Sodium Thiosulfate: The Complete Chemistry, Formula, and Dechlorination Guide

What Sodium Thiosulfate Actually Is

Sodium thiosulfate is the sodium salt of thiosulfuric acid — an inorganic crystalline solid with formula Na₂S₂O₃ whose defining feature is two inequivalent sulfur atoms, one at an oxidation state of +5 and one at −1, arranged as a tetrahedral [S–SO₃]²⁻ anion. That structure is why the compound is a mild but versatile reducing agent, a facile chelator of soft metal cations like silver and gold, and a near-neutral chlorine quencher. It is not a sulfite, not a sulfate, and definitely not a sulfide — though it is related to all three by oxidation state.

In a bottle, the common form you will touch is the pentahydrate, Na₂S₂O₃·5H₂O. These are glassy, colorless-to-white, chunky crystals that look a little like rock salt with a softer, slightly translucent edge. They dissolve in water endothermically — the solution noticeably cools as the crystals break down — which is why sodium thiosulfate has a long history as a lecture-demonstration "supercooled liquid" reusable hand warmer.

Commercial sodium thiosulfate traces back to 1799, when French chemist Louis-Joseph Proust first characterized it, but its modern history begins in 1840 with John Herschel's discovery that it dissolves unexposed silver halides. That discovery made photography practical. For the next 160 years, "hypo" — a nineteenth-century abbreviation of "sodium hyposulfite," the old and incorrect name — was inseparable from the darkroom. The name persists in photography even though chemists moved to the correct "thiosulfate" long ago.

Today the dominant volume applications are water treatment and iodometric titration, with photography, gold leaching, pharmaceutical antidote, and textile processing rounding out the market. Alliance Chemical has supplied sodium thiosulfate pentahydrate to all of these sectors — municipal dechlorination plants, aquaculture farms, analytical and pharmaceutical labs, metal-finishing shops, and textile mills — with a Certificate of Analysis on every lot.

Chemistry and Structure

Sodium thiosulfate consists of two sodium cations paired with a thiosulfate anion, [S₂O₃]²⁻. The anion is essentially a sulfate ion (SO₄²⁻) with one of the oxygens replaced by a sulfur. That single substitution drops the symmetry, breaks the four-fold equivalence, and creates the asymmetric reactivity that all of sodium thiosulfate's uses hinge on.

The bonding picture is usually drawn with a central sulfur surrounded tetrahedrally by three oxygens and one terminal sulfur. The S–O bonds have significant double-bond character (average length ≈ 1.47 Å); the central-to-terminal S–S bond is longer (≈ 2.01 Å) and polarizable. Formal oxidation states are conventionally assigned as +5 on the central sulfur and −1 on the terminal sulfur, giving an average oxidation state of +2 — the unusual number that tripped up nineteenth-century chemists and led to the obsolete name "sodium hyposulfite."

Three reaction families dominate the chemistry. Oxidation to tetrathionate is the soft-oxidation pathway used in iodometric titration: two thiosulfate ions couple at their terminal sulfurs as they donate electrons, forming the S₄O₆²⁻ tetrathionate anion and two iodide ions. Oxidation to sulfate is the full-oxidation pathway used in chlorine dechlorination, where each sulfur atom ends up fully oxidized to SO₄²⁻ while the chlorine reduces all the way down to chloride. Reaction with acid is the decomposition pathway that is the heart of every undergraduate kinetics demo: thiosulfate + HCl → colloidal sulfur + SO₂ + chloride + water, with the sulfur making the solution turn milky white.

A fourth, distinctly more specialized behavior is soft-metal complexation. Silver and gold cations form very stable bis-thiosulfate complexes — [Ag(S₂O₃)₂]³⁻ and [Au(S₂O₃)₂]³⁻. That is the exact reason sodium thiosulfate fixes photographs (silver) and can leach gold from ore (gold), and it is why aurothiosulfate chemistry has been studied as an environmentally cleaner alternative to cyanide extraction.

Physical Properties at a Glance

All values below are for the pentahydrate Na₂S₂O₃·5H₂O unless otherwise noted, because that is the form shipped in ninety percent of industrial and laboratory orders. When the anhydrous salt differs meaningfully, both numbers are shown.

