Phosphoric acid

Phosphoric acid is produced industrially by two general routes.[14] In the wet process a phosphate-containing mineral such as calcium hydroxyapatite is treated with sulfuric acid.[15]

${\displaystyle {\ce {Ca5(PO4)3OH + 5H2SO4 -> 3H3PO4 + 5CaSO4v + H2O}}}$

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.

The phosphoric acid from both processes may be further purified by removing compounds of arsenic and other potentially toxic impurities.

All three hydrogens are acidic, with dissociation constants pK_a1 = 2.14, pK_a2 = 7.20, and pK_a3 = 12.37. It follows that, in water solutions, phosphoric acid is mostly dissociated into some combination of its three anions, except at very low pH. The equilibrium equations are:

H₃PO₄   + H₂O ⇌ H₃O⁺ + H₂PO₄⁻      K_a1= 7.25×10⁻³ [pK_a1 = 2.14]

H₂PO₄⁻+ H₂O ⇌ H₃O⁺ + HPO₄²⁻       K_a2= 6.31×10⁻⁸ [pK_a2 = 7.20]

HPO₄²⁻+ H₂O ⇌ H₃O⁺ +  PO₄³⁻        K_a3= 3.98×10⁻¹³ [pK_a3 = 12.37]

The dominant use of phosphoric acid is for fertilizers, consuming approximately 90% of production.[16]

Food-grade phosphoric acid (additive E338[17]) is used to acidify foods and beverages such as various colas and jams, providing a tangy or sour taste. Soft drinks containing phosphoric acid, which would include Coca-Cola, are sometimes called phosphate sodas or phosphates. Phosphoric acid in soft drinks has the potential to cause dental erosion.[18] Phosphoric acid also has the potential to contribute to the formation of kidney stones, especially in those who have had kidney stones previously.[19]

Specific applications of phosphoric acid include:

• In anti-rust treatment by phosphate conversion coating or passivation
• As an external standard for phosphorus-31 nuclear magnetic resonance.
• In phosphoric acid fuel cells.
• In activated carbon production.[20]
• In compound semiconductor processing, to etch Indium gallium arsenide selectively with respect to indium phosphide.[21]
• In microfabrication to etch silicon nitride selectively with respect to silicon dioxide.[22]
• As a pH adjuster in cosmetics and skin-care products.[23]
• As a sanitizing agent in the dairy, food, and brewing industries.[24]

A link has been shown between long-term regular cola intake and osteoporosis in later middle age in women (but not men).[25] This was thought to be due to the presence of phosphoric acid, and the risk for women was found to be greater for sugared and caffeinated colas than diet and decaffeinated variants, with a higher intake of cola correlating with lower bone density.

At moderate concentrations phosphoric acid solutions are irritating to the skin. Contact with concentrated solutions can cause severe skin burns and permanent eye damage.[26]

• Phosphate fertilizers, such as ammonium phosphate fertilizers

• Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. ISBN 978-1439855119.

Wikimedia Commons has media related to phosphoric acid.

• National pollutant inventory – Phosphoric acid fact sheet
• NIOSH Pocket guide to chemical hazards