Phosphoric acid

Phosphoric acid
Other namesOrthophosphoric acid
CAS number7664-38-2
Molecular formulaH3PO4
Molar mass98.0
Appearancewhite solid or
colourless, viscous liquid (>42°C)
Density1.685 g/ml (liquid)
Melting point 42.35 °C, 107.6°F, 567.27°R
Boiling point 158 °C, 415.4°F, 875.1°R decomp.
Acidity (pKa)2.12, 7.21, 12.67
Viscosity85% aqueous solution
? cP
EU classificationCorrosive(C)
S-phrasesS1/2, S26, S45
Related Compounds
Other anionsNitric acid
Arsenic acid
Other cationsAmmonium phosphate
Trisodium phosphate
Related Phosphorus acidHypophosphorous acid
Phosphorous acid
Pyrophosphoric acid
Tripolyphosphoric acid
Hypophosphoric acid
Perphosphoric acid
Permonophosphoric acid
Except where noted otherwise, data are given for
materials in their standard state
(at 25 C, 100 kPa)

Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a mineral (inorganic) acid having the chemical formula H3PO4. Alternatively, orthophosphoric acid molecules can combine with themselves to form a variety of compounds referred to as phosphoric acids in a more general way. The term "phosphoric acid" can also refer to a chemical or reagent consisting of phosphoric acids, usually mostly orthophosphoric acid.

Orthophosphoric acid chemistry

Pure anhydrous phosphoric acid is a white solid that melts at 42.35 °C to form a colorless, viscous liquid.

Most people and even chemists simply refer to orthophosphoric acid as "phosphoric acid", which is the IUPAC name for this compound. The prefix ortho- usually is used when one wants to distinguish it from other phosphoric acids called polyphosphoric acids. Orthophosphoric acid is a non-toxic, inorganic, rather weak triprotic acid which, when pure, is a solid at room temperature and pressure. The chemical structure of orthophosphoric acid is shown above in the data table. Orthophosphoric acid is a very polar molecule, therefore it is highly soluble in water. The oxidation state of phosphorus (P) in ortho- and other phosphoric acids is +5; the oxidation state of all the oxygens (O) is -2 and all the hydrogens (H) is +1. Triprotic means that an orthophosphoric acid molecule can dissociate up to three times, giving up an H+ each time, which typically combines with a water molecule, H2O, as shown in these reactions:

H3PO4(s)   + H2O(l) H3O+(aq) + H2PO4(aq)       Ka1= 7.5×10−3

H2PO4(aq)+ H2O(l) H3O+(aq) + HPO42–(aq)       Ka2= 6.2×10−8

HPO42–(aq)+ H2O(l) H3O+(aq) +  PO43–(aq)        Ka3= 2.14×10−13

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 Ka1, Ka2, and Ka3 given at 25°C. Associated with these three dissociation constants are corresponding pKa1=2.12 , pKa2=7.21 , and pKa3=12.67 values at 25°C. Even though all three hydrogen (H ) atoms are equivalent on an orthophosphoric acid molecule, the successive Ka 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 generally 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 and 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.[1] 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 only be accomplished by means of an extremely strong desiccant (and not by heating alone). It produces phosphoric anhydride 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-aborbing and is used as a desiccant.

Phosphoric acid as a chemical reagent

Pure 75-85% aqueous solutions (the most common) are clear, colourless, odourless, non-volatile, rather viscous, syrupy liquids, but still pourable. Phosphoric acid is very commonly used as an aqueous solution of 85% phosphoric acid or H3PO4. Because it is a concentrated acid, an 85% solution can be corrosive, although not toxic when diluted. Because of the high percentage of phosphoric acid in this reagent, at least some of the orthophosphoric acid is condensed into polyphosphoric acids in a temperature-dependent equilibrium, but for the sake of labeling and simplicity, the 85% represents H3PO4 as if it were all orthophosphoric acid. Other percentages are possible too, even above 100%, where the phosphoric acids and water would be in an unspecified equilibrium, but the overall elemental mole content would be considered specified. When aqueous solutions of phosphoric acid and/or phosphate are dilute, they are in or will reach an equilibrium after a while where practically all the phosphoric/phosphate units are in the ortho- form.

Preparation of hydrogen halides

Phosphoric acid reacts with halides to form the corresponding hydrogen halide gas (steamy fumes are observed on warming the reaction mixture). This is a common practice for the laboratory preparation of hydrogen halides.

