CHM3120L

Introduction to Analytical Chemistry: Laboratory

Determination of Ascorbic Acid by Redox Titration: Procedure

Preparation of standard 0.02 M KIO₃ solution.

1. Obtain from the instructor a weighing bottle containing dry KIO₃.
2. Accurately weigh by difference about 2.0 g of the solid into a small beaker and transfer it quantitatively to a 500 mL volumetric flask.
3. Add water and stir to dissolve solid. Dilute to the mark and mix thoroughly.
4. Calculate and record the molar concentration of the solution.

Preparation and standardization of 0.1 M Na₂S₂O₃.

1. Boil about 500 mL of distilled water for 10 to 15 min. Allow the water to cool to room temperature; then add about 12 g of Na₂S₂O₃·5H₂O and 0.05 g of Na₂CO₃.
2. Stir until the solid has dissolved. Transfer the solution to a clean glass or plastic bottle and store in a dark place.
3. Pipet 25 mL of the KIO₃ solution into each of three 250 mL Erlenmeyer flasks. From this point treat each of the flasks separately until the titration is finished.
4. Add 2 g of solid KI and 10 mL of 0.5 M H₂SO₄ to the first flask and immediately begin titrating with the Na₂S₂O₃ solution. When the titration solution has lost almost all of its color (only a pale yellow remains), add 2 mL of starch indicator. Continue titrating until the blue color disappears.
5. Repeat for the other two flasks. If the range of titration volumes is greater than about 0.20 mL perform another standardization titration.
6. Calculate and record the molar concentration of the solution.

Determination of Ascorbic Acid.

1. Crush a Vitamin C table and accurately weigh about 100 mg of the powder into each of three Erlenmeyer flasks. Treat each individually as follows.
2. Add 40 mL of 0.5 M H₂SO₄ and 20 mL of water to dissolve the tablet. (Use a stirring rod to break up any large particles but some solid material used as a binder in the tablet may not dissolve.)
3. When the tablet is dissolved, add 2 grams of KI and, using a transfer pipet, 25 mL of the KIO₃ solution.
4. Titrate with the thiosulfate solution just as you did in standardizing the thiosulfate solution. The samples may be colored, making it somewhat more difficult to judge when to add the starch indicator. From the first titration and the weight of sample calculate how much thiosulfate can be added to each of your remaining samples before the indicator is added.
5. Repeat for the remaining samples.
6. For each sample find the weight percent of ascorbic acid in the tablet.