Sodium tetrathionate

Sodium tetrathionate is a salt of sodium and tetrathionate with the formula Na₂S₄O₆^.xH₂O. The salt normally is obtained as the dihydrate (x = 2). It is a colorless, water-soluble solid. It is a member of the polythionates, which have the general formula [S_n(SO₃)₂]^2-. Other members include trithionite (n = 1), pentathionate (n = 3), hexathionate (n = 4).^[1]

Sodium tetrathionate dihydrate

Names
IUPAC name Sodium (sulfonatodisulfanyl)sulfonate dihydrate
Identifiers
CAS Number	13721-29-4 dihydrate 10101-86-7 anhydrous
3D model (JSmol)	Interactive image
ECHA InfoCard	100.208.917
PubChem CID	203055
CompTox Dashboard (EPA)	DTXSID50160108
InChI 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 Key: HAEPBEMBOAIUPN-UHFFFAOYSA-L
SMILES O.O.[O-]S(=O)(=O)SSS(=O)(=O)[O-].[Na+].[Na+]
Properties
Chemical formula	Na2S4O6
Molar mass	306.2665 g/mol (dihydrate)
Appearance	white powder
Density	2.1 g/mL (25 °C)
Solubility in water	30.6 g/L (20 °C)
Hazards
NFPA 704 (fire diamond)	2 0 0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Sodium tetrathionate is formed by the oxidation of sodium thiosulfate (Na₂S₂O₃), e.g. by the action of iodine:^[1]

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:^[1]

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.^[2]

Structure of tetrathionate as its dihydrated sodium salt.^[2]