The tetrathionate anion, \(S _4 O _6^{2-}\), is a sulphur oxoanion derived from the compound tetrathionic acid, \(H _2 S _4 O _6\). Two of the sulphur atoms present in the ion are in oxidation state \(0\) and two are in oxidation state \(+5\).
$H_3PO_4 + 3OH^{-} \rightarrow PO_4^{3-} + 3H_2O$