3NaCl(s) + H3PO4(l) → NaH2PO4(s) + HCl(g)
3NaBr(s) + H3PO4(l) → NaH2PO4(s) + HBr(g)
3NaI(s) + H3PO4(l) → NaH2PO4(s) + HI(g)

Rust removal

Phosphoric acid may be used by direct application to rusted iron, steel tools or surfaces to convert iron(III) oxide (rust) to a water soluble phosphate compound. It is usually available as a greenish liquid, suitable for dipping (acid bath), but is more generally used as a component in a gel, commonly called naval jelly. As a thick gel, it may be applied to sloping, vertical, or even overhead surfaces. Care must be taken to avoid acid burns of the skin and especially the eyes, but the residue is easily diluted with water. When sufficiently diluted it can even be nutritious to plant life, containing the essential nutrients phosphorus and iron. It is sometimes sold under other names, such as "rust remover" or "rust killer". It should not be directly introduced into surface water such as creeks or into drains, however. After treatment, the reddish-brown iron oxide will be converted to a black iron phosphate compound coating that may be scrubbed off. Multiple applications of phosphoric acid may be required to remove all rust. The resultant black compound can provide further corrosion resistance (such protection is somewhat provided by the superficially similar Parkerizing and blued electrochemical conversion coating processes.) After application and removal of rust using phosphoric acid compounds, the metal should be oiled (if to be used bare, as in a tool) or appropriately painted, most durably by using a multiple coat process of primer, intermediate, and finish coats.

Processed food use

Food grade phosphoric acid is used to acidify foods and beverages such as various colas, but not without controversy as to its health effects. It provides a tangy taste, and being a mass-produced chemical, is available cheaply and in large quantities. The low cost and bulk availability is unlike more expensive natural seasonings that give comparable flavors, such as ginger for tangyness, or citric acid for sourness, obtainable from lemons and limes. (However most citric acid in the food industry is not extracted from citrus fruit, but fermented by Aspergillus niger mold from scrap molasses, waste starch hydrolysates and phosphoric acid.) It is labeled as E number E338.

Biological effects on bone calcium

Phosphoric acid, used in many soft drinks (primarily cola), has been linked to lower bone density in epidemiological studies. For example a study[2] using dual-energy X-ray absorptiometry rather than a questionnaire about breakage, provides reasonable evidence to support the theory that drinking cola results in lower bone density. This study was published in the American Journal of Clinical Nutrition. A total of 1672 women and 1148 men were studied between 1996 and 2001. Dietary information was collected using a food frequency questionnaire that had specific questions about the number of servings of cola and other carbonated beverages and that also made a differentiation between regular, caffeine-free, and diet drinks. The paper finds statistically significant evidence to show that women who consume cola daily have lower bone density. Total phosphorus intake was not significantly higher in daily cola consumers than in nonconsumers; however, the calcium-to-phosphorus ratios were lower. The study also suggests that further research is needed to confirm the findings.

On the other hand, a study funded by Pepsi suggests that low intake of phosphorus leads to lower bone density. The study does not examine the effect of phosphoric acid, which binds with magnesium and calcium in the digestive tract to form salts that are not absorbed, but rather, it studies general phosphorus intake.[3]

However, a well-controlled clinical study by Heaney and Rafferty using calcium-balance methods found no impact of carbonated soft drinks containing phoshporic acid on calcium excretion.[4] The study compared the impact of water, milk and various soft drinks (two with caffeine and two without; two with phosphoric acid and two with citric acid) on the calcium balance of 20- to 40-year-old women who customarily consumed ~3 or more cups (680 ml) of a carbonated soft drink per day. They found that, relative to water, only milk and the two caffeine-containing soft drinks increased urinary calcium, and that the calcium loss associated with the caffeinated soft drink consumption was about equal to that previously found for caffeine alone. Phosphoric acid without caffeine had no impact on urine calcium, nor did it augment the urinary calcium loss related to caffeine. Because studies have shown that the effect of caffeine is compensated for by reduced calcium losses later in the day[5], Heaney and Rafferty concluded that the net effect of carbonated beverages – including those with caffeine and phosphoric acid -- is negligible and that the skeletal effects of carbonated soft drink consumption are likely due primarily to milk displacement.

Other chemicals such as caffeine (also a significant component of popular common cola drinks) were also suspected as possible contributors to low bone density, due to the known effect of caffeine on calciuria. One other study, comprised of 30 women over the course of a week suggests that phosphoric acid in colas has no such effect, and postulates that caffeine has only a temporary effect which is later reversed. The authors of this study conclude that the skeletal effects of carbonated beverage consumption are likely due primarily to milk displacement.[6] (Another possible confounding factor may be an association between high soft drink consumption and sedentary lifestyle.)

Medical use

Phosphoric acid is used in dentistry and orthodontics as an etching solution, to clean and roughen the surfaces of teeth where dental appliances or fillings will be placed. Phosphoric acid is also an ingredient in over the counter anti-nausea medications which also contain high levels of sugar (glucose and fructose). It should not be used by diabetics without consultation with a doctor. Phosphoric acid is also used as a catalyst in the synthesis of aspirin because it provides a larger number of hydrogen ions with less contamination when compared to hydrochloric acid and sulfuric acid.[7]

Preparation of phosphoric acid

Phosphoric acid can be prepared by two routes - the Thermal Process and the Wet Process.

