The term polyphosphoric acid refers to concentrated grades of phosphoric acid (H
3PO
4) above 95%. At these high concentrations, the separate phosphate units are polymerized. Thermphos supplies a wide range of high-purity grades between 97 and 118, where the number designates the apparent H
3PO
4 concentration.
Each grade contains several species of different chain length which are in equilibrium with each other, and higher concentrations have a longer average chain length. The product is made by concentration of orthohosphoric acid, or by dissolution of P2O5 in water.
The product has similar applications to P2O5 but has the advantage that it is a liquid, which allows for easier handling and dosing.
All grades of PPA are highly viscous, colourless transparent liquids. Temperature has a strong influence on supercooling and viscosity.