Do Oranges Lose or Gain Vitamin C After Being Picked?

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p043.shtml#summary

• Orange Juice Titration Kit (1). Includes:
  • Juicer for extracting juice from oranges
  • Cheesecolth
  • Vitamin C tablets, 250-mg
  • Masking tape
  • Permanent marker
  • Chemical splash goggles
  • 2% Lugol's iodine solution (30 mL); also available from Amazon.com
  • Soluble starch (30 g)
  • Small funnel (do not use for food after using it for chemistry)
  • 50 mL graduated cylinder
  • 500 mL graduated cylinder
  • 50 mL Ehrlenmeyer flask
  • 50 mL burette
  • Ring stand
  • Burette clamp
  • Lab apron
  • Eyedropper (a transfer pipette or medicine dropper would work too)
  • Nitrile gloves (rubber or latex would work too)
  • Measuring spoons (a balance accurate to the 0.1 gram would also work)
  • 100 mL beaker
  • Glass bottle, amber. Iodine is light sensitive and needs to be stored, once mixed, in an amber glass bottle or in an aluminum foil covered bottle.

You will also need to gather these items:

• About 10–12 juicing oranges (or other citrus fruit)
  • Ideally, you would have access to a citrus tree with ripe fruit for the duration of the project.
  • 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.
  • You'll get better juice yield if you buy juicing oranges, not eating oranges.
• Distilled water; available in the bottled water section of most grocery stores

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. Rinse and dry the 500 mL graduated cylinder.
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 1 g (which is equivalent to 1/4 teaspoon) of soluble starch to 200 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. Rinse and dry the 500 mL graduated cylinder.
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 50 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 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 50 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 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 a 20 mL 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 mg/mL) * (20 mg)/(8.5 mL) = 16.0 mg
       
11. From your results, which oranges had the most vitamin C?