There is a need to be able to measure small traces of selenium in ground water and river water quickly and accurately, because of the damage that a potential toxic level can cause to the environment. In the past selenium has been detected by electroanalytical techniques, such as anodic stripping procedures and more recently by cathodic stripping, which is based on the preconcentration and subsequent reduction of mercury and copper selenides.

Now, researchers at the New Mexico State University, Las Cruces, U.S.A., have discovered that the sensitivity of selenium detection by a cathodic stripping voltammetric procedure can be greatly improved by adding rhodium instead of copper selenide to the sample mixture (J. Wang and J. Lu, “Ultratrace Measurements of Selenium by Cathodic Stripping Voltammetry in the Presence of Rhodium ”, Anal. Chim. Acta, 1993, 274, (2),219-224).

This extremely sensitive cathodic stripping voltammetric method is based on the accumulation and subsequent reduction of a rhodium selenide layer on the hanging mercury drop electrode. Insoluble layers of rhodium selenide are formed on the mercury surface during the preconcentration step. The reduction of the accumulated rhodium selenide gives a sharp cathodic stripping response which is around ten to fifty times larger then the analogous copper or mercury selenide peaks, respectively. The best assaying results have been obtained for a sample containing 0.1 M sulphuric acid solution, 10 µg/l rhodium and preconcentration at - 0.2 V. A fast linear scan follows which gives a high signal to background characteristic and faster assay. Under optimum conditions, an extremely low detection limitof 0.5 ng/1 of selenium can be achieved, following a three minute preconcentration.

This method is fast, ultrasensitive, giving high selectivity and precision, and can be applied forselenium measurements in complex environmental and biological samples.