0.1 N Sodium Thiosulfate Standardization
Introduction
Sodium thiosulfate (Na2S2O3) is a strong reducing agent commonly used as a titrant in analytical chemistry, particularly in the determination of iodine, bromine, and chlorine concentrations. To ensure accurate results, it is essential to standardize the sodium thiosulfate solution. In this article, we will discuss the standardization of 0.1 N sodium thiosulfate solution.
Theory
The standardization of sodium thiosulfate is based on its reaction with potassium iodate (KIO3) in acidic medium. The reaction is as follows:
Na2S2O3 + 2KIO3 → 2NaIO3 + K2S2O3
The reaction involves the reduction of iodate ions (IO3-) to iodine (I2) by thiosulfate ions (S2O32-). The liberated iodine is then titrated with a standard sodium thiosulfate solution.
Apparatus
- Conical flask: 250 mL
- Burette: 50 mL
- Pipette: 25 mL
- Stirring rod
- Erlenmeyer flask: 100 mL
Reagents
- 0.1 N sodium thiosulfate solution: (Na2S2O3)
- 0.1 N potassium iodate solution: (KIO3)
- Sulfuric acid: (H2SO4)
- Starch indicator: 1%
Procedure
Preparation of Sodium Thiosulfate Solution
- Weigh accurately about 25 g of sodium thiosulfate pentahydrate (Na2S2O3·5H2O) and transfer it to a 1000 mL volumetric flask.
- Add about 500 mL of distilled water to the flask and stir until the salt is completely dissolved.
- Make up the volume to 1000 mL with distilled water.
- Mix well and label the solution as 0.1 N sodium thiosulfate solution.
Standardization of Sodium Thiosulfate Solution
- Weigh accurately about 0.25 g of potassium iodate (KIO3) and transfer it to a 250 mL conical flask.
- Add about 25 mL of distilled water to the flask and stir until the salt is completely dissolved.
- Add 10 mL of 1 M sulfuric acid (H2SO4) to the flask and stir well.
- Add 25 mL of the prepared sodium thiosulfate solution to the flask and stir well.
- Add 1 mL of starch indicator to the flask and stir well.
- Titrate the liberated iodine with the sodium thiosulfate solution until the blue color disappears.
- Record the burette reading.
- Repeat steps 1-7 until concordant results are obtained.
Calculation
- Calculate the normality of the sodium thiosulfate solution using the following formula:
Normality of Na2S2O3 = (Weight of KIO3 × Normality of KIO3) / (Volume of Na2S2O3 used × Equivalent weight of I2)
where Normality of KIO3 = 0.1 N, Equivalent weight of I2 = 126.9 g/mol
Tips and Precautions
- Handle the sodium thiosulfate solution with care, as it is a strong reducing agent.
- Ensure that the potassium iodate solution is freshly prepared to avoid any errors.
- Use a high-quality starch indicator to ensure accurate results.
- Perform the titration in a well-ventilated area to avoid inhaling iodine vapors.
By following this procedure, you can accurately standardize 0.1 N sodium thiosulfate solution, which is essential for various analytical applications.
