Phosphorus oxoacids

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Template:Chembox header | Phosphenous acid
Template:Chembox header | General
IUPAC name Phosphenous acid
Molecular formula HPO2
Molar mass 63.980501 g/mol
Appearance and smell colorless, acrid
CAS numbers 6303-21-5
Template:Chembox header | Properties
Density and phase 1.274 g/cm3, liquid
Solubility in water miscible
Melting point 26.5 °C (299.5 K)
Boiling point 106 °C (379 K) decomp.
Acidity (pKa) 1.2
Viscosity ? cP at ? °C
Template:Chembox header | Structure
Molecular shape
Dipole moment ? D
Template:Chembox header | Hazards
MSDS External MSDS
EU classification not listed
NFPA 704 Template:NFPA 704
Flash point non-flammable
RTECS number ?
Template:Chembox header | Supplementary data page
Structure and
n, εr, etc.
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Template:Chembox header | Related compounds
Related oxoacids Hypophosphorous acid
Phosphinic acid
Related compounds Sodium phosphenite
Barium phosphenite
Template:Chembox header | Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
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Editor-In-Chief: Henry A. Hoff


Phosphorus forms a variety of compounds with oxygen and a number of these fit the description of an oxoacid. A phosphorus oxoacid contains phosphorus and oxygen, at least one hydrogen atom bound to oxygen, and forms an ion by the loss of one or more protons.

Phosphenous acid

Phosphorous acids

Phosphenous acid (HOPO) is structurally based on σ2λ3 phosphorus.[1] CID 22497.[2] With only one hydroxyl radical per molecule, phosphenous acid can form the dimer, diphosphenous acid, with the release of one H2O. But, phosphenous acid does not yield H2O when it forms an oligomer with more molecules than diphosphenous acid. Instead the double bonded oxygen is shared with the adjacent phosphorus atom. In this way phosphenous acid can form polymers, including cyclic, cyclooligophosphenous acid, and branched, ultraoligophosphenous acid. An easily obtained cyclic trimer of phosphenous acid is composed of (HOPO)x repeat units where x=3, and OH is replaced by 2,6-di-tert-butyl-4-methylphenyl (Ar).[3]

Phosphinic acid

Phosphinic acid, HOP(O)H2, is a tautomer that exists in equilibrium with the minor tautomer hypophosphorous acid, HP(OH)2. Phosphinic acid is IUPAC name hydroxy(oxo)phosphanium, CAS Number 6303-21-5, CID 6326996, H2O2P+,[4] and CID 3085127, CAS Number 57583-56-9, H3O2P, with IUPAC name phosphinic acid. Both tautomers are described on the same page: Hypophosphorous acid. Usually, hypo and ous when added signify that the acid contains two less oxygen atoms than the "ic", phosphoric acid. Phosphinic acid is monoprotic as highlighted by its formula written as HOP(O)H2 and monobasic. Salts derived from these acids are called phosphinates or hypophosphites.

Hypophosphorous acid

The minor tautomer, HP(OH)2, does not have as yet a CID number, CAS number, or IUPAC name, but has been shown to be a stable tautomer.[5][6][7]


Hydroperoxyphosphane (the IUPAC name) has the same formula, H3PO2, exists as H2POOH, CID 5260047, but has no CAS number.

Phosphorous acid

Phosphorous acid is sometimes called phosphorus acid, orthophosphorous acid, or monophosphorous acid. Phosphorous acid is a minor tautomer that exists in equilibrium with phosphonic acid. Both tautomers have their own pages: Phosphonic acid and Phosphorous acid, but because preparation and uses of 'phosphorous acid' actually pertain more to the major tautomer, phosphonic acid, and it is more often referred to as 'phosphorous acid', this added information can be found under phosphorous acid. Phosphorous acid has the chemical formula H3PO3, which is best expressed as P(OH)3 to show its triprotic character. P(OH)3 (IUPAC: phosphorous acid) has CAS number 10294-56-1 and CID: 107909[8]. It has been shown to be a stable tautomer.[5][9]

Diphosphorous acid is made by condensation:

2P(OH)3 <=> (OH)2POP(OH)2 + H2O

For phosphorous acid, an oligophosphorous acid refers to a few molecules condensed into a molecule with the loss of H2O as each unit of P(OH)3 is added on. The general formula is


where n = 2, 3, 4, etc., oligo-. Here as with phosphonic acid the repeat unit is (HPO2)n-2. This repeat unit (HPO2) is phosphenous acid.

