Sodium+Thiosulfate

The [|thiosulfate] [|anion] is tetrahedral in shape and is notionally derived by replacing one of the [|oxygen] atoms by a [|sulfur] atom in a [|sulfate] anion. The S-S distance indicates a single bond, implying that the sulfur bears significant negative charge and the S-O interactions have more double bond character. The first protonation of thiosulfate occurs at sulfur.
 * Sodium thiosulfate** ([|Na]2[|S]2[|O]3), also spelled **sodium thiosulphate**, is a colorless [|crystalline] compound that is more familiar as the [|pentahydrate], [|Na]2[|S]2[|O]3•5[|H]2[|O], an [|efflorescent], [|monoclinic] crystalline substance also called sodium hyposulfite or “hypo.”

Thiosulfate anion characteristically reacts with [|dilute] [|acids] to produce sulfur, [|sulfur dioxide] and [|water] Na2S2O3 + 2 HCl → 2 NaCl + S + SO2 + H2O This reaction is known as a "[|clock reaction]", because when the sulfur reaches a certain concentration the solution turns from colourless to a pale yellow. This reaction has been employed to generate colloidal sulfur. When the protonation is conducted at low temperatures, H2S2O3 ([|thiosulfuric acid]) can be obtained. It is a somewhat strong acid with pKas of 0.6 and 1.7 for the first and second dissociation respectively.