noun
Etymology: French acide phosphorique
1.
a. : a syrupy or deliquescent crystalline tribasic acid H 3 PO 4 that is obtained by hydration of phosphorus pentoxide or by decomposition of phosphates (as phosphate rock) by leaching with sulfuric acid, that is converted to pyrophosphoric acid when heated above 150°C and to metaphosphoric acid when heated until dense white fumes appear, and that is used chiefly in making fertilizers and other phosphates, in rust-proofing metals, in sugar refining, and as a flavoring agent in soft drinks — called also orthophosphoric acid
b. : a compound (as pyrophosphoric acid or metaphosphoric acid) consisting of phosphate groups linked directly to each other by oxygen
2. : phosphorus pentoxide in combined form as determined by analysis especially in soils and fertilizers