POLYPHOSPHORIC ACID is a thick, viscous liquid consisting of linear molecules of various chain lengths. It is formed by condensing orthophosphoric acid to eliminate water between two or more molecules. If two molecules of phosphoric acid are heated to remove one molecule of water, pyrophosphoric acid is formed. Theoretically pure tripoly-phosphoric acid is formed with a P2O5 content of 82.6%. However, an acid with this P2O5 concentration actually contains only 17% tripoly-phosphoric acid with the remainder a mixture of various chain lengths.

Condensed phosphoric acids are not pure compounds. They are a mixture of polyphosphoric acids in equilibrium. The P2O5 content controls the equilibrium constituent composition. The polymer composition of polyphosphoric acid concentrations can be found below.

Polyphosphoric acid chains consist of PO4 tetrahedrons joined by a shared oxygen atom at the corner of the tetrahedral and connected by hydrogen bonds. These hydrogen bonds in concentrated solutions are responsible for the viscous nature of the acid. In dilute solutions the phosphate ions are hydrogen bonded to the water rather than the PO4 ions. Thus the viscous nature of concentrated phosphoric causes stratification when two strengths are mixed due to hydrogen bonding. It requires vigorous mixing and an indefinite time for the mixture to reach equilibrium.

Polyphosphoric acids of 82-84% P2O5 concentration are used as a catalyst in the chemical and the petroleum industries for polymerization, alkylation, dehydration, condensation, and isomerization processes. One advantage of polyphosphoric acids is that they retain their acid characteristics at a much higher temperature than most other acids.