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|Section2= |Section3= |Section4= |Section7= |Section8= }} Phosphoric acid (also known as orthophosphoric acid or phosphoric(V) acid) is a mineral (inorganic) acid having the chemical formula H3PO4. Orthophosphoric acid refers to ''phosphoric acid'', which is the IUPAC name for this compound. The prefix ''ortho'' is used to distinguish the acid from related phosphoric acids, called polyphosphoric acids. Orthophosphoric acid is a non-toxic acid, which, when pure, is a solid at room temperature and pressure. The conjugate base of phosphoric acid is the dihydrogen phosphate ion, , which in turn has a conjugate base of hydrogen phosphate, , which has a conjugate base of phosphate, . Phosphates are nutritious for all forms of life. In addition to being a chemical reagent, phosphoric acid has a wide variety of uses, including as a rust inhibitor, food additive, dental and orthop(a)edic etchant, electrolyte, flux, dispersing agent, industrial etchant, fertilizer feedstock, and component of home cleaning products. Phosphoric acids and phosphates are also important in biology. The most common source of phosphoric acid is an 85% aqueous solution; such solutions are colourless, odourless, and non-volatile. The 85% solution is a syrupy liquid, but still pourable. As a strong acid, it is corrosive. Because of the high percentage of phosphoric acid in this reagent, at least some of the orthophosphoric acid is condensed into polyphosphoric acids. For the sake of labeling and simplicity, the 85% represents H3PO4 as if it were all orthophosphoric acid. Dilute aqueous solutions of phosphoric acid exist in the ortho- form. ==Reactions== Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way. Anhydrous phosphoric acid, a white low melting solid, is obtained by dehydration of 85% phosphoric acid by heating under a vacuum.〔Klement, R. (1963) "Orthophosphoric Acid" in ''Handbook of Preparative Inorganic Chemistry'', 2nd ed., G. Brauer (ed.), Academic Press, NY. Vol. 1. p. 543.〕 Orthophosphoric acid ionizes upon dissolving in water, mainly to give H2PO4- and protons: :H3PO4(s) + H2O(l) H3O+(aq) + H2PO4−(aq) ''K''a1= 7.25×10−3 :H2PO4−(aq)+ H2O(l) H3O+(aq) + HPO42−(aq) ''K''a2= 6.31×10−8 :HPO42−(aq)+ H2O(l) H3O+(aq) + PO43−(aq) ''K''a3= 4.80×10−13 The anion after the first dissociation, H2PO4−, is the ''dihydrogen phosphate'' anion. The anion after the second dissociation, HPO42−, is the ''hydrogen phosphate'' anion. The anion after the third dissociation, PO43−, is the phosphate or orthophosphate anion. For each of the dissociation reactions shown above, there is a separate acid dissociation constant, called ''K''a1, ''K''a2, and ''K''a3 given at 25 °C. Associated with these three dissociation constants are corresponding p''K''a1=2.12, p''K''a2=7.21, and p''K''a3=12.67 values at 25 °C. Even though all three hydrogen (H) atoms are equivalent on an orthophosphoric acid molecule, the successive ''K''a values differ since it is energetically less favorable to lose another H+ if one (or more) has already been lost and the molecule/ion is more negatively charged. Because the triprotic dissociation of orthophosphoric acid, the fact that its conjugate bases (the phosphates mentioned above) cover a wide pH range, and, because phosphoric acid/phosphate solutions are, in general, non-toxic, mixtures of these types of phosphates are often used as buffering agents or to make buffer solutions, where the desired pH depends on the proportions of the phosphates in the mixtures. Similarly, the non-toxic, anion salts of triprotic organic citric acid are also often used to make buffers. Phosphates are found pervasively in biology, especially in the compounds derived from phosphorylated sugars, such as DNA, RNA, and adenosine triphosphate (ATP). There is a separate article on phosphate as an anion or its salts. Upon heating orthophosphoric acid, condensation of the phosphoric units can be induced by driving off the water formed from condensation. When one molecule of water has been removed for each two molecules of phosphoric acid, the result is pyrophosphoric acid (H4P2O7). When an average of one molecule of water per phosphoric unit has been driven off, the resulting substance is a glassy solid having an empirical formula of HPO3 and is called metaphosphoric acid.〔(phosphoric acid ). The Columbia Encyclopedia, Sixth Edition.〕 Metaphosphoric acid is a singly anhydrous version of orthophosphoic acid and is sometimes used as a water- or moisture-absorbing reagent. Further dehydrating is very difficult, and can be accomplished only by means of an extremely strong desiccant (and not by heating alone). It produces ''phosphoric anhydride'' (phosphorus pentoxide), which has an empirical formula P2O5, although an actual molecule has a chemical formula of P4O10. Phosphoric anhydride is a solid, which is very strongly moisture-absorbing and is used as a desiccant. In the presence of superacids (acids stronger than ), reacts to form mystery products, perhaps corrosive, acidic salts of the hypothetical tetrahydroxylphosphonium ion, which is isoelectronic with orthosilicic acid. The suspected reaction with , for example, is supposed to go: :H3PO4 + → () ()− 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Phosphoric acid」の詳細全文を読む スポンサード リンク
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