phosphomolybdate

Phosphomolydate refers to the phosphomolydate anion, [PMo12O40]3−, a member of the polyoxometalate family. It is the conjugate base of phosphomolybdic acid, H3PMo12O40, and forms a variety of salts (for example, ammonium, sodium, and potassium phosphomolydate) that are typically water-soluble under acidic conditions. In aqueous solution the anion adopts a Keggin-type structure consisting of a central PO4 tetrahedron surrounded by twelve MoO6 octahedra, giving a highly symmetric, robust framework.

The compound is usually prepared by combining orthophosphoric acid with molybdate under strongly acidic conditions to yield the [PMo12O40]3− anion. The solid salts are stable and can be crystallized with various countercations. In solution the anion remains highly polar and resistive to hydrolysis, retaining the characteristic Keggin structure.

Phosphomolydate exhibits notable redox chemistry. It can be reduced by suitable reducing agents to form phosphomolydenum blue, a deeply colored mixed-valence Mo(V)/Mo(VI) species. This color change underpins several analytical and qualitative tests. The phosphomolydate system also serves as a catalytic component in certain acid- and oxidation-reaction processes, often as part of broader heteropoly acid catalysts.

Applications of phosphomolydate span catalysis and analytical chemistry. As a component of heteropoly acid reagents, it is used in oxidation catalysis and in colorimetric assays for reducing substances and antioxidants, where the reduced blue species is monitored spectrophotometrically. In qualitative assays, its derivative reagents contribute to established procedures such as the Folin–Ciocalteu method for determining total phenolics.