Ribbon seaweeds contain iodine compounds which they obtain by extracting iodide ions from seawater. In this demonstration or class experiment, students observe seaweed being heated to an ash to produce iodide, which can then be oxidised to iodine and recovered by solvent extraction and evaporation

This practical has to be a teacher demonstration if the iodine produced is to be isolated as a solid. Under suitable circumstances it could be considered for a class experiment, using a smaller quantity of seaweed and up to the point of seeing the iodine colour in aqueous solution. Safety considerations involving the use of the flammable solvent rule out the solvent extraction of the iodine as a class experiment for larger groups.

However, with suitable small groups, extraction of the aqueous iodine into a non-polar solvent to show its characteristic purple colour could be carried out on a test tube scale, using only a few cm3 of the solvent. Solid iodine would not be isolated.

All solvent residues must be placed in a container in a fume cupboard for subsequent disposal.

The demonstration is rather lengthy, possibly taking 30–45 minutes to the point where the extracted iodine solution can be left to evaporate. Teachers may wish to consider commencing the heating of the seaweed prior to the start of the lesson, which may perhaps enable 15 minutes of lesson time to be saved.

It takes about 30–45 minutes to set up.



  • Eye protection
  • Access to a fume cupboard
  • Beakers, 250 cm3, x2
  • Beaker tongs
  • Filter funnel
  • Filter paper
  • Evaporating basin, 75 cm3 or similar
  • Separating funnel, 100 cm3
  • Bunsen burner
  • Tripod
  • Heat resistant mat
  • Large tin lid (size to fit across the top of the tripod)


  • Hydrogen peroxide solution, ‘20 volume’ (about 1.6 M) (IRRITANT), 10 cm3
  • Distilled or deionised water
  • Sulfuric acid, 1 M (IRRITANT), 5 cm3
  • Kelp or ribbon seaweed (Laminaria), about a dozen 50 cm lengths (note 7)

Health, safety and technical notes

  1. Read our standard health and safety guidance.
  2. Wear eye protection.
  3. Carry out the first stage (Procedure, step 1) in a fume cupboard.
  4. Hydrogen peroxide, H2O2(aq), (IRRITANT) – see CLEAPSS Hazcard HC050 and CLEAPSS Recipe Book RB045.

    Note that in the original version of this experiment, the solvent used was tetrachloromethane, but with restrictions on the use of chlorinated hydrocarbons this compound and related solvents should not be used in this experiment even if stocks are still held.
  6. Sulfuric acid, H2SO4(aq), (IRRITANT) – see CLEAPSS Hazcard HC098a and CLEAPSS Recipe Book RB098.
  7. Laminaria seaweed: this is the flat, brown, ribbon-like seaweed commonly called kelp, and any good source will suffice. Freshly collected material is likely to be the best, but some biological suppliers may be able to provide. Allow to dry, and keep dry, until ready to use.


  1. In a fume cupboard, 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. Turn off the Bunsen burner.
  4. 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.
  5. 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 seconds. With the separating funnel inverted, release any pressure that has built by opening the tap briefly.
  6. 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.
  7. Run off the lower aqueous layer and discard down the sink with running water.
  8. Run the purple cyclohexane layer into an evaporating basin, and set aside to evaporate in the fume cupboard. DO NOT HEAT.
  9. Iodine crystals will form slowly, probably in time to be shown to the class in the next lesson.

Teaching notes

The seaweed contains a wide range of elements drawn from seawater by the living algae, giving concentrations in the organism that may be considerably higher than in seawater itself. In the past, this natural concentration of useful elements, for which there was then no other economic source, gave rise to a large scale cottage industry network around the west coasts of Britain and Ireland, employing thousands in gathering and burning the kelp on the shore. The story of these past times can be used to enliven the waiting times at the different stages of the demonstration – see, for example, Orkneyjar for historical details of kelp-burning communities.

The two principal products were soda ash (sodium carbonate) for the alkali industry in the 18th and early 19th centuries, and iodine in the later 19th century. Kelp was also – and still is – used as an excellent fertiliser. Soda ash production from kelp ceased when the chemical industry itself found ways of producing sodium carbonate from sodium chloride, and iodine production from kelp ceased when the extraction of iodine from Chile saltpetre became dominant. In both events, thousands were put out of work, leading to large numbers emigrating from Scotland and Ireland to America and elsewhere. Nowadays, a small-scale industry continues, providing the raw material for expensive kelp-derived health supplements containing several ‘essential minerals’ including iodine. A typical kelp capsule may contain 0.1% iodine. The main kelp product at present, though, is for alginates used in food and drink, cosmetics and dozens of other everyday products.

Burning the kelp converts the iodine compounds in the kelp into iodide anions. Bromide and chloride ions are also present. The cations in the ash include sodium and potassium, so the leached solution in effect contains sodium and potassium halides.

Adding hydrogen peroxide in acid solution preferentially oxidises the iodide anions to iodine:

2I(aq) + 2H+(aq) + H2O2(aq) → 2H2O(l) + I2(aq)

Although iodine is sparingly soluble in water, it is much more soluble in the presence of more iodide ions, and the brown colour is due to this effect, though some of the iodine may be present as a dark brown precipitate.

On shaking the aqueous mixture with cyclohexane, iodine, being much more soluble in organic solvents than in water, passes mainly into the cyclohexane. The change to a purple colour shows that the iodine is now present as iodine molecules, I2.

When the cyclohexane evaporates at room temperature, the less volatile iodine is left behind and forms shiny black crystals in the evaporating basin. Solid iodine sublimes, so in time the iodine would also evaporate if not kept in the closed container.

There is a wealth of information available on the internet as background to this experiment.

A search for ‘kelp’ produces hundreds of health food supplement suppliers, whose websites give information about the mineral contents of kelp capsules and their claimed benefits. These may be useful in promoting discussion about scientific evidence and how it is used and misused in such contexts.