REDUCTION OF LOW CONCENTRATION CR(VI) IN ACID-SOLUTIONS

The speciation of chromium in natural waters presents problems of sampling, transport and storage, especially when the species are in low concentration. Typical sampling procedures involve filtration of aliquots of the water into acid-cleaned polyethylene bottles and storage at pH 2 for subsequent speciation. This study shows that, at low concentration, Cr-51(VI) is reduced to Cr-51(III) in acid solution at rates which depend strongly on both the chromium concentration and on the acid concentration. Thus, 20% of 10(-6) mol/l Cr-51(VI) is reduced in 10 days at pH 4 while 45% is reduced at pH 2. More than 10% of Cr-51(VI) is reduced in 1 day at pH 2 and in 1 h at pH 1. However, if the Cr-51(VI) concentration is 10(-4) mol/l or higher, only very small amounts are reduced in one day at ambient temperatures. All reductions produce a mixture of Cr-51(III) products, as shown by ion exchange chromatography. Such results suggest that Cr(VI)/Cr(III) speciation measurements of sampled waters may be compromised at very low chromium concentrations by the storage procedures commonly adopted and that alternative procedures should be considered for accurate speciation at low concentrations.