Phosphoric acid

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Phosphoric acid
Phosphoric-acid-2D-dimensions.svg
Ball-and-stick model Phosphoric-acid-3D-balls.png
Ball-and-stick model
Space-filling model Phosphoric-acid-3D-vdW.png
Space-filling model
Names
IUPAC name
Phosphoric acid
Other names
Orthophosphoric acid, hydrogen phosphate
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.028.758 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 231-633-2
E number E338 (antioxidants, ...)
KEGG
PubChem CID
RTECS number
  • TB6300000
UNII
UN number 1805
  • InChI=1S/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4) Yes check.svgY
    Key: NBIIXXVUZAFLBC-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4)
    Key: NBIIXXVUZAFLBC-UHFFFAOYAI
  • OP(=O)(O)O
Properties
H3PO4
Molar mass 97.994 g·mol−1
AppearanceColorless solid
Odor Odorless
Density 1.6845 g/cm3 (25 °C, 85%), [1] 1.834 g/cm3 (solid) [2]
Melting point 42.35 °C (108.23 °F; 315.50 K) anhydrous [3]
29.32 °C (84.78 °F; 302.47 K) hemihydrate [4]
Boiling point
  • 212 °C (414 °F) [5] (only water evaporates) [6]
  • 392.2 g/(100 g) (−16.3 °C)
  • 369.4 g/(100 mL) (0.5 °C)
  • 446 g/(100 mL) (15 °C) [7]
  • 548 g/(100 mL) (20 °C) [8]
Solubility Soluble in ethanol
log P −2.15 [9]
Vapor pressure 0.03 mmHg (20 °C) [10]
Conjugate base Dihydrogen phosphate
−43.8·10−6 cm3/mol [11]
  • 1.3420 (8.8% w/w aq. soln.) [12]
  • 1.4320 (85% aq. soln) 25 °C
Viscosity 2.4–9.4  cP (85% aq. soln.)
147 cP (100%)
Structure
Monoclinic
Tetrahedral
Thermochemistry [13]
145.0 J/(mol⋅K)
Std molar
entropy (S⦵298)
150.8 J/(mol⋅K)
−1271.7 kJ/mol
−1123.6 kJ/mol
Hazards
GHS labelling:
GHS-pictogram-acid.svg [14]
Danger
H290, H314 [14]
P280, P305+P351+P338, P310 [14]
NFPA 704 (fire diamond)
NFPA 704.svgHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
3
0
0
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
1530 mg/kg (rat, oral) [15]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 1 mg/m3 [10]
REL (Recommended)
TWA 1 mg/m3 ST 3 mg/m3 [10]
IDLH (Immediate danger)
1000 mg/m3 [10]
Safety data sheet (SDS) ICSC 1008
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula H 3 P O 4. 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. [16]

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. [17] [18] [19]

Wet process

In the wet process, a phosphate-containing mineral such as calcium hydroxyapatite and fluorapatite are treated with sulfuric acid. [20]

Ca5(PO4)3OH + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + H2O
Ca5(PO4)3F + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + HF

Calcium sulfate (gypsum, CaSO4) is a by-product, which is removed as phosphogypsum. The hydrogen fluoride (HF) gas is streamed into a wet (water) scrubber producing hydrofluoric acid. 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. [21] 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. [22] 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. [23]

Purification

Phosphoric acids produced from phosphate rock or thermal processes often requires purification. A common purification methods is liquid-liquid extraction, which involves the separation of phosphoric acids 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. [24]

Fractional crystallization can achieve highest 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. [25]

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.

Aqueous solutions

Aqueous solutions up to 62.5% H3PO4 are eutectic, 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; the monohydrate). Beyond this the phase 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 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 the hemihydrate 2H3PO4•H2O, freezing at 29.32°C. [26] [27] 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 to supercool before crystallization occurs, and may be relatively resistant to crystallisation even when stored below the freezing point. [4]

Self condensation

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 with pyrophosphoric 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. [28]

As the concentration is increased higher acids are formed, culminating in the formation of polyphosphoric acids. [29] It is not possible to fully dehydrate phosphoric acid to phosphorus 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. [4] [3]

Uses

The dominant use of phosphoric acid is for fertilizers, consuming approximately 90% of production. [30]

