Extracting Iodine from Kelp

Ribbon seaweeds (sp. laminaria) contain iodine compounds which they obtain by extracting iodide ions from seawater. This iodine can be extracted from the seaweed by heating it in air to an ash, in which the iodine is present as iodide. The iodide is dissolved out of the residue by boiling water, the solution filtered and the iodide oxidised to iodine using hydrogen peroxide. The iodine colours the solution brown at this stage. Isolating the iodine from this solution can be done by solvent extraction, followed by evaporation of the solvent 1.

1/ Pile the seaweed up on the tin lid supported on a tripod and begin heating with a strong Bunsen flame. It may have to be added a portion at a time during the heating, given the quantity to be reduced to ash. When all has been turned to ash, about a dessertspoonful of residue will remain.
2/ Boil the ash with about 20 cm³ of purified water in a beaker, and filter while hot. Collect the clear filtrate in a second beaker and allow to cool.
3/ Add about 2 cm³ of dilute sulfuric acid to the solution, followed by hydrogen peroxide solution. A deep brown colour of iodine is formed as hydrogen peroxide oxidises the iodide ions present to iodine.
4/ Transfer the mixture to a separating funnel and add 10 - 20 cm³ of cyclohexane. Stopper the separating funnel, secure it with your thumb, and shake vigorously for about 30 s. With the separating funnel inverted, release any pressure that has built by opening the tap briefly.
5/ Clamp the funnel and allow the layers to separate. The cyclohexane will form a layer on top of the aqueous layer, and be coloured purple by the iodine now dissolved in it.
6/ Run off the lower aqueous layer and discard down the sink with running water.
7/ Run the purple cyclohexane layer into an evaporating basin, and set aside to evaporate in the fume cupboard. DO NOT HEAT!
8/ Iodine crystals will form slowly.

1. http://www.practicalchemistry.org/experiments/extracting-iodine-from-seaweed,256,EX.html