|Molar mass||82.00 g/mol|
|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 alcohol|
|Acidity (pKa)||1.3, 6.7|
|Main hazards||skin irritant|
|H3PO4 (i.e., PO(OH)3)
H3PO2 (i.e., H2PO(OH))
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
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.
Nomenclature and tautomerism
H3PO3 is more clearly described with the structural formula HPO(OH)2. This species exists in equilibrium with a minor tautomer P(OH)3. IUPAC recommendations, 2005, are that the latter be called phosphorous acid, whereas the dihydroxy form is called phosphonic acid. Only the reduced phosphorus compounds are spelled with an "ous" ending.
The P(OH)3 tautomer has been observed as a ligand bonded to molybdenum. 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.
Structure and oxidation state
In the solid state, HP(O)(OH)2 is tetrahedral with one shorter P=O bond of 148 pm and two longer P-O(H) bonds of 154 pm. The central phosphorus atom is assigned an oxidation state of +3.
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).
- PCl3 + 3 H2O → HPO(OH)2 + 3 HCl
Potassium phosphite is also a convenient precursor to phosphorous acid:
- K2HPO3 + 2 HCl → 2 KCl + H3PO3
In practice aqueous potassium phosphite is treated with excess hydrochloric acid. By concentrating the solution and precipitations with alcohols, the pure acid can be separated from the salt.
- 4 H3PO3 → 3 H3PO4 + PH3
In practice this reaction yields a number of undefined phosphorus suboxides as well.
Phosphorous acid is a moderately strong dibasic acid. It reacts with alkalis forming acid phosphites and normal phosphites. Thus, reaction with sodium hydroxide gives sodium dihydrogen phosphite and disodium hydrogen phosphite, but not trisodium phosphite, Na3PO3 as the third (P-bound) hydrogen is not acidic.
- H3PO3 + NaOH → NaH2PO3 + H2O
- H3PO3 + 2 NaOH → Na2HPO3 + 2H2O
- 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
Phosphorous acid is a diprotic acid, since the hydrogen bonded directly to the central phosphorus atom is not readily ionizable. Chemistry examinations often test students' appreciation of the fact that all three hydrogen atoms are not acidic under aqueous conditions, in contrast with H3PO4, phosphoric acid. The hydrogenphosphite ion, HP(O)2(OH)− is a moderately strong acid.
- HP(O)(OH)2 → HP(O)2(OH)− + H+ pKa = 1.3
- HP(O)2(OH)− → HPO32− + H+ pKa = 6.7
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.
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.
In industry and agriculture
The most important use of phosphorous acid is the production of phosphonates which are used in water treatment. Phosphorous acid is also used for preparing phosphite salts, such as potassium phosphite. These salts, as well as aqueous solutions of pure phosphorous acid, have shown effectiveness in controlling a variety of microbial plant diseases, in particular, treatment using either trunk injection or foliar containing phosphorous acid salts is indicated in response to infections by phytophthora and pythium-type plant pathogens (both within class oomycetes, known as water molds), such as dieback/root rot and downy mildew. Anti-microbial products containing salts of phosphorous acid are marketed in Australia as 'Yates Anti-Rot'; and in the United States of America, for example, aluminum salts of the monoethyl ester of phosphorous acid (known generically as 'Fosetyl-Al') are sold under the trade name 'Aliette'. Phosphorous acid and its salts, unlike phosphoric acid, are somewhat toxic and should be handled carefully.
As a chemical reagent
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- Chanjuan Xi, Yuzhou Liu, Chunbo Lai, Lishan Zhou (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.
- M. N. Sokolov, E. V. Chubarova, K. A. Kovalenko, I. V. Mironov, A. V. Virovets1, E. V. Peresypkina,V. P. Fedin (2005). "Stabilization of tautomeric forms P(OH)3 and HP(OH)2 and their derivatives by coordination to palladium and nickel atoms in heterometallic clusters with the Mo3MQ44+ core (M = Ni, Pd; Q = S, Se)". Russian Chemical Bulletin 54 (3): 615. doi:10.1007/s11172-005-0296-1.
- CRC Handbook of Chemistry and Physics, 87th Ed. 8-42
- Josef Novosad, 1994, Encyclopedia of Inorganic Chemistry, John Wiley and Sons, ISBN 0-471-93620-0
- Organic Labs. Product label for 'Exel LG,' Retrieved April 9, 2007.
- Yates, a Division of Orica Australia Pty Ltd. “MSDS ('Yates Anti Rot Phosacid Systemic Fungicide').” Version 1. SH&E Shared Services, Orica. Homebush, NSW (Australia): April 4, 2005 (retrieved from www.orica.com April 9, 2007).
- US EPA. “Fosetyl-Al (Aliette): Reregistration Eligibility Decision (RED) Fact Sheet.” Office of Pesticide Programs, US EPA. Washington, DC (USA): 1994 (retrieved from www.epa.gov April 9, 2007).
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- D. E. C. Corbridge. “Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology.” 5th ed. Elsevier: Amsterdam. ISBN 0-444-89307-5.
- Concise Inorganic Chemistry J.D.Lee Oxford University Press ISBN 978-81-265-1554-7