Phosphoric Acid
Phosphoric acid, also termed orthophosphoric acid, is a substance with the chemical formula H3PO4. At high concentrations in aqueous solution, it is a relatively weak acid. The pH value at which a one molar solution exists as half orthophosphoric acid and half dihydrogen phosphate ion, H2PO4-, the pK1, is 2.15. At a pH of 7.20, the pK2 a one molar solution of phosphoric acid exists as equal concentrations of dihydrogen phosphate ion and monohydrogen phosphate ion, H2PO42- and at pH 12.37, the pK3 the solution consists of equal concentrations of monohydrogen phosphate and phosphate ion, PO43-. At lower concentrations, a greater proportion of deprotonation occurs. Salts of all three anions can be prepared by using solutions of appropriate pH. Phosphoric acid is not an oxidizing acid: the phosphorus atom in the phosphate ion is fully oxidized.
Orthophosphoric acid is produced in quantities of millions of tons each year, principally from a process in which rocks containing phosphate minerals such as calcium fluoride orthophosphate, Ca5(PO4)3F, are crushed and dissolved in aqueous sulfuric acid solutions to produce solutions of phosphoric acid and, in this case, hydrofluoric acid, leaving behind insoluble portions of the rock and gypsum (hydrated calcium sulfate, CaSO4.2H20). Most of the phosphoric acid that is commercially produced is converted to salts used in fertilizers. Much purer solutions of orthophosphoric acid are produced by roasting phosphorpentoxide or phosphorus (IV) oxide, P4O10, in the presence of water. This process is used to make the orthophosphoric acid used in soft drinks (the major use), detergents, and other consumer items. Phosphoric acid gives soft drinks a slightly tart taste.
Phosphoric acid and its phosphate derivatives are essential in human nutrition. The so-called "backbone" of DNA as well as the linkages in RNA are phosphate diesters. The "energy-storage" molecule prevalent in all living systems is adenosine triphosphate (ATP). Adenosine triphosphate is the nucleotide adenine found in DNA and RNA with an extended phosphate chain. When the terminal unit of the triphosphate chain is hydrolyzed in a cell to form adenosine diphosphate (ADP) and phosphate ion energy is released due to hydration of the phosphate ion. Energy is stored when ADP and hydrated phosphate ion are converted by a coupled biochemical reaction to form ATP. A common misconception is that there is a weak phosphate bond in ATP. Not so, the energy is released and absorbed in hydrating and dehydrating the highly charged phosphate ion.
Industrially, phosphoric acid and its derivatives are used in metal cleaning and treatment. The phosphate ion is strongly attracted to metal ions, particularly 2+ and 3+ ions, due to its high negative charge, so it is good at removing ferric and ferrous ions in rust, calcium and magnesium salts that are impurities and contaminants, and so forth. It is also used as a catalyst in the production of ethylene and in the manufacture of numerous commercial products.
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