Related Links

• Science Fair Project Guide

---

• Chemistry Safety Guide

Project Summary

Difficulty	7  –  8
Time required	Long (a couple of weeks)
Prerequisites	Ideally you'd have your own citrus tree with ripe fruit for this project. The second-best option is to use citrus fruit from a store.
Material Availability	Specialty items
Cost	Average ($50 - $100)
Safety	Adult supervision required

Share this Project Idea! |

Donate to Science Buddies

Sponsor

Sponsored by a generous grant from the Camille and Henry Dreyfus Foundation

Abstract

Objective

The goal of this project is to determine whether the amount of vitamin C in oranges (or other citrus fruit) decreases after the fruit is picked.

Introduction

In this project, you'll learn an interesting method for determining the amount of vitamin C in a solution. The technique you will be using is called titration.

Titration is used to determine the unknown concentration of a chemical in a solution. In a titration, a carefully measured amount of a second chemical is gradually added to the solution. The added chemical reacts with the original chemical, whose concentration is unknown. The original chemical whose concentration is unknown is called the titrant, and the added chemical whose concentration is known is called the titration solution. The titration solution reacts with the titrant, and the progress of this reaction is carefully monitored. When 100% of the original compound has reacted with the added chemical, the titration is complete. Now the concentration of the original chemical can be determined from the amount of titration solution that was added.

So you can better understand how a titration works, let's look at the specific example of determining the concentration of vitamin C. Vitamin C is the titrant in this case (because its concentration is unknown). You start with a measured volume of the titrant. The titrating solution that will added to the titrant is iodine. You will start out by using your iodine solution to titrate a known amount of vitamin C, using a solution prepared from a vitamin C tablet. You will carefully measure the amount of iodine solution neeed to titrate the known amount of vitamin C. You will know when the titration is complete because you will add a third chemical—soluble starch—to the solution. The starch acts as an indicator: the starch changes the color of the solution when the iodine/vitamin C reaction is complete. As soon as the solution changes color, you will stop adding iodine solution. Once you have calibrated your iodine solution with a known amount of vitamin C, you can then repeat the procedure to determine how much vitamin C there is in samples of fresh-squeezed orange juice.

The chemical reaction of iodine with vitamin C is called an oxidation-reduction reaction. (Chemists often use the shorthand "redox reaction" to refer to this type of reaction.) The ascorbic acid is oxidized to dehydroascorbic acid, and the iodine is reduced to iodide ions. Oxidation-reduction reactions always occur in pairs like this. The molecule that loses electrons is oxidized, and the molecule that accepts the electrons is reduced.

How will different storage times affect the amount of vitamin C in juice squeezed from oranges? Get ready to do some titrations to find out for yourself.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Titration
• Titrant
• Titration solution
• Indicator solution
• Stoichiometry
• Vitamin C
• Iodine

More advanced students should also study:

• Oxidation-reduction (redox) reactions
• Redox potential

Questions

• What happens when iodine is added to a starch solution?
• What happens when iodine is added to a starch solution that also contains vitamin C?

Bibliography

• For a good, basic reference on chemistry, see:
  Andrew Rader Studios, 1997–2007. "Chem4Kids," Chem4Kids.com [accessed July 18, 2007] http://www.chem4kids.com.
• The Materials Safety Data Sheet (MSDS) for Lugol's solution has important chemical safety information:
  Home Science Tools, n.d. "MSDS for Lugol's Iodine Solution," Home Science Tools [accessed July 18, 2007] http://www.hometrainingtools.com/catalog/file/104/CH-IODINE.pdf.
• This project is based on:
  
  • University of Canterbury, n.d. "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, and
  • Ganong, B., n.d. "Determination of Vitamin C in Orange Juice," Mansfield University [accessed July 18, 2007] http://faculty.mansfield.edu/bganong/biochemistry/vitaminc.htm.
• Another useful reference on the vitamin C titration procedure is:
  Helmenstine, A.M., 2007. "Vitamin C Determination by Iodine Titration," About.com: Chemistry [accessed July 18, 2007] http://chemistry.about.com/od/demonstrationsexperiments/ss/vitctitration.htm.
• For more information on titration techniques see Chemistry Lab Techniques.
• More advanced students should do background research on stoichiometry and oxidation-reduction (redox) reactions. Here is a helpful review of high school chemistry that can be a good starting point:
  
