Monocalcium phosphate

Monocalcium phosphate is an inorganic compound with the chemical formula Ca(H₂PO₄)₂ ("ACMP" or "CMP-A" for anhydrous monocalcium phosphate). It is commonly found as the monohydrate (""MCP" or "MCP-M"), Ca(H₂PO₄)₂·H₂O (CAS# 10031-30-8). Both salts are colourless solids. They are used mainly as superphosphate fertilizers and are also popular leavening agents.

Preparation

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

Ca(OH)₂ + 2 H₃PO₄ â†' Ca(H₂PO4)2 + 2 H2O</sub>

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

Ca(H₂PO₄)₂ â†' Ca(HPO₄) + H₃PO₄

Applications

Use in fertilizers

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

Ca₅(PO₄)₃F + 7 H₃PO₄ â†' 5 Ca(H₂PO₄)₂ + HF

This solid is called triple superphosphate. Several million tons are produced annually for use as fertilizers. Residual HF typically reacts with silicate minerals co-mingled with the phophate ores to produce hydrofluorosilicic acid (H₂SiF₆). The majority of the hexafluorosilicic acid is converted to aluminium fluoride and cryolite for the processing of aluminium. 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.

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 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).

See also

• Dicalcium phosphate

References

Further reading

• Havlin, J.L., J.D. Beaton, S.L. Tisdale, and W.L. Nelson. 2005. Soil Fertility and Fertilizers. 7th edn. Pearson Prentice Hall, N.J., ISBN 0130278246