Determination of Vitamin C in Orange Juice

This is a very appropriate exercise for this course and one of the few that illustrates basic chemical properties of a vitamin. It is included in many lab manuals in various forms, especially for Organic & Biochemistry courses, and can be applied to many foods besides orange juice. Titration of vitamin C is usually done with either iodine or an organic dye like DCIP. Iodine is less expensive and works fine for us. I have students compare vitamin C contents of fresh, reconstituted frozen, and canned orange juices. Although they are similar, there is an interesting and counterintuitive trend we see every year.

Students begin by standardizing the iodine solution (0.125% iodine, 1% potassium iodide) with 1.00 mg/mL ascorbic acid (made fresh). They use a 25-mL volumetric pipet to transfer 25.00 mL of ascorbic acid to a 125-mL flask and add 2 mL of 6 M acetic acid and 3 mL of 1% starch. They then titrate to a blue endpoint with iodine solution from a 50-mL buret. I caution them about bubbles, show them how to read the miniscus, and describe the approach to endpoint. From the volume of titrant they used, they calculate the "volume of iodine that reacts with 1 mg of ascorbic acid."

They then use the same method to determine the amount of iodine needed to titrate 25-mL samples of fresh, reconstituted frozen, and canned orange juice. The orange juices were previously strained through cheesecloth. The endpoint isn't as obvious as with the ascorbic acid, but even with that subjective variation, students find similar trends. Using the number ("volume of iodine...") they obtained above, they calculate the concentration of vitamin C in each orange juice, generally finding values ranging from 0.2 to 0.6 mg/mL.

The trend is that canned orange juice seems to have the highest concentration of vitamin C; fresh orange juice usually has the lowest. Students find this counterintuitive because they are taught that heating, as in the canning process, destroys vitamins. I use their surprise to lead into the question of what they conclude, from the chemistry they learned here, to be the most important cause of ascorbic acid loss in processed orange juice.

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