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Phosphoric acid is commonly encountered in chemical laboratories as an 85% aqueous solution, which is a colourless, odourless, and non-volatile syrupy liquid. Although phosphoric acid does not meet the strict definition of a strong acid, the 85% solution can still severely irritate the skin and damage the eyes.
Phosphoric acid is produced industrially by two general routes. In the wet process a phosphate-containing mineral such as calcium hydroxyapatite is treated with sulfuric acid.
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 pKa1 = 2.14, pKa2 = 7.20, and pKa3 = 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:
Food-grade phosphoric acid (additive E338) is used to acidify foods and beverages such as various colas and jams, providing a tangy or sour taste. The phosphoric acid also serves as a preservative. 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. Phosphoric acid also has the potential to contribute to the formation of kidney stones, especially in those who have had kidney stones previously.
Christensen, J. H. & Reed, R. B. (1955). "Design and Analysis Data--Density of Aqueous Solutions of Phosphoric Acid Measurements at 25 °C". Ind. Eng. Chem. 47 (6): 1277-1280. doi:10.1021/ie50546a061.