  • Shodor Education Foundation, Inc., 1998a. "Stoichiometry," UNC-Chapel Hill Chemistry Fundamentals Program [accessed July 18, 2007] http://www.shodor.org/unchem/basic/stoic/index.html.
  • Shodor Education Foundation, Inc., 1998b. "Redox Reactions," UNC-Chapel Hill Chemistry Fundamentals Program [accessed July 18, 2007] http://www.shodor.org/unchem/advanced/redox/index.html.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• About 10–12 juicing oranges (or other citrus fruit)
  1. Ideally, you would have access to a citrus tree with ripe fruit for the duration of the project.
  2. The next-best option is to use a big batch of store-bought citrus fruit. Use the procedure described below to measure the vitamin C content of citrus fruit stored for various lengths of time.
  3. You'll get better juice yield if you buy juicing oranges, not eating oranges.
• Juicer for extracting juice from oranges (manual or electric is fine)
• Cheesecloth
• Vitamin C tablets
• Distilled water
• Transfer pipettes
• Masking tape
• Permanent marker
• Small funnel (do not use for food after using it for chemistry)
• The following items can be ordered from Science Kit & Boreal Laboratories or Home Science Tools:
  • Chemical safety goggles
  • Lab apron
  • Rubber (latex) gloves
  • Iodine solution, use one of the following:
    • Lugol's iodine solution (part # CH-iodine from Home Science Tools) or
    • iodine solution for starch test (part # WW9704606 from Science Kit & Boreal Laboratories.
  • Soluble starch
  • 50 mL graduated cylinder
  • 250 mL gradulated cylinder
  • 500 mL graduated cylinder
  • 50 mL Ehrlenmeyer flask
  • 50 mL buret
  • Ring stand
  • Buret clamp
• Electronic kitchen balance (accurate to 0.1 g)
• Glass jars for iodine (300 mL) and starch solutions.
• Optional: you can obtain more accurate results if you use a 250 mL volumetric flask for making your vitamin C standard solution.

Disclaimer: Science Buddies occasionally provides information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. Science Buddies receives no consideration, financial or otherwise, from suppliers for these listings. (The sole exception is any Amazon.com or Barnes&Noble.com link.) If you have any comments (positive or negative) related to purchases you've made for science fair projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.

Experimental Procedure

1. Do your background research so that you are knowledgeable about the terms, concepts, and questions, above.
2. Wear gloves, chemical safety goggles, and a lab coat or apron when using the iodine solutions in this experiment.
3. Dilute the Lugol's solution 1:10 in distilled water to make your iodine titration solution. (Note: if you purchased the iodine solution for starch test, you can skip this step.)
   1. Pour the 30 mL Lugol's solution into the 500 mL graduated cylinder.
   2. Add enough distilled water to bring the total fluid volume to 300 mL and mix.
   3. Store the solution in a clean, tightly covered glass jar that is clearly labeled. Store it in a location that is protected from light.
4. Make a starch indicator solution.
   1. This can be anywhere from 0.5 to 1.0%. The exact amount of starch is not critical.
   2. For a 0.5% solution, add 0.25 g of soluble starch to 50 mL of near-boiling distilled water.
   3. Stir to dissolve, and allow to cool.
   4. When cool, store the starch solution in a clean, tightly covered glass jar that is clearly labeled.
5. Make a fresh vitamin C standard solution (1 mg/mL). Do this on each day that you make vitamin C measurements from oranges.
   1. You will use this solution to "standardize" your iodine titration solution. You will measure how much of your iodine solution it takes to oxidize a known amount of vitamin C. You can then use your iodine titration solution to determine the amount of vitamin C from test samples of juice from oranges.
   2. Crush a 250 mg vitamin C tablet, and dissolve it in 100 mL of distilled water.
   3. Pour into a graduated cylinder and add distilled water to bring the total volume to 250 mL.
6. Titrate 25 mL of vitamin C standard solution
   1. Use a clean 25 mL graduated cylinder to measure 20 mL of vitamin C standard solution.
   2. Pour this into a 50 mL Ehrlenmeyer flask (the shape of this flask allows you to swirl the solution to mix it without spilling).
   3. Add 10 drops of starch indicator solution.
   4. Set up the 50 mL buret on the the ringstand.
   5. Use a funnel to carefully fill the buret with your iodine titration solution. Tip: the fluid level should not be past the graduated markings on the buret.
   6. Write down the initial volume of the iodine titration solution in the buret.
   7. Place the Ehrlenmeyer flask (containing the vitamin C and starch solutions) under the buret.
   8. Carefully release the spring clamp of the buret to add iodine solution drop by drop.
   9. Swirl the flask to mix in the iodine solution after each addition.
   10. The titration is complete when the iodine creates a blue-back color in the solution that lasts for longer than 20 seconds.
   11. Record the final volume of the iodine solution in the buret.
   12. The difference between the initial volume and the final volume is the amount of iodine titration solution needed to oxidize the vitamin C.
   13. Repeat this step three times. You should get results that agree within about 0.1 mL.
7. Pick (or buy) 10–12 juice oranges. You will measure the vitamin C content of two oranges on the day of picking (day 1) and on days 2, 4, 8, and 14.
8. Prepare fresh-squeezed orange juice samples.
   1. Use a juicer to squeeze orange juice from two oranges.
   2. You need 20 mL of juice per titration, and you should do at least three titrations per storage condition, for a total of 60 mL.
   3. Filter the orange juice through cheesecloth to remove any pulp and seeds.
9. Titrating an orange juice sample is quite similar to titrating the vitamin C standard. Here are the steps:
   1. Use a clean 25 mL graduated cylinder to measure 20 mL of the fresh-squeezed juice.
   2. Pour this into a 50 mL Ehrlenmeyer flask (the shape of this flask allows you to swirl the solution to mix it without spilling).
   3. Add 10 drops of starch indicator solution.
   4. Set up the 50 mL buret on the the ringstand.
   5. Fill the buret nearly full with your iodine titration solution.
   6. Write down the initial volume of the iodine titration solution in the buret.
   7. Place the Ehrlenmeyer flask (containing the vitamin C and starch solutions) under the buret.
   8. Carefully release the spring clamp of the buret to add iodine solution drop by drop.
   9. Swirl the flask to mix in the iodine solution after each addition.
   10. The titration is complete when the iodine creates a distinct color change in the juice/starch solution. This color change will be harder to see than with the vitamin C solution, since the juice starts out orange. The color will change from orange to grayish brown when the endpoint is reached. If you continue to add iodine, the color will darken further. You want to note the volume of iodine added when the color first changes.
   11. Record the final volume of the iodine solution in the buret.
   12. The difference between the initial volume and the final volume is the amount of iodine titration solution needed to oxidize the vitamin C.
   13. Repeat this step three times. You should get results that agree within about 0.1 mL.
10. You can calculate the amount vitamin C in your samples by setting up a proportion. Here's an example (with made-up numbers) to show you how:
    1. Let's say that it took 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.
    2. Let's also say it takes 6.8 mL of iodine solution to titrate 20 mL of a test sample of orange juice.
    3. We'll call the amount of vitamin C in the orange juice sample x. You can find what x is with the following equation:
       x/6.8 mL = 20 mg/8.5 mL = 16.0 mg
       
