What is sodium thiosulfate?
The short answer: sodium thiosulfate (sodium thiosulphate) is an inorganic sodium salt with the formula Na2S2O3. It is a mild reducing agent that is very soluble in water. Industry uses it to remove chlorine from water, to fix photographic film, as a titrant in iodometric analysis, as an “antichlor” in textile bleaching and in leaching and extraction work. It is usually sold as the pentahydrate, Na2S2O3·5H2O.
The old trade name hypo comes from “sodium hyposulfite”, an outdated name for the same compound. Do not confuse it with sodium hydrosulfite (sodium dithionite, Na2S2O4), a much stronger and self-heating reducing agent used in vat dyeing. When you order, quote the CAS number so there is no mix-up.
Sodium thiosulfate formula and molar mass
The thiosulfate ion, S2O32−, is a sulphate ion in which one oxygen atom has been replaced by sulphur. Two sodium ions balance its charge, which gives the formula Na2S2O3. In trade you meet two forms:
| Item | Anhydrous | Pentahydrate |
|---|---|---|
| Formula | Na2S2O3 | Na2S2O3·5H2O |
| Molar mass | 158.11 g/mol | 248.18 g/mol |
| CAS number | 7772-98-7 | 10102-17-7 |
| Appearance | White powder | Colourless to white crystals or granules |
| HS code (6-digit) | 2832.30 (thiosulphates) | |
| Other names | Sodium thiosulphate, hypo, sodium hyposulfite (old name), disodium thiosulfate | |
The molar mass of the anhydrous salt adds up from two sodiums at 22.99, two sulphurs at 32.06 and three oxygens at 16.00: 45.98 + 64.12 + 48.00 = 158.10, rounded to 158.11 g/mol with more precise atomic weights. Five waters of crystallisation add 90.08, giving about 248.18 g/mol for the pentahydrate. That difference matters when you calculate doses: 1 kg of pentahydrate contains only about 637 g of Na2S2O3.
Properties of sodium thiosulfate pentahydrate
- Solubility: very soluble in water, practically insoluble in ethanol.
- Heat: the pentahydrate melts in its own water of crystallisation at about 48 °C and loses that water on further heating. This is why it is used in some reusable heat packs.
- Acids: in acid the thiosulfate ion breaks down, releasing sulphur dioxide gas and a pale yellow precipitate of sulphur. That is the classic “disappearing cross” rate experiment in schools, and the reason it must be stored away from acids.
- Oxidisers: it reacts with chlorine, hypochlorite, iodine and other oxidising agents, which is the basis of most of its uses.
Is sodium thiosulfate a base? Not in the way caustic soda is. It is a salt, and its water solutions are close to neutral. It is chosen for its reducing power, not for adjusting pH.
What is sodium thiosulfate used for?
Dechlorination of water
The largest everyday use is as a chlorine neutraliser. Thiosulfate reduces free chlorine and hypochlorite to harmless chloride, and is itself oxidised to sulphate. It is used to dechlorinate treated effluent before discharge, to neutralise chlorine in water samples before microbiological testing, to make tap water safe for fish in aquariums, and to bring down chlorine in swimming pools and hot tubs after a shock dose. Some pool products also sell it for removing chloramine.
Photography (“hypo” fixer)
In black-and-white film and paper processing, sodium thiosulfate dissolves the unexposed silver halide so the image no longer reacts to light. This is the original “hypo” fixer, and the reason the nickname stuck.
Analytical chemistry: iodometric titration
Standardised thiosulfate solution is the titrant in iodometry. It reacts with iodine as 2 S2O32− + I2 → S4O62− + 2 I−, with starch as the indicator. Laboratories use it to measure available chlorine in bleach, dissolved oxygen (Winkler method), copper content and many other oxidising agents. Analytical work calls for a reagent grade.
Textiles, leather and paper
After bleaching with chlorine or hypochlorite, textile and pulp processors use it as an antichlor to stop residual bleach from damaging the fibre. It is also used in leather processing.
Mining and gold extraction
Thiosulfate leaching has been studied and used as an alternative to cyanide for extracting gold from some ores, because it forms a soluble gold–thiosulfate complex.
Medicine
Pharmaceutical-grade sodium thiosulfate is used by clinicians, for example as part of cyanide poisoning treatment. Medical use is a matter for doctors and the relevant medicine regulator; this guide covers industrial and laboratory supply only.
How much sodium thiosulfate neutralises chlorine?
It depends on how much chlorine is in the water, the water volume and the strength of the product you use, so there is no single figure. On paper, the full reaction of thiosulfate with chlorine to sulphate needs a little under 1 mg of pentahydrate for every 1 mg of chlorine. In real water the reaction and losses vary, so the practical method is:
- Measure the free chlorine level with a test kit.
- Work out the dose from the product label or your plant’s procedure for that water volume.
- Add it in steps, mix, and test again before adding more.
