Speciation of Chromium in Waters by Coprecipitation-AAS

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Abstract
An analytical scheme for chromium speciation by coprecipitation with lead salts-AAS is described. It allows the partitioning of the total concentration of the metal into four fractions: Cr(VI), free Cr(III), complexed Cr(III), and particulate metal. First, the sample is filtered through 0.45 μ; filters are treated with diluted nitric acid to dissolve and determine the particulate chromium. The filtrate was used for three determinations: one of them on the acidified sample, without any treatment; another one after a coprecipitation with lead phosphate at pH 6–7 (both valency states are scavenged); and finally, the other one after a coprecipitation with lead sulphate at pH 3 (only Cr(VI) is collected). GFAAS has been used in all measurements using standards of K2Cr2O7 in 0.1 N HNO3. It is confirmed that the responses of both valency states do not show significative statistical differences, using coated as well as ordinary tubes. In our opinion, this scheme shows some important advantages: the concentration of Cr(VI), the species whose determination has more interest, is calculated directly, not by difference; in both coprecipitations the carriers do not interfere in the later measurement by GFAAS; the same standard solutions are used in the four necessary determinations for each sample, and it is not necessary to apply the standard additions method because the cation has been already scavenged from the original matrix; finally, the analytical procedure is relatively simple in comparison with the usual tedious laboratory work that is required by this kind of study. The scheme described above has been successfully applied to 33 samples from the Sau reservoir.