Author: Brittland DeKorver
Institute for Chemical Education and Nanoscale Science and Engineering Center
University of Wisconsin
– Madison
Purpose: To learn about titrations and vitamin C content in fruits.
Learning Objectives:
1. Different fruits contain different amounts of vitamins.
2. The amount of vitamins in a product (as appears on the label) can be determined by chemical tests.
National Science Education Standards:

Content Standard C: Life Science

Diversity and adaptations of organisms

Structure and function in living systems

Content Standard F: Science in Personal and Social Perspectives:

Science and Technology in Society
Grade Level: 2-8
Time: Preparation 2 hours, Activity 1 hour
Materials:

Tincture of Iodine

Cornstarch

Various fruit and vegetable juices, such as: o Broccoli o Apple o Grape o Parsley o Orange o Grapefruit o Lemon o Carrots o Red pepper

Demonstration samples: o Starch indicator in water o Ascorbic acid with starch indicator

Titration kits – 1 per student

Paper bag containing: o Goggles

o Nitrile Gloves o Plastic pipet or eye dropper o Plastic test tube o Small plastic tub o Instruction sheet o 3 or 4 toothpicks

30 Small vials containing starch (will need one per titration)

30 Small vials containing iodine (will need approximately one per titration)

Poster board or white board for displaying results
 “C” and “I” placards for model

Saran wrap and black cloth pieces for model
Safety:
Iodine will stain skin and clothing. Although frequently used as a disinfectant and sanitizer, iodine is toxic when ingested. Some students may have an iodine allergy; so all students should take care to not get it on their skin. Students should wash hands after the activity.
Preparation ahead of time:
Make starch indicator 1 by heating 100mL distilled water to near boiling, adding 1 gram of starch, mix well and let cool. To prepare fruit and vegetable juices, measure out about 100g of the peeled fruit or vegetable, chop into pieces, place in blender with 50mL of water and blend until smooth. Fruit juices should not be prepared too far in advance, as Vitamin C content decreases over time.
Prepare titration kits for students with equipment listed above in brown paper bags. Fill about 30 vials 1/3 with starch indicator solution (for about ten students to do 3 samples each). Fill 30 vials with iodine.
Introduction:
Begin with a demonstration of the indicator. Show the students that iodine turns black in the presence of starch by adding one or two drops of iodine to the starch only sample. Then, show them that if Vitamin C (ascorbic acid) is present, it does not turn black by adding one or two drops of iodine. Ask the students what might cause this. Solicit answers, but continue to add iodine drop by drop. When the solution turns black (endpoint) explain to the students that the ascorbic acid can react with (“use up”) any iodine added to the solution. Because there is only a limited amount of Vitamin C, when you add a lot of iodine, not all of it will be reacted. The left over iodine causes the solution to turn black. If we know how much iodine it takes to turn the mixture black, we will know how much Vitamin C was in the juice.
1 Helmenstine, Anne Marie. “Vitamin C Determination by Iodine Titration.” http://chemistry.about.com/od/demonstrationsexperiments/ss/vitctitration.htm
Accessed 10/16/08

Procedure:
1. Student Titration Act-It-Out a. Define one part of the room to be the “test tube”. This is where the reaction will take place. b. Designate some of the students to act as starch molecules, some to act as iodine molecules, and some to act as Vitamin C molecules. c. The Vitamin C-students should be instructed to stand next to or hold hands with any iodine-students. However, a Vitamin C-student can only be paired with one iodine-student at a time, and vice versa. d. The starch students should each have a placard that is white on one side and black on the other. These students will hold the placard so the white side is visible when a lone iodine-student is present in the “test tube”. When they see an iodine-student is present in the test tube and not paired up with a Vitamin-C student, the starch-students will hold up the black side of the placard instead of the white side. e. Begin the model with two Vitamin C-students and one starch student in the “test tube”. f. Allow one iodinestudent to enter the “test tube”. Have the students keep count of how many iodine students are in the test tube. g. Ask the students remaining in the audience to confirm the results, e.g. did the Vitamin-C student do the right thing, is the starchstudent showing us the right thing? h. Allow one more iodinestudent to enter the “test tube.” i. Ask the students what will happen when another iodine-student enters the test tube. j. Allow another iodinestudent to enter the “test tube.” The starch student should now change colors. k. Begin the model again, but start with four Vitamin-C students instead. l. Ask the students to draw a conclusion about what it means about the Vitamin C if 15 students would be required before the starch changes colors.
2. Titrating Fruit Samples a. Tell the students they will each do a titration of a different juice sample. They must keep track of how much iodine they add so that the amounts of vitamin C in various juices can be compared. b. Remind the students that nothing in the lab is edible, and they cannot taste the solutions in their kits. Tell them that since iodine stains skin, they should wear gloves. Warn them to not get iodine on their clothing. c. Pass out the titration kits and help the students to follow the directions included in the kit.

d. As students determine the amount of drops of iodine required to reach the endpoint, help them record their results on a bar graph poster. e. Students who complete their titration may begin a second trial.
Discussion:
Have the students examine the results displayed on the graph. Ask them if they are surprised by any of the food items. One surprising result may be fresh apple has much less vitamin C than applesauce; applesauce has vitamin C added as a preservative. Students may also be surprised to learn that several foods have more vitamin C than oranges.
If time permits, talk to the students about scurvy, the illness that results from a deficiency in vitamin C.
Evaluation:
Which food had the most vitamin C?
Which food had the least amount of vitamin C?
Why is vitamin C important?
Which has more vitamin C, applesauce or apples?
What type of information can you obtain from a titration experiment?
This lesson is the product of the Institute for Chemical Education and the
Nanoscale Science and Engineering Center at the University of Wisconsin-Madison.
This Material is based upon work supported by the National
Science Foundation under grant number DMR-0425880.
SCIENCountErs Lessons are licensed under a Creative Commons Attribution-
NonCommercial 4.0 International License. Permissions beyond the scope of this license may be available by emailing ice@chem.wisc.edu.