Potassium orthophosphates

The normal salt, potassium ortho-phosphate, K₃PO₄, is obtained by heating basic slag or native calcium phosphate with charcoal and potassium sulphate. The potassium sulphide simultaneously formed is converted into phosphate by addition of phosphoric acid, or the potassium phosphate is precipitated by addition of alcohol. The aqueous solution of the salt is very alkaline in reaction, owing to hydrolytic dissociation. The heat of formation from the elements in solution is 483.6 Cal.

The interaction of the calculated quantities of phosphoric acid and potassium carbonate in aqueous solution yields dipotassium hydrogen phosphate, K₂HPO₄. It is only known in solution, and in this form has a slightly alkaline reaction to litmus, but is neutral to phenolphthalein. Its heat of formation in aqueous solution is 429.2 Cal.

Calcium phosphate reacts with potassium hydrogen sulphate and sulphuric acid to form potassium dihydrogen phosphate, KH₂PO₄, the most important of the potassium phosphates. It forms doubly refracting crystals melting at 96° C., of density 2.3325 at 9.2° C., and 2.338 at 20° C., and of specific heat 0.208 between 17° and 48° C. The heat of formation in aqueous solution is 374.4 Cal. When heated at 244° C., it loses water, yielding the acid pyrophosphate, K₂H₂P₂O₇.