Analytical chemistry of chromium species in the environment, and interpretation of results

Valence-selective species determination of chromium in environmental and biological material presents intricacies primarily due to the complex nature of the samples and the instability of the oxidation states of chromium. In particular organic matter ubiquitously present in environmental samples introduces the risk of chromium(VI) reduction during analysis which takes place preferably in acidic solution. Also chromium(III) oxidation may occur in alkaline medium. Hence analytical methods should be adequately adapted to the reliable determination of chromium(VI) in blood, natural water, effluents, sediments and soil. It can be shown that polarography is most effectually suitable to the determination of chromium(VI) compounds. Chromium(VI) is electrochemically active over the entire pH range, so that medium pH can be selected for measuring, thus protecting samples most effectively from undergoing redox reactions during the analytical procedure. Other established methods such as spectrophotometry or valence-specific extraction are more crucial, because these generally require acidic medium. In some cases sample pre-treatment can be employed to eliminate reductants prior to final measurement. Very few methods are available to selectively measure chromium(III). Indirect determination is therefore preferred. Optimum conditions for the determination of total chromium by AAS are shortly outlined.