Phosphorous acid

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Phosphorous acid
Wireframe model of phosphorous acid
Ball and stick model of phosphorous acid
IUPAC name
phosphonic acid
Other names
Dihydroxyphosphine oxide

Orthophosphorous acid

Oxo-λ5-phosphonous acid
3D model (JSmol)
ECHA InfoCard 100.033.682 Edit this at Wikidata
RTECS number
  • SZ6400000
  • InChI=1S/H3O3P/c1-4(2)3/h4H,(H2,1,2,3) checkY
  • InChI=1/H3O3P/c1-4(2)3/h4H,(H2,1,2,3)
  • OP(=O)O
  • OP(O)O
Molar mass 81.99 g/mol
Appearance white solid
Density 1.651 g/cm3 (21 °C)
Melting point 73.6 °C (164.5 °F; 346.8 K)
Boiling point 200 °C (392 °F; 473 K) (decomposes)
310 g/100 mL
Solubility soluble in ethanol
Acidity (pKa) 1.1, 6.7
−42.5·10−6 cm3/mol
Main hazards skin irritant
Safety data sheet[1]
R-phrases (outdated) 22-35
S-phrases (outdated) 26-36/37/39-45
NFPA 704 (fire diamond)
Related compounds
Related compounds
H3PO4 (i.e., PO(OH)3)
H3PO2 (i.e., H2PO(OH))
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Phosphorous acid is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), 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.

Nomenclature and tautomerism[edit]

Tautomerism of H3PO3.png

H3PO3 is more clearly described with the structural formula HPO(OH)2. In the solid state, HP(O)(OH)2 is tetrahedral with a P–H bond of 1.32 pm, one shorter P=O bond of 148 pm and two longer P–O(H) bonds of 154 pm. This species exists in equilibrium with an extremely minor tautomer P(OH)3. IUPAC recommends that the latter be called phosphorous acid, whereas the dihydroxy form is called phosphonic acid.[2] Only the reduced phosphorus compounds are spelled with an "ous" ending.

Other important oxyacids of phosphorus are phosphoric acid (H3PO4) and hypophosphorous acid (H3PO2). The reduced phosphorus acids are subject to similar tautomerism involving shifts of H between O and P.


HPO(OH)2 is the product of the hydrolysis of its acid anhydride:

P4O6 + 6 H2O → 4 HPO(OH)2

(An analogous relationship connects H3PO4 and P4O10).

On an industrial scale, the acid is prepared by hydrolysis of phosphorus trichloride with water or steam:[3]

PCl3 + 3 H2O → HPO(OH)2 + 3 HCl


Acid–base properties[edit]

Phosphorous acid has a pKa in the range 1.26–1.3.[4][5]

HP(O)(OH)2 → HP(O)2(OH) + H+            pKa = 1.3

It is a diprotic acid, the hydrogenphosphite ion, HP(O)2(OH) is a weak acid:

+ H+            pKa = 6.7

The conjugate base HP(O)2(OH) is called hydrogen phosphite, and the second conjugate base, HPO2−
, is the phosphite ion.[6] (Note that the IUPAC recommendations are hydrogen phosphonate and phosphonate respectively).

The hydrogen atom bonded directly to the phosphorus atom is not readily ionizable. Chemistry examinations often test students' appreciation of the fact that not all three hydrogen atoms are acidic under aqueous conditions, in contrast with H3PO4.

Redox properties[edit]

On heating at 200 °C, phosphorous acid disproportionates to phosphoric acid and phosphine:[7]

4 H3PO3 → 3 H3PO4 + PH3

This reaction is used for laboratory-scale preparations of PH3.

