Oxidation-reduction reactions are clearly one of the major controlling factors in the geochemical and environmental behavior of Se. Predictions of its solubility and aqueous speciation must include some measure of the redox status (Eh or pH) of the system. However, laboratory experiments show that even at high concentrations of Se, and over a wide range of pH, the Pt electrode is completely insensitive to the relative abundances of dissolved Se (VI) and Se (IV). For Se, as for many other multivalent elements, predictions concerning aqueous speciation cannot be based on a measured Eh and equilibrium geochemical calculations; instead, chemical analyses must be used to measure the abundances of the predominant dissolved species.

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