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Phosphonic acid

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

Dihydroxy(oxo)-λ5-phosphane
Dihydroxy-λ5-phosphanone
Orthophosphorous acid
Oxo-λ5-phosphanediol

Oxo-λ5-phosphonous acid
Identifiers
13598-36-2 YesY
3D model (Jmol) Interactive image
ChEBI CHEBI:44976 YesY
ChemSpider 10449259 YesY
10459438 (17O3) YesY
ECHA InfoCard 100.033.682
KEGG C06701 YesY
RTECS number SZ6400000
UNII 35V6A8JW8E N
Properties
H3PO3
Molar mass 82.00 g/mol
Appearance white solid
deliquescent
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
−42.5·10−6 cm3/mol
Structure
tetrahedral
Hazards
Main hazards skin irritant
Safety data sheet http://www.sigmaaldrich.com/MSDS/
R-phrases 22-35
S-phrases 26-36/37/39-45
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calcium Special hazards (white): no codeNFPA 704 four-colored 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).
N  (what is YesYN ?)
Infobox references

Dihydroxy(oxo)-λ5-phosphane
Dihydroxy-λ5-phosphanone
Orthophosphorous acid
Oxo-λ5-phosphanediol

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.

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.

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:

(An analogous relationship connects H3PO4 and P4O10).

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

Potassium phosphite is also a convenient precursor to phosphorous acid:

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.


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