Critical examination of some common reagents for reducing selenium species in chemical analysis

In an attempt to resolve apparently conflicting statements in the literature, a study has been made of the action of various reductants on selenium(VI) and selenium(IV). Chloride in hot non-oxidizing acid medium will reduce Se(VI) to Se(IV) but not further. Sulphur dioxide will reduce Se(IV) to Se, but has no effect on Se(VI). Hydrazinium salts reduce both Se(VI) and Se(VI) to Se, but hydroxylammonium salts reduce only Se(IV) to Se. Hydrogen peroxide partially reduces Se(VI) to Se(IV) but not further. If chloride is also present [to reduce Se(VI) to Se(IV)], sulphur dioxide or hydroxylammonium salts can then reduce the Se(IV) to Se, and this combined effect has led to some confusion in interpretation of experimental observations.