Disodium hydrogen orthophosphate, Na₂HPO₄

Disodium hydrogen orthophosphate, Na₂HPO₄, is obtained in the form of dodecahydrate by neutralizing phosphoric acid with sodium carbonate. It is the ordinary sodium phosphate of commerce, and is a white, crystalline salt, its solubility at 10° C. per. 100 grams of water being 3.9 grams, and at 30° C. 24.1 grams.

Solubility of Disodium Hydrogen Orthophosphate (Shiomi)

Temperature, °C	10.26	25.15	40.2	60.23	99.77
Grams Na2HPO4 in 100 g. water	3.55	12.02	54.88	83.00	102.15

There are three breaks in the curve: at 36.45° C., corresponding with the transition from dodecahydrate to heptahydrate; at 48° C. (probably heptahydrate to dihydrate); and at 95.2° C. (probably dihydrate to anhydrous salt). Later work has indicated the dodecahydrate to exist in two forms, α and β, their transition-temperature being 29.6° C. The transition-temperature of the α-hydrate to the heptahydrate was found to be 35.0° C. The solubilities of the two dodecahydrates were also determined.

For the density of the dodecahydrate Clarke gives 1.535, and at the temperature of liquid air Dewar gives the value 1.545. For the specific heat of the crystalline salt between -20° and 2° C. Person found 0.454, and for the fused salt between 44° and 97° C. 0.758; for the solid dodecahydrate Nernst and Lindemann give 0.3723, and for the heptahydrate 0.3230. Thomsen gives 50.04 Cal. as the heat of formation of the dihydrate from sodium hydroxide and phosphoric acid. The latent heat of fusion of the dodecahydrate at 36.1° C. is given by Person as 23.9 Cal.

For the transition-points of the dodecahydrate to the heptahydrate and of the heptahydrate to the dihydrate d'Ans and Schreiner give

Na₂HPO₄,12H₂O(35.4° C.) Na₂HPO₄,7H₂O(48.35° C.) Na₂HPO₄,2H₂O.

At a pressure of 0.1 atm. Biltz gives the transition-points on the absolute scale as

Na₂HPO₄,12H₂O(317°)→Na₂HPO₄,7H₂O(319°)→Na₂HPO₄,2H₂O(346°) Na₂HPO₄.

The transition-point of the dodecahydrate into the heptahydrate is given by Person as 36.4° C., Baur 36.6° C., and Tilden 35° C.; and the transition-point into the anhydrous salt by Muller as 43.5° C.

When heated, disodium hydrogen phosphate gives up its water of crystallization, and then decomposes, forming sodium pyrophosphate.

Other investigations include the refraction constants, and the dissociation-pressure of the water of crystallization of the dodecahydrate at the transition-point; and such properties of solutions as density, specific heat, boiling-point, vapour-pressure, molecular depression of the freezing-point, electric conductivity, and hydrolytic dissociation.

Treatment of disodium hydrogen orthophosphate with phosphoric acid converts it into sodium dihydrogen orthophosphate, NaH₂PO₄, which crystallizes as monohydrate and dihydrate. For the density of the monohydrate Schiff gives 2.040, and for that of the dihydrate Dufet gives 1.9096. On heating, the salt loses its water of crystallization at 100° C., and at 200° C. is converted by further dehydration into disodium dihydrogen pyrophosphate, Na₂H₂P₂O₇, and at a higher temperature this substance is transformed into sodium metaphosphate, NaPO₃. At 180° C. transformation into the pyrophosphate is very slow, about 178 hours being required for complete conversion.

At 0° C. 100 grams of water dissolve 59.9 grams, and at 18° C. 84.6 grams of the salt, yielding an acid solution. For the heat of formation from the elements Berthelot gives 355.0 Cal.; from the interaction of phosphoric acid and sodium hydroxide he gives 14.7 Cal., and Thomsen 14.829 Cal.

Attention has also been directed to the refractivity of the salt; and to such properties of its solution as vapour-pressure, molecular depression of the freezing-point, electric conductivity, and hydrolytic dissociation.