Determination of Vitamin C
Description
The amount of vitamin C is determined in juices from fruits, and vegetables.
Requires
- Starch (Cornstarch, Maizena, cornflour – but not maize flour)
- Water
- Kettle and heatproof container, or saucepan and somewhere to heat it
- Tincture of iodine.
- Pipette (eye dropper or similar)
- Vitamin C (soluble tablets are best for this.)
These are the same as required for the Iodine-Starch Indicator experiment. Tincture of iodine may be available in your local pharmacy, otherwise there are various alternatives (such as Betadine or Povidone) but they may not work as well in that the colour may not be so obviously and completely removed. It may be necessary to obtain supplies of this from the internet.
Method
First, prepare the starch indicator following the instructions in the Iodine-Starch Indicator experiment. Then, dissolve one of your vitamin C tablets in a known amount of water. One tablet often contains 1 g of vitamin C. This dissolved in one litre of water gives a concentration of one gram per litre. This is a known concentration with which we will calibrate our iodine.
Take a small (but measured) amount of the known solution, add some starch indicator, then add the iodine solution one drop at a time (dropwise) with gentle stirring. Count the number of drops it takes to produce the deep blue colouration so that it persists for at least 30 seconds. Let us call that number of drops N. Take the same amount of the unknown solution (such as orange juice) and do exactly the same. Add some starch indicator, then count the number of drops it takes to produce a persistent deep blue colour. Let us call that number of drops n.
It is likely that n will be around a factor of ten smaller than N, so if you have N = 94, n is often going to be less than ten drops. If the known solution was a concentration of 1 g per litre, and the two volumes of solution that were tested were the same, then the concentration of vitamin C in the unknown solution is simply n / N g L-1.
This is an example of quantitative analytical chemistry because we are determining how much (the quantity) is present, not just what type of thing is present. The latter is determining the “quality” of what is present and is known as a qualitative analysis.
Principle
This experiment relies on the fact that one molecule of vitamin C will be reduced by one molecule of iodine, turning it into iodide. While there is still vitamin C in the solution any iodine that is added will be converted to iodide so there is no iodine to produce the deep blue colour in the solution. When all the vitamin C has been used up the next molecules of iodine that are added stay as iodine and produce a deep colour in solution.
Tip
Many of the fruit juices and solutions that are going to be tested already have their own colour. It is sometimes worthwhile having a sample that has had too much iodine added to it as a comparison in order to be able to spot the point at which the colour changes.
Safety
Take care with boiling water. Allow water to cool before pouring or mixing.
Take care with iodine. It will stain most things it comes into contact with. The colour can often be removed, however, by treating the surface with vitamin C solution.