DETERMINATION OF CHROMIUM(III) AND CHROMIUM(VI) BY GRAPHITE-FURNACE ATOMIC-ABSORPTION SPECTROMETRY AFTER COPRECIPITATION WITH HAFNIUM HYDROXIDE

A coprecipitation method with hafnium hydroxide was studied for the de termination of chromium(III) and chromium(VI) in water by graphite-fur nace atomic absorption spectrometry. Both 0.04-1.6 mu g of chromium(II I) and 0.04-1.6 mu g of chromium(VI) in a 200 cm(3) water sample were quantitatively coprecipitated with hafnium hydroxide at pH 5.7; 0.04-1 .6 mu g of chromium(III) alone was done at pH 11. Each of the precipit ates obtained at pH 5.7 and 11 was dissolved in nitric acid; the solut ions were then diluted to 10 cm(3) and subjected to determinations of the total amount of chromium(III) and chromium(VI) and the amount of c hromium(III) alone, respectively. The detection limits were 0.02 ng cm (-3) of chromium(III) and chromium(VI) in 200 cm(3) of the initial sam ple solutions. The thirty four matrix ions tested did not produce any serious interference effects in the chromium(III) determination. Altho ugh the presence of fluoride, phosphate, and sulfate tended to reduce the recovery of chromium(VI), they could coexist up to amounts of 1 mg , 1 mg, and 50 mg, respectively.