Phosphorous acid slowly oxidizes in air to phosphoric acid.[3]

Both phosphorous acid and its deprotonated forms are good reducing agents, although not necessarily quick to react. They are oxidized to phosphoric acid or its salts. It reduces solutions of noble metal cations to the metals. When phosphorous acid is treated with a cold solution of mercuric chloride, a white precipitate of mercurous chloride forms:

H3PO3 + 2 HgCl2 + H2O → Hg2Cl2 + H3PO4 + 2 HCl

Mercurous chloride is reduced further by phosphorous acid to mercury on heating or on standing:

H3PO3 + Hg2Cl2 + H2O → 2 Hg + H3PO4 + 2 HCl

As a ligand[edit]

Upon treatment with metals of d6 configuration, phosphorous acid is known to coordinate as the otherwise rare P(OH)3 tautomer. Examples include Mo(CO)5(P(OH)3) and [Ru(NH3)4(H2O)(P(OH)3)]2+.[8][9]

Structure of Mo(CO)5P(OH)3.[10]


The most important use of phosphorous acid (phosphonic acid) is the production of basic lead phosphite, which is a stabilizer in PVC and related chlorinated polymers.[3]

It is used in the production of basic lead phosphonate PVC stabilizer, aminomethylene phosphonic acid and hydroxyethane diphosphonic acid. It is also used as a strong reducing agent and in the production of phosphorous acid, synthetic fibres, organophosphorus pesticides, and the highly efficient water treatment agent ATMP.

Ferrous materials, including steel, may be somewhat protected by promoting oxidation ("rust") and then converting the oxidation to a metalophosphate by using phosphoric acid and further protected by surface coating. (See: Passivation (chemistry)).

Organic derivatives[edit]

The IUPAC (mostly organic) name is phosphonic acid. This nomenclature is commonly reserved for substituted derivatives, that is, organic group bonded to phosphorus, not simply an ester. For example, (CH3)PO(OH)2 is "methylphosphonic acid", which may of course form "methylphosphonate" esters.


  1. ^ "MSDS - 215112". Retrieved 12 April 2018.
  2. ^ International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSCIUPAC. ISBN 0-85404-438-8. Electronic version..
  3. ^ a b c Bettermann, Gerhard; Krause, Werner; Riess, Gerhard; Hofmann, Thomas (2000). "Phosphorus Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_527..
  4. ^ Larson, John W.; Pippin, Margaret (1989). "Thermodynamics of ionization of hypophosphorous and phosphorous acids. Substituent effects on second row oxy acids". Polyhedron. 8 (4): 527–530. doi:10.1016/S0277-5387(00)80751-2.
  5. ^ CRC Handbook of Chemistry and Physics (87th ed.). p. 8–42.
  6. ^ Novosad, Josef (1994). Encyclopedia of Inorganic Chemistry. John Wiley and Sons. ISBN 0-471-93620-0.
  7. ^ Gokhale, S. D.; Jolly, W. L. (1967). "Phosphine". Inorganic Syntheses. 9: 56–58. doi:10.1002/9780470132401.ch17.
  8. ^ Sernaglia, R. L.; Franco, D. W. (2005). "The ruthenium(II) center and the phosphite-phosphonate tautomeric equilibrium". Inorg. Chem. 28 (18): 3485–3489. doi:10.1021/ic00317a018.CS1 maint: uses authors parameter (link)
  9. ^ Xi, Chanjuan; Liu, Yuzhou; Lai, Chunbo; Zhou, Lishan (2004). "Synthesis of molybdenum complex with novel P(OH)3 ligand based on the one-pot reaction of Mo(CO)6 with HP(O)(OEt)2 and water". Inorganic Chemistry Communications. 7 (11): 1202. doi:10.1016/j.inoche.2004.09.012.
  10. ^ Xi, Chanjuan; Liu, Yuzhou; Lai, Chunbo; Zhou, Lishan (2004). "Synthesis of molybdenum complex with novel P(OH)3 Ligand based on the One-Pot Reaction of Mo(CO)6 with HP(O)(OEt)2 and Water". Inorganic Chemistry Communications. 7 (11): 1202–1204. doi:10.1016/j.inoche.2004.09.012.

Further reading[edit]

  • Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5.
  • Corbridge., D. E. C. (1995). Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology (5th ed.). Amsterdam: Elsevier. ISBN 0-444-89307-5.
  • Lee, J.D. (3 January 2008). Concise Inorganic Chemistry. Oxford University Press. ISBN 978-81-265-1554-7.