Measuring vitamin C

The experiment outlined below allows measurement of the levels of vitamin C in any solutions. The experiments will measure the levels of the vitamin in aqueous solutions (i.e. samples made up in water) because vitamin C is usually quite soluble in water.

To understand the important principles behind this step, it is not essential to do this experiment at this stage or, in fact, at all. If you are not able to carry out this experiment, read through the details below and try to work out what you would expect to see. You could set up a discussion about your expectation using the Comments boxes.

If you are studying this course with other people (e.g. within a school class), you might want to think about doing the experiment together

If you do plan to carry out this experiment, you will need to read the instructions carefully and do some planning before starting it.

Materials required

For this experiment you will need:

• Cornflour (corn starch, Maizena or equivalent)
• Tincture of iodine
• Soluble vitamin C tablet
• Fruit or vegetable juice (ideally something that is not highly coloured).
• Pipette or eye dropper
• Boiling water
• Measuring device (e.g. a syringe, or a small vessel that can be reliably filled to the same point)
• Cup (for boiling water to make the indicator)
• A range of other small containers to make measurements in

First of all, you should prepare your starch indicator. To do this, put about quarter of a teaspoon of cornflour in a cup and add about half a cup of boiling water. The mixture needs to be runny afterwards, not a paste and this is your starch indicator. Test it by adding a few drops of indicator to a few drops of tincture of iodine dissolved in water: if it has been prepared properly the indicator solution should develop a blue-black colour upon addition of the iodine.

Calibration experiment

Before we can measure the amount of vitamin C in our samples, we need to “calibrate” our measurements. This means that we need to find out how the experiment works using samples containing known amounts of vitamin C.

Make a note of how much vitamin C is in one of the tablets and dissolve this in one litre (1 L) of water. Take a small (known) amount – 50 mL for example – of the vitamin C solution and add five drops of starch indicator.

To your mixture of vitamin C and starch indicator, add drops of iodine, carefully counting the drops, until the solution develops a blue-black colour that does not disappear on stirring. To get a more accurate number you might repeat the experiment and take the average of a number of readings.

Now you know the number of drops of iodine that react with the vitamin C in our volume of solution. If that volume is a known number of millilitres we can work out how many drops of iodine we require per milligram of vitamin C. If the volume is unknown, but reproducible, we can work the quantity of vitamin C in the unknown by using the ratio of drops. Please ensure that the same equipment is used to measure the levels of vitamin C in the next stage of the experiment (the same dropper, etc.).

Measuring vitamin C levels in an experimental sample

Here, you will perform the same experiment on a solution that contains an unknown amount of vitamin C. For this you will use fruit or vegetable juice.

Measure out a known volume of your unknown juice. Use the same method that you used for the calibration above. Add some starch indicator and count the number of drops of iodine required before a stable colour is formed. It is likely to be far fewer drops than in the calibration experiment.

In the next step we will outline how to analyse the results from this experiment and there will be an opportunity to discuss what they mean.

If you have the opportunity to conduct this experiment, feel free to post comments about what you observe.