Popular LabKit · Recipe
0.1 N Sodium Thiosulphate: Preparation and Standardisation
How to prepare 0.1 N sodium thiosulphate and standardise it iodometrically with potassium iodate – quantities, reactions, method, formula and a worked example.
Preparation: 0.1 N (0.1 M) sodium thiosulphate, 1000 mL
| Component | Amount |
|---|---|
| Sodium thiosulphate pentahydrate, Na₂S₂O₃·5H₂O | 25 g (24.82 g theoretical) |
| Anhydrous sodium carbonate (stabiliser) | 0.2 g |
| Freshly boiled and cooled purified water | to 1000 mL |
Thiosulphate reacts with one electron per molecule in iodometry, so 0.1 N = 0.1 M.
Preparation
- Boil purified water for 5 minutes and cool it covered (removes CO₂ and kills bacteria that decompose thiosulphate).
- Dissolve 25 g sodium thiosulphate pentahydrate and 0.2 g sodium carbonate in it and make up to 1000 mL.
- Store in an amber bottle and let it stand for a day before standardising.
Standardisation with potassium iodate
- Dry potassium iodate at 110 °C to constant weight.
- Weigh accurately about 0.10 g into an iodine flask and dissolve in 25 mL of water.
- Add 2 g of potassium iodide and 10 mL of 1 M hydrochloric acid; stopper and let stand for 1 minute. Iodine is liberated.
- Titrate with the thiosulphate until the colour is pale straw-yellow, add 1 mL of starch solution and continue until the blue colour just disappears.
- Repeat twice more and use the mean factor.
1 mol KIO₃ ≡ 6 mol thiosulphate
Molarity = (weight of KIO₃ in g ÷ 214.00) × 6 ÷ (titre in mL ÷ 1000)
Worked example
0.1015 g KIO₃ needed 28.30 mL. Molarity = 0.1015 ÷ 214.00 × 6 ÷ 0.02830 = 0.1006 M, factor 1.0056.
Calculate your factor (pre-filled) →
Potassium dichromate is also used: 1 mol K₂Cr₂O₇ ≡ 6 mol thiosulphate. Choose “Sodium thiosulphate vs potassium dichromate” in the calculator.
For pharmacopoeial work, use the primary standard and quantities your pharmacopoeia specifies (some use potassium bromate or potassium dichromate). For better precision you can weigh about 1 g of potassium iodate, dissolve it to 250 mL in a volumetric flask and titrate 25 mL aliquots.
Recipes are calculated with the Popular LabKit buffer model (ionic strength and temperature corrected) and checked against published buffer tables. For a pharmacopoeial test, follow the reagent or buffer recipe in your monograph. Always confirm pH with a calibrated pH meter.
Frequently asked questions
Why add sodium carbonate?
A little alkali stops slow decomposition of thiosulphate to sulphur in slightly acidic water, so the strength stays steady for longer.
When should I add the starch?
Near the end point, when the solution is pale yellow. Added too early, starch binds iodine strongly and the end point is sluggish.
How stable is 0.1 N sodium thiosulphate?
Reasonably stable for a few weeks when made with boiled water and stored in amber glass, but restandardise regularly – weekly for routine use.