Grade Level: 7th - 10th; Type: Biology
Using a starch-iodine solution, students will measure the amount of vitamin C in ripe, half-ripe and unripe oranges. They will learn about the instability of vitamin C.
- Is there more vitamin C in ripe oranges or unripe oranges?
- Is there more vitamin C in oranges that have been sitting out on the counter vs oranges stored in the fridge?
- What is the consequence of purchasing old produce?
- How can I measure the vitamin C in foods?
Vitamin C (also known as ascorbic acid) is highly unstable and has short shelf-life. While unripe fruits have high concentrations of vitamin C, this diminishes as the fruit ripens. As ripe fruits age, the vitamin C continues to disappear.
Students can evaluate these phenomena using a starch and iodine solution. Iodine reacts to starch by becoming purple unless vitamin C is present which is why the deep purple starch iodine solution turns clear when students drip it into the orange juice. While the clarity dissipates by the adding additional iodine starch solution, higher concentrations of vitamin C will require more iodine-starch solution to prevent the mixture from clearing.
- Triple beam or scale balance
- Corn starch
- 2 gallons distilled water
- Two one-gallon glass jugs (such as apple cider is sold in)
- 50 ml concentrated HCl
- 13 g iodine
- 20 g potassium iodide
- One 250 or 500 ml Erlenmeyer
- Graduated cylinder
- 1 liter beaker (other types of glass container will do)
- 250 ml beaker
- Clamp and stand for holding burette
- Ripe, half ripe and unripe oranges
- Juicer (optional)
Although reagents and glassware can be obtained from a lab supply house, hopefully students can borrow these from their schools. Distilled water is available in large grocers or from your school lab. Cornstarch is available from grocery stores. It may be easier to get unripe oranges if you live in Florida or California where you can get them from a tree.
Before starting the experiment, make two solutions: (1) the starch reaction solution and (2) 0.01 N iodine.These two solutions are made as directed below.
- Weigh out 1.2 g cornstarch powder and transfer it to a small saucepan.
- Make slurry of the cornstarch powder and 20 ml of distilled water.
- Add 180 ml to the slurry.
- Bring to a boil and stir (this can be done over the kitchen stove).
- Fill a one-gallon glass jug with a half gallon of distilled water.
- Add 47 ml of concentrated HCl to the one-gallon jug.
- Add 20 ml of the cornstarch solution to the one-gallon jug.
- Fill the jug with another half gallon of water.
- Stir. Cover with a lid and label.
0.01 N Iodine Solution
- Measure 12.7 g iodine and 20 g KI (potassium iodide) and transfer them to a 1 l beaker.
- Dissolve the reagents in 100 ml distilled water. Stir.
- Bring the total volume of the solution up to 1000 ml.
- Using a graduated cylinder, measure 378 ml of the solution made in steps 1 – 3 and transfer to a gallon glass jug. Cover with a lid and label.
- Measure 50 ml of the starch solution and transfer to an Erlenmeyer flask.
- Measure 50 ml of the iodine solution and transfer to the Erlenmeyer flask, stirring it together with the starch solution until thoroughly mixed. Cover with cork. Label
- Clamp your burette to a ring stand. Place a 250 ml beaker under the burette. Make sure the stopcock on the burette is closed.
- Using a funnel, add the starch-iodine solution to the burette, making sure that it does not over-fill. Practice opening and closing the stopcock so that you can easily count drops that come out when the stopcock is opened.
- Prepare your first orange sample by peeling the unripe orange and putting it in a juicer or manually squeezing the juice from the unripe orange into a beaker. Make sure you have at least 100 ml of juice in a beaker, using additional unripe oranges if necessary.
- Place the orange juice beaker under the burette and very gradually add the starch-iodine solution, counting the number of drops as you go. With each drop, the solution will react with the vitamin C and temporarily become clear. Once all the ascorbic acid is neutralized, the addition of the starch-iodine solution will not become clear, but will instead retain its dark color. Record the total number of drops it take to reach this point.
- Repeat this experiment using a half-ripe orange and a ripe orange. Consider repeating the experiment to compare ripe fruit that has been left on the counter with ripe refrigerated fruit.
Terms/Concepts:Vitamin C (ascorbic acid, Using a burette and making solutions, Concentration of vitamin C as a function of ripeness and age of fruit; Chemical stability
Hobbs, Christopher & Elson Haas. Vitamins for Dummies. For Dummies Press (1999)
Katherine M. Phillips et al. “Stability of Vitamin C in Frozen Raw Fruit and Vegetable
Homogenates” Journal of Food Composition and Analysis. 23 253–259 (2010)
Linus Pauling Institute: Vitamin C