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 H2PO−4, 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.
Production Phosphoric acid is produced industrially by one of two routes, wet processes and dry.
Wet process In the wet process, phosphate-containing minerals such as calcium hydroxyapatite or fluorapatite are treated with sulfuric acid.
Ca5(PO4)3OH + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + H2O Ca5(PO4)3F + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + HF 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.
Dry process 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. However, this process is more expensive and energy-intensive than the wet process, which produces phosphoric acid with a lower concentration of P2O5 (about 26–52%) and a higher level of impurities. The wet process is the most common method of producing phosphoric acid for fertilizer use. Even in China, where the thermal process is still used quite widely due to relatively cheap coal as opposed to the sulfuric acid, over 7/8 of phosphoric acid is produced with wet process.
Purification Phosphoric acid produced from phosphate rock or thermal processes often requires purification. A common purification method is liquid–liquid extraction, which involves the separation of phosphoric acid from water and other impurities using organic solvents, such as tributyl phosphate (TBP), methyl isobutyl ketone (MIBK), or n-octanol. Nanofiltration involves the use of a premodified nanofiltration membrane, which is functionalized by a deposit of a high molecular weight polycationic polymer of polyethyleneimines. Nanofiltration has been shown to significantly reduce the concentrations of various impurities, including cadmium, aluminum, iron, and rare earth elements. The laboratory and industrial pilot scale results showed that this process allows the production of food-grade phosphoric acid. Fractional crystallization can achieve higher purities typically used for semiconductor applications. Usually a static crystallizer is used. A static crystallizer uses vertical plates, which are suspended in the molten feed and which are alternatingly cooled and heated by a heat transfer medium. The process begins with the slow cooling of the heat transfer medium below the freezing point of the stagnant melt. This cooling causes a layer of crystals to grow on the plates. Impurities are rejected from the growing crystals and are concentrated in the remaining melt. After the desired fraction has been crystallized, the remaining melt is drained from the crystallizer. The purer crystalline layer remains adhered to the plates. In a subsequent step, the plates are heated again to liquify the crystals and the purified phosphoric acid drained into the product vessel. The crystallizer is filled with feed again and the next cooling cycle is started.
Properties
Acidic properties In aqueous solution phosphoric acid behaves as a triprotic acid.
H3PO4 ⇌ H2PO−4 + H+, pKa1 = 2.14 H2PO−4 ⇌ HPO2−4 + H+, pKa2 = 7.20 HPO2−4 ⇌ PO3−4 + H+, pKa3 = 12.37 The difference between successive pKa values is sufficiently large so that salts of either monohydrogen phosphate, HPO2−4 or dihydrogen phosphate, H2PO−4, can be prepared from a solution of phosphoric acid by adjusting the pH to be mid-way between the respective pKa values.
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