Iodine / starch indicator


A cloudy liquid is added to a yellow solution and the yellow solution immediately turns black. Vitamin C is then added to the black solution which turns colourless.


  • Startch (Cornstarch, Maizena, cornflour – but not maize flour)
  • Water
  • Kettle and heatproof container or saucepan and somewhere to heat it
  • Tincture of iodine
  • Vitamin C


Add a little hot (boiling is best) water to a small quantity of cornstarch. A teaspoonful is usually sufficient for several experiments. Heat this in a heatproof container in a kettle or surrounded by water in a saucepan. The water needs to boil for a few minutes so that the grains of starch break open. The mixture will thicken and become translucent when it is ready. If it becomes too thick simply add more warm water. This is your indicator.

Add a few drops of tincture of iodine to water in a transparent container. When the indicator is added to the iodine a deep blue/black colour will develop. This colour will persist until the iodine is turned into iodide. That happens when vitamin C is added.


Iodine, in its elemental form, combines with starch to form a range of coloured solutions. Sometimes the solution can appear brown, sometimes a dark blue/black colour. The starch often has a structure like that shown in the gallery of interactive molecules. This can be rotated to show channels, like tunnels, running through the molecule. When iodine gets into these channels the blue black colour is formed.

Vitamin C is an anti-oxidant. Anti-oxidants are also known as reducing agents. Vitamin C is enough of a reducing agent that it can turn iodine into iodide. Iodide does not have the same interaction with starch as iodine, which is why the starch loses its colour.

One molecule of vitamin C is needed to turn one atom of iodine into iodide. This means we can use this starch indicator and iodine to measure the vitamin C content of foods and fruit juices.


This reaction can be used to do some chemistry magic by turning water into “wine”.


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.

Tags: Colour change, indicator, starch, iodine, vitamin C.