ABSTRACT
Chromium forms several oxides. This chapter lists their relevant characteristics and properties. All of them, except α -chromium(III) oxide, are thermodynamically unstable in contact with water. The acid-base dissolution behavior of chromium(III) oxides depends more on the degree of oxolation than on the degree of crystallinity of the solid. Freshly precipitated hydrous chromium oxide readily dissolves in both acid and base, as expected from equilibrium solubility considerations. In fact, the limiting factor is solubility; in the case of basic dissolution, strongly alkaline media must be provided or dissolved anionic chromium(III) must be scavenged (e.g., by oxidants). The usage of acid solutions has a distinct advantage when mixed chromium-iron oxides must be dissolved because the oxidizing alkaline treatment generates, in such a case, a very stable iron oxide growing layer that makes subsequent chromium dissolution controlled by slow diffusion processes. The phase transfer of chromium is brought about by the decomposition of surface complexes.