A polyphosphorous acid can have dozens of units condensed in a row. Regardless of the value of n, both polyphosphorous acid and polyphosphonic acid have the same chemical formula for any specific n, e.g., triphosphosphonic acid is H5P3O7 and triphosphorous acid is H5P3O7 for n=3.

In oligophosphorous acids of sufficient size, there are multiple OH that can result in the condensation of a cyclophosphorous acid that does not have multiple (HPO3) metaphosphoric acid units.

However, the usual referral to a cyclophosphorous acid (cyclophosphites) may be misnomers wherein the cyclic portion is carbon-based with a phosphorous acid side chain of one or more molecules, or one or a limited number of either of the two tautomers included in the ring but as a minority contributor.

For example the effect of varying ring size on the phosphonate-phosphite tautomerism of cyclophosphorous acids has been shown.[10] But the cyclophosphorous acids are biheteroorganic.

Branching can also occur in either oligophosphorous or polyphosphorous acid. These are ultraoligophosphorous or ultrapolyphosphorous acids, or ultraoligophosphites and ultrapolyphosphites, respectively.

Phosphoric acid

Phosphoric acid, orthophosphoric acid, or monophosphoric acid has its own page: Phosphoric acid. Other acids commonly called 'phosphoric acid' are on page: Phosphoric acids and phosphates. Two of the oligophosphoric acids also have their own pages: diphosphoric acid or Pyrophosphoric acid and Triphosphoric acid or tripolyphosphoric acid. Generally, oligophosphoric and polyphosphoric acids are discussed on the page: Phosphoric acids and phosphates. Phosphoric acids can occur in cyclic form, cyclooligophosphoric acids, branched, ultraphosphoric acids, or combinations of both, usually referred to as polyphosphoric acids. See the page: Phosphoric acids and phosphates for more information, especially phosphoric anhydride.

Metaphosphoric acid

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.[11] Metaphosphoric acid is a singly anhydrous version of orthophosphoric acid and is sometimes used as a water- or moisture-absorbing reagent. Metaphosphoric acid has phosphorus in the formal oxidation state of +5 and is CID 3084658[12]. Although it has several CAS numbers: 10343-62-1, 135306-83-1, 13566-25-1, and 37267-86-0, only the last two are associated with the IUPAC name phosphenic acid.

Phosphoric acid units can be bonded together in rings (cyclic structures), chains (catena), or branched by condensation yielding water as each acid is added. These structures consist of linked metaphosphoric acid molecules. For example, trimetaphosphoric acid on the page: Phosphoric acids and phosphates#Cyclo- or metaphosphoric acids and metaphosphates is the simplest cyclophosphoric acid. Metaphosphoric acid would need to share the doubly bonded oxygen with neighboring phosphorus atoms to form polymers itself.

Paraphosphoric acid

Paraphosphoric acid (H5PO5) is obtained by burning phosphorus in dry air, or oxygen gas, appears in small crystals, like snow, and is formed, combined with water, by heating to redness pyrophosphoric and phosphoric acids; when fused, it cools into a brittle and transparent solid, resembling ice.[13] Paraphosphoric acid has phosphorus in the formal oxidation state of +5. It occurs as P(OH)5, CID 517655 with IUPAC name pentahydroxyphosphorane.

Thiophosphoric acid

Thiophosphoric acids have one or more oxygens replaced by sulfur. An example of a thiophosphoric acid ester (thiophosphorate) is on the page: Diazinon. Monothiophosphoric acid has the molecular formula H3PO3S, P(O)(OH)2(SH), is SID 50009326 and CID 167254 114 Da.[14] Dithiophosphoric acid has the molecular formula H3PO2S2, P(S)(OH)2(SH), is CAS number 15834-33-0, SID 729652 and CID 152119 130 Da.[15] A trithiophosphoric acid is phosphorotrithioic acid with the molecular formula H9C3OPS3, actually the methyl ester S,S,S,-trimethyl phophorotrithioate, H3CSP(=O)(SCH3)SCH3, CID 120321, IUPAC Name: bis(methylsulfanyl)phosphorylsulfanylmethane, 188 Da, CAS number 681-71-0.[16]

Perphosphoric acid

Permonophosphoric Acid

The use of per before the phosphoric indicates that the acid contains one more oxygen than phosphoric acid. For example, permonophosphoric acid has the formula H3PO5 instead of H3PO4 for orthophosphoric acid (monophosphoric acid).

