A successful theory systematizing or correlating the color of inorganic complexes has not yet been advanced. Linus Pauling in his Richards Medal address (7) listed such a theory as one of the puzzling unsolved problems of structural chemistry . The colors developed by solutions containing the same element in different valence states is particularly interesting, in that the color of the mixture may be radically different from that of either component. Cuprous chloride in concentrated hydrochloric acid is colorless, cupric chloride in a similar solution is green; present together they produce a dark brown or black solution (3). Similarly, a hydrochloric acid solution of antimony trichloride is colorless, of antimony pentachloride a pale yellow; a mixture of the two, however, possesses an intense red-brown color. Although ferrous hydroxide is white and ferric hydroxide brown, the ferrous-ferric hydroxide resulting from partial air oxidation of freshly precipitated ferrous hydroxide is black. Again, colorless tervalent ytterbium on reduction with metallic zinc to green, bivalent ytterbium passes through a purple stage, again probably a mixed valence complex(2). In the present paper a more detailed study is reported of the antimonous chloride-antimonic chloride-hydrochloric acid system
