Aluminium phosphate

Aluminium phosphate (AlPO4) is a chemical compound. It is used in cake mixes and in some baking powders as a leavening agent to help baked goods rise.

It is also used industrially as a high-temperature dehydrating agent. When strongly heated, aluminium phosphate decomposes into aluminium oxide and phosphorus pentoxide, the latter of which is very effective at absorbing water. Thus, aluminium phosphate can be used to dehydrate acetic acid through the following series of reactions:


 * 4AlPO4 = 2Al2O3 + P4O10
 * 6CH3COOH + P4O10 = 4H3PO4 + 6CH2CO (ketene)
 * CH2CO + CH3COOH = (CH3CO)2O (acetic anhydride)

Occurrence
Although it is unstable to heat, in geologically stable regions like Australia and Southern Africa, alumiunium phosphate is generally the most common form of phosphorus in soils, as it forms when phosphate in rainwater reacts with dissolved aluminium in the soil. Although it is not as insoluble as many other components of soils in those regions, aluminium phosphate can form exceedingly insoluble double salts known as taranakites with many essential elements for plant growth that are normally highly soluble in water, such as potassium and nitrogen.

In coastal areas of South Africa and Namibia, the combination of exceedingly old soils and a high input of phosphate from very fertile oceans due to the Benguela Current causes aluminium phosphate to accumulate to form a concentrated mineral, known as aluminium phosphate rock. Only in recent years has there been any interest in the mining of these considerable deposits as a source of phosphorus for agriculture, but there could be considerable economic value if low-cost treatment becomes possible.