Sodium tetrathionate is a salt of sodium and tetrathionate with the formula Na<sub>2</sub>S<sub>4</sub>O<sub>6</sub><sup>.</sup>H<sub>2</sub>O. The salt normally is obtained as the dihydrate ( = 2). It is a colorless, water-soluble solid. It is a member of the polythionates, which have the general formula [S<sub></sub>(SO<sub>3</sub>)<sub>2</sub>]<sup>2-</sup>. Other members include trithionate ( = 1), pentathionate ( = 3), hexathionate ( = 4).
Sodium tetrathionate is formed by the oxidation of sodium thiosulfate (Na<sub>2</sub>S<sub>2</sub>O<sub>3</sub>), e.g. by the action of iodine:
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:
The ion has ideal C<sub>2</sub> symmetry, like H<sub>2</sub>S<sub>2</sub>. The S-S-S dihedral angle is nearly 90ð. The central S-S distance is 2.115 à, 0.01 àlonger than the two other S-S distances as well as those distances in most polysulfanes.