Disodium phosphate

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.[1]

Disodium phosphate
IUPAC name
Sodium hydrogen phosphate
Other names
Disodium hydrogen orthophosphate
Sodium hydrogen phosphate
Sodium phosphate dibasic
disodium phosphate
3D model (JSmol)
ECHA InfoCard 100.028.590
EC Number
  • 231-448-7
E number E339(ii) (antioxidants, ...)
RTECS number
  • WC4500000
Molar mass 141.96 g/mol (anhydrous)
177.99 g/mol (dihydrate)
268.07 g/mol (heptahydrate)
Appearance White crystalline solid
Odor odorless
Density 1.7 g/cm3
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
log P -5.8
Acidity (pKa) 2.15, 6.82, 12.35
56.6·10−6 cm3/mol
1.35644..1.35717 at 20°C
Main hazards Irritant
Safety data sheet ICSC 1129
NFPA 704 (fire diamond)
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
17000 mg/kg (rat, oral)
Related compounds
Other anions
sodium phosphite
Other cations
Dipotassium phosphate
Diammonium phosphate
Related compounds
Monosodium phosphate
Trisodium phosphate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

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

HPO42− + H2O H2PO4 + OH

Production and reactions

It can be generated by neutralization of phosphoric acid with sodium hydroxide:

H3PO4 + 2 NaOH → HNa2PO4 + 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 → 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.[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 HNa2PO4 → Na4P2O7 + H2O

Monobasic and dibasic sodium phosphate are used as a saline laxative to treat constipation or to clean the bowel before a colonoscopy.[4]


  1. "Physical data (pdf)" (PDF). Archived from the original (PDF) on 2011-07-17. Retrieved 2007-03-06.
  2. 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
  3. "MSDS". Archived from the original on 2017-10-28. Retrieved 2011-09-26.
  4. "Sodium Phosphate, Dibasic, Sodium Phosphate, Monobasic Oral solution". Krames Patient Education. Archived from the original on 2013-01-28. Retrieved 2009-10-29.
  • Media related to Disodium phosphate at Wikimedia Commons solubility in Prophylaxis alcohol
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