Monocalcium phosphate

Monocalcium phosphate
Names
	IUPAC name Calcium bis(dihydrogen phosphate)
	Other names Acid calcium phosphate; Calcium acid phosphate; Calcium diorthophosphate; Calcium biphosphate; Calcium superphosphate; Monobasic calcium phosphate; Monocalcium orthophosphate; Phosphoric acid, calcium salt (2:1)
Identifiers
CAS Number	7758-23-8 ; 10031-30-8 (monohydrate) ;
3D model (JSmol)	Interactive image;
ChemSpider	22862 ;
ECHA InfoCard	100.028.943
E number	E341(i) (antioxidants, ...)
PubChem CID	24454;
UNII	701EKV9RMN ; 0N4E6L5449 (monohydrate) ;
CompTox Dashboard (EPA)	DTXSID2044262 ;
	InChI InChI=1S/Ca.2H3O4P/c;21-5(2,3)4/h;2(H3,1,2,3,4)/q+2;;/p-2 Key: YYRMJZQKEFZXMX-UHFFFAOYSA-L ; InChI=1/Ca.2H3O4P/c;21-5(2,3)4/h;2(H3,1,2,3,4)/q+2;;/p-2 Key: YYRMJZQKEFZXMX-NUQVWONBAY;
	SMILES OP(=O)(O)[O-].OP(=O)(O)[O-].[Ca+2];
Properties
Chemical formula	CaH4P2O8
Molar mass	234.05 g/mol
Appearance	White powder
Density	2.220 g/cm3
Melting point	109 °C (228 °F; 382 K)
Boiling point	203 °C (397 °F; 476 K) (decomposes)
Solubility in water	2 g/100 mL
Refractive index (nD)	1.5176
Structure
Crystal structure	Triclinic
Hazards
NFPA 704 (fire diamond)	1 0 0
Flash point	Non-flammable
Related compounds
Other anions	Calcium pyrophosphate
Other cations	Magnesium phosphate; Dicalcium phosphate; Tricalcium phosphate; Strontium phosphate
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Monocalcium phosphate is an inorganic compound with the chemical formula Ca(H₂PO₄)₂ ("AMCP" or "CMP-A" for anhydrous monocalcium phosphate). It is commonly found as the monohydrate ("MCP" or "MCP-M"), Ca(H₂PO₄)₂·H₂O. Both salts are colourless solids. They are used mainly as superphosphate fertilizers and are also popular leavening agents.^[1]

Preparation

Material of relatively high purity, as required for baking, is produced by treating calcium hydroxide with phosphoric acid:

Ca(OH)₂ + 2 H₃PO₄ → Ca(H₂PO₄)₂ + 2 H₂O

Samples of Ca(H₂PO₄)₂ tend to convert to dicalcium phosphate:

Ca(H₂PO₄)₂ → Ca(HPO₄) + H₃PO₄

Applications

Use in fertilizers

Superphosphate fertilizers are produced by treatment of phosphorite with acids ("acidulation"). Using phosphoric acid, fluorapatite is converted to Ca(H₂PO₄)₂:

Ca₅(PO₄)₃F + 7 H₃PO₄ → 5 Ca(H₂PO₄)₂ + HF

This solid is called triple superphosphate. Several million tons are produced annually for use as fertilizers. Using sulfuric acid, fluorapatite is converted to a mixture of Ca(H₂PO₄)₂ and CaSO₄. This solid is called single superphosphate.

Residual HF typically reacts with silicate minerals co-mingled with the phosphate ores to produce hexafluorosilicic acid (H₂SiF₆). The majority of the hexafluorosilicic acid is converted to aluminium fluoride and cryolite for the processing of aluminium.^[1] These materials are central to the conversion of aluminium ore into aluminium metal.

When sulfuric acid is used, the product contains phosphogypsum (CaSO₄·2H₂O) and is called single superphosphate. ^[2]

Use as leavening agent

Calcium dihydrogen phosphate is used in the food industry as a leavening agent, i.e., to cause baked goods to rise. Because it is acidic, when combined with an alkali carbonate ingredient, commonly sodium bicarbonate (baking soda) or potassium bicarbonate, it reacts to produce carbon dioxide and a salt. Outward pressure of the carbon dioxide gas causes the rising effect. When combined in a ready-made baking powder, the acid and alkali ingredients are included in the right proportions such that they will exactly neutralize each other and not significantly affect the overall pH of the product. AMCP and MCP are fast acting, releasing most carbon dioxide within minutes of mixing. It is popularly used in pancake mixes. In double-acting baking powders, MCP is often combined with the slow-acting acid sodium acid pyrophosphate (SAPP).^[3]

References

^ ^a ^b Schrödter, Klaus; Bettermann, Gerhard; Staffel, Thomas; Wahl, Friedrich; Klein, Thomas; Hofmann, Thomas (2008). "Phosphoric Acid and Phosphates". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_465.pub3. ISBN 978-3527306732.
^ Kongshaug, Gunnar; Brentnall, Bernard A.; Chaney, Keith; Gregersen, Jan-Helge; Stokka, Per; Persson, Bjørn; Kolmeijer, Nick W.; Conradsen, Arne; Legard, Torbjørn; Munk, Harald; Skauli, Øyvind; Kiiski, Harri; Solheim, Kai Roger; Legard, Torbjörn; Brentnall, Bernard A.; Rauman-Aalto, Paulina (2014). "Phosphate Fertilizers". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. pp. 1–49. doi:10.1002/14356007.a19_421.pub2. ISBN 978-3527306732.
^ John Brodie, John Godber "Bakery Processes, Chemical Leavening Agents" in Kirk-Othmer Encyclopedia of Chemical Technology 2001, John Wiley & Sons. doi:10.1002/0471238961.0308051303082114.a01.pub2