Metaphosphates

Metaphosphates are a class of inorganic phosphorus compounds consisting of condensed phosphate units connected by P–O–P bridges. They include cyclic and linear polyphosphates built from repeating PO3 units. The chemistry is organized around metaphosphoric acid derivatives and the salts derived from them, which can exist as various polymer lengths depending on synthesis and conditions.

The best-known metaphosphates are cyclic species such as trimetaphosphate [P3O9]3− and hexametaphosphate [P6O18]6−, as well as longer linear polyphosphates. Commercial salts include sodium trimetaphosphate and sodium hexametaphosphate (SHMP, Na6P6O18). In solution, these species interconvert with orthophosphate and other polyphosphates, their distribution influenced by pH, temperature, and metal ions. Metaphosphates can form complexes with metal cations and often appear as useful food additives, water-treatment agents, and buffering/plasticizing components in polymers.

Preparation typically involves dehydration or condensation of phosphoric acid or shorter phosphate units under controlled heating or acid–base conditions, promoting P–O–P linkage formation and water removal. Hydrolysis of metaphosphates yields orthophosphate, with the rate and products dependent on pH and the specific polyphosphate structure. In aqueous environments, metaphosphates act as phosphate sources and can function as chelating or sequestering agents due to their affinity for multivalent cations.

Applications include water-softening and corrosion-inhibition in detergents, food processing as thickeners and stabilizers, and industry uses in ceramics and glazes. Environmental considerations focus on eutrophication potential when metaphosphates are discharged.