sodium tetrathionate
{{Chembox
| Name = Sodium tetrathionate dihydrate
| ImageFile = Sodium tetrathionate.png
| ImageSize =
| ImageAlt =
| IUPACName = Sodium (sulfonatodisulfanyl)sulfonate dihydrate
| OtherNames =
|Section1={{Chembox Identifiers
| InChI = 1S/2Na.H2O6S4.2H2O/c;;1-9(2,3)7-8-10(4,5)6;;/h;;(H,1,2,3)(H,4,5,6);2*1H2/q2*+1;;;/p-2
| InChIKey = HAEPBEMBOAIUPN-UHFFFAOYSA-L
| SMILES = O.O.[O-]S(=O)(=O)SSS(=O)(=O)[O-].[Na+].[Na+]
| CASNo = 13721-29-4
| CASNo_Comment = dihydrate
| CASNo1 = 10101-86-7
| CASNo1_Comment = anhydrous
| PubChem = 203055
}}
|Section2={{Chembox Properties
| Formula = Na2S4O6
| MolarMass = 306.2665 g/mol (dihydrate)
| Appearance = white powder
| Density = 2.1 g/mL (25 °C)
| MeltingPt =
| BoilingPt =
| Solubility = 30.6 g/L (20 °C)
}}
|Section3={{Chembox Hazards
| NFPA-H = 2
| NFPA-F = 0
| NFPA-R = 0
| MainHazards =
| FlashPt =
| AutoignitionPt = }}
}}
Sodium tetrathionate is a salt of sodium and tetrathionate with the formula Na2S4O6.{{mvar|x}}H2O. The salt normally is obtained as the dihydrate ({{mvar|x}} = 2). It is a colorless, water-soluble solid. It is a member of the polythionates, which have the general formula [S{{mvar|n}}(SO3)2]2-. Other members include trithionate ({{mvar|n}} = 1), pentathionate ({{mvar|n}} = 3), hexathionate ({{mvar|n}} = 4).{{cite book|last1=Greenwood|first1=N. N.|author-link=Norman Greenwood|last2=Earnshaw|first2=A.|date=2 January 1991|title=Chemistry of the Elements - 3rd Edition|url=https://www.elsevier.com/books/comparative-inorganic-chemistry/moody/978-0-7131-3679-1|access-date=2022-02-15|publisher=Elsevier|doi=10.1016/C2013-0-11881-8|isbn=978-1-4832-8008-0|oclc=1040594550}}
Sodium tetrathionate is formed by the oxidation of sodium thiosulfate (Na2S2O3), e.g. by the action of iodine:
:2 Na2S2O3 + I2 → Na2S4O6 + 2 NaI
The reaction is signaled by the decoloration of iodine. This reaction is the basis of iodometric titrations.
Other methods include the coupling of sodium bisulfite with disulfur dichloride:
:2 NaHSO3 + S2Cl2 → Na2S4O6 + 2 HCl
The ion has ideal C2 symmetry, like H2S2. The S-S-S dihedral angle is nearly 90°. The central S-S distance is 2.115 Å, 0.01 Å longer than the two other S-S distances as well as those distances in most polysulfanes.
