Disodium phosphate (DSP), or sodium hydrogen phosphate, or sodium phosphate dibasic, is the inorganic compound with the formula Na2HPO4. It is one of several sodium phosphates. The salt is known in anhydrous form as well as forms with 2, 7, 8, and 12 hydrates. All are water-soluble white powders; the anhydrous salt being hygroscopic.
Sodium hydrogen phosphate
Disodium hydrogen orthophosphate
Sodium hydrogen phosphate
Sodium phosphate dibasic
3D model (JSmol)
|E number||E339(ii) (antioxidants, ...)|
CompTox Dashboard (EPA)
|Molar mass||141.96 g/mol (anhydrous)|
177.99 g/mol (dihydrate)
268.07 g/mol (heptahydrate)
|Appearance||White crystalline solid|
|Melting point||250 °C (482 °F; 523 K) decomposes|
|7.7 g/100 ml (20 °C) |
11.8 g/100 mL (25 °C, heptahydrate)
|Solubility||insoluble in alcohol|
|Acidity (pKa)||2.15, 6.82, 12.35|
Refractive index (nD)
|1.35644..1.35717 at 20°C|
|Safety data sheet||ICSC 1129|
|NFPA 704 (fire diamond)|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|17000 mg/kg (rat, oral)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
- HPO42− + H2O ⇌ H2PO4− + OH−
Production and reactions
- H3PO4 + 2 NaOH → HNa2PO4 + 2 H2O
- CaHPO4 + NaHSO4 → NaH2PO4 + CaSO4
In the second step, the resulting solution of monosodium phosphate is partially neutralized:
- NaH2PO4 + NaOH → HNa2PO4 + 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. 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 HNa2PO4 → Na4P2O7 + H2O
- "Physical data (pdf)" (PDF). Archived from the original (PDF) on 2011-07-17. Retrieved 2007-03-06.
- Klaus Schrödter, Gerhard Bettermann, Thomas Staffel, Friedrich Wahl, Thomas Klein, Thomas Hofmann "Phosphoric Acid and Phosphates" in Ullmann’s Encyclopedia of Industrial Chemistry 2008, Wiley-VCH, Weinheim. doi:10.1002/14356007.a19_465.pub3
- "MSDS". Archived from the original on 2017-10-28. Retrieved 2011-09-26.
- "Sodium Phosphate, Dibasic, Sodium Phosphate, Monobasic Oral solution". Krames Patient Education. Archived from the original on 2013-01-28. Retrieved 2009-10-29.
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