Phosphoric acid (also known as orthophosphoric acid or phosphoric(V) acid) is a weak acid with the chemical formula H3PO4. Orthophosphoric acid refers to phosphoric acid, which is the IUPAC name for this compound. The prefix ortho- is used to distinguish the acid from related phosphoric acids, called polyphosphoric acids. Orthophosphoric acid is a non-toxic acid, which, when pure, is a solid at room temperature and pressure. The conjugate base of phosphoric acid is the dihydrogen phosphate ion, H
2PO−
4, which in turn has a conjugate base of hydrogen phosphate, HPO2−
4, which has a conjugate base of phosphate, PO3−
4. Phosphates are essential for life.[8]
The most common source of phosphoric acid is an 85% aqueous solution; such solutions are colourless, odourless, and non-volatile. The 85% solution is a syrupy liquid, but still pourable. Although phosphoric acid does not meet the strict definition of a strong acid, the 85% solution is acidic enough to be corrosive. Because of the high percentage of phosphoric acid in this reagent, at least some of the orthophosphoric acid is condensed into polyphosphoric acids; for the sake of labeling and simplicity, the 85% represents H3PO4 as if it were all in the ortho form. Dilute aqueous solutions of phosphoric acid exist in the ortho form.
Contents
1 Manufacture
2 Acidic properties
3 Uses
4 Safety
5 See also
6 References
7 External links
Manufacture
Phosphoric acid is produced industrially by two general routes.[9] In the wet process a phosphate-containing mineral such as calcium hydroxyapatite is treated with sulfuric acid.[10]
{\displaystyle {\ce {Ca5(PO4)3OH + 5H2SO4 -> 3H3PO4 + 5CaSO4v + H2O}}} {\displaystyle {\ce {Ca5(PO4)3OH + 5H2SO4 -> 3H3PO4 + 5CaSO4v + H2O}}}
Fluoroapatite is an alternative feedstock, in which case fluoride is removed as the insoluble compound Na2SiF6. The phosphoric acid solution usually contains 23–33% P2O5 (32–46% H3PO4). It may be concentrated to produce commercial- or merchant-grade phosphoric acid, which contains about 54–62% P2O5 (75–85% H3PO4). Further removal of water yields superphosphoric acid with a P2O5 concentration above 70% (corresponding to nearly 100% H3PO4. Calcium sulfate (gypsum) is produced as a by-product and is removed as phosphogypsum.
To produce food-grade phosphoric acid, phosphate ore is first reduced with coke in an electric arc furnace, to make elemental phosphorus. Silica is also added, resulting in the production of calcium silicate slag. Elemental phosphorus is distilled out of the furnace and burned with air to produce high-purity phosphorus pentoxide, which is dissolved in water to make phosphoric acid.