Disodium phosphate

Disodium phosphate (DSP), or disodium hydrogen phosphate, or sodium phosphate dibasic, is an inorganic compound with the chemical formula Na2HPO4. It is one of several sodium phosphates. The salt is known in anhydrous form as well as hydrates Na2HPO4*nH2O, where n is 2, 7, 8, and 12. All are water-soluble white powders. The anhydrous salt is hygroscopic.

The pH of disodium hydrogen phosphate water solution is between 8.0 and 11.0, meaning it is moderately basic:
 * HPO4(2−) + H2O ⇌ H2PO4− + OH−

Production and reactions
It can be generated by neutralization of phosphoric acid with sodium hydroxide:
 * H3PO4 + 2 NaOH → Na2HPO4 + 2 H2O

Industrially It is prepared in a two-step process by treating dicalcium phosphate with sodium bisulfate, which precipitates calcium sulfate:
 * CaHPO4 + NaHSO4 → NaH2PO4 + CaSO4

In the second step, the resulting solution of monosodium phosphate is partially neutralized:
 * NaH2PO4 + NaOH → Na2HPO4 + H2O

Uses
It is used in conjunction with trisodium phosphate in foods and water softening treatment. In foods, it is used to adjust pH. Its presence prevents coagulation in the preparation of condensed milk. Similarly, it is used as an anti-caking additive in powdered products. It is used in desserts and puddings, e.g. Cream of Wheat to quicken cook time, and Jell-O Instant Pudding for thickening. In water treatment, it retards calcium scale formation. It is also found in some detergents and cleaning agents.

Heating solid disodium phosphate gives the useful compound tetrasodium pyrophosphate:
 * 2 Na2HPO4 → Na4P2O7 + H2O

Laxative
Monobasic and dibasic sodium phosphate are used as a saline laxative to treat constipation or to clean the bowel before a colonoscopy.