Three numbers deserve emphasis for process engineers. First, solubility rises sharply with temperature, from about 70 g per 100 mL at 20°C to well over 230 g per 100 mL at 60°C. That makes it trivially easy to prepare concentrated dosing stock solutions in warm water. Second, the dissolution is distinctly endothermic; a kilo of pentahydrate mixed into a liter of water will visibly chill the beaker. Third, the pentahydrate melts at 48°C, and the "melting" is actually the solid dissolving in the water of crystallization — if you keep a drum of pentahydrate above 50°C in a warehouse, it will liquefy into a sulfur-syrup that is much harder to handle.

Keep pentahydrate drums below 40°C. At 48°C, they melt in their own water.

Pentahydrate vs. Anhydrous: Which Form to Order

The two commercial forms are chemically the same salt but behave differently in packaging, storage, and dosing. Nine of ten laboratory and industrial orders ship as pentahydrate. The anhydrous form is specified where added water would interfere with the process.

Choose pentahydrate for general water-treatment dechlorination, aquarium and aquaculture use, photography, iodometric titration (using freshly standardized solutions), textile mordanting, and metal-finishing rinse water. The five built-in water molecules dissolve along with the salt, so for any aqueous application they are chemically neutral.

Choose anhydrous when the water content would cause a problem. That comes up mostly in three situations: non-aqueous process chemistry (you are adding thiosulfate to a solvent system where water interferes), precision dry-weight reagent preparation (you want to weigh out a known mass of active ingredient without the hydrate correction), and long-term moisture-sensitive blends (you are pre-mixing thiosulfate into a solid formulation where hydrate water could phase-separate over time).

Regulatory and Food-Grade Status

Sodium thiosulfate is unusually well-positioned on the regulatory side compared with most inorganic reducing agents. It holds FDA GRAS status for direct food addition, it has an active USP and Food Chemicals Codex (FCC) monograph, and it is an EPA-approved dechlorinator in drinking-water residual work.

The federal regulatory anchors that matter for buyers:

• 21 CFR 184.1807 — FDA affirmation of sodium thiosulfate as Generally Recognized as Safe (GRAS) for direct food use, chiefly as a formulation aid, processing aid, and reducing agent.
• USP monograph (Sodium Thiosulfate) — pharmacopeial specification for pentahydrate, covering identity, assay ≥ 99.0%, heavy metals, sulfate and sulfite contaminants, chloride, iron, and loss on drying limits.
• Food Chemicals Codex (FCC) — the food-grade analog of USP, with slightly different limits on arsenic, heavy metals, and lead. Use FCC-grade for direct food-contact dechlorination (bottled water, beverage plants).
• EPA-accepted dechlorinator — widely used in municipal drinking-water and wastewater discharge permits to remove residual chlorine before effluent release.
• WHO Model List of Essential Medicines — listed as the antidote for cyanide poisoning (paired with sodium nitrite).
• NSF/ANSI 60 — certified grades are available for drinking-water treatment chemical applications.

For regulated applications, the decision between grades is straightforward: use USP or FCC grade whenever the finished product or treated stream could enter pharmaceutical, food, or potable-water use. Technical grade sodium thiosulfate is sufficient for textile processing, photographic-lab effluent, gold-leaching trials, non-potable water dechlorination, and academic research, and is typically twenty to thirty-five percent less expensive per kilogram than USP grade.

On the transport and storage side, sodium thiosulfate pentahydrate is not regulated as DOT hazardous material, is not RCRA-listed, and is not on the CERCLA reportable-quantity list. For international shipments it carries no UN number for the pentahydrate; the anhydrous form is likewise non-hazardous for transport. Bulk domestic shipments move on standard palletized freight.

Clinical use — which is the one regulatory corner most industrial buyers will never touch — is governed by the FDA drug approval framework. Pedmark (sodium thiosulfate injection) is an FDA-approved drug for prevention of cisplatin-induced ototoxicity in pediatric oncology, a 2022 approval that revived clinical interest in the compound and brought sodium thiosulfate back into the WHO Essential Medicines list.

Synonyms and Alternative Names

Sodium thiosulfate shows up under a long list of trade and historical names. Almost all of them refer to the same compound — but a couple are worth double-checking before ordering substitute material.

