Fruits vs. Iodine

Find ascorbic acid in fruits using iodine!

Difficulty:
Danger:
Duration:
20 minutes
Fruits vs. Iodine

Reagents

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the plastic tray.
General safety rules
  • Do not allow chemicals to come into contact with the eyes or mouth.
  • Keep young children, animals and those not wearing eye protection away from the experimental area.
  • Store this experimental set out of reach of children under 12 years of age.
  • Clean all equipment after use.
  • Make sure that all containers are fully closed and properly stored after use.
  • Ensure that all empty containers are disposed of properly.
  • Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
  • Do not replace foodstuffs in original container. Dispose of immediately.
General first aid information
  • In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
  • If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
  • In case of inhalation: Remove person to fresh air.
  • In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
  • In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
  • In case of injury always seek medical advice.
Advice for supervising adults
  • The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
  • This experimental set is for use only by children over 12 years.
  • Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
  • The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
  • The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
  • Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.

FAQ and troubleshooting

What kinds of fruits and vegetables should I use?

You can try many different fruits and vegetables! But you can likely expect the most interesting results from red sweet peppers, lemons, kiwis, and bananas. We also recommend trying tangerines, grapes, and apples.

I don’t see any spots on the iodine paper. What’s wrong?

Perhaps the fruit wasn’t actually resting on the iodine paper. Try asking a parent or a supervising adult to help you cut your fruits as flat as possible.

Can I eat the fruit slices after the experiment?

No! Even though the slices you used in the experiment might still look edible, they have been in contact with chemical reagents. Dispose of them with household garbage or compost.

How can I figure out which fruit contains the most vitamin C?

The less iodine staining you see on the paper under a fruit slice, the more vitamin C is present in the slice. You can learn more about what happens in the scientific description of this experiment.

Step-by-step instructions

First, prepare some fruit samples. Lemon and bell pepper are good examples.

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To perform the test, obtain some iodine using a reaction between potassium iodide KI and copper sulfate CuSO4.

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A sheet of paper stained with iodine is a good ascorbic acid indicator.

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When the juice from the fruit slices comes into contact with the iodine, a chemical reaction starts, turning the iodine into colorless iodide.

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When you remove the fruits, you can see white spots on the paper. These spots indicate the presence of ascorbic acid.

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Disposal

Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.

Scientific description

What is ascorbic acid and why do we need it?

Did you know that oxygen is actually pretty toxic? It even killed almost all life on Earth some 2.5 billion years ago, in the course of what is sometimes called "The Great Oxygenation Event." That's because oxygen is a strong oxidant, meaning that it is really keen to take electrons from other compounds. This can be a good thing, as it allows many organisms, including us, to obtain energy from the oxidation of organic compounds. On the other hand, living things are made of organic compounds that can be ruined by oxidation.

To mitigate this problem, some organisms devised special molecules that are oxidized way easier than most, and hence "take the heat" of aggressive oxidizing agents like oxygen. Ascorbic acid in plants is one such compound.

To demonstrate that ascorbic acid is indeed good at dealing with oxidants, we use iodine I2, which is also a decent oxidant, but, unlike oxygen, has a rich brown color. When iodine takes electrons from ascorbic acid, it turns into iodide ions I (iodine atoms with extra electrons), which are colorless. The more ascorbic acid a fruit or a vegetable contains, the more iodine it can turn into iodide ions and the less brown iodine will be left on the paper.

Ascorbic acid, also known as vitamin C, has other important functions besides being an antioxidant. For instance, vitamin C plays a crucial role in the functioning of some enzymes in our bodies. The human body can't produce vitamin C on its own; it must get it from food.

When food is lacking this molecule, the body can contract a particularly nasty disease called scurvy, which has all sorts of unpleasant symptoms like weakness and tooth loss. Before we discovered vitamin C’s ability to prevent this disease in the 20th century, people who had no access to fruits or vegetables, like sailors, got scurvy all the time. Next time you see an image of a pirate's toothless grin, remember, chances are he didn't lose his teeth in a fight, but just didn't eat enough vegetables!

How do we obtain iodine in this experiment?

Iodine forms when copper sulfate CuSO4 meets the solution of potassium iodide KI. This leads to the following reaction:

2CuSO4 + 2KI → Cu2SO4 + K2SO4 + I2

This is iodine, which makes the solution brownish.