Thermal phosphoric acid: This very pure phosphoric acid is obtained by burning elemental phosphorus to produce phosphorus pentoxide and dissolving the product in dilute phosphoric acid. This produces a very pure phosphoric acid, since most impurities present in the rock have been removed when extracting Phosphorus from the rock in a furnace. The end result is food grade, thermal phosphoric acid; however, for critical applications additional processing to remove arsenic compounds may be needed.

Wet phosphoric acid: Wet process phosphoric acid is prepared by adding sulfuric acid to calcium phosphate rock.

The simplified reaction is:
3 H2SO4 + Ca3(PO4)2 + 6 H2O ↔ 2 H3PO4 + 3 CaSO4.2H2O + 6 H2O

Wet process acid can be purified by removing fluorine to produce animal grade phosphoric acid or by solvent extraction and arsenic removal to produce food grade phosphoric acid.

Other applications

It is used as the electrolyte in phosphoric-acid fuel cells. It is also used as an external standard for phosphorus-31 nuclear magnetic resonance (NMR).

Phosphoric acid is used as a cleaner by construction trades to remove mineral deposits, cementitious smears, and hard water stains. It is also used as an ingredient in some household cleaners aimed at similar cleaning tasks.

Hot phosphoric acid is used in microfabrication to etch silicon nitride (Si3N4). It is highly selective in etching Si3N4 instead of SiO2, silicon dioxide.

Phosphoric acid is used as a flux by hobbyists (such as model railroaders) as an aid to soldering.

Phosphoric acid is also used in hydroponics pH solutions to lower the pH of nutrient solutions. While other types of acids can be used, phosphorus is a nutrient used by plants, especially during flowering, making phosphoric acid particularly desirable. General Hydroponics pH Down liquid solution contains phosphoric acid in addition to citric acid and ammonium bisulfate with buffers to maintain a stable pH in the nutrient reservoir.

Phosphoric acid is used as a pH adjuster in cosmetics and skin care products.[1]

Phosphoric acid is used as a chemical oxidizing agent for activated carbon production[8].


1. ^ phosphoric acid. The Columbia Encyclopedia, Sixth Edition. 2001-05
2. ^ Tucker et al. Am. J Clin. Nut. Oct 2006. Colas, but not other carbonated beverages, are associated with low bone mineral density in older women: The Framingham Osteoporosis Study.
3. ^ Elmståhl S, Gullberg B, Janzon L, et al. Increased incidence of fractures in middle-aged and elderly men with low intakes of phosphorus and zinc. Osteoporos Int 1998;8:333–40.
4. ^ Robert P Heaney and Karen Rafferty, Carbonated beverages and urinary calcium excretion, American Journal of Clinical Nutrition, Vol. 74, No. 3, 343-347, September 2001
5. ^ Barger-Lux MJ, Heaney RP, Stegman MR.[2], Effects of moderate caffeine intake on the calcium economy of premenopausal women. American Journal of Clinical Nutrition, Vol. 52:722–725
6. ^ Robert P Heaney and Karen Rafferty, Carbonated beverages and urinary calcium excretion, American Journal of Clinical Nutrition, Vol. 74, No. 3, 343-347, September 2001
7. ^ Abdullah Rathur
8. ^ C. Toles, S. Rimmera and J. C. Hower (1996). "Production of activated carbons from a Washington lignite using phosphoric acid activation". Carbon 34 (11): 1419-1426. DOI:10.1016/S0008-6223(96)00093-0. 

External links

This article compares various kinds of phosphoric acids and phosphates.

