Preparation and Standardization of Sodium Thiosulphate
Aim: To perform the standardization of sodium thiosulphate.
Requirements:
- Apparatus: Beaker, Funnel, Pipette, Burrette.
- Chemicals: Potassium bromate, Sodium thiosulphate, HCl, Potassium iodide, Starch indicator.
Principle:
Potassium bromate is the primary standard. Potassium bromate can be used as an oxidising agent for inorganic reducers determination. For direct titration with potassium bromate are used specific redox indicators or various azo-dyes, are destroyed by bromate ion surplus (disappearance of colour). A mixture of potassium bromate and potassium bromide in acidified solution applies for back titration of organic compounds. This mixture produces bromine, which reacts with aryl radicals in organic compounds structures. The endpoint in bromatometric back titration is established in iodometric-like – titration with sodium thiosulfate, with starch serving as the indicator.
Procedure:
(A) 0.1 M sodium thiosulphate: Dissolve 25 g of sodium thiosulphate and 0.2 g of sodium carbonate in carbon dioxide-free water and dilute to 1000 ml with the same solvent.
(B) Standardisation: Dissolve 0.200 g of potassium bromate, weighed accurately, in sufficient water to produce 250.0 ml. To 50.0 ml of this solution add 2 g of potassium iodide and 3 ml of 2 M hydrochloric acid and titrate with the sodium thiosulphate solution using the starch solution, added towards the end of the titration, as an indicator until the blue colour is discharged.
1 ml of 0.1 M sodium thiosulphate is equivalent to 0.002784 g of KBrO3·
Observation Table of Preparation and Standardization of Sodium Thiosulphate
| Sr. No. | Parameter | Reading |
| 1. | Burette solution | Sodium thiosulphate solution |
| 2. | Conical flask solution | Potassium iodide, potassium bromate, HCl |
| 3. | Indicator | Starch solution |
| 4. | Endpoint | Blue to colourless |
| Burette reading B.R (ml) | Trial 1 | Trial 2 | Trial 3 | Average (BR) |
Reaction: Iodine is generated by the reaction between a known volume of standard potassium bromate and an unmeasured excess of potassium iodide:
BrO3−+ 6I− + 6H+ → Br− + 3I2 + 3H2O
The iodine produced is titrated with sodium thiosulfate solution.
Result: The molarity of sodium thiosulphate is …… M.
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