:Thiophosphoric acid
{{chembox
| ImageFile1 = Thiophosphoric acid.svg
| ImageSize1 = 150
| OtherNames = Monothiophosphoric acid
|Section1={{Chembox Identifiers
| CASNo = 13598-51-1
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = TYM4M7EWCW
| PubChem = 167254
| ChemSpiderID = 146330
| SMILES = OP(=S)(O)O
| StdInChI = 1S/H3O3PS/c1-4(2,3)5/h(H3,1,2,3,5)
| StdInChIKey = RYYWUUFWQRZTIU-UHFFFAOYSA-N
}}
|Section2={{Chembox Properties
| Formula = {{chem2|H3PO3S}}
| MolarMass = 114.061
| Appearance = colorless
| pKa = 1.79, 5.42, 10.08{{cite book|title=Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution|editor-first=D. D.|editor-last=Perrin|edition=2nd|series=IUPAC Chemical Data|issue=29|publisher=Pergamon|location=Oxford|year=1982|publication-date=1984|orig-date=1969|lccn=82-16524|isbn=0-08-029214-3|at=Entry 238}}
}}
}}
Thiophosphoric acid is an inorganic compound with the chemical formula {{chem2|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: {{chem2|S\dP(\sOH)3}} and {{chem2|O\dP(\sOH)2(\sSH)}}.
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:R. Klement "Phosphorus" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 570, 568.
:{{chem2|P2S5 + 6 NaOH → 2 Na3PO2S2 + H2S + 2 H2O}}
:{{chem2|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:
:{{chem2|Ba3(PO2S2)2 + 3 H2SO4 → BaSO4 + 2 H3PO2S2}}
Under controlled conditions, dithiophosphoric acid hydrolyses to give the monothioderivative:
:{{chem2|H3PO2S2 + H2O → H3PO3S + H2S}}