Acid Phosphates

On account of the relatively higher degree of dissociation of sulphuric acid it is capable of liberating phosphoric acid from phosphates, and the reaction is favoured by the low solubility of calcium sulphate.

Ca₃P₂O₈ + 3H₂SO₄ = 3CaSO₄ + 2H₃PO₄

This probably is the first stage of the reaction by which superphosphates are produced. It is succeeded by

Ca₃P₂O₈ + 4H₃PO₄ = 3CaH₄P₂O₈

When two mols of sulphuric acid are added to one of the tribasic phosphate more than 96 per cent, of the phosphate is rendered freely soluble in water, superphosphate being formed as follows:—

Ca₃P₂O₈ + 2H₂SO₄ = 2CaSO₄ + CaH₄P₂O₈

In practice it is arranged that a little of the tribasic phosphate is left undecomposed, and this then reacts according to the equation

Ca₃P₂O₈ + CaH₄P₂O₈ = 4CaHPO₄

The amount of freely soluble phosphate is thus reduced. When the superphosphate is brought into contact with water the insoluble part is increased by the production of a solution containing from 12 to 5½ times as much phosphoric acid as lime, or about 4 times as much when CaHPO₄ is present as a solid. The presence of CaSO₄ in commercial phosphate does not much alter these ratios, as is shown by the following solubilities:—

Solid Phases	P2O5, grams/litre.	CaO grams/litre
CaSO4.2H2O, CaHPO4.2H2O, CaH4P2O8	317	77
CaSO4.2H2O, CaSO4, CaH4P2O8.H2O	545	38

The ratio P₂O₅/CaO is 4.1 in the solutions which are in contact with solid " reverted " phosphate and sulphate of lime, and 14 in the solutions which are in contact with solid superphosphate and sulphate of lime.