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 Hn+2PnO3n+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.
