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Reduction of Hexavalent Chromium in Water Samples Acidified for Preservation¹

ABSTRACT

Reduction of hexavalent chromium, Cr(VI), in water samples, preserved by standard techniques, was investigated. The standard preservation technique for water samples that are to be analyzed for Cr(VI) consists of filtration through a 0.45-µm membrane, acidification to a pH < 2, and storage in plastic bottles. Batch experiments were conducted to evaluate the effect of H⁺ concentration, NO₂, temperature, and dissolved organic carbon (DOC) on the reduction of Cr(VI) to Cr(III). The rate of reduction of Cr(VI) to Cr(III) increased with increasing NO₂, DOC, H⁺, and temperature. Reduction of Cr(VI) by organic matter occurred in some samples even though the samples were unacidified. Reduction of Cr(VI) is inhibited to an extent by storing the sample at 4°C. Stability of Cr(VI) in water is variable and depends on the other constituents present in the sample. Water samples collected for the determination of Cr(VI) should be filtered (0.45-µm membrane), refrigerated, and analyzed as quickly as possible. Water samples should not be acidified. Measurement of total Cr in addition to Cr(VI) can serve as a check for Cr(VI) reduction. If total Cr is greater than Cr(VI), the possibility that Cr(VI) reduction has occurred needs to be considered.

Key Words: chemical reactions • stability • dissolved organic matter

¹ Contribution of the U.S. Geological Survey, Water Resources Division, Denver, CO 80225.

² Geochemist and chemical engineer, U.S. Geological Survey, MS 413, Box 25046, Federal Center, Denver, CO 80225.

Received for publication November 21, 1983.