Orthophosphoric acid

The simplest compound of a series of phosphoric acids is sometimes called by its common name, orthophosphoric acid, but more often called by its IUPAC name, simply
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CAS registry numbers are unique numerical identifiers for chemical compounds, polymers, biological sequences, mixtures and alloys. They are also referred to as CAS numbers, CAS RNs or CAS #s.
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A chemical formula is a concise way of expressing information about the atoms that constitute a particular chemical compound. A chemical formula is also a short way of showing how a chemical reaction occurs.
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Molar mass, symbol M,[1] is the mass of one mole of a substance (chemical element or chemical compound).[2] It is a physical property which is characteristic of each pure substance.
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In physics, density is mass m per unit volume V—how heavy something is compared to its size. A small, heavy object, such as a rock or a lump of lead, is denser than a lighter object of the same size or a larger object of the same weight, such as pieces of
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The melting point of a crystalline solid is the temperature range at which it changes state from solid to liquid. Although the phrase would suggest a specific temperature and is commonly and incorrectly used as such in most textbooks and literature, most crystalline compounds
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boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.[1][2][3][4]
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An acid dissociation constant, denoted by Ka, is an equilibrium constant for the dissociation of a weak acid. According to the Brønsted-Lowry theory of acids and bases an acid is only recognised by its reaction with a base.
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Viscosity is a measure of the resistance of a fluid to deform under either shear stress or extensional stress. It is commonly perceived as "thickness", or resistance to flow.
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The poise (symbol P; IPA: /pwɑːz/) is the unit of dynamic viscosity in the centimetre gram second system of units. It is named after Jean Louis Marie Poiseuille.
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Council Directive 67/548/EEC of 27 June 1967 on the approximation of laws, regulations and administrative provisions relating to the classification, packaging and labelling of dangerous substances (as amended) is the main European Union law concerning chemical safety.
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A corrosive substance is one that will destroy or irreversibly damage a substance, including living tissue, by chemical action (rapid corrosion of living tissue). The main hazards to people include damage to eyes, skin and tissue under the skin, but inhalation or ingestion of a
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R-phrases (short for Risk Phrases) are defined in Annex III of European Union Directive 67/548/EEC: Nature of special risks attributed to dangerous substances and preparations.
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S-phrases are defined in Annex IV of European Union Directive 67/548/EEC: Safety advice concerning dangerous substances and preparations. The list was consolidated and republished in Directive 2001/59/EC , where translations into other EU languages may be found.
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ion is an atom or molecule which has lost or gained one or more electrons, making it positively or negatively charged. A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion
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The chemical compound nitric acid (HNO3), also known as aqua fortis and spirit of nitre, is an aqueous solution of hydrogen nitrate (anhydrous nitric acid). It is a highly corrosive and toxic acid that can cause severe burns.
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Arsenic acid is the chemical compound with the formula H3AsO4. More descriptively written as AsO(OH)3, this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly.
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ion is an atom or molecule which has lost or gained one or more electrons, making it positively or negatively charged. A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion
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Ammonium phosphate - Molar Mass = 149.12g/mol. The normal ammonium phosphate, (NH4)3PO4, is obtained as a crystalline powder, on mixing concentrated solutions of ammonia and phosphoric acid, or on the addition of excess of ammonia to the acid
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Trisodium phosphate (TSP), available at most hardware stores in white powder form, is a cleaning agent, stain remover and degreaser, commonly used to prepare surfaces for painting.
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Hypophosphorous acid is a phosphorus oxoacid and a powerful reducing agent. Inorganic chemists refer to the free acid by this name (also as "HPA") although its official IUPAC name is phosphinic acid.
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The compound phosphorous acid, with formula H3PO3, is one of the oxoacids of phosphorus. The other important members of this family are phosphoric acid, H3PO4, and hypophosphorous acid, H3PO2.
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Pyrophosphoric acid, also known under the name diphosphoric acid, is a syrupy liquid or a needle-like crystalline solid. Pyrophosphoric acid is colorless, odorless, hygroscopic and is soluble in water, diethyl ether, and ethyl alcohol.
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Triphosphoric acid, also tripolyphosphoric acid, with formula H5P3O10, is a condensed form of phosphoric acid.

In polyphosphoric acids, it is the next after pyrophosphoric acid, H4P2O7
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standard state of a material is its state at 1 bar (100 kilopascals exactly). This pressure was changed from 1 atm (101.325 kilopascals) by IUPAC in 1990.[1] The standard state of a material can be defined at any given temperature, most commonly 25 degrees Celsius,
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A mineral acid is an acid derived by chemical reaction from inorganic minerals, as opposed to organic acids. Examples include:
  • Hydrochloric acid
  • Nitric acid
  • Phosphoric acid
  • Sulfuric acid
  • Boric acid
  • Hydrofluoric acid


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A chemical formula is a concise way of expressing information about the atoms that constitute a particular chemical compound. A chemical formula is also a short way of showing how a chemical reaction occurs.
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1, −1
(amphoteric oxide)
Electronegativity 2.20 (Pauling scale) More

Atomic radius 25 pm
Atomic radius (calc.) 53 pm
Covalent radius 37 pm
Van der Waals radius 120 pm

Thermal conductivity (300 K) 180.
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5, 4
(mildly acidic oxide)
Electronegativity 2.19 (Pauling scale)
Ionization energies
(more) 1st: 1011.8 kJmol−1
2nd: 1907 kJmol−1
3rd: 2914.1 kJmol−1

Atomic radius 100 pm
Atomic radius (calc.
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2, −1
(neutral oxide)
Electronegativity 3.44 (Pauling scale)
Ionization energies
(more) 1st: 1313.9 kJmol−1
2nd: 3388.3 kJmol−1
3rd: 5300.5 kJmol−1

Atomic radius 60 pm
Atomic radius (calc.
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