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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 PhasesP2O5, grams/litre.CaO grams/litre
CaSO4.2H2O, CaHPO4.2H2O, CaH4P2O831777
CaSO4.2H2O, CaSO4, CaH4P2O8.H2O54538

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.

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