Sodium thiosulfate (Na2S2O3) is a chemical compound used in various chemical analyses, including iodometric and titrimetric methods. Here’s how to prepare and standardize both molar (1.0 M) and normal (1.0 N) sodium thiosulfate solutions.
Preparation of 1.0 M Sodium Thiosulfate Solution:
A 1.0 M sodium thiosulfate solution contains one mole of Na2S2O3 in one liter.
Materials Required:
– Sodium thiosulfate pentahydrate (Na2S2O3·5H2O)
– Distilled water
– A balance
– A glass stirring rod
– A glass or plastic container
– A 1.0 L volumetric flask
Procedure:
1. Weigh out the required amount of sodium thiosulfate pentahydrate (248.18 g/mol) to make a 1.0 M solution. To prepare 1.0 L of 1.0 M Na2S2O3 solution, you’ll need 248.18 grams of Na2S2O3·5H2O. Ensure that the substance is accurately weighed.
2. Transfer the weighed Na2S2O3·5H2O into a clean, dry glass or plastic container.
3. Add distilled water to the container and stir the mixture with a glass stirring rod until the Na2S2O3·5H2O is completely dissolved. The final volume of the solution should be less than 1.0 L at this stage.
4. Transfer the solution into a 1.0 L volumetric flask. Rinse the container with distilled water to ensure all the Na2S2O3·5H2O is transferred to the flask.
5. Fill the volumetric flask to the 1.0 L mark with distilled water while maintaining the temperature at a consistent level (usually 20°C). The solution is now a 1.0 M sodium thiosulfate solution.
Standardization of 1.0 M Sodium Thiosulfate Solution to Prepare 1.0 N Solution:
To standardize the 1.0 M Na2S2O3 solution and prepare a 1.0 N solution, you’ll need to determine its exact molar concentration.
Materials Required:
– A primary standard reagent, such as potassium iodate (KIO3)
– Starch indicator
– A burette
– A balance
– A 1.0 L volumetric flask
Procedure:
1. Weigh out a sample of a primary standard reagent, such as potassium iodate (KIO3) of known purity. Accurately weigh about 0.3913 g of KIO3. The molar mass of KIO3 is 214.00 g/mol.
2. Dissolve the weighed KIO3 in distilled water and transfer it into a 1.0 L volumetric flask.
3. Add a few drops of starch indicator to the KIO3 solution. Starch will form a blue-black complex with iodine at the endpoint of the titration.
4. Prepare the burette with the 1.0 M Na2S2O3 solution.
5. Titrate the KIO3 solution with the Na2S2O3 solution until the blue-black color disappears, indicating complete reduction of all the iodine.
6. Record the volume of Na2S2O3 solution used for the titration. Use this volume to calculate the exact molar concentration of the Na2S2O3 solution.
7. If the calculated molarity closely approximates 1.0 M, you can consider the solution as a 1.0 N sodium thiosulfate solution. If not, adjust the concentration and re-standardize if necessary.
