chemistry

Sodium thiosulfate

Sodium thiosulfate is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Sodium thiosulfate rather than just read about it. In short: Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula Na2S2O3·xH2O. Typically it is available as the white or colorless pentahydrate (x = 5), which is a white solid that dissolves well in water.

Key takeaways

Sodium thiosulfate belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Sodium thiosulfate to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sodium thiosulfate from memory before moving on to harder problems.

Reference excerpt

Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula Na2S2O3·xH2O. Typically it is available as the white or colorless pentahydrate (x = 5), which is a white solid that dissolves well in water. The compound is a reducing agent and a ligand, and these properties underpin its applications. Historically known as "hyposulphite of soda", sodium thiosulfate is used in medicine as a treatment for cyanide poisoning. It is also a reliable photographic fixer and played a key role in the development of photography in the 19th century.

Uses Sodium thiosulfate is used predominantly in dyeing. It converts some dyes to their soluble colorless "leuco" forms. It is also used to bleach "wool, cotton, silk, soaps, glues, clay, sand, bauxite, and edible oils, edible fats, and gelatin."

Medical uses

Sodium thiosulfate is used to treat cyanide poisoning. It is on the World Health Organization's List of Essential Medicines. Other uses include topical treatment of ringworm and tinea versicolor, and treating some side effects of hemodialysis and chemotherapy. In September 2022, the U.S. Food and Drug Administration (FDA) approved sodium thiosulfate under the trade name Pedmark to lessen the risk of ototoxicity and hearing loss in infant, child, and adolescent cancer patients receiving the chemotherapy medication cisplatin.

Photographic processing

In photography, sodium thiosulfate is used in both film and photographic paper processing as a photographic fixer. It is sometimes still called "hypo" from the original chemical name, hyposulphite of soda. It functions to dissolve silver halides, e.g., AgBr, components of photographic emulsions. Ammonium thiosulfate is typically preferred to sodium thiosulfate for this application. The ability of thiosulfate to dissolve silver ions is related to its ability to dissolve gold ions.

Neutralizing chlorinated water It is used to dechlorinate tap water including lowering chlorine levels for use in aquariums, swimming pools, and spas (e.g., following superchlorination) and within water treatment plants and within ECS BWTS to treat settled backwash water prior to release into rivers. The reduction reaction is analogous to the iodine reduction reaction. In pH testing of bleach substances, sodium thiosulfate neutralizes the color-removing effects of bleach and allows one to test the pH of bleach solutions with liquid indicators. The relevant reaction is akin to the iodine reaction: thiosulfate reduces the hypochlorite (the active ingredient in bleach) and in so doing becomes oxidized to sulfate. The complete reaction is:

4 NaClO + Na2S2O3 + 2 NaOH → 4 NaCl + 2 Na2SO4 + H2O Similarly, sodium thiosulfate reacts with bromine, removing the free bromine from the solution. Solutions of sodium thiosulfate are commonly used as a precaution in chemistry laboratories when working with bromine and for the safe disposal of bromine, iodine, or other strong oxidizers.

Structure

Two polymorphs are known as pentahydrate. The anhydrous salt exists in several polymorphs. In the solid state, 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 terminal sulfur holds a significant negative charge and the S-O interactions have more double-bond character.

Production Sodium thiosulfate is prepared by oxidation of sodium sulfite with sulfur. It is also produced from waste sodium sulfide from the manufacture of sulfur dyes. This salt can also be prepared by boiling aqueous sodium hydroxide and sulfur according to the following equation. However, this is not recommended outside of a laboratory, as exposure to hydrogen sulfide can result if improperly handled.

6 NaOH + 4 S → 2 Na2S + Na2S2O3 + 3 H2O

Principal reactions Upon heating to 300 °C, it decomposes to sodium sulfate and sodium polysulfide:

4 Na2S2O3 → 3 Na2SO4 + Na2S5 Thiosulfate salts characteristically decompose upon treatment with acids. Initial protonation occurs at sulfur. When the protonation is conducted in diethyl ether at −78 °C, 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 dissociations, respectively. Under normal conditions, acidification of solutions of this salt excess with even dilute acids results in complete decomposition to sulfur, sulfur dioxide, and water:

8 Na2S2O3 + 16 HCl → 16 NaCl + S8 + 8 SO2 + 8 H2O

Coordination chemistry Thiosulfate forms complexes with transition metal ions. One such complex is [Au(S2O3)2]3−.

Iodometry Some analytical procedures exploit the oxidizability of thiosulfate anion by iodine. The reaction produces tetrathionate:

2 S2O2−3 + I2 → S4O2−6 + 2 I− Due to the quantitative nature of this reaction, as well as because Na2S2O3·5H2O has an excellent shelf-life, it is used as a titrant in iodometry. Na2S2O3·5H2O is also a component of iodine clock experiments. This particular use can be set up to measure the oxygen content of water through a long series of reactions in the Winkler test for dissolved oxygen. It is also used in estimating volumetrically the concentrations of certain compounds in solution (hydrogen peroxide, for instance) and in estimating the chlorine content in commercial bleaching powder and water.

Organic chemistry Alkylation of sodium thiosulfate gives S-alkylthiosulfates, which are called Bunte salts. The alkylthiosulfates are susceptible to hydrolysis, affording the thiol. This reaction is illustrated by one synthesis of thioglycolic acid:

ClCH2CO2H + Na2S2O3 → Na[O3S2CH2CO2H] + NaCl Na[O3S2CH2CO2H] + H2O → HSCH2CO2H + NaHSO4

Safety Sodium thiosulfate has low toxicity. LDLo for rabbits is 4000 mg/kg.

References

Illustrations

Worked examples

Example 1 — a first encounter with Sodium thiosulfate

Start with the simplest possible case. Write down what Sodium thiosulfate claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sodium thiosulfate before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sodium thiosulfate ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sodium thiosulfate

In research: Sodium thiosulfate appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sodium thiosulfate in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sodium thiosulfate is common in secondary-school and first-year university syllabi. It links to neighbouring topics Antidotes, Antifungals for dermatologic use, Orphan drugs, so understanding it makes those chapters shorter.
In everyday life: Look for Sodium thiosulfate outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Sodium thiosulfate” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Sodium thiosulfate in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Sodium thiosulfate means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Sodium thiosulfate out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Sodium thiosulfate in simple terms?

Why does Sodium thiosulfate matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sodium thiosulfate?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sodium thiosulfate.