Sodium Thiosulfate
Properties
| State | Solid at room temperature |
| Color | White crystalline powder (anhydrous); colorless monoclinic crystals (pentahydrate) |
| Solubility | Very soluble in water (70 g/100 mL at 20 °C) |
| Melting Point | 48 °C (pentahydrate); 220 °C (anhydrous) |
| Boiling Point | Decomposes above 100 °C |
About Sodium Thiosulfate
Sodium thiosulfate (Na2S2O3, 158.108 g/mol; pentahydrate Na2S2O3.5H2O at 248.17 g/mol) is a salt that punches well above its weight in chemistry — three completely unrelated fields rely on it as a workhorse reagent. The thiosulfate anion S2O3 2- is structurally one sulfate oxygen replaced by a sulfur, giving an unsymmetrical tetrahedral geometry with one terminal S in the -2 state and a central S in the +5 state. In black-and-white photography, that structure is the basis of the fixing step: thiosulfate forms the soluble complex [Ag(S2O3)2] 3- with unreacted silver halide grains, washing them out of the emulsion and leaving only the developed silver image — Sir John Herschel discovered this in 1819 and 'hypo' (from the obsolete name 'hyposulfite') has been in continuous photographic use ever since. In analytical chemistry, sodium thiosulfate is the standard titrant for iodometry: it reduces iodine quantitatively in a 2:1 stoichiometry (2S2O3 2- + I2 -> S4O6 2- + 2I-), and the sharp endpoint with starch indicator (deep blue starch-iodine complex disappears at the equivalence point) makes it the go-to method for determining oxidizing agents like Cu 2+, ClO-, IO3 -, and dissolved O2. In emergency medicine, it is FDA-approved as the second component of the cyanide antidote kit — sulfur from thiosulfate is transferred to cyanide by the mitochondrial enzyme rhodanese, converting toxic CN- into the relatively harmless thiocyanate SCN- that is excreted renally.
Where you'll encounter it
If you have ever taken an analytical chemistry lab course, the standardized 0.1 N sodium thiosulfate solution you used in iodometric titrations was almost certainly prepared from the pentahydrate, standardized against potassium iodate or potassium dichromate, and stored away from light because S2O3 2- slowly decomposes in acid and bacterial action. The starch indicator added near the endpoint forms the famous deep blue starch-triiodide inclusion complex — and the moment that blue snaps to colorless is one of the cleanest visual endpoints in analytical chemistry. In a hospital ED treating cyanide poisoning from smoke inhalation or industrial exposure, the protocol since the 1930s has been amyl nitrite by inhalation, then IV sodium nitrite (10 mL of 3% solution) to convert hemoglobin to methemoglobin which sequesters cyanide, then 50 mL of 25% sodium thiosulfate IV over 10 minutes to provide the sulfur that rhodanese needs to detoxify the bound cyanide as thiocyanate. Newer protocols use hydroxocobalamin (Cyanokit) but thiosulfate remains a backup.
Common Uses
- Photographic fixer that dissolves unreacted silver halide as soluble [Ag(S2O3)2] 3- complex
- Standard titrant in iodometric analysis at 0.1 N for determining Cu, ClO-, IO3-, and dissolved O2
- IV antidote for cyanide poisoning at 12.5 g per 50 mL dose, providing sulfur for rhodanese-catalyzed CN- to SCN-
- Dechlorination agent for aquarium water and laboratory dechlorination at 1.6 mg per 1 mg residual Cl2
- Topical treatment for tinea versicolor as 25% solution (Tinver) for the superficial Malassezia infection
- IV treatment for calciphylaxis in chronic kidney disease patients to dissolve calcium-phosphate vascular deposits
- Reductant for cisplatin-induced nephrotoxicity in oncology
- Leaching agent for gold and silver extraction in environmentally friendlier alternatives to cyanide leaching
Safety Information
Low toxicity. GHS: not classified as hazardous. No OSHA PEL specifically for sodium thiosulfate; standard nuisance-dust limits apply. Acute oral LD50 in rats is 6.4 g/kg — extremely low toxicity for an inorganic salt. The major hazard is generation of SO2 and elemental sulfur if mixed with strong acids: S2O3 2- + 2H+ -> S(s) + SO2(g) + H2O, the famous 'sulfur clock' reaction used in kinetics teaching. SO2 has an OSHA PEL of 5 ppm TWA. Standard PPE (safety glasses, nitrile gloves) is sufficient for routine handling. The pentahydrate effloresces in dry air, losing water of crystallization and converting to a powdery residue, so seal containers tightly. Solutions should be protected from light and refrigerated to slow bacterial decomposition that produces tetrathionate and bisulfite.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.