ApplicationDemand (2006) in thousands of tonsMain phosphate derivatives
Soaps and detergents1836 STPP
Food industry309 STPP (Na5P3O10), SHMP, TSP, SAPP, SAlP, MCP, DSP (Na2HPO4), H3PO4
Water treatment 164SHMP, STPP, TSPP, MSP (NaH2PO4), DSP
Toothpastes 68 DCP (CaHPO4), IMP, SMFP
Other applications287 STPP (Na3P3O9), TCP, APP, DAP, zinc phosphate (Zn3(PO4)2), aluminium phosphate (AlPO4), H3PO4

Food-grade phosphoric acid (additive E338 [31] ) is used to acidify foods and beverages such as various colas and jams, providing a tangy or sour taste. The phosphoric acid also serves as a preservative. [32] Soft drinks containing phosphoric acid, which would include Coca-Cola, are sometimes called phosphate sodas or phosphates. Phosphoric acid in soft drinks has the potential to cause dental erosion. [33] Phosphoric acid also has the potential to contribute to the formation of kidney stones, especially in those who have had kidney stones previously. [34]

Specific applications of phosphoric acid include:

Phosphoric acid may also be used for chemical polishing (etching) of metals like aluminium or for passivation of steel products in a process called phosphatization. [40]

Safety

Phosphoric acid is not a strong 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. [41]

A link has been shown between long-term regular cola intake and osteoporosis in later middle age in women (but not men). [42]

See also

Related Research Articles

<span class="mw-page-title-main">Acid</span> Chemical compound giving a proton or accepting an electron pair

An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen ion, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid.

<span class="mw-page-title-main">Phosphorus</span> Chemical element with atomic number 15 (P)

Phosphorus is a chemical element; it has symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Earth. It has a concentration in the Earth's crust of about 0.1%, less abundant than hydrogen but more than manganese. In minerals, phosphorus generally occurs as phosphate.

<span class="mw-page-title-main">Phosphate</span> Anion, salt, functional group or ester derived from a phosphoric acid

In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid, a.k.a. phosphoric acid H3PO4.

<span class="mw-page-title-main">Fertilizer</span> Substance added to soil to enhance plant growth

A fertilizer or fertiliser is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from liming materials or other non-nutrient soil amendments. Many sources of fertilizer exist, both natural and industrially produced. For most modern agricultural practices, fertilization focuses on three main macro nutrients: nitrogen (N), phosphorus (P), and potassium (K) with occasional addition of supplements like rock flour for micronutrients. Farmers apply these fertilizers in a variety of ways: through dry or pelletized or liquid application processes, using large agricultural equipment, or hand-tool methods.

<span class="mw-page-title-main">Phosphorite</span> Sedimentary rock containing large amounts of phosphate minerals

Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxide (P2O5). Marketed phosphate rock is enriched ("beneficiated") to at least 28%, often more than 30% P2O5. This occurs through washing, screening, de-liming, magnetic separation or flotation. By comparison, the average phosphorus content of sedimentary rocks is less than 0.2%.

<span class="mw-page-title-main">Pyrophosphoric acid</span> Chemical compound

Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H4P2O7 or, more descriptively, [(HO)2P(O)]2O. Colorless and odorless, it is soluble in water, diethyl ether, and ethyl alcohol. The anhydrous acid crystallizes in two polymorphs, which melt at 54.3 and 71.5 °C. The compound is a component of polyphosphoric acid, an important source of phosphoric acid. Anions, salts, and esters of pyrophosphoric acid are called pyrophosphates.

Tributyl phosphate, known commonly as TBP, is an organophosphorus compound with the chemical formula (CH3CH2CH2CH2O)3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of phosphoric acid with n-butanol.

<span class="mw-page-title-main">Phosphorous acid</span> Chemical compound (H3PO3)

Phosphorous acid is the compound described by the formula H3PO3. This acid is diprotic, not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula RPO3H2, are called phosphonic acids.

<span class="mw-page-title-main">Phosphoric acids and phosphates</span> Class of chemical species; phosphorus oxoacids and their deprotonated derivatives

In chemistry, a phosphoric acid, in the general sense, is a phosphorus oxoacid in which each phosphorus (P) atom is in the oxidation state +5, and is bonded to four oxygen (O) atoms, one of them through a double bond, arranged as the corners of a tetrahedron. Two or more of these PO4 tetrahedra may be connected by shared single-bonded oxygens, forming linear or branched chains, cycles, or more complex structures. The single-bonded oxygen atoms that are not shared are completed with acidic hydrogen atoms. The general formula of a phosphoric acid is Hn+2−2xPnO3n+1−x, where n is the number of phosphorus atoms and x is the number of fundamental cycles in the molecule's structure, between 0 and ⁠n + 2/2⁠.

