Vitamin C

Measure the vitamin C content of a juice!

Difficulty:
Danger:
Duration:
30 minutes
Experiment's video preview

Safety

  • Put on protective gloves and eyewear.
  • Conduct the experiment on the 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

How to calculate vitamin C content in a juice?

For those who can’t wait to see the answer, here is the formula:

100 mL of juice contains 3.3 mg * (number of drops) = X mg of vitamin C.

Now, if you are curious to see where the number “3.3” came from, below are the calculations. They aren’t super easy. Are you ready? Then let’s start!

Iodine solution concentration is 0.05 mole per liter. One milliliter contains 0.05/1000=0.00005 mol of iodine. Now, 1 mL contains about 33 drops. Therefore, 1 drop contains 0.00005/33=0.0000015 mol, or 0.0015 mmol of iodine. Next, iodine reacts with vitamin C (ascorbic acid) in 1:1 proportion, i.e. one drop of iodine reacts with 0.0015 mmol of acid. Molar mass of the acid =176 g/mol. Thus, the mass of ascorbic acid = 0.0015 mmol*176 g/mol =0.27 mg.

Normally, nutrient content in food is expressed per 100 mL volume. The volume of our test sample is 8 mL, i.e. we have to multiply the obtained number by 100/8=12.5. Thus, we get 0.27 mg*12.5=3.3 mg.

Then, the final formula is: 100 mL of juice contents 3.3 mg * (number of drops) = X mg of vitamin C.

The pipette is clogged with shredded potato, and now I can’t draw solution into it.

Wash the pipette under water stream to remove clogging. When drawing the solution in, slightly tilt the beaker to avoid drawing in shredded potato. And even if small particles of potato do get inside the pipette, they won’t ruin the experiment.

What did we prepare the second sample for?

For titration, we have to have an initial sample as a reference. We will compare the analyzed solution with it after adding each next drop into the test solution. Indeed, our eyes get tired of monotonous work and lose sensitivity to slight color changes. In order to catch the end point of titration, after adding each drop of iodine solution, make sure to mix the contents in the cup and compare its color with the reference (initial sample). When the solution in the cup turns dirty-green, record the total number of drops added.

What should I do if color of the juice changed instantly, and I couldn’t count the exact number of drops added?

Prepare another test sample: take a clean vial and use it to measure citrus juice. Pour it into a clean disposable cup. Continue starting from step 7. This time, thoroughly mix the contents in the cup after adding each next drop and remember to compare color of the test solution with the reference sample in the vial.

Step-by-step instructions

  1. Take a grater and grate a raw peeled potato. Fill a beaker with potato shreds up to 50 mL mark.

  2. Top the potato in the beaker with water, up to 50 mL mark.

  3. Fill the vial with citrus juice.

  4. Pour the juice from the vial into a disposable cup.

  5. Now, prepare a reference: fill the vial with some more citrus juice.

  6. Seal the reference vial.

  7. With a pipette, measure 1 mL of liquid from the beaker.

  8. Pour the liquid from the pipette into the cup with citrus juice.

  9. Add 1 drop of 0.05M iodine solution into the cup.

  10. Gently swirl the cup from side to side to mix the contents.

  11. Compare the contents in the cup and in the vial. If the color of these liquids remained the same after you added iodine I2 solution, repeat steps 9 – 11 and count how many times you have added iodine solution into the cup!

  12. As soon as the color of solution in the cup turns dirty green, write down the number of I2 drops added and finish the experiment.

Graphical step-by-step instruction

Disposal

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

Scientific description

How do you measure the amount of vitamin C in a juice?

Carefully count the number of drops which were necessary to produce color change. Multiply this number by 3.3 (this number is the correct multiplier if you have followed all of the instructions closely). This way you will have calculated the number of milligrams of vitamin C in 100ml of juice.

Learn more

As a matter of fact, this experiment is not as simple as it may seem. Titration – the method we used, is a quantitative one for the determination of a substance. The idea of titration in our case is to drip the iodine solution of a specific concentration into a juice solution with an unknown concentration of vitamin C (which is also known as ascorbic acid) until the chemical reaction ceases. Quite often to determine the moment when the reaction stops we add an indicator to the solution which changes color at this point (in our case this is the starch from the potato).

We conducted specific calculations so that everyone can determine the concentration of vitamin C in their juice. In these calculations we have taken into account the amount of iodine in each drop, the volume of the juice used and how each molecule of iodine interacts with molecules of ascorbic acid. This is where the “magical” 3.3 multiplier in the determination of the amount of vitamin C comes from.

