Sodium thiosulphate: Preparation and standardization of molar and normal solutions
Sodium thiosulfate (Na2S2O3) is commonly used in the preparation of molar and normal solutions. Here is a general procedure for the preparation and standardization of a 0.1 M sodium thiosulfate solution and a 0.1 N sodium thiosulfate solution:
Preparation of 0.1 M sodium thiosulfate solution:
- Weigh 15.51 g of sodium thiosulfate pentahydrate (Na2S2O3·5H2O) using an analytical balance and transfer it to a 1-liter volumetric flask.
- Add about 800 mL of distilled water to the flask and swirl gently to dissolve the sodium thiosulfate.
- Once the sodium thiosulfate is completely dissolved, add more distilled water until the solution reaches the 1-liter mark on the flask.
- Cap the flask and mix the solution thoroughly to ensure uniformity.
Standardization of 0.1 M sodium thiosulfate solution:
- Weigh about 0.5 g of primary standard potassium iodate (KIO3) using an analytical balance and transfer it to a clean, dry Erlenmeyer flask.
- Add about 50 mL of distilled water to the flask and swirl gently to dissolve the KIO3.
- Add a few drops of starch indicator to the solution.
- Titrate the KIO3 solution with the 0.1 M sodium thiosulfate solution until the blue color disappears.
- Record the volume of the sodium thiosulfate solution used for the titration.
Calculation:
The molar mass of KIO3 is 214.00 g/mol. The equation for the reaction between KIO3 and sodium thiosulfate is:
2Na2S2O3 + KIO3 → Na4S2O6 + 2NaI + KCl
From the balanced equation, we know that 1 mole of KIO3 reacts with 2 moles of Na2S2O3. Therefore, the number of moles of Na2S2O3 can be calculated from the volume of sodium thiosulfate solution used in the titration and the molarity of the solution.
The weight of KIO3 used in the titration can also be used to calculate the molarity of the sodium thiosulfate solution.
Standardization of 0.1 N sodium thiosulfate solution:
- Weigh about 1.58 g of potassium iodide (KI) using an analytical balance and transfer it to a clean, dry Erlenmeyer flask.
- Add about 50 mL of distilled water to the flask and swirl gently to dissolve the KI.
- Add a few drops of starch indicator to the solution.
- Titrate the KI solution with the 0.1 N sodium thiosulfate solution until the blue color disappears.
- Record the volume of the sodium thiosulfate solution used for the titration.
Calculation:
The molar mass of KI is 166.00 g/mol. The equation for the reaction between KI and sodium thiosulfate is:
2Na2S2O3 + 2KI → Na2S4O6 + 2KCl + I2
From the balanced equation, we know that 2 moles of Na2S2O3 react with 2 moles of KI. Therefore, the number of moles of Na2S2O3 can be calculated from the volume of sodium thiosulfate solution used in the titration and the normality of the solution.
