by

Sodium thiosulphate (Na2S2O3) is an inorganic salt compound that is chiefly used as an antidote for cyanide poisoning. Recognised as one of the most important medicines in the health system, it has several medical uses and a range of interesting properties.

Properties

Also known as sodium hyposulfite or simply as hypo, this compound appears in two forms: anhydrous and pentahydrate. However, it is most commonly found as a white or colourless pentahydrate (Na2S2O3 . 5H2O).

Physically, sodium thiosulphate is a crystalline solid at room temperature:

  • It dissolves well and is very soluble with water
  • It is insoluble with ethanol
  • It loses water readily as it is an efflorescent compound
  • It decomposes at 300°C to form sodium sulphate and sodium polysulfide
  • It produces toxic fumes of disodium oxide and sulphur oxides when heated
  • It is a sulphur donating agent

Chemically, sodium thiosulphate is an ionic compound that exists in a monoclinic crystal structure. In crystallography, this is one of the 7 crystal systems. The chemical formula of sodium thiosulphate (Na2S2O3) also shows us that it is composed of two sodium cations (Na+) and a thiosulphate anion (S2O3–) where the central sulphur atom is bonded to another sulphur atom as well as three oxygen atoms.

Production

Sodium thiosulphate is made in a number of different ways. Industrially, it is synthesised from the liquid by-products of sodium sulphide. The solution this gives contains sodium carbonate, and this is then mixed with sulphur dioxide before getting re-crystallised.

In the laboratory, it can be prepared either by:

  1. Heating an aqueous solution of sodium sulphite with sulphur, or
  2. Boiling an aqueous solution of sodium hydroxide and sulphur

How is Sodium Thiosulphate Used?

While this compound has a range of applications, it is most commonly found in the medical industry. Its most important use here is as an antidote for cyanide poisoning. Because sodium thiosulphate can donate one of its sulphur atoms, it helps convert cyanide into thiocyanate which can then be safely excreted from the body. Other medical applications include:

  • As an antifungal agent for the treatment of things like ringworm or tinea versicolour
  • To treat pityriasis versicolour
  • To prevent alkylation and tissue destruction in chemotherapy patients
  • As an anti-calcification treatment for the rare but serious illness, calciphylaxis
  • To protect against cisplatin toxicity by binding to free platinum

In vitro testing has shown that sodium thiosulphate even has neuroprotective properties. This has led many to speculate that, in the future, it has the potential to treat neurodegenerative diseases like Alzheimer or Parkinson disease.

But this compound is not restricted to the medical industry. Some other uses include:

  • In iodometry as an analytical reagent for chlorine, bromine, sulphide etc.
  • In photography in the manufacture of photographic fixers
  • In leather tanning as a dichromate reducing agent
  • In gold extraction as a component of a lixiviant
  • In water treatment to dechlorinate or neutralise tap water, pool, aquariums, etc.
A macro shot of a crystal with pink and blue light around it
Sodium thiosulphate structure is a monoclinic crystal system. It is also composed of two sodium cations (Na+) and a thiosulphate anion (S2O3-). This is what gives it the chemical formula Na2S2O3

Reactions

Sodium thiosulphate is reactive with a number of compounds and is used in many reactions across a range of industries. Here are some of the main ones:

  1. Clock Reactions

A clock reaction is a unique type of chemical reaction that results in a colourless solution rapidly changing colour after a period of delay. This happens because there is a clockwise series of 2 reactions taking place.

This experiment is used to show how chemical kinetics work in action. The best example is the iodine clock reaction, which involves 2 solutions getting mixed together.

First, hydrogen peroxide is mixed with a dilute acid (e.g. sulphuric acid or acetic acid) in a beaker. In a second container, potassium iodide is mixed with sodium thiosulphate and starch. Both of these solutions appear slightly cloudy and colourless.

When added together, two reactions occur: the main reaction and a secondary reaction. The main reaction involves the production of iodine, while the second reaction sees the iodine getting reconverted to iodide. This is where the clock reaction takes place as these reactions continue as a cycle, giving rise to an ‘induction period’ where nothing appears to be happening to the solution.

This cycle of iodide-iodine-iodide continues until the thiosulphate has ran out. When this happens, the iodine instead reacts with the starch. This is recognised by the solution suddenly turning a black-blue colour. This dramatic colour change tells us that the reaction is complete.

  1. Hydrochloric Acid & Sodium Thiosulphate

This is another reaction that is used to demonstrate kinetics in chemistry. When sodium thiosulphate is added to a solution of hydrochloric acid, one of the products that are formed is elemental sulphur.

The presence of sulphur acts as a colloid and gradually clouds the solution. The time it takes to turn completely opaque is dependent on the concentration of sodium thiosulphate. Strong concentrations will mean more particles which will in turn create more collisions and increase the rate of reaction.

This experiment is commonly used in schools to show students how the rate of a reaction can be predicted and measured. If you would like to know more about it, check out this blog post.

  1. Other Reactions

In the textile and paper industries, sodium thiosulphate pentahydrate is often oxidised by chlorine to create sodium sulphate (Na2SO4), sulphuric acid (H2SO4) and hydrochloric acid (HCl).

In analytical chemistry, it is oxidised by iodine to form sodium tetrathionate (Na2S4O6). This happens because of the stoichiometric reaction that takes place between the thiosulphate anion and iodine. This makes sodium thiosulphate useful in an iodometric titration.

Safety Hazards

Although sodium thiosulphate is classified as a non-toxic compound, it is important to note that when it is heated to decomposition it releases toxic fumes, namely sulphur oxides (SOx).

If you are exposed to or accidentally ingest these fumes, they could irritate your eyes, skin and mucous membranes, as well as cause breathing problems. When working with sodium thiosulphate under heat, it is important to wear the appropriate PPE and to ensure you’re in a ventilated area.

Sodium thiosulphate can also react very erratically with some oxidising agents. For that reason, it should always be stored away from substances like potassium permanganate, nitrate or chlorate.

An IV drip. Sodium thiosulphate can be used intravenously like this.
Sodium thiosulphate has a range of medical uses. It can be used to treat very rare diseases, like cyanide poisoning or calciphylaxis; to act as a topical anti-fungal for ringworm and tinea versicolour; it can even be used intravenously in some medical applications.

About the author

Jessica Clifton

Director

Jessica is a Director at ReAgent and leads a variety of growth projects. She has an extensive background in marketing, and has worked in the chemical manufacturing industry since 2019. When she’s not writing articles for ReAgent, Jessica can be found on a run, in her campervan, building LEGO, or watching Star Wars.
View all posts by Jessica Clifton

Disclaimer

The blog on chemicals.co.uk and everything published on it is provided as an information resource only. The blog, its authors and affiliates accept no responsibility for any accident, injury or damage caused in part or directly from following the information provided on this website. We do not recommend using any chemical without first consulting the Material Safety Data Sheet which can be obtained from the manufacturer and following the safety advice and precautions on the product label. If you are in any doubt about health and safety issues please consult the Health & Safety Executive (HSE).