Thiophosphoric acid

Thiophosphoric acid
Names
	Other names Monothiophosphoric acid
Identifiers
CAS Number	13598-51-1 ;
3D model (JSmol)	Interactive image;
ChemSpider	146330;
PubChem CID	167254;
UNII	TYM4M7EWCW ;
CompTox Dashboard (EPA)	DTXSID8074948 ;
	InChI InChI=1S/H3O3PS/c1-4(2,3)5/h(H3,1,2,3,5) Key: RYYWUUFWQRZTIU-UHFFFAOYSA-N;
	SMILES OP(=S)(O)O;
Properties
Chemical formula	H3PO3S
Molar mass	114.061
Appearance	colorless
Acidity (pKa)	1.79, 5.42, 10.08
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Thiophosphoric acid is an inorganic compound with the chemical formula H₃PO₃S. Structurally, it is the acid derived from phosphoric acid with one oxygen atom replaced by sulfur atom, although it cannot be prepared from phosphoric acid. It is a colorless compound that is rarely isolated in pure form, but rather as a solution. The structure of the compound has not been reported, but two tautomers are reasonable: S=P(−OH)₃ and O=P(−OH)₂(−SH).

Preparation

The compound has been prepared in a multistep process starting with the base hydrolysis of phosphorus pentasulfide to give dithiophosphate, which is isolated as its barium salt:^[2]

P₂S₅ + 6 NaOH → 2 Na₃PO₂S₂ + H₂S + 2 H₂O

2 Na₃PO₂S₂ + 3 BaCl₂ → 2 Ba₃(PO₂S₂)₂ + 6 NaCl

In a second stage, the barium salt is decomposed with sulfuric acid, precipitating barium sulfate and liberating free dithiophosphoric acid:

Ba₃(PO₂S₂)₂ + 3 H₂SO₄ → BaSO₄ + 2 H₃PO₂S₂

Under controlled conditions, dithiophosphoric acid hydrolyses to give the monothioderivative:

H₃PO₂S₂ + H₂O → H₃PO₃S + H₂S

References

^ Perrin, D. D., ed. (1982) [1969]. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). Entry 238. ISBN 0-08-029214-3. LCCN 82-16524.
^ R. Klement "Phosphorus" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 570, 568.

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[P82db-1] Perrin, D. D., ed. (1982) [1969]. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). Entry 238. ISBN 0-08-029214-3. LCCN 82-16524.

[2] R. Klement "Phosphorus" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 570, 568.

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