Rapid spectrophotometric determination of chromium(III) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol in the presence of benzoate and sodium dodecyl sulphate

Abstract

The reaction between chromium(III) and 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) is accelerated by sodium dodecyl sulphate (SDS), which also acts as a solubilising and sensitising agent, and sodium benzoate causes a further increase in the absorbance of the complex. The optimum pH range for the reaction is 5.0–5.8 (benzoate buffer). The intensely red complex exhibits maximum absorbance at 595 nm, obeys Beer's law over the concentration range 0.02–0.52 µg ml^–1 of chromium(III), has a molar absorptivity of 7.93 × 10⁴ l mol^–1 cm^–1 and a Sandell sensitivity of 0.65 ng cm^–2 and the coefficient of variation of the determination is 0.92%. The metal to ligand ratio is 1 : 2 in the absence of SDS and 1 : 1 in its presence. A procedure for the determination of chromium(III) and chromium(VI), when present together, is described. The method has been applied to the determination of the chromium content of an alloy. The method is the most rapid and sensitive for the spectrophotometric determination of chromium(III).