METAPHOSPHORIC ACID

Metaphosphoric acid is a singularly anhydrous version of orthophosphoric acid and is sometimes used as a water- or moisture-absorbing reagent. Metaphosphoric acid has phosphorus in the formal oxidation state of ＋5. Although it has several CAS numbers: 10343-62-1, 135306-83-1, 13566-25-1, and 37267-86-0, only the last two are associated with the IUPAC name “phosphenic acid”.
Metaphosphoric acid units can be bonded together in rings (cyclic structures), chains (catena), or branched by condensation yielding water as each acid unit is added. These structures consist of linked metaphosphoric acid molecules. For example, trimetaphosphoric acid is the simplest cyclophosphoric acid. Metaphosphoric acid would need to share the doubly bonded oxygen with neighboring phosphorus atoms to form polymers itself. Many oligamers are possible with the alkaline earths but the trimetaphosphates and hexametaphosphates seem to be the most prevalent.

transparent, glass-like solid or soft silky masses; hygroscopic; volatilizes at red heat (HPO3)n [MER06]

ChEBI: Phosphonate(1-) is a monovalent inorganic anion obtained by deprotonation of one of the two OH groups in phosphonic acid. It is a phosphorus oxoanion and a monovalent inorganic anion. It is a conjugate base of a phosphonic acid. It is a conjugate acid of a phosphonate(2-).