New Cation-Exchange Method for Field Speciation of Hexavalent Chromium
U.S. Geological Survey:
Chromium (Cr) is usually present in surface and ground water in either the +3 or the +6 oxidation state. Because Cr(III) is considered an essential nutrient and Cr(VI) is considered a toxin and a carcinogen, it is important to distinguish between oxidation states of Cr. Hexavalent chromium [Cr(VI)] concentration limits for drinking water now being considered by the California Environmental Protection Agency present analysts with the need for quantitation limits in the sub-parts-per-billion (ppb or µg/L) range. Existing methods for accurately determining Cr(VI) in natural waters have one or more of the following weaknesses: (1) poor sensitivity, (2) time-instability of the Cr redox species, (3) necessity for expensive instrumentation that is not commonly available. A new cation-exchange method for field speciation of Cr(VI) has been developed to meet the stringent regulatory standards and to improve upon the weaknesses of existing methods. The capability of the new technique compares avourably with existing methods such as the diphenylcarbazide direct colorimetric and EPA 218.6 ion-chromatographic/colorimetric methods.