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Phosphoric acid

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Phosphoric acid

Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula H3PO4. It is commonly encountered as an 85% aqueous solution, which is a colourless, odourless, and non-volatile syrupy liquid. It is a major industrial chemical, being a component of many fertilizers.

The compound is an acid. Removal of all three H+ ions gives the phosphate ion PO3−4. Removal of one or two protons gives dihydrogen phosphate ion H2PO4, and the hydrogen phosphate ion HPO2−4, respectively. Phosphoric acid forms esters, called organophosphates.

The name "orthophosphoric acid" can be used to distinguish this specific acid from other "phosphoric acids", such as pyrophosphoric acid. Nevertheless, the term "phosphoric acid" often means this specific compound; and that is the current IUPAC nomenclature.

Phosphoric acid is produced industrially by one of two routes, wet processes and dry.

In the wet process, phosphate-containing minerals such as calcium hydroxyapatite or fluorapatite are treated with sulfuric acid.

By-products include calcium sulfate (CaSO4) and hydrogen fluoride (HF). The HF gas may be recovered by streaming it into a wet (water) scrubber producing hydrofluoric acid. CaSO4 is better known as gypsum, which is commonly used in the construction industry, however the CaSO4 produced from phosphoric acid production can contain trace levels of radioactive elements such as radium. This makes it unsuitable for commercial use, and it is called phosphogypsum to distinguish it. It is typically stored indefinitely.

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

To produce food-grade phosphoric acid, phosphate ore is first reduced with coke in an electric arc furnace, to give elemental phosphorus. This process is also known as the thermal process or the electric furnace process. 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 thermal process produces phosphoric acid with a very high concentration of P2O5 (about 85%) and a low level of impurities.

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