OCCURRENCES, USES, AND PROPERTIES OF CHROMIUM

Chromium is the 21st most abundant element in the Earth's crust with a mean concentration in United States soils of about 40 mg/kg. Although it exists in several oxidation states, the zero, trivalent, and hexav alent states are the most important in commercial products and the env ironment. Nearly all naturally occurring chromium is in the trivalent state, usually in combination with iron or other metal oxides. Althoug h only about 15% of the chromium mined is used in the manufacture of c hemicals, most applications of chromium utilize the chemistry of chrom ium. For instance, the ''stainless'' nature of stainless steel is due to the chemical properties of the chromium oxides which form on the su rface of the alloy. Similarly, the protective properties of chrome pla ting of metals, chromated copper arsenate (CCA) treatment of wood, and chrome tanning of leather are all dependent on chromium chemistry. Th e key to these uses is that under typical environmental and biological conditions of pH and oxidation-reduction potential, the most stable f orm of chromium is the trivalent oxide. This form has very low solubil ity and low reactivity resulting in low mobility in the environment an d low toxicity in living organisms. In this paper the chemical propert ies of chromium are discussed for the major commercial products in the context of the Eh-pH diagram for chromium. (C) 1997 Academic Press.