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a bright blue molybdenum-containing substance formed by the action of reducing agents (SO2, H2S, Zn, and glucose) on solutions of molybdic acid or acid solutions of molybdates. The term “molybdenum blue” is used to denote various compounds in which molybdenum occurs with an oxidation number of +5 or +6. Such substances are obtained as amorphous precipitates of varying composition—for example, Mo8O23·xH2O and Mo4O11O·xH2O. Molybdenum blue is readily adsorbed from colloidal solutions by plant and animal fibers, tinting them blue; this principle, in particular, is the basis of the silk-dyeing process. The reactions involved in the formation of molybdenum blue are widely used in analytical chemistry.