Purpose
In this experiment we will determine the amount of vitamin C (ascorbic acid) in different fruit juices by titration of the juice with a solution of iodine. The iodine reacts rapidly with the vitamin C. If you have a juice you would like to analyze, bring about 125 ml to lab. Compare at least two juices.
Special Equipment: Things for each group to use and return on the same day.
| 25 ml pipet (~$15), Pipet pump, Buret and brush (~$75) |
| Be sure to rinse all the glassware with water and return it to the box when you are done. |
Procedure
Preparation of the Sample: Add about 1 g of oxalic acid (to stabilize the ascorbic acid) to about 100 ml of the juice in a 250 ml beaker.
The Titration: Rinse your clean 50 ml buret with a little iodine solution and fill it with the iodine solution using a funnel. Drain it to just below the first volume mark. Be certain that there are no air bubbles in the tip. Read the initial volume to ±0.01 mL.
Pipette 25.0 ml of your filtered fruit juice into a clean Erlenmeyer flask. Add a little (the tip of your spatula) of the thyodene indicator. Add iodine solution from the buret with constant swirling until a definite color change is observed throughout the solution.
Sometimes the color will fade on standing, so take the first time it changes as your endpoint. Record the final level of your iodine solution. Repeat this section twice. You will have three trials for each of two juice samples.
Calculations
One mL of the iodine solution is equivalent to 0.30 mg of vitamin C. Calculate the number of milligrams of vitamin C in 25 ml of your sample by multiplying the volume in mL of iodine solution you used by the vitamin C equivalent ( 0.30 mg) of the iodine solution. Record this in your notebook for each of your trials. Calculate the average value, as well, and record that. Do three trials for each juice. For your Conclusion compare the vitamin C content of the two juices you analyzed.
Wastes: All of the reagents and wastes in this experiment can be put down the drain with the water running.