Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorlessphosphorus-containingsolid, andinorganic compound with thechemical formulaH3PO4. 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 name "orthophosphoric acid" can be used to distinguish this specific acid from other "phosphoric acids", such aspyrophosphoric acid. Nevertheless, the term "phosphoric acid" often means this specific compound; and that is the currentIUPAC nomenclature.
By-products includecalcium sulfate (CaSO4) andhydrogen fluoride (HF). The HF gas may be recovered by streaming it into awet (water) scrubber producinghydrofluoric acid.CaSO4 is better known as gypsum, which is commonly used in the construction industry, however theCaSO4 produced from phosphoric acid production can contain trace levels of radioactive elements such asradium. This makes it unsuitable for commercial use, and it is calledphosphogypsum 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 producecommercial- ormerchant-grade phosphoric acid, which contains about 54–62%P2O5 (75–85%H3PO4). Further removal of water yieldssuperphosphoric acid with aP2O5 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 withcoke in anelectric arc furnace, to give elementalphosphorus. This process is also known as the thermal process or the electric furnace process. Silica is also added, resulting in the production ofcalcium silicate slag. Elemental phosphorus is distilled out of the furnace and burned with air to produce high-purityphosphorus pentoxide, which is dissolved in water to make phosphoric acid.[22] The thermal process produces phosphoric acid with a very high concentration ofP2O5 (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 ofP2O5 (about 26–52%) and a higher level of impurities. The wet process is the most common method of producing phosphoric acid for fertilizer use.[23] 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.[24]
Phosphoric acid produced fromphosphate rock or thermal processes often requires purification. A common purification method isliquid–liquid extraction, which involves the separation of phosphoric acid from water and other impurities using organic solvents, such astributyl phosphate (TBP),methyl isobutyl ketone (MIBK), orn-octanol.Nanofiltration involves the use of a premodified nanofiltration membrane, which is functionalized by a deposit of a high molecular weight polycationicpolymer ofpolyethyleneimines. Nanofiltration has been shown to significantly reduce the concentrations of various impurities, includingcadmium,aluminum,iron, andrare earth elements. The laboratory and industrial pilot scale results showed that this process allows the production of food-grade phosphoric acid.[25]
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.[26]
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 successivepKa 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 thepH to be mid-way between the respective pKa values.
Aqueous solutions up to 62.5%H3PO4 areeutectic, exhibiting freezing-point depression as low as −85 °C. When the concentration of acid rises above 62.5% the freezing-point increases, reaching 21 °C by 85%H3PO4 (w/w; themonohydrate). Beyond this thephase diagram becomes complicated, with significant local maxima and minima. For this reason phosphoric acid is rarely sold above 85%, as beyond this adding or removing small amounts of moisture risks the entire mass freezing solid, which would be a major problem on a large scale. A local maximum at 91.6% which corresponds to thehemihydrate 2H3PO4•H2O, freezing at 29.32 °C.[27][28] There is a second smaller eutectic depression at a concentration of 94.75% with a freezing point of 23.5 °C. At higher concentrations the freezing point rapidly increases. Concentrated phosphoric acid tends tosupercool before crystallization occurs, and may be relatively resistant to crystallisation even when stored below the freezing point.[13]
Phosphoric acid is commercially available as aqueous solutions of various concentrations, not usually exceeding 85%. If concentrated further it undergoes slow self-condensation, forming an equilibrium withpyrophosphoric acid:
2 H3PO4 ⇌ H2O + H4P2O7
Even at 90% concentration the amount of pyrophosphoric acid present is negligible, but beyond 95% it starts to increase, reaching 15% at what would have otherwise been 100% orthophosphoric acid.[29]
As the concentration is increasedhigher acids are formed, culminating in the formation ofpolyphosphoric acids.[30] It is not possible to fully dehydrate phosphoric acid tophosphorus pentoxide, instead the polyphosphoric acid becomes increasingly polymeric and viscous. Due to the self-condensation, pure orthophosphoric acid can only be obtained by a careful fractional freezing/melting process.[13][12]
Food-grade phosphoric acid (additiveE338[32]) is used to acidify foods and beverages such as variouscolas and jams, providing a tangy or sour taste. The phosphoric acid also serves as apreservative.[33] Soft drinks containing phosphoric acid, which would includeCoca-Cola, are sometimes calledphosphate sodas or phosphates. Phosphoric acid in soft drinks has the potential to cause dental erosion.[34] Phosphoric acid also has the potential to contribute to the formation ofkidney stones, especially in those who have had kidney stones previously.[35]
Phosphoric acid is not astrong acid. However, at moderate concentrations phosphoric acid solutions are irritating to the skin. Contact with concentrated solutions can cause severe skin burns and permanent eye damage.[42]
^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.
^Edwards, O. W.; Dunn, R. L.; Hatfield, J. D. (1964). "Refractive Index of Phosphoric Acid Solutions at 25 C.".J. Chem. Eng. Data.9 (4):508–509.doi:10.1021/je60023a010.
^abGreenwood, N. N.; Thompson, A. (1959). "701. The mechanism of electrical conduction in fused phosphoric and trideuterophosphoric acids".Journal of the Chemical Society (Resumed): 3485.doi:10.1039/JR9590003485.
^Ross, William H.; Jones, Russell M. (August 1925). "The Solubility and Freezing-Point Curves of Hydrated and Anhydrous Orthophosphoric Acid".Journal of the American Chemical Society.47 (8):2165–2170.Bibcode:1925JAChS..47.2165R.doi:10.1021/ja01685a015.
^Jameson, R. F. (1 January 1959). "151. The composition of the "strong" phosphoric acids".Journal of the Chemical Society (Resumed):752–759.doi:10.1039/JR9590000752.
^Qaseem, A; Dallas, P; Forciea, MA; Starkey, M; et al. (4 November 2014). "Dietary and pharmacologic management to prevent recurrent nephrolithiasis in adults: A clinical practice guideline from the American College of Physicians".Annals of Internal Medicine.161 (9):659–67.doi:10.7326/M13-2908.PMID25364887.S2CID3058172.
