Highly pure chlorine gas is then given out at this stage. The sodium ions are discharged and dissolve in the flowing mercury, which then is ran into a second container called the decomposer (below the electrolyser). This ensures that the products are never mixed so therefore cannot react together.
The membrane cell
In this method an ion exchange membrane separates the two products. This membrane is made from copolymers of tetrafluroethene.
The membrane works by allowing cations, Na+ (aq) to pass through it but not anions, Cl- (aq) or water molecules. Therefore when the brine enters the anode side the chlorine ions are discharged producing chlorine gas and sodium ions pass through the membrane to the cathode side. The chlorine gas, which cannot pass through the membrane, is then collected.
The sodium ions then join with the circulating dilute sodium hydroxide solution and as the concentration of the solution increases, due to the newly formed hydroxide ions not being able to pass through the membrane, it is given out through a different outlet. As the chloride ions could not pass through the membrane the sodium hydroxide solution has a high purity and is not contaminated with salt.
Fig. 1: the membrane cell
The halogens
The methods for producing chlorine and iodine work well due to the chemistry of the elements. Chlorine and iodine both belong to group 7, the halogens. All halogens have seven electrons in the outer shell. The outer shell has to be full to convert ions into molecules, this is why the methods of extracting chlorine and iodine include the transfer of electrons as redox reactions.
It is harder for the ions to convert to molecules as iodine is less reactive than chlorine (due to it being larger as it has one extra full shell of electrons). This is why the process of extracting iodine is a slower process then that of chlorine. Iodine is produced by a displacement reaction the chlorine molecules readily lose electrons to the iodide ions. This is not possible in the production of chlorine as the only element that will displace it is fluorine. Fluorine is the most reactive halogen and is not reactive to use in the chlor-alkali industry, therefore chlorine is produced by electrolysis.
Extracting iodine
In the nineteenth century iodide was extracted from kelp (seaweed). The kelp contained iodide ions absorbed by the seawater and become more concentrated by burning the kelp. Sulphuric acid was then added and had enough oxidising power to discharge the iodide ions into iodide molecules. The sulphuric acid is reduced to sulphur dioxide and hydrogen sulphide.
This method was not very efficient as it took twenty tonnes of kelp to produce just one tonne of ash so it was discovered that by drying the seaweed under cover and burning it in a closed kiln increased the yield of iodide.
However other methods of obtaining iodide were soon discovered. A way had been found to extract the element from huge deposits of sodium nitrate discovered in Northern Chile years before. This method was much more preferred as it was shown to contain around 640 ppm of iodide. The sodium nitrate is put in to water at 40°C and then allowed to cool to let the sodium nitrate crystallise. After cooling a spray of the solution is into sulphur dioxide which reduces the iodate(V) ions to iodide ions. Iodide is liberated by adding a small amount of the original iodate(V) to the resulting solution. The iodide then separates as a solid and is extracted with a hydrocarbon solvent.
Another method used today is from extracting iodine from brine using a redox reaction. The brine is reacted with chlorine after purification and acidification with sulphuric acid. This solution is dilute in iodine and is concentrated by blowing a stream of air in to the solution causing the iodine to vaporise. The iodine rich air passes into an absorbing tower containing acid; here adding sulphur dioxide reduces the iodine. When this solution is chlorinated again the concentration of iodine is greater then the solubility so therefore crystals of iodine separate and these can be obtained through filtration.
These methods are a lot easier to control then the original way of extracting iodine by using kelp. Iodine can be obtained quicker as there is no need for drying. This is why the kelp industry stopped and the other methods became more popular.
The uses of chlorine, sodium hydroxide, hydrogen and iodine in present day industry.
References.
Article one – Napoleon’s legacy: riches from the seas from Salters open book paper 2002
Article two – The salt of the Earth from Salters open book paper 2002
Salters Chemical Ideas book (second edition) Page number 204
Chemistry in Action (second edition) Michael Freemantle Pages 498-500
