Iron(III) phosphate, also ferric phosphate, is the inorganic compound with the formula FePO4. Several related materials are known, including four polymorphs of FePO4 and two polymorphs of the dihydrate FePO4·(H2O)2. These materials find several technical applications as well as occurring in the mineral kingdom.
Ferric orthophosphate, Ferric phosphate
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||150.815 g/mol (anhydrous)|
|Density||3.056 g/cm3 (anhydrous)|
2.87 g/cm3 (20 °C, dihydrate)
|Melting point|| 250 °C (482 °F; 523 K) |
0.642 g/100 mL (100 °C)
Heat capacity (C)
|180.5 J/mol·K (dihydrate)|
|171.3 J/mol·K (dihydrate)|
Std enthalpy of
|-1888 kJ/mol (dihydrate)|
|GHS Signal word||Warning|
|H315, H319, H335|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
The most common form of FePO4 adopts the structure of α-quartz. As such the P and Fe have tetrahedral molecular geometry. At high pressures, a phase change occurs to a more dense structure with octahedral Fe centres. Two orthorhombic structures and a monoclinic phase are also known. In the two polymorphs of the dihydrate, the Fe centre is octahedral with two mutually cis water ligands.
Iron(III) phosphate can be used in steel and metal manufacturing processes. When bonded to a metal surface, iron phosphate prevents further oxidation of the metal. Its presence is partially responsible for the corrosion resistance of the Iron pillar of Delhi.
Iron phosphate coatings are commonly used in preparation for painting or powder coating, in order to increase adhesion to the iron or steel substrate, and prevent corrosion, which can cause premature failure of subsequent coating processes. It can also be used for bonding fabrics, wood, and other materials to iron or steel surfaces.
Anhydrous iron phosphate has been investigated as an intercalation electrode in a lithium-ion battery despite having low electronic conductivity.
Pesticide pellets containing iron phosphate plus a chelating agent, such as EDTA, leach heavy metals from soil into groundwater. The Research Institute of Organic Agriculture (FiBL) reported the EDTA content and stated products were likely to be no safer than metaldehyde baits. Ferric phosphate slug and snail baits marketed in the U.S. contain EDTA.
Iron(III) phosphate is not allowed as food additive in the European Union. It was withdrawn from the list of allowed substances in the directive 2002/46/EC in 2007.
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