• Hypo — universal photography-darkroom abbreviation.
• Sodium hyposulfite — obsolete nineteenth-century name, chemically incorrect but still printed on older labels.
• Antichlor — industrial shorthand for its chlorine-quenching role in textile and paper processing.
• Sodium thiosulphate — British spelling; identical chemistry.
• Na₂S₂O₃ / Na₂S₂O₃·5H₂O — formula shorthand.
• Ametox — older pharmaceutical trade name for sodium thiosulfate injection.
• Pedmark — current FDA-approved sodium thiosulfate injection brand (pediatric oncology).
• Caution: Do not confuse with sodium dithionite (Na₂S₂O₄, a much stronger reducing agent), sodium tetrathionate (Na₂S₄O₆, the oxidation product), or sodium persulfate (Na₂S₂O₈, an oxidizer) — the formulas look similar, the chemistries are opposite.

Industrial Applications

Sodium thiosulfate is one of the most versatile single-anion inorganic reagents on the market. Its mild reducing power, near-neutral pH, low toxicity, and ability to complex soft metals make it the default chemistry across a surprisingly broad set of industries.

For an analogous inorganic reducing agent with a very different pH and odor signature, see our sodium bisulfite guide, which covers the closely related (but stronger and lower-pH) NaHSO₃ chemistry used in boiler-water oxygen scavenging, food preservation, and wine stabilization. And for oxidizing-side chemistry, our hydrogen peroxide guide walks through the titrant side of many iodometric determinations.

Two less-visible but commercially meaningful uses round out the market. Mining assay labs use sodium thiosulfate to quantify silver during fire-assay workflows. Leather tanneries use it as an antichlor after chromium bleaching. Neither application is volume-dominant, but together they account for a steady portion of industrial pentahydrate demand.

Dechlorination: How to Calculate the Dose

The most common question we get about sodium thiosulfate is also the simplest to answer once the stoichiometry is clear: 2.85 grams of pentahydrate neutralizes 1 gram of free available chlorine in water. That single number, applied correctly, covers pool and spa superchlorination recovery, aquarium and aquaculture tap-water conditioning, municipal effluent dechlorination, and emergency water-system rinse-out.

The full balanced equation (for the hypochlorite form most plants dose) is:

The dosing worksheet then becomes a three-line calculation:

Three operational notes matter for every one of these applications. First, pre-dissolve before dosing. Sodium thiosulfate pentahydrate is highly water-soluble (70 g / 100 mL at 20°C) and dissolves in seconds in warm water; a pre-dissolved slug mixes faster and evenly than dry crystals. Second, never add thiosulfate directly to a concentrated bleach tank. Localized over-stoichiometry can release sulfur compounds and foul equipment. Dose into the treated stream downstream of the bleach, or into a well-mixed basin with residence time. Third, always verify with a post-dose free-chlorine test. The reaction is fast, but real-world dose calculations are rarely perfect and sensitive systems demand confirmation.

Two point eight five. That is the number. Memorize it.

For chloraminated municipal water (the monochloramine NH₂Cl now used by most U.S. utilities for distribution disinfection), the picture is slightly more nuanced. Thiosulfate reduces the chloramine but releases free ammonia as a byproduct, which can be toxic to fish at aquarium concentrations. Aquarists and aquaculturists working with chloraminated supply typically combine a thiosulfate dechlorinator with an ammonia-binding conditioner (sodium hydroxymethanesulfonate or similar), or use a dedicated chloramine-neutralizing product. For bulk utility work, this is usually handled by a parallel aeration or biological step downstream.

A final note on reaction speed: at typical water-treatment concentrations and room temperature, the dechlorination reaction is essentially instantaneous — completed in under ten seconds of mixing. That is an order of magnitude faster than sulfur dioxide sparging or most activated-carbon contactors. Where a pump-dosed liquid dechlorinant has to hit a flowing stream and be done before the next unit operation, thiosulfate pentahydrate is hard to beat.

Sodium Thiosulfate vs. Sodium Bisulfite

This is the most frequently muddled comparison in inorganic reducing chemistry. Both salts neutralize chlorine. Both are widely available at food and technical grades. Both are reasonably inexpensive. But they behave differently, and the wrong choice can cost you a dye batch, a boiler, or a fish tank.

The decision rule: pick thiosulfate when pH stability matters, pick bisulfite when cost per active kilogram or odor-neutral is not a concern. Aquariums and potable-water dechlorination almost always favor thiosulfate because the near-neutral product pH is the whole point. Boilers and cooling towers usually favor bisulfite because the mild pH drop is inside the normal operating window and the raw material is cheaper. For detailed bisulfite process chemistry, including oxygen scavenging in steam systems, see our complete sodium bisulfite guide.

