Phosphoric acid, H3PO4 (orthophosphoric acid), is a white crystalline substance which melts at 108°F (42°C). It is most commonly found in aqueous form (dissolved in water), where it forms a colorless, thick liquid. Phosphoric acid is widely used in the manufacturing of phosphate detergents and fertilizers. Because of increased algae growth in lakes with high levels of phosphate in them the use of phosphate detergents has been dramatically curtailed in many areas. Phosphoric acid is added to some foods (especially colas) to give a tart flavor to the final product. Since phosphoric acid can donate three protons (hydrogen ions) to other substances, it is known as a triprotic acid.
Phosphoric acid is a weak acid, with only a small percentage of the molecules in solution ionizing. Phosphoric acid is manufactured by the reaction of sulfuric acid upon phosphate rocks (commonly found in Florida), most notably calcium phosphate, as shown below:
The other product of the reaction, calcium sulfate dihydrate is gypsum and is used in drywall in the construction industry.
In addition to using calcium phosphate as a starting material, fluorapatite Ca5(PO4)3 may also be used. The two processes shown above are known as wet processes, which may give impure phosphoric acid as a product. Much higher levels of purity may be obtained by using the furnace process, in which phosphate containing minerals react with coke and silica at high temperatures. The resulting product is then dissolved in water to produce very pure phosphoric acid.
Alternatively, phosphoric acid may be produced by reacting tetraphosphorous decoxide with water:
Phosphoric acid is used as an acidulant in the food industry (It is the second most common acidulant used, behind citric acid). As an acidulant it serves as a preservative and buffer, provides tartness, and modifies the viscosity (or resistance to flow) of liquids.
When pure phosphoric acid is heated, two molecules may condense (release water from a reaction between them) to form a polyphosphoric acid. Salts of polyphosphoric acids are used in the manufacturing of detergents to help bind calcium and magnesium ions from hard water.