3D model (JSmol)
CompTox Dashboard (EPA)
|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|
|Main hazards||skin irritant|
|Safety data sheet||http://www.sigmaaldrich.com/MSDS/|
|NFPA 704 (fire diamond)|
|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).
|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. Organic derivatives of phosphorous acid, compounds with the formula RPO3H2, are called phosphonic acids.
Nomenclature and tautomerism
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. 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).
- PCl3 + 3 H2O → HPO(OH)2 + 3 HCl
- 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:
- HP(O)2(OH)− → HPO2−
3 + H+ pKa = 6.7
The conjugate base HP(O)2(OH)− is called hydrogen phosphite, and the second conjugate base, HPO2−
3, is the phosphite ion. (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.
- 4 H3PO3 → 3 H3PO4 + PH3
This reaction is used for laboratory-scale preparations of PH3.
Phosphorous acid slowly oxidizes in air to phosphoric acid.
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
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)).
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.
- "MSDS - 215112". www.sigmaaldrich.com. Retrieved 12 April 2018.
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- 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.
- 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.
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