Molybdates

Molybdates are inorganic compounds that contain the molybdate anion MoO4^2−, derived from molybdic acid (H2MoO4). In aqueous solution, speciation is pH-dependent: at pH higher than about 4, MoO4^2− predominates; in mildly acidic solutions HMoO4− is common; in strongly acidic conditions H2MoO4 is present. At higher concentrations and lower pH, condensed polyoxomolybdate species such as Mo7O24^6− can form.

Common molybdate salts include sodium molybdate (Na2MoO4), ammonium molybdate ((NH4)2MoO4), and the ammonium heptamolybdate ((NH4)6Mo7O24·4H2O), which can crystallize from molybdate solutions. In nature, molybdate minerals such as powellite CaMoO4 and wulfenite PbMoO4 occur, and oxidation of molybdenite MoS2 yields molybdate in solution.

Industrial production typically begins with molybdenite ore, which is roasted and treated with alkali to form sodium molybdate, a precursor to other salts. Molybdates serve as sources of the essential trace element molybdenum in fertilizers and act as catalysts in petrochemical processing (notably hydrodesulfurization). They are also used in pigments for glass and ceramics, and in analytical chemistry, where the molybdate blue reaction is employed for phosphate determination.

Safety and handling considerations for molybdate salts indicate relatively low to moderate toxicity; they are generally nonhazardous to humans in small amounts but can be harmful if ingested in large quantities or released to the environment in high concentrations. Standard inorganic chemical hygiene practices and local regulations apply.