The Principal Methods of Determining Oxidation-Reduction Conditions

Abstract

Many chemical elements have the peculiar attribute, depending on existing conditions, of changing their valence and, with this, their properties. For example, iron may have zero valence or may be divalent or trivalent; manganese may be 0, +2, + 3, +4, and more; sulfur may be -2, 0, +6; and so forth. The remarkable change in properties of chemical elements with change in valence determines one of the decisive factors in the migration and concentration of these elements. For example, the expressed features of alkalinity in bivalent manganese and iron lend these elements a broader migrational capacity in the sedimentary shell of the earth than tervalent iron or quadrivalent manganese. Elemental sulfur is many times less mobile than its oxidized or reduced forms. The great significance of valence change in the migration and concentration of chemical elements was pointed out by Goldschmidt [1933], Fersman [1955], Shcherbina [1939, 1949, 1956, 1965], Pustovalov [1940], Perel’man [1965], and others. Fersman [1955] noted that the oxidation — reduction potential has a very special meaning in explaining the order of coprecipitation in the supergene environment. On the basis of normal potentials, Shcherbina [1939] divided the elements of different valences into groups, one of which manifested a tendency to coprecipitate with the ferrous ion, the other with the ferric. Elements that are concentrated under oxidizing conditions are disseminated under reducing conditions, and the reverse [Shcherbina, 1949, 1956]. Specific features manifested in trace elements during valence changes lead to concentrations chiefly under strongly oxidizing conditions or under strongly reducing conditions [Mason, 1949]. It is no exaggeration to state that without knowledge of the oxidation — reduction state during the process of sedimentary rock formation one cannot seriously approach a solution to the problems of concentration and dissemination of elements in the sedimentary shell of the earth.