Vitamin C is a vital nutrient and we all need enough of it in our diet if we are to stay healthy. Along with some other animals, we have to eat foods containing vitamin C as we cannot synthesise it in our bodies. Without enough vitamin C humans develop a disease known as scurvy, which used to be a deadly disease for sailors and soldiers, who often had to go for long periods without fresh fruit and vegetables.
Vitamin C is a term for a number of molecules that all have the same biological action. The one that is most closely associated with vitamin C is ascorbic acid and you can look at the structure of the molecule in detail in our Gallery of Molecules.
In the previous step we highlighted that vitamin C molecules act as anti-oxidants. To identify how much ascorbic acid is present in a sample, we are going to use this property, which means that it can act as a reducing agent. By participating in such reactions the ascorbic acid, in turn, is oxidised itself. To summarise the reaction:
- Ascorbic acid readily gives up its electrons to a powerful enough oxidising agent.
- The loss of these electrons reduces the oxidising agent and oxidises the reducing agent.
In the experiment that we outline in the next step, the oxidising agent we are going to use is iodine. The iodine starts off electrically neutral and has a blue-black colour in the presence of starch. The vitamin C will be used to reduce the iodine, which will then become iodide (I-). As iodide does not make the blue-black colour in the presence of starch, once all the iodine has been converted to iodide the blue-black colour will disappear. Analysis of the reaction between ascorbic acid and iodine has shown that it takes one molecule of ascorbic acid to reduce one molecule of iodine.
If we make a solution containing vitamin C and starch and then add iodine, initially the iodine will be converted to iodide and the colour will disappear. At some point all of the vitamin C will have been used up and we will see a blue-black colour that does not disappear on stirring. If we know how much iodine we have added to the solution, then we can tell how much vitamin C was in the original solution.
In order to use this experiment we will first turn the process on its head. By making a solution of vitamin C, using a soluble vitamin C tablet, the concentration will be known. The amount of iodine that is required to observe a stable colour in this solution can then be measured.
For example, dissolving a tablet with 1000 mg (1 g) of vitamin C in a litre of water gives a solution with a vitamin C concentration of 1 g per litre. If 10 mL of this solution is used for a measurement then there is 10 mg of vitamin C in the solution. If some starch is added and then tincture of iodine is added drop by drop we might find that 80 drops of iodine have to be added before the solution becomes blue-black (and the colour does not disappear when the solution is stirred). So 10 mg of vitamin C have converted 80 drops of iodine. If we then test an unknown and have to add 8 drops of the same iodine solution before a stable colour is formed we will know that there was 1 mg of vitamin C in the solution.
This is the experiment we are going to outline in the next step.