Spectrophotometric determination of osmium based on its catalytic effect on the oxidation of carminic acid by hydrogen peroxide

A highly sensitive spectrophotometric method is described for the determination of trace amounts of osmium(VIII), based on its catalytic effect on the oxidation of carminic acid by hydrogen peroxide. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of carminic acid at 540 nm after 3 min of mixing the reagents. The optimum reaction conditions were 1×10−4 mol l−1 carminic acid, 0.013 mol l−1 hydrogen peroxide and pH 10 at 25°C. By using the recommended procedure, the calibration graph was linear from 0.1 to 1.5 ng ml−1 of osmium; the detection limit was 0.02 ng ml−1; the RSD for five replicate determinations of 0.2–1.4 ng ml−1 was in the range of 1.8–4.7%. The influence of several foreign ions on osmium determination were studied and the effect of interfering ions were removed by extracting osmium into isobuthyl methyl ketone and back extracting into sodium hydroxide solution.