Disodium phosphate

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Disodium phosphate
Disodium hydrogen phosphate.png
CAS number 7558-79-4 YesY
10028-24-7 (dihydrate)
7782-85-6 (heptahydrate)
10039-32-4 (dodecahydrate)
PubChem 24203
ChemSpider 22625 YesY
EC number 231-448-7
ChEBI CHEBI:34683 YesY
RTECS number WC4500000
Jmol-3D images Image 1
Molecular formula Na2HPO4
Molar mass 141.96 g/mol (anhydrous)
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
Solubility in water 7.7 g/100 ml (20 °C)
11.8 g/100 mL (25 °C, heptahydrate)
Acidity (pKa) 12.35
EU Index Not listed
Main hazards Irritant
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point Non-flammable
Related compounds
Other anions sodium phosphite
Other cations Dipotassium phosphate
Diammonium phosphate
Related compounds Monosodium phosphate
Trisodium phosphate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Disodium hydrogen phosphate 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]

Acid-base properties[edit]

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

HPO42− + H2O \overrightarrow{\leftarrow} H2PO4 + OH

Production and reactions[edit]

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 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)
  2. ^ a b 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
  4. ^ [1]