Does sodium thiosulfate lower pH? The dechlorination reaction produces some acid, so pH can drop slightly, especially after a large dose. Check pH after treatment and adjust if needed. Overdosing also leaves thiosulfate in the water, which will consume the next chlorine you add.
Sodium thiosulfate grades
| Grade | Typical use | What to check |
|---|---|---|
| Technical / industrial (pentahydrate crystals) | Effluent dechlorination, textiles, leather, photography, pools | Assay, insolubles, iron, sulphide, crystal size |
| Extra pure / laboratory reagent (AR) | Iodometric titration, water testing labs | Assay and impurity limits on the COA, clean solution appearance |
| Pharmacopoeia (IP / BP / USP) | Pharmaceutical manufacture | Compliance with the named monograph, and the documents your regulator needs |
| Anhydrous | Where water content or weight per dose matters | Assay on a dry basis, moisture |
Grades are made to an agreed specification. Ask for the Certificate of Analysis (COA) of the batch you will receive, not only a typical specification sheet.
Safety, storage and shipping
Sodium thiosulfate is a low-hazard chemical compared with most water treatment chemicals, but it still needs sensible handling:
- Avoid creating dust and wear eye protection and gloves when handling bulk material.
- Keep it in closed bags or drums in a cool, dry store. The pentahydrate can cake in humid conditions and lose water in warm, dry air.
- Store it away from acids (which release sulphur dioxide) and strong oxidising agents.
- When strongly heated it decomposes and can give off sulphur oxides.
Most safety data sheets do not classify it as dangerous goods for transport, but always confirm the classification on the SDS for the exact grade you buy. Each Scapex shipment comes with a 16-section GHS MSDS and a COA.
Where to buy sodium thiosulfate: questions to ask a supplier
- Is it the pentahydrate or the anhydrous form, and what is the CAS number on the documents?
- Which grade is it, and what do the assay and impurity limits on the batch COA show?
- What crystal or particle size is supplied?
- Which packing is offered (bags with liners, drums) and how is it protected from moisture?
- Which documents come with the shipment: COA, MSDS, certificate of origin, packing list and commercial invoice?
Enquire with Scapex Exports
Scapex Exports is an exporter and supplier of industrial and fine chemicals from India. Thiosulfate products in our catalogue include sodium thiosulphate pentahydrate (hypo), sodium thiosulfate 5-hydrate extra pure for water treatment, sodium thiosulfate 5-hydrate extra pure laboratory reagent and potassium thiosulfate. Each product page links to its COA and MSDS.
For related reading, see our guides to sodium hypochlorite grades and handling, the chlorine source thiosulfate is most often used to neutralise, and sodium metabisulphite, another sulphur-based reducing agent used for dechlorination.
Send the form and grade you need, the quantity, packing and destination port, and we will reply with a quotation.
Frequently asked questions
What is sodium thiosulfate?
Sodium thiosulfate is an inorganic salt with the formula Na2S2O3. It is a water-soluble, mild reducing agent used to neutralise chlorine, fix photographic film, titrate iodine in laboratories and act as an antichlor in textile bleaching. It is usually sold as the pentahydrate, Na2S2O3·5H2O.
What is the formula and molar mass of sodium thiosulfate?
The formula is Na2S2O3, with a molar mass of 158.11 g/mol (CAS 7772-98-7). The pentahydrate, Na2S2O3·5H2O, has a molar mass of about 248.18 g/mol (CAS 10102-17-7).
What is sodium thiosulfate used for?
Mainly for dechlorinating water (effluent, pools, hot tubs, aquariums and water samples), as the hypo fixer in photography, as the titrant in iodometric analysis, as an antichlor in textile and pulp bleaching, in leather processing and in thiosulfate gold leaching.
How much sodium thiosulfate neutralizes chlorine?
It depends on the chlorine level, the water volume and the product strength. On paper, fully reducing chlorine to sulphate takes a little under 1 mg of pentahydrate per 1 mg of chlorine, but real doses vary, so dose from the product label, add in steps and retest with a chlorine test kit.
Is sodium thiosulfate a base?
No. It is a salt, and its water solutions are close to neutral. It is used for its reducing power, not to raise pH.
Does sodium thiosulfate lower pH?
Slightly. The reaction with chlorine produces some acid, so pH can fall a little after a large dose. Test pH after dechlorinating and adjust if needed.
Is sodium thiosulfate the same as sodium hydrosulfite?
No. Hypo is an old name for sodium thiosulfate (Na2S2O3). Sodium hydrosulfite is sodium dithionite (Na2S2O4), a stronger reducing agent used in vat dyeing. Quote the CAS number when ordering.
Sources: PubChem (US National Library of Medicine) compound records for sodium thiosulfate, CAS 7772-98-7, and sodium thiosulfate pentahydrate, CAS 10102-17-7; IUPAC standard atomic weights; World Customs Organization Harmonized System, heading 28.32.