Calcium citrate

Calcium citrate is the calcium salt of citric acid. It is commonly used as a food additive (E333), usually as a preservative, but sometimes for flavor. In this sense, it is similar to sodium citrate. Calcium citrate is also found in some dietary calcium supplements (e.g. Citracal or Caltrate). Calcium makes up 24.1% of calcium citrate (anhydrous) and 21.1% of calcium citrate (tetrahydrate) by mass. The tetrahydrate occurs in nature as the mineral Earlandite.

Calcium citrate

2D structure of calcium citrate

Calcium citrate tetrahydrate[1]

Calcium citrate tetrahydrate
IUPAC name
2-hydroxy-1,2,3-propane-tricarboxylic acid calcium salt (2:3)
Other names
E333, tricalcium dicitrate
3D model (JSmol)
ECHA InfoCard 100.011.265
EC Number
  • 212-391-7
E number E333 (antioxidants, ...)
  • InChI=1S/2C6H8O7.3Ca/c2*7-3(8)1-6(13,5(11)12)2-4(9)10;;;/h2*13H,1-2H2,(H,7,8)(H,9,10)(H,11,12);;;/q;;3*+2/p-6 Y
  • InChI=1/2C6H8O7.3Ca/c2*7-3(8)1-6(13,5(11)12)2-4(9)10;;;/h2*13H,1-2H2,(H,7,8)(H,9,10)(H,11,12);;;/q;;3*+2/p-6
  • [Ca+2].[Ca+2].[Ca+2].O=C([O-])CC(O)(C([O-])=O)CC(=O)[O-].[O-]C(=O)C(O)(CC([O-])=O)CC([O-])=O
Molar mass 498.4334 g/mol (anhydrous)
570.4945 g/mol (tetrahydrate)
Appearance White powder
Odor odorless
Density 1.63 g/cm3 (anhydrous)
2.00 g/cm3 (tetrahydrate)[1]
Melting point Decomposes
Boiling point Decomposes
0.85 g/L (18 °C)
0.95 g/L (25 °C)
Solubility insoluble in alcohol
Triclinic (tetrahydrate)
P1, No. 2
a = 0.59466(4) nm, b = 1.02247(8) nm, c = 1.66496(13) nm
α = 72.213(7)°, β = 79.718(7)°, γ = 89.791(6)°[1]
Occupational safety and health (OHS/OSH):
Main hazards
NFPA 704 (fire diamond)
Related compounds
Other cations
Magnesium citrate
Strontium citrate
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

In 2020, calcium was the 204th most commonly prescribed medication in the United States, with more than 2 million prescriptions.[2][3]

Chemical properties

Calcium citrate is sparingly soluble in water. Needle-shaped crystals of tricalcium dicitrate tetrahydrate [Ca3(C6H5O7)2(H2O)2]·2H2O were obtained by hydrothermal synthesis. The crystal structure comprises a three-dimensional network in which eightfold coordinated Ca2+ cations are linked by citrate anions and hydrogen bonds between two non-coordinating crystal water molecules and two coordinating water molecules.[1]


Calcium citrate is an intermediate in the isolation of citric acid from the fungal fermentation process by which citric acid is produced industrially.[4] The citric acid in the broth solution is neutralized by limewater, precipitating insoluble calcium citrate. This is then filtered off from the rest of the broth and washed to give clean calcium citrate.

3 Ca(OH)2(s) + 2 C6H8O7(l) → Ca3(C6H5O7)2(s) + 6 H2O(l)

The calcium citrate thus produced may be sold as-is, or it may be converted to citric acid using dilute sulfuric acid.

Biological role

In many individuals, bioavailability of calcium citrate is found to be equal to that of the cheaper calcium carbonate.[5] However, alterations to the digestive tract may change how calcium is digested and absorbed. Unlike calcium carbonate, which is basic and neutralizes stomach acid, calcium citrate has no effect on stomach acid.[6] Calcium carbonate is harder to digest than calcium citrate,[6] and calcium carbonate carries a risk of "acid rebound" (the stomach overcompensates by producing more acid),[6] so individuals who are sensitive to antacids or who have difficulty producing adequate stomach acid may choose calcium citrate over calcium carbonate for supplementation. According to recent research into calcium absorption after gastric bypass surgery,[7] calcium citrate may have improved bioavailability over calcium carbonate in Roux-en-Y gastric bypass patients who are taking calcium citrate as a dietary supplement after surgery. This is mainly due to the changes related to where calcium absorption occurs in the digestive tract of these individuals.


  1. Herdtweck, Eberhardt; Kornprobst, Tobias; Sieber, Roland; Straver, Leo; Plank, Johann (2011). "Crystal Structure, Synthesis, and Properties of tri-Calcium di-Citrate tetra-Hydrate [Ca3(C6H5O7)2(H2O)2]·2H2O". Zeitschrift für anorganische und allgemeine Chemie. 637 (6): 655–659. doi:10.1002/zaac.201100088.
  2. "The Top 300 of 2020". ClinCalc. Retrieved 7 October 2022.
  3. "Calcium - Drug Usage Statistics". ClinCalc. Retrieved 7 October 2022.
  4. "Use of Lime in the Chemical Industry". National Lime Association. Archived from the original on 2006-09-29. Retrieved 2006-11-25.
  5. Heaney RP, Dowell MS, Bierman J, Hale CA, Bendich A (2001). "Absorbability and cost effectiveness in calcium supplementation". Journal of the American College of Nutrition. 20 (3): 239–46. doi:10.1080/07315724.2001.10719038. PMID 11444420. S2CID 206488.
  6. "What you need to know about calcium". Harvard Health Publications. 9 June 2009. Retrieved 4 August 2014.
  7. Tondapu P, Provost D, Adams-Huet B, Sims T, Chang C, Sakhaee K (June 2009). "Comparison of the Absorption of Calcium Carbonate and Calcium Citrate after Roux-en-Y Gastric Bypass". Obesity Surgery. 19 (9): 1256–1261. doi:10.1007/s11695-009-9850-6. PMC 4469176. PMID 19437082.
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