As stated under Phosphorus, phosphoric oxide, P 2 0 5, combines with water in three proportions to form H 2 O P 2 0 5 or HP03, metaphosphoric acid; 2H 2 O P 2 0 5 or H4P207, pyrophosphoric acid; and 3H 2 O P 2 0 5 or H 3 PO 4, orthophosphoric or ordinary phosphoric acid.
On long heating the syrup is partially converted into pyrophosphoric and metaphosphoric acids, but on adding water and boiling the ortho-acid is re-formed.
Pyrophosphoric acid, 'H' 4 P 2 0 7, is a tetrabasic acid which may be regarded as derived by eliminating a molecule of water between two molecules of ordinary phosphoric acid; its constitution may therefore be written (HO) 2 0P O PO(OH) 2.
The first is formed when 30% hydrogen peroxide reacts with phosphorus pentoxide or metaor pyrophosphoric acids at low temperatures and the mixture diluted with ice-cold water.
Perphosphoric acid is formed when pyrophosphoric acid is treated with a large excess of hydrogen peroxide.
Pyrophosphoryl chloride, P 2 0 3 C1 4, corresponding to pyrophosphoric acid, was obtained by Geuther and Michaelis (Ber., 1871, 4, P. 766) in the oxidation of phosphorus trichloride with nitrogen peroxide at low temperature; it is a colourless fuming liquid which boils at about 212° with some decomposition.