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Sodium tetrathionate is a 
salt
In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
of sodium
Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
and tetrathionate
The tetrathionate anion, , is a sulfur oxyanion derived from the compound tetrathionic acid, H2S4O6. Two of the sulfur atoms present in the ion are in oxidation state 0 and two are in oxidation state +5. Alternatively, the compound can be vi ...
with the formula Na2S4O6.H2O. The salt normally is obtained as the dihydrate
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
( = 2). It is a colorless, water-soluble solid. It is a member of the polythionates
Polythionates are oxyanions with the formula (''n'' ≥ 0). They occur naturally and are the products of redox reactions of thiosulfate. Polythionates are readily isolable, unlike the parent polythionic acids.
Preparation
Many members of the poly ...
, which have the general formula (SO3)2sup>2-. Other members include trithionate ( = 1), pentathionate ( = 3), hexathionate ( = 4).
Sodium tetrathionate is formed by the oxidation of sodium thiosulfate
Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula . Typically it is available as the white or colorless pentahydrate (x = 5), which is a white solid that dissolves well in water. The compound is a reducing agent an ...
(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
The bisulfite ion (IUPAC-recommended nomenclature: hydrogensulfite) is the ion . Salts containing the ion are also known as "sulfite lyes". Sodium bisulfite is used interchangeably with sodium metabisulfite (Na2S2O5). Sodium metabisulfite diss ...
with disulfur dichloride
Disulfur dichloride (or disulphur dichloride by the British English spelling) is the inorganic compound of sulfur and chlorine with the Chemical formula, formula . It is an amber oily liquid.
Sometimes, this compound is incorrectly named ''sulfur ...
:
: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.
References
Tetrathionates {{inorganic-compound-stub