Seaweed contains a wide range of elements drawn from sea-water by the living algae, giving concentrations in the
organism that may be considerably higher than in sea-water 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 two principal products were soda ash (sodium carbonate) for the alkali industry in the 18th and early 19th
centuries, and iodine in the later nineteenth century. Kelp was also used, and still is, 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.
Aim
You are going to extract iodine from some laminaria seaweed (kelp) using the process of solvent extraction
Equipment
•
Laminaria seaweed (kelp)
•
Bunsen burner
•
Tongs
•
Beaker x 2
•
Funnel
•
Filter paper
•
Conical flask
•
Dilute sulphuric acid
•
1.5M hydrogen peroxide (25ml)
•
20ml Cyclohexane
•
Tap funnel
•
Retort stand, boss and clamp
•
Basin
•
Kettle
Safety
• Wear eye protection at all times
• Use of fume cupboard is vital
• Caution when shaking the tap
funnel – pressure will build so
remove stopper at regular intervals
• No naked flames near cyclohexane
Method for TEACHER DEMO
Assemble all the apparatus you will need. You will need to
follow the procedure below:
1. In a fume cupboard, burn a quantity of Laminaria
seaweed (kelp) to ash
2. Gently heat a quantity of the ash in a beaker of water to
dissolve the iodine
3. Filter this into a conical flask using a funnel and filter
paper
4. Add a few drops of dilute sulphuric acid and
approximately 20ml of the 1.5M hydrogen peroxide
5. Next, put 20ml cyclohexane in a tap funnel
6. Add the filtrate from the conical flask to the
cyclohexane
7. Place a stopper in the tap funnel and shake, periodically
removing the stopper to relieve pressure build up
8. When the water has become clear and the cyclohexane
layer has turned red place the tap funnel into a clamp
on a retort stand and allow layers to settle
9. Run off the water layer into a beaker and discard
10. Next, run off the cyclohexane layer into a second flask
11. Leave the cyclohexane layer to evaporate in the fume
cupboard (DO NOT USE NAKED FLAME)
12. Evaporation rate can be increased by immersing the
flask in a basin of hot water
Observations
Why did we burn the kelp to ash?
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Why was the use of cyclohexane important to obtaining the iodine?
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Research: When and how was iodine discovered?
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Further Context
How does your experiment link to the real world?
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How could this experiment be altered to be conducted on a larger scale?
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