Disodium phosphate

Disodium phosphate (DSP), or disodium hydrogen phosphate, or sodium phosphate dibasic, is an inorganic compound with the chemical formulaNa₂HPO₄. It is one of several sodium phosphates. The salt is known in anhydrous form as well as hydratesNa₂HPO₄·nH₂O, where n is 2, 7, 8, and 12. All are water-soluble white powders. The anhydrous salt is hygroscopic.^[1]

The pH of disodium hydrogen phosphate water solution is between 8.0 and 11.0, meaning it is moderately basic:

HPO2−4 + H₂O ⇌ H₂PO−4 + OH⁻

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:^[2]

CaHPO₄ + NaHSO₄ → NaH₂PO₄ + CaSO₄

In the second step, the resulting solution of monosodium phosphate is partially neutralized:

NaH₂PO₄ + NaOH → Na₂HPO₄ + H₂O

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.^[3] 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.^[2]

Heating solid disodium phosphate gives the useful compound tetrasodium pyrophosphate:

2 Na ₂ HPO ₄ → Na ₄ P ₂ O ₇ + H ₂ O

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