PYROPHOSPHATES

Are those with the anion PO3; all are alkaline.

Vary in toxicity.

The salts of pyrophosphoric acid or diphosphoric acid, with ammonia or sodium or potassium hydroxide are known as pyrophosphates. When phosphoric acid is concentrated above 54% P₂O₅, superphosphoric acid (or pyrophosphoric acid) is formed that contains 30% or more of P₂O₅ in the form of condensed or polyphosphoric acids of general formula H_n+2P_nO_3n+1.Essentially, the production of superphosphoric acid first involves the removal of physical water from the weaker acid [by vacuum concentration of merchant-grade phosphoric acid (54% P₂O₅)], and then the removal of chemical bound water. These steps can be represented as follows:
The simplest polyphosphoric acid is pyrophosphoric acid (H₄P₂O₇). Pyrophosphoric acid is dehydrated phosphoric acid. Superphosphoric acids are suitable for the production of clear, liquid fertilizers with micronutrients because of the ability to sequester metal ions.