<span class="mw-page-title-main">Ammonium phosphate</span> Chemical compound

Ammonium phosphate is the inorganic compound with the formula (NH4)3PO4. It is the ammonium salt of orthophosphoric acid. A related "double salt", (NH4)3PO4.(NH4)2HPO4 is also recognized but is impractical to use. Both triammonium salts evolve ammonia. In contrast to the unstable nature of the triammonium salts, the diammonium phosphate (NH4)2HPO4 and monoammonium salt (NH4)H2PO4 are stable materials that are commonly used as fertilizers to provide plants with fixed nitrogen and phosphorus.

<span class="mw-page-title-main">Phosphorus pentoxide</span> Chemical compound

Phosphorus pentoxide is a chemical compound with molecular formula P4O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrating agent.

<span class="mw-page-title-main">Monocalcium phosphate</span> Chemical compound

Monocalcium phosphate is an inorganic compound with the chemical formula Ca(H2PO4)2 ("AMCP" or "CMP-A" for anhydrous monocalcium phosphate). It is commonly found as the monohydrate ("MCP" or "MCP-M"), Ca(H2PO4)2·H2O. Both salts are colourless solids. They are used mainly as superphosphate fertilizers and are also popular leavening agents.

<span class="mw-page-title-main">Hexafluorosilicic acid</span> Octahedric silicon compound

Hexafluorosilicic acid is an inorganic compound with the chemical formula H
2SiF
6. Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless.

<span class="mw-page-title-main">Urea phosphate</span> Chemical compound

Urea phosphate is a 1:1 combination of urea and phosphoric acid that is used as a fertilizer. It has an NPK formula of 17-44-0, and is soluble in water, producing a strongly acidic solution.

<span class="mw-page-title-main">Fractional crystallization (chemistry)</span> Method for refining substances based on differences in their solubility

In chemistry, fractional crystallization is a stage-wise separation technique that relies on the liquid-solid phase change. fractional crystallisation. This is an old established method which is nevertheless capable of excellent results and a description is available in "The practical Methods of organic Chemistry" by Ludwig Gattermann in 1898 and a translation was available in English by WILLIAM B. SHOBER who was a Lehigh University, USA.

<span class="mw-page-title-main">Disodium phosphate</span> Chemical compound

Disodium phosphate (DSP), or disodium hydrogen phosphate, or sodium phosphate dibasic, is an inorganic compound with the chemical formula Na2HPO4. It is one of several sodium phosphates. The salt is known in anhydrous form as well as hydrates Na2HPO4·nH2O, where n is 2, 7, 8, and 12. All are water-soluble white powders. The anhydrous salt is hygroscopic.

<span class="mw-page-title-main">Copper(II) phosphate</span> Chemical compound

Copper(II) phosphate are inorganic compounds with the formula Cu3(PO4)2. They can be regarded as the cupric salts of phosphoric acid. Anhydrous copper(II) phosphate and a trihydrate are blue solids.

<span class="mw-page-title-main">Ammonium dihydrogen phosphate</span> Chemical compound

Ammonium dihydrogen phosphate (ADP), also known as monoammonium phosphate (MAP) is a chemical compound with the chemical formula (NH4)(H2PO4). ADP is a major ingredient of agricultural fertilizers and dry chemical fire extinguishers. It also has significant uses in optics and electronics.

<span class="mw-page-title-main">Phosphorus deficiency</span> Plant disorder

Phosphorus deficiency is a plant disorder associated with insufficient supply of phosphorus. Phosphorus refers here to salts of phosphates, monohydrogen phosphate, and dihydrogen phosphate. These anions readily interconvert, and the predominant species is determined by the pH of the solution or soil. Phosphates are required for the biosynthesis of genetic material as well as ATP, essential for life. Phosphorus deficiency can be controlled by applying sources of phosphorus such as bone meal, rock phosphate, manure, and phosphate-fertilizers.

<span class="mw-page-title-main">Peroxymonophosphoric acid</span> Oxyacid of phosphorus

Peroxymonophosphoric acid is an oxyacid of phosphorus. It is a colorless viscous oil. Its salts are called peroxymonophosphates. Another peroxyphosphoric acid is peroxydiphosphoric acid, H4P2O8.

References

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