You can try to obtain this number yourself. The concentration of I2 is denoted on its bottle (0.05 M, this means 0.05 moles of iodine in 1 liter, which is 1000 milliliters of solution). The molar mass of vitamin C is 176 grams per mole. Let’s assume that all of the vitamin C has reacted with the iodine and let’s say that each molecule of iodine uses up a molecule of ascorbic acid (vitamin C). We assume that the volume of each drop is 0.03 ml (this means that 33 drops add up to only 1 ml!). We would love to discuss your calculations if you send them to us at support@melscience.com; please add #vitaminC in the subject of your letter. Good luck!

Why do we use a potato in the experiment?

Potatoes have lots of starch in them. This is why they are not only nutritious and sweet in their cooked form, but can also be used in our experiment! Starch is able to react with iodine I2, which we add to the juice. This happens almost straight away and a dark-blue compound is formed. You can see this for yourself by dripping some iodine onto an uncooked potato. Our goal in the experiment is to identify the point in time when molecularly free iodine I2 appears in the mixture. It can only appear as soon we have run out of vitamin C (that’s the previously mentioned ascorbic acid).

Learn more

However, this interaction between the iodine and ascorbic acid doesn't happen immediately. This is why you can observe the change in color as you add the drops of iodine. Iodine and starch form a rather unstable compound, it is normally called clathrate or an inclusion compound.

This interaction has a rather simple explanation. Starch is made up of pretty big molecules, they have a complex spatial structure. The molecule is long and flexible. These molecules easily interweave; their structure can be compared to that of when your earphones get entangled in your pocket. When this structure builds up it leaves little cavities which the iodine can easily get into and stay in for a long time. However, if you offer the iodine a more interesting deal then it’ll readily leave the starch. The vitamin C offers exactly this sort of “interesting deal”. As a result, the iodine leaves the cavity so that it can react with the vitamin C, this is when the blue coloring disappears.

Why do we add iodine to the juice?

You will probably already have heard that citrus juices are rich in vitamin C – which is ascorbic acid C6H8O6. Vitamin C is an antioxidant; this means that when oxidants enter our organisms the ascorbic acid is the one taking the hit for us. This is one of the reasons why citrus juices are good for us (in sensible amounts!). Iodine I2 is also an oxidant which means that the ascorbic acid can react with it. That’s exactly what happens in our case! Upon adding iodine to the citrus juice the vitamin C, which means that it gives its electrons to the iodine: C6H8O6 + I2 → C6H6O6 + 2HI

Or, from the iodine’s perspective:

I2 + 2e- → 2I-

Learn more

What are antioxidants? These can be various natural chemical substances which can have completely different structures. But they all have the following properties.

• First of all, they can be found in the tissue of living organisms (synthesized in plants, animals and humans or enter our organisms with food that we eat).

• Secondly, they take part in many chemical processes which are part our daily living activities.

• Thirdly, they easily interact with aggressive or dangerous oxidants that either enter the organism naturally or are a by-product of organic processes.

The third property is the reason why they are called antioxidants (literally: anti-oxidant). From our organism’s point of view, the most aggressive and dangerous substances are radicals. These are typically produced when cells breathe or are affected by sunlight. Our organism is always ready to neutralize these dangerous by-products of everyday life. However, sometimes radicals from other sources enter the organism, this is when vitamin C and other antioxidants take the hit for us.

So what are other substances that have the properties of an antioxidant (apart from ascorbic acid)? Well, some other vitamins are. Such as tocopherol (vitamin E), beta-carotene (vitamin A) and other carotenoids which are similar in structure, these can be found in red and orange fruits. A close relative of carotenoids is lycopene which gives tomatoes and watermelons their red coloring, it also has antioxidant properties. Anthocyans – these are pigments which give flowers, fruits and leaves various colors (such as lilac, pink, purple and blue), are also antioxidant agents. You shouldn’t pass up on eating plenty of fruits, vegetables, berries and greens to keep up your antioxidant levels.

With MEL Chemistry you can get acquainted with the chemical properties of such antioxidants as «anthocyans» and «lycopene»!

Can you use other juices apart from citrus ones in the experiment?

The juices that fit the task best are orange, grapefruit and lemon. But can you use others? Yes, of course. As long as the juice has enough vitamin C and it’s not blue or a very similar color – remember, this is the signal for when the vitamin C has run out. Peach and apricot juices are examples of these. What else should you look at when choosing the juice (or drink) for this experiment? Check it out in the Follow up section!