Phosphotungstate

Phosphotungstate is a class of heteropoly anions that combines a central phosphate group with twelve tungsten-oxide units, arranged in the Keggin structure. The most prominent member is the anion [PW12O40]3−, which in acidic or protonated form appears as H3PW12O40 (phosphotungstic acid) in the solid state or in concentrated solution.

Its structure comprises a central PO4 tetrahedron surrounded by twelve WO6 octahedra, giving a highly symmetrical cluster of approximately one nanometer in diameter. Tungsten atoms are in the +6 oxidation state, and the entire assembly is stabilized by extensive oxide bridges. The species exist in various protonation states depending on pH.

Preparation and salts: Phosphotungstate is prepared by condensation of phosphoric acid with sources of tungsten(VI) in strongly acidic aqueous media; the resulting anion can be isolated as salts with alkali or ammonium cations (for example Na3[PW12O40], other counterions) or kept as the free acid H3PW12O40. The compound is a representative member of the broader class of polyoxometalates.

Properties: It is a very strong Brønsted acid and a relatively stable, water-soluble catalyst. The Keggin anion also displays redox activity associated with tungsten centers and serves as a versatile component in oxidation reactions and other acid-catalyzed processes. Its stability in acidic media makes it useful under hydrolytic conditions, and many derivatives exist by substituting tungsten with other addenda atoms (e.g., molybdenum) or by heteroatom variation.

Applications: Phosphotungstate and related heteropoly acids are widely used as catalysts in organic synthesis, including selective oxidations with hydrogen peroxide, hydrolysis, and acid-catalyzed transformations. They also find use in electrochemistry, photocatalysis, and materials science as components of catalysts, membranes, and sensors.