- What it is. This sodium thiosulfate solution is a 1.0 N (1.0 M) concentrated titrant and stock solution, supplied in a 1 quart size. It is used for iodometric titrations of concentrated oxidants and for preparing weaker thiosulfate standards by dilution.
- How iodometry works. In iodometric methods, an oxidizing analyte reacts with excess potassium iodide to release iodine, which is then titrated with thiosulfate: 2 S2O3²⁻ + I2 → S4O6²⁻ + 2 I⁻. Starch indicator, added near the endpoint, turns blue-black with iodine and goes colorless when all iodine is consumed.
- Molar equals normal. Each thiosulfate ion gives up one electron in this reaction, so for iodometry its normality equals its molarity, and its equivalent weight equals its formula weight.
- Standardization. Standardize thiosulfate against primary standards that release a known amount of iodine from iodide, such as potassium iodate or potassium dichromate. Restandardize periodically, since thiosulfate solutions slowly change strength.
- Stability. Thiosulfate solutions decompose slowly due to bacterial action, light, and acidity, which can deposit sulfur and reduce strength. Clean storage and occasional restandardization keep results reliable.
- Bleach assays. Commercial bleach contains several percent available chlorine, so titrating a diluted aliquot with 1.0 N thiosulfate keeps titration volumes practical. Acidify the sample with excess potassium iodide, then titrate the released iodine to the starch endpoint.
- What each milliliter titrates. One milliliter of 1.0 N thiosulfate reacts with 1.0 milliequivalent, equal to 126.9 mg of iodine or 35.45 mg of chlorine as Cl2 (calculated). That strength suits concentrated samples such as commercial bleach and strong oxidant solutions.
- Calculating results. Available chlorine as Cl2 equals the titrant volume multiplied by the standardized normality and by 35.45 mg per milliequivalent, divided by the sample mass or volume. Express results as percent or g/L as your method requires.
- Stock for dilutions. Diluting 100 mL to 1 liter with freshly boiled, cooled deionized water gives approximately 0.1 N, which should then be standardized. For ready-to-use 0.1 N titrant, see Lab Pro's Sodium Thiosulfate 0.1 N Solution, 1 Gallon.
- Where it is used. Laboratories use 1.0 N sodium thiosulfate to determine available chlorine in concentrated bleach and disinfectants, assay strong oxidizers, prepare working thiosulfate standards, and dechlorinate samples or solutions when a concentrated reductant is needed.
Technical Specifications
| Chemical Name | Sodium thiosulfate, aqueous volumetric solution |
| CAS Numbers | 10102-17-7 (sodium thiosulfate pentahydrate) or 7772-98-7 (anhydrous), per SDS; 7732-18-5 (water) |
| Synonyms | Sodium hyposulfite, hypo |
| Formula (active) | Na2S2O3 (pentahydrate Na2S2O3·5H2O) |
| Formula Weight | 248.18 g/mol (pentahydrate); 158.11 g/mol (anhydrous) |
| Equivalent Weight | Formula weight (molar = normal in iodometry) |
| Concentration | 1.0 N (1.0 M) |
| Content | About 248.2 g/L as pentahydrate (calculated) |
| Titration Equivalence | 1.00 mL = 126.9 mg I2; 35.45 mg Cl2 (calculated) |
| Indicator | Starch, added near endpoint |
| Primary Standards | Potassium iodate; potassium dichromate |
| Appearance | Clear, colorless liquid |
| Stability Note | Protect from light, acid, and microbial contamination |
| Package Size | 1 quart |
Applications / Common Uses
- Bleach assay: Titrates available chlorine in concentrated bleach.
- Strong oxidant assays: Measures concentrated oxidizers.
- Working standard preparation: Dilutes to 0.1 N and weaker titrants.
- Dechlorination: Neutralizes chlorine in samples and solutions.
- Teaching labs: Demonstrates iodometric titration.
Storage, Handling & Shipping
Store tightly closed at room temperature, protected from light, acids, and oxidizers. Thiosulfate solutions slowly decompose through bacterial action and in acidic conditions, depositing sulfur, so keep them clean, avoid contamination, and restandardize periodically.
- Incompatible materials: Acids (release sulfur dioxide and sulfur), strong oxidizers, and iodine.
- Personal protection: Safety glasses and gloves.
- Spill response: Wipe up and rinse the area with water.
- First aid: Eyes: rinse with water for several minutes.
Related Categories & Services
- Shop Buffers, Standards & Solutions for related products in this category.
- Explore Inorganic Chemicals and ACS Grade Laboratory Chemicals.
- Vendor Managed Inventory (VMI) Program: Lab Pro can stock, track, and replenish this item on site so it never runs out. Contact info@labproinc.com or 888-452-2776 to set up a VMI assessment.
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FAQs
When should I use 1.0 N thiosulfate instead of 0.1 N?
Use 1.0 N for concentrated oxidants, such as commercial bleach or strong disinfectant solutions, to keep titration volumes practical. It also serves as a stock for preparing weaker thiosulfate standards. For residual chlorine and dissolved oxygen, the 0.1 N or weaker titrants give better resolution.
How do I dilute 1.0 N thiosulfate to 0.1 N?
Pipette 100 mL into a 1 liter volumetric flask and dilute to the mark with freshly boiled, cooled deionized water, which reduces dissolved carbon dioxide and microbes. Mix well, then standardize the diluted solution against potassium iodate before use, since dilution can introduce small errors.
How much chlorine does 1 mL of 1.0 N thiosulfate measure?
One milliliter of 1.0 N sodium thiosulfate corresponds to 35.45 mg of chlorine as Cl2. For bleach assays, multiply the titrant volume by this factor and divide by the sample mass or volume to report available chlorine, using the standardized normality. Subtract a reagent blank.
Why is thiosulfate used to remove chlorine?
Thiosulfate reduces chlorine and hypochlorite to chloride, neutralizing their oxidizing power. That is why it is added to water samples collected for bacteriological testing and used to dechlorinate solutions. In titrations, the same chemistry applies to iodine released by oxidants. Check your method's dose.
How should 1.0 N thiosulfate be stored?
Keep it tightly closed at room temperature, protected from light, acids, and contamination. Concentrated solutions are somewhat more stable than dilute ones, but they still decompose slowly over time. Re-standardize before critical work and discard solution that turns cloudy. Label the opening date on the container.
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