Permonophosphoric acid is an acid formed when pyrophosphoric acid is treated with a large excess of hydrogen peroxide.

H4P2O7 + 2H2O2 <=> 2H3PO5 + H2O

Permonophosphoric acid 114 Da is a triprotic acid, yielding PO53- (permonophosphate ion) with its H+s removed. The preparation of permonophosphoric acid, H3PO5, was obtained by J. Schmidlin and P. Massini (Ber., 1910, 43, 1162).[17] Monoperphosphoric acid can be obtained when phosphorus pentoxide is treated with 30 % hydrogen peroxide, while cooled with ice-water.[18] But there are other forms of H3PO5.

When two oxygens are linked as in hydrogen peroxide, the permonophosphoric acid formed is peroxomonophosphoric acid (PMPA), (OH)2O=P-O-O-H, H3PO5. PMPA can be prepared by the perchloric acid hydrolysis of potassium peroxodiphosphate.[19] The species H3PO5, H2PO5-, HPO52-, and PO53- become predominant in this order with increasing pH and the electrophilic character of these species decreases.[19] PMPA exists as H2PO5- and HPO52- in the pH range 4-7.[19]

Monoperphosphoric acid and permonophosphoric acid are the same acid. Should the phosphoric acid be either an oligophosphoric acid or polyphosphoric acid as discussed below, the location of per may indicate different acids. For instance, perdiphosphoric acid would be H4P2O8, one more oxygen added to pyrophosphoric acid. But diperphosphoric acid could refer to two permonophosphoric acid molecules condensed with the loss of H2O to yield H4P2O9.

Perphosphates have been produced from perphosphoric acid.[20]

Superphosphoric acid

Superphosphoric acid is a blend of orthophosphoric and polyphosphoric acid, CAS Number 8017-16-1. Superphosphoric acids can be in the concentration range of 62-85% P2O5 (86-117% H3PO4), but at about 90% H3PO4 the amount of condensed phosphate species becomes significant.[21] The concentration range of the superphosphoric acids is a range of increasing importance in fertilizer technology.[21]

Superphosphoric acid is sometimes called multiphosphoric acid or polyphosphoric acid.

Superphosphates (salts or esters) can result from the use of superphosphoric acid. Superphosphate has been produced by acidulating phosphate rock with superphosphoric acid, where the superphosphoric acid contained about 49% of its phosphate as orthophosphate, 42% as pyrophosphate, and the remainder as polyphosphates.[22]

Hyperphosphoric acid

Hyperphosphoric acid is concentrated phosphoric acid. It can be prepared in two ways by:

(1) producing phosphorus pentoxide electrothermically and dissolving this oxide in a small quantity of water or

(2) concentrating phosphoric acid obtained by wet treatment with hot gas at temperatures above 700°C.[23]

It has also been called oleophosphoric acid.[23]

The first process produces hyperphosphoric acid of high purity and concentration but is very expensive. The second process produces a cheaper acid but introduces several impurities, among which are fluorine and sulphuric acid, and various metals, e.g. aluminium and iron. The metallic impurities exist as complexes in the form of polyphosphoric salts.[23]

Hyperphosphoric acid can be added to blood to release CO2.[24]

Hyperphosphates (salts or esters) or oleophosphates can result from the use of hyperphosphoric acid. Hyperphosphate, when used to induce hyperphosphatemia, may directly promote calcium deposition and osteocalcin expression.[25]


The content on this page was first contributed by: Henry A. Hoff.

Initial content for this page in some instances came from Wikipedia.


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  19. 19.0 19.1 19.2 Panigrahi GP, Panda R (1980). "Oxidation studies with peroxomonophosphoric acid. III. A kinetic and mechanistic study of oxidation of dialkyl sulfides". Bull Chem Soc Jpn. 53 (8): 2366–70.
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  23. 23.0 23.1 23.2 "Process for obtaining concentrated phosphoric acid".
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  25. Lu W, Wang X, Zhao X (2007). "Study of vascular smooth muscle cell calcification induced by hyperphosphate and intervented by phosphonoformic acid". J Nanjing Med Univ. 21 (6): 377–81. doi:10.1016/S1007-4376(07)60082-3. Unknown parameter |month= ignored (help)


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