How do we know that the moles of ascorbic acid originally present in the juice is equal to the moles of I2 that we added when the color change occurred?
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To determine if the moles of ascorbic acid (vitamin C) in the juice are equal to the moles of iodine (I2) added when the color change occurs, you will need to perform a redox titration known as the iodine titration method. Here's how you can do it:
1. Prepare a solution of iodine by dissolving a known amount of potassium iodide (KI) in water and adding iodine (I2) to it. This solution will be used as the titrant.
2. Take a known volume of the juice and add it to a flask or beaker.
3. Next, add a few drops of starch indicator solution to the juice. The starch indicator will form a blue-black complex when it reacts with iodine.
4. Titrate the juice solution with the iodine solution (titrant) gradually, swirling the flask or beaker after each addition. The iodine will react with the ascorbic acid in the juice and turn the solution blue-black due to the formation of iodine-starch complex.
5. Continue adding the iodine solution until the blue-black color persists for at least 30 seconds. This indicates that all the ascorbic acid in the juice has reacted with the iodine.
6. Record the volume of the iodine solution (titrant) that was required to produce the color change.
7. Finally, using the stoichiometric ratio between ascorbic acid and iodine, calculate the moles of ascorbic acid originally present in the juice based on the volume of iodine solution used.
The stoichiometric ratio is determined from the balanced chemical equation representing the reaction between ascorbic acid and iodine. In this titration, the reaction between ascorbic acid and iodine is as follows:
C6H8O6 (ascorbic acid) + I2 → C6H6O6 + 2HI
Based on the equation, every mole of ascorbic acid reacts with one mole of iodine. Therefore, the moles of ascorbic acid in the juice should be equal to the moles of iodine added when the color change occurred.
It's important to note that the accuracy of this method relies on precise measurement and careful titration technique. Proper lab safety precautions should also be followed throughout the process.