11. From your results, which oranges had the most vitamin C?

Variations

• You can extend the experiment to compare different storage methods for juice oranges. What effect does refrigerated storage have on vitamin C levels? Is refrigeration better at preserving vitamin C than room-temperature storage? Design an experiment to find out.
• Measure the amount of vitamin C in other fruits and vegetables. Which ones have the highest vitamin C content? Which ones have the lowest? Puree a 100 g sample of the fruit or vegetable, and strain it through cheesecloth with about 50 mL of distilled water. Add distilled water to bring the total volume up to 100 mL. From your measurement, you can figure out how much vitamin C there is per 100 g of the fruit or vegetable you sampled. You can use typical weights of the fruit or vegetable to calculate how much total vitamin C there is in each fruit or vegetable you test.
• Measure the vitamin C content in fruit juices. Do the amounts correspond to what is on the label?
• Does the amount of vitamin C in fruit juice decrease during refrigerated storage? Does the type of container (e.g., glass bottle, or paperboard carton) matter?
• Does orange juice from frozen concentrate have the same amount of vitamin C as not-from-concentrate juice?

• For more science project ideas in this area of science, see Chemistry Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

This project is based on:

• University of Canterbury, date unknown. "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.

Last edit date: 2007-10-05 11:30:00

Career Focus

If you like this project, you might enjoy exploring careers in Chemistry.

	Chemist Everything in the environment, whether naturally occurring or of human design, is composed of chemicals. Chemists search for and use new knowledge about chemicals to develop new processes or products.			Food Science Technician Good taste, texture, quality, and safety are all very important in the food industry. Food science technicians test and catalog the physical and chemical properties of food to help ensure these aspects.
	Agricultural Technician As the world's population grows larger, it is important to improve the quality and yield of food crops and animal food sources. Agricultural technicians work in the forefront of this very important research area by helping scientists conduct novel experiments. If you would like to combine technology with the desire to see things grow, then read further to learn more about this exciting career.			Food Scientist or Technologist There is a fraction of the world's population that doesn't have enough to eat or doesn't have access to food that is nutritionally rich. Food scientists or technologists work to find new sources of food that have the right nutrition levels and that are safe for human consumption. In fact, our nation's food supply depends on food scientists and technologists that test and develop foods that meet and exceed government food safety standards. If you are interested in combining biology, chemistry, and the knowledge that you are helping people, then a career as a food scientist or technologist could be a great choice for you!

Join Science Buddies Become a Science Buddies member! It's free! As a member you will be the first to receive our new and innovative project ideas, news about upcoming science competitions, science fair tips, and information on other science related initiatives. Support Science Buddies If this website has helped you, won't you consider a small gift so we may continue developing resources to help teachers and students?