A related comparison some buyers raise: sodium thiosulfate vs. sodium metabisulfite (Na₂S₂O₅). Metabisulfite is essentially a dry, storage-stable form of bisulfite (it hydrolyzes to bisulfite in water), so all the bisulfite tradeoffs apply — pH drop, SO₂ odor, food-preservation use. For industrial dechlorination you will rarely choose metabisulfite over thiosulfate unless the dry form is specifically required. See our sodium metabisulfite food-grade guide for the food preservation and wine chemistry details.

Safety and Handling

Sodium thiosulfate is one of the least hazardous industrial salts we stock. Oral LD₅₀ in rats runs above 5,600 mg/kg, it is not DOT-regulated, not RCRA-listed, and the FDA affirms it as GRAS for direct food use. None of that is a reason to skip basic hygiene and engineering controls — but the risk profile is genuinely mild compared with acids, strong bases, or oxidizing chemistries.

Standard PPE and engineering controls are straightforward:

• Dust masks (N95 or equivalent) for handling bulk crystals or powder; bulk transfer dust can irritate airways and eyes at high concentrations.
• Chemical splash goggles for solution preparation; nitrile gloves for prolonged skin contact.
• Local exhaust ventilation at dry-transfer points on high-throughput lines; no special ventilation needed for aqueous dosing downstream of the mix tank.
• Segregate storage from strong acids, strong oxidizers (bleach, chlorates, chromates, peroxides, permanganate), and ignition sources.
• Keep dry stock in tightly closed bags or drums; pentahydrate effloresces in very dry air (loses some crystal water) and clumps in humid air.

Spill response is simple: sweep up dry spills and reuse or bag for routine solid-waste disposal. Aqueous spills can be diluted and sent to sanitary sewer under most municipal permits (confirm with local authority). There is no evacuation, no neutralization, and no special decontamination requirement at typical lab and industrial quantities.

Incompatibility awareness matters more than toxicity. The three chemistries to keep separate: strong acids (SO₂ evolution), strong oxidizers (rapid exothermic oxidation to sulfate, can be vigorous with concentrated hypochlorite or permanganate), and soft-metal salts like silver nitrate (complexation; useful in photography chemistry but an unplanned precipitation in a process tank).

Occupational exposure limits: there is no OSHA PEL, no ACGIH TLV, and no NIOSH REL for sodium thiosulfate itself. Exposure limits on particulates not otherwise classified (15 mg/m³ total dust, 5 mg/m³ respirable) serve as the default. Fresh pentahydrate is essentially odorless; any SO₂ odor in a storage area is a warning that solid has come into contact with acid somewhere upstream.

First-aid guidance follows standard inorganic salt practice: flush eyes with water for 15 minutes for splashes, wash skin thoroughly with soap and water, rinse mouth and drink water for ingestion, and seek medical attention only if symptoms are severe or persistent. Clinical toxicology data is extensive because sodium thiosulfate is itself an FDA-approved antidote; acute human toxicity is low.

Grades We Stock at Alliance Chemical

Alliance Chemical stocks sodium thiosulfate pentahydrate as our primary commercial grade. The material meets USP pentahydrate monograph limits and is suitable for water treatment, aquaculture, iodometric titration, photography, metal finishing, and textile antichlor use.

Every Alliance Chemical lot of sodium thiosulfate pentahydrate ships with a Certificate of Analysis showing actual measured assay, heavy metals, sulfate, sulfite, chloride, iron, and loss-on-drying values. An SDS is provided on receipt. For FCC food-grade or NSF/ANSI 60 drinking-water-certified shipments, specify the required grade at time of order. Bulk drop-ships and large-volume municipal contracts are supported.

Sources and Further Reading

The data points in this guide are drawn from primary pharmacopeial, regulatory, and reference sources. Where a number is a range rather than a single value, it reflects real lot-to-lot variation in commercial product.

• PubChem CID 24477 — Sodium thiosulfate (anhydrous) (National Library of Medicine)
• PubChem CID 61472 — Sodium thiosulfate pentahydrate
• FDA 21 CFR 184.1807 — Direct Food Substance Affirmed as Generally Recognized as Safe: Sodium thiosulfate
• USP – NF monograph, Sodium Thiosulfate (current edition), United States Pharmacopeia
• Food Chemicals Codex (FCC) monograph, Sodium Thiosulfate
• WHO Model List of Essential Medicines — listing for sodium thiosulfate (cyanide antidote)
• EPA Method 330 — Chlorine, Total Residual (standard reference for free available chlorine quantitation)