Polyphosphoric acid is a mixture of orthophosphoric acid and linear phosphoric acids. It is described empirically as a wt % of P2O5 in water. The distribution of phosphoric acids that are found in PPA is dependent upon the wt % of P2O5. Commercially available PPA contains 82-85% P2O5, with 83% P2O5 considered to be the standard. Polyphosphoric acid (PPA) is a thick, viscous liquid consisting of linear molecules of various chain lengths. The powerful dehydrating properties of PPA, low nucleophilicity of the phosphoric acid media, and moderate acidity makes this reagent very widely used.
Polyphosphoric acid is produced by condensation polymerization of ortho-phosphoric acid as follows:
2 H3PO4 → H4P2O7 + H2O
Polyphosphoric acid is used as a strong drying and dehydrating agent.
Polyphosphoric acid is used to make phosphate esters, acid phosphates and pharmaceuticals. It is a good ring closing reagent.
It can be used as a reaction medium and solvent.
It is used in metal treatment and as an asphalt and bitumen additive.