Indirect Spectrophotometric Determination of Chromium

A sensitive spectrophotometric method for determining trace amounts of chromium(VI) is described. In the presence of acetate buffer, chromium(VI) oxidizes hydroxylamine quantitatively to nitrite at pH 4.0 ± 0.5. The nitrite diazotizes p-nitroaniline to form a diazonium salt that, in acidic medium, couples with N-(1-naphthyl)ethylenediamine dihydrochloride to form an azo dye with a molar absorptivity of 4.1 × 104 L/mol-cm at 545 nm. The color is stable for 5 h, and the system obeys Beer's law in the range 0-8 μg chromium(VI) in a final volume of 10 mL. The detection limit of chromium(VI) is 0.6 μg. Chromium(III) can be determined after it is oxidized with bromine water in alkaline medium to chromium(VI). Extraction of the azo dye in alkaline medium with methyl isobutyl ketone followed by addition of methanolic hydrochloric acid permitted determination of chromium(VI) down to 10 ng/mL. The method has been used to determine chromium in alloy steels, pharmaceutical preparations, and industrial effluents.