Materials & Procedure

Materials:

1. Three different samples of orange juice:
-home-made fresh-squeezed
-premium not-from concentrate juice
-made from frozen concentrate
2. juicer for extracting juice
3. cheesecloth
4. vitamin c tablets
5. distilled water
6. transfer pipettes
7. masking tape
8. permanent marker
9. small funnel
10. iodine solution
11. soluble starch
12. 50 mL graduated cylinder
13. 250 mL graduated cylinder
14. 500 mL graduated cylinder
15. 50 mL Ehrlenmeyer flask
16. 50 mL buret
17. ring stand
18. buret clamp
19. plastic transfer pipettes or an eyedropper
20. electronic kitchen balance
21. glass jars for iodine


Procedure:

1. Dilute the Lugol's solution in distilled water to make your iodine titration solution. Pour the 30 mL solution into the 500 mL graduated cylinder. Add enough distilled water to bring the total fluid volume to 300 mL and mix. Store the solution in a clean, tightly covered glass jar that is clearly labeled. Store it in a location that is protected from light.

2. Make a starch solution. Stir and dissolve it and allow it to cool. When it is cooled, store the starch solution in a clean, tightly covered glass jar that is clearly labeled.

3. Make a fresh vitamin C standard solution (1 mg/mL). Do this on each day that you make vitamin C measurements from oranges. Crush a 250 mg vitamin C tablet, and dissolve it in 100 mL of distilled water. Pour into a graduated cylinder and add distilled water to bring the total volume to 250 mL.

4. Titrate 25 mL of vitamin C standard solution. Use a clean 25 mL graduated cylinder to measure 20 mL of vitamin C standard solution. Pour this into a 50 mL Ehrlenmeyer flask. Add 10 drops of starch indicator solution. Set up the 50 mL buret on the ringstand. Use a funnel to carefully fill the buret with your iodine titration solution. Write down the initial volume of the iodine titration solution in the buret. Place the Ehrlenmeyer flask under the buret. Carefully release the spring clamp of the buret to add iodine solution drop by drop. Swirl the flask to mis in the iodine solution after each addition. The titration is complete when the iodine creates a blue-black color in the solution that lasts for longer than 20 seconds. Record the final volume of the iodine solution in the buret. The difference between the initial volume and the final volume is the amount of iodine titration solution needed to oxidize the vitamin C. Repeat this step three times.

5. Use a juicer to squeeze orange juice from two or more oranges. You will need 20 mL of juice per titration, and you should do at lesat three titrations per storage condition for a total of 60 mL. Filter the orange juice through a cheesecloth to remove any pulp and seeds.

6. Next, you will titrate the orange juice. Use a clean 25 mL graduated cylinder to measure 20 mL of the fresh-squeezed juice. Pour this into a 50 mL Ehrlenmeyer flask. Add 10 drops of starch indicator solution. Set up the 50 mL buret on the ringstand. Fill the buret nearly full with your iodine titration solution. Write downthe inital volume of the iodine titration solution in the buret. Place the Ehrlenmeyer flask under the buret. Carefully release the spring clamp of the buret to add iodine solution drop by drop. Swirl the flask to mix in the iodine solution after each addition. When the iodine creates a distinct color change in the juice solution the titration is completed. The color will change from orange to grayish brown when the endpoint is reached. Record the final volume of the iodine solution in the buret. The difference between the initial volume and the final volume is the amount of iodine titration solution needed to oxidize the vitamin C. Repeat this step three times.

7. Calculate the average amount of iodine needed to titrate a 20 mL sample for each juice.

8. Determine if one of the juices had more vitamin C than the others.