Thiosulfate

Formation

Thiosulfate ion is produced by the reaction of sulfite ion with elemental sulfur, and by incomplete oxidation of sulfides (e.g. pyrite oxidation). Sodium thiosulfate can be formed by disproportionation of sulfur dissolving in sodium hydroxide.

Reactions

Thiosulfate ions reacts with acids to give sulfur dioxide and various sulfur rings:

8 S₂O₃²⁻ + 16 H⁺ → → 8 SO₂ + S₈ + 8 H₂O

This reaction may be used to generate sulfur colloids and demonstrate the Rayleigh scattering of light. If white light is shone from below, blue light is seen from sideways and orange light from above, due to the same mechanisms that color the sky at midday and dusk.

Thiosulfate ions react with iodine to give tetrathionate ions:

2 S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2 I⁻

This reaction is key for iodometry. With bromine (X = Br) and chlorine (X = Cl), thiosulfate ions are oxidized to sulfate ions:

S₂O₃²⁻ + 4 X₂ + 5 H₂O → 8 X⁻ + 10 H⁺ + 2 SO₄²⁻

Reactions with metals and metal ions

Thiosulfate ion extensively forms diverse complexes with transition metals.

Also reflecting its affinity for metals, thiosulfate ion rapidly corrodes metals in acidic conditions. Steel and stainless steel are particularly sensitive to pitting corrosion induced by thiosulfate ions. Molybdenum improves the resistance of stainless steel toward pitting. In alkaline aqueous conditions and medium temperature (60 °C (140 °F)), carbon steel and stainless steel are not attacked, even at high concentration of base (30%/w potassium hydroxide), thiosulfate ion (10%/w) and in the presence of fluoride ion (5%/w potassium fluoride).

In film photography, thiosulfate salts are consumed on a large scale as a fixer reagent. This application exploits thiosulfate ion's ability to form coordination complexes with silver. Sodium thiosulfate, commonly called hypo (from "hyposulfite"), was widely used in photography to fix black and white negatives and prints after the developing stage; modern "rapid" fixers use ammonium thiosulfate as a fixing salt because it acts three to four times faster.

Thiosulfate salts have been used to extract or leach gold and silver from their ores as a less toxic alternative to cyanide ion.

Reactions with other salts

Inorganic nitrites detonate violently when heated with thiosulfates. This combination is notably used in emergency medicine in the cyanide antidote kit, though as separate medicines.

Biochemistry

The enzyme rhodanase (thiosulfate sulfurtransferase) catalyzes the detoxification of cyanide ion by thiosulfate ion by transforming them into thiocyanate ion and sulfite ion:

CN⁻ + S₂O₃²⁻ → SCN⁻ + SO₃²⁻

Sodium thiosulfate is used as a combination with amyl nitrite and sodium nitrite, along with hydroxocobalamin. It is most effective in a pre-hospital setting, since immediate administration by emergency personnel is necessary to reverse rapid intracellular hypoxia caused by the inhibition of cellular respiration, at complex IV.

It activates thiosulfate sulfurtransferase (TST) in mitochondria. TST is associated with protection against obesity and type II (insulin resistant) diabetes.

Thiosulfate can also work as electron donor for growth of bacteria oxidizing sulfur, such as Chlorobium limicola forma thiosulfatophilum. These bacteria use electrons from thiosulfate (and other sources) and carbon from carbon dioxide to synthesize carbon compounds through reverse Krebs cycle.

Some bacteria can metabolise thiosulfates.

Minerals

Thiosulfate ion is a component of the very rare mineral sidpietersite Pb₄(S₂O₃)O₂(OH)₂.

Nomenclature

Thiosulfate is an acceptable common name and almost always used.

The functional replacement IUPAC name is sulfurothioate; the systematic additive IUPAC name is trioxidosulfidosulfate(2−) or trioxido-1κ³O-disulfate(S—S)(2−).

Thiosulfate also refers to the esters of thiosulfuric acid, e.g. O,S-dimethyl thiosulfate CH₃−O−S(=O)₂−S−CH₃. Such species are rare.