why we add acid to the standardization of sodium thiosulphate solution?

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In the standardization of sodium thiosulphate solution, acid is added to maintain a stable pH and ensure accurate titration results. Here's why:

1. Reaction conditions: The standardization of sodium thiosulphate involves titrating it against a known solution of iodine. This reaction occurs in an acidic medium, as indicated by the balanced chemical equation:

2S2O3²¯ + I2 + 2H+ → S4O6²¯ + 2HI

By adding acid, you ensure that the reaction proceeds in the required acidic conditions.

2. pH control: The addition of acid helps maintain a stable pH throughout the titration process. A stable and known pH is crucial for accurate measurement of the sodium thiosulphate solution.

3. Reaction completion: Acidic conditions accelerate the reaction between thiosulphate ions (S2O3²¯) and iodine (I2). This makes the reaction faster, ensuring faster completion and allowing for more precise measurement.

To add acid during the standardization of sodium thiosulphate solution, you can follow these steps:

1. Prepare the setup: Set up your titration apparatus, including a burette containing the sodium thiosulphate solution and a conical flask with the iodine solution.

2. Add indicator: Before adding any acid, add a suitable indicator to the iodine solution. Typically, starch is used as an indicator, which gives a blue color that disappears when the iodine is fully reacted.

3. Add acid: As you begin the titration, slowly and carefully add a few drops of a strong acid (usually hydrochloric acid or sulphuric acid) to the conical flask containing the iodine solution. Swirl the flask to mix the acid and iodine solution.

4. Titrate: Begin titrating by adding the sodium thiosulphate solution gradually into the flask, swirling continuously. The addition of sodium thiosulphate will gradually decolorize the blue iodine-starch complex.

5. Endpoint detection: Observe the disappearance of the blue color, indicating the full reaction of iodine with sodium thiosulphate. This is your endpoint, so stop adding the sodium thiosulphate and record the volume used.

By following these steps, you ensure the proper acidification of the iodine solution during the standardization of sodium thiosulphate solution, which enables accurate and reliable results.