Determination of chromium(VI) in waste sludge containing chromium(III) by atomic absorption spectrometry

Abstract

A quantitative analysis of sludge for chromium (VI) is very difficult because the sludge contains slightly soluble chromates together with organic substances or other reducing materials. The method proposed here aimed at solving the problem menthioned and involved the following processes: (i) Dissolve chromates by treating the sample with EDTA in an ammoniacal medium, and then mask excess EDTA with calcium. As excess EDTA impedes the formation of Cr(III)-DDTC, the EDTA must be masked by adding calcium to the sample slurry prior to neutralization, (ii) Allow the dissolved Cr(VI) to react with sodium diethyldithiocarbamate(DDTC) at pH5~7, and then extract the Cr(III)-DDTC into methylisobutylketone(MIBK) immediately. It is necessary to make DDTC and MIBK co-exist with the sample slurry to effect the process (ii). Also, this is useful to remove the effect of reducing materials. In this method chromium(III) in waste sludge does not interfere with the determination, and Gr(VI) should be determined by means of a fuel-lean flame to reduce the effect of co-existing elements. Procedure: Transfer the finely pulverized sample, which contained (5~200) μg Cr(VI), to a separatory funnel in which 20 ml of 10% DDTC solution is placed. Add 20 ml of 0.05 M EDTA solution and 10 ml of conc. ammonia, and then shake the mixture 10 min. Add 25 ml of 0.05 M calcium chloride solution and mix. Adjust the pH of the mixture to 5.5 by adding acetic acid-ammonium acetate buffer solution. Shake the mixture for a few min. with (50~100) ml MIBK. Filter the residue off on a small filter paper and then allow the phases to separate. Evapotate the organic layer to dryness, and decompose the residue with nitric acid. The solution obtained is analyzed for chromium by atomic absorption spectrometry. A coefficient of variation was (5~10)%. © 1977, The Japan Society for Analytical Chemistry. All rights reserved.