[The change of valence of chromium by acid treatment]

There are many kinds of chromium compounds in nature, and most of them exist as Cr6+- and Cr3+-compounds. Generally, these two kinds of compounds are stable in normal circumstances. When the chromium in biomaterial or in atmospheric substances is analyzed, the samples are usually ashed with strong acids at high temperatures to destroy the organic components, and chromium compounds are changed to water soluble salts in this ashing process. And then chromium is determined in various methods. The relation between the used acids and the behavior of chromium were investigated, and the following facts have been cleared. Though the total amount of chromium in the samples remains constant, the valence of chromium is apt to change easily in the ashing process (Cr6+ in equilibrium Cr3+). As Cr3+ is more stable than Cr6+ in acid solution, the reductive reaction proceeds mainly as (Cr6+ leads to Cr3+). But each acid has the peculiar reductive action on Cr6+. Even if the same acid is used, its reductive power changes very sensitively as the used conditions are altered. The reductive action of mixed acid differs from that of original acid remarkably. Certain acid has the oxidizing action on Cr3+. Thus, the mechanisms and reactions between strong acids and chromium are very complicated. So, for the determination of the amounts of individual Cr6+ and Cr3+, the oxidizing or reductive action of the used acid must be considered cautiously.