Sodium orthophosphates

Normal sodium orthophosphate, Na₃PO₄, is obtained by the interaction of equimolecular proportions of disodium hydrogen phosphate and sodium hydroxide. It is known in the anhydrous form, and as heptahydrate, decahydrate, and dodecahydrate. For the density of the anhydrous salt at 17.5° C. Clarke gives 2.536, and for that of the dodecahydrate at ordinary temperature he gives 1.620, and Dufet 1.6445. The solution in water is strongly alkaline; at 15° C. 100 grams of water dissolve 10.5 grams of the dodecahydrate. The heat of formation from the elements is 452.4 Cal.; that from phosphoric acid and sodium hydroxide is given by Thomsen as 34.029 Cal., and by Berthelot as 33.6 Cal. At 18° to 20° C. the heat of solution of the dodecahydrate is -14.5 Cal.

References are appended to work on the transition-point, and refractivity; and to properties of solutions, such as hydrolytic dissociation, density, vapour-pressure, and electric conductivity.