BPR.The analytical procedure was as follows. A sample solution containing (235) μg tungsten (VI) is taken into a 25 ml measuring flask. Suitable amount of 3 N hydrochloric acid, 2.0 ml of 2 w/v% gum arabic, 2.0 ml of 10-2M zephiramine and 1.0 ml of 0.03 w/v% BPR are added to the flask, and the solution is made up to 25 ml with water, the final acidity being (1.01.3) N. The mixed solution is heated in a water bath of (55±2) °C for 10 minutes. After cooling for 20 minutes, the absorbance of the colored solution is measured at 621 nm against the reagent blank. Mercury (II), bismuth (III), antimony (III), tin (IV), vanadium (V), chromium (VI), molybdenum (VI) and nitrite interfered with the determination even in the same amount as the tungsten (VI). The permissible amount of chromium (VI) could be increased by addition of 1 w/v% L-ascorbic acid and 0.1 M EDTA.