Grade Level: 8th - 9th; Type: Chemistry
Identify vitamin C in fruit juices and to determine which of the following juices contain the most vitamin C, orange, apple, cranberry or grape
- What is ascorbic acid?
- How is ascorbic acid used by the human body?
- What is an indicator?
- How is iodine used as an indicator?
- How does titration assist us in quantifying our data?
- What happens during an oxidation/reduction reaction?
On the information level, this experiment serves to acquaint students with basic Information on how to determine the amount of ascorbic acid, another name for vitamin C, in various fruit juices namely orange, apple, cranberry and grape. The human body is incapable of producing vitamin C and is therefore dependent on obtaining this vitamin from food. Since this is a quantitative study, it would be ideal to use the titration method. Titration enables us tomeasure the unknown amount of a chemical in a solution. It reacts with the titrant (the unknown solution) and enables us to calculate the concentration from the amount of titrating solution which we added and the proportion or the ratio of the two chemicals in the chemical reaction.
In this experiment, the student will titrate vitamin C (ascorbic acid) by using iodine. He/she use an oxidation /reduction reaction. The ascorbic acid is oxidized and the iodine is reduced to iodine ions. The student starts with a known concentration of iodine and carefully measures the amount being added. He/she can readily calculate how much vitamin C is present. Color changes such as when iodine comes in contact with starch and the solution and starch turn blue black, serve to alert us as to when all of the ascorbic acid has been oxidized. Once all of the ascorbic acid is oxidized, added iodine will alert us by reacting with the starch and turning blue black.
This science fair experiment also serves to acquaint students with the essential processes of using the scientific method, of clearly delineating the project`s objective, of formulating a hypothesis as to the anticipated outcomes of the project as well as recording and analyzing, synthesizing data and formulating valid results within the parameters of the obtained data.
- Vitamin C tablets
- Distilled water
- Transfer pipettes
- Masking tape
- Permanent marker
- Small funnel
- Lugol's iodine solution
- Soluble starch
- 50 mL graduated cylinder
- 250 mL graduated cylinder
- 500 mL graduated cylinder
- 50 mL Ehrlenmeyer flask
- 50 mL buret ring stand
- Electronic kitchen balance (accurate to 0.1 g)
- Glass jars for iodine (300 mL)
- Starch solutions
- Orange, apple, cranberry and grape juices.
All of these can be purchased from Science Kit and some of the materials may be borrowed from the school science lab.
- Gather all the materials you will need for this project. These include vitamin C tablets, distilled water, pipettes, masking tape, a permanent marker, small funnel, Lugol's iodine solution, soluble starch, a 50 mL, graduated cylinder, 250 mL graduated cylinder, 500 mL graduated cylinder, 50 mL Ehrlenmeyer flask, 50 mL buret ring stand, an electronic kitchen balance, glass jars for iodine (300 mL) and starch solutions, and orange, apple, cranberry and grape juices. Use orange juice without pulp.
- Put on your safety gloves, goggles, and a lab coat or apron when using the iodine solutions in this experiment. Dilute the Lugol's solution 1:10 in distilled water to make your iodine titration solution. Pour the solution into the 500 mL graduated cylinder. Add distilled water to bring the total fluid volume to 300 mL and mix. Store the solution in a labeled, clean, tightly covered glass jar .Store in a location that is protected from light. Make a starch indicator solution (0.5 to 1.0%) and place in a labeled, clean, tightly covered glass jar.
- Make a fresh vitamin C standard solution (1 mg/mL). You will use this solution to "standardize" your iodine titration solution and measure how much of your iodine solution it takes to oxidize a known amount of vitamin C. Crush a 250 mg vitamin C tablet; dissolve it in 100 mL of distilled water. Titrate 25 mL of vitamin C standard solution. Use a clean 25 mL graduated cylinder to measure 20 mL of vitamin C standard solution and pour into a 50 mL Ehrlenmeyer flask.
- Add 10 drops of starch indicator solution. Set up the 50 mL buret on the ring stand. Use a funnel to fill the buret with the iodine titration solution. Write down the initial volume of the iodine titration solution in Release the spring clamp of the buret and add iodine solution drop by drop.
- The titration is complete when the iodine creates a blue-back color 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.
- Now, titrate each sample of orange juice, apple juice, grape juice and cranberry juice first. Use a graduated cylinder and add 10 drops of starch indicator. Set up the 50 mL buret on the ring stand and fill with iodine solution. Write down the initial volume of the iodine solution in the buret. Carefully release the spring clamp and add iodine solution drop by drop. The titration is complete when the iodine creates a distinct color change in the juice/starch solution. Record the final volume of the solution in the buret. Record the difference between the initial volume and the final volume. This is the amount of iodine titration solution needed to oxidize the vitamin C. Repeat this step three times and average your results. Do this for all of the juices.
- Now, calculate the amount vitamin C in your samples by setting up a proportion. Here's an example: Let's say that it took an average of 8.5 mL of iodine solution to titrate 20 mL of 1 mg/mL Vitamin C standard solution, which means 20 mg vitamin C total. Let's say it takes an average of 6.8 mL of iodine solution to titrate a 20 mL test sample of orange juice. We'll call the amount of vitamin C in each of the juice samples x. You can find what x is with the following equation:X = (6.8 mg/ml)*(20 mg)/(8.5ml) = 16.0 mg .
- Record all of your data in the Data Chart. Analyze your data. Write up your report. Enclose your responses to the research questions as well as any photos you have taken and make certain to include your bibliography.
Terms/Concepts: Vitamin C; ascorbic acid; titration; titrate; titrating solute; oxidation reduction reaction; indicators
- University of Canterbury, "Determination of vitamin C Concentration by Titration," Science Outreach Programme, College of Science, University of Canterbury, Christchurch, New Zealand [accessed July 18, 2007] http://www.outreach.canterbury.ac.nz/chemistry/documents/vitaminc_iodine.pdf.