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Life's Essential Elements: Chlorine & Iodine.

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Life's Essential Elements: Chlorine & Iodine Chlorine and iodine, both halogens, have important uses in everyday life particularly in industries. Their uses are described on pages 6-8. Both these elements had been discovered around 200 years ago and have been used ever since however methods of obtaining these elements have changed throughout the years. Extracting Chlorine Several processes can be used to obtain chlorine the main one being electrolysis of sodium chloride solution known as brine, a method used by chlor-alkali industries. Electrolysis can be achieved using three types of electrolytic cells namely the diaphragm cell, mercury cell and membrane cell. `The main difference in these technologies lies in the manner by which the chlorine gas and the sodium hydroxide are prevented from mixing with each other.'^(1) [image002.jpg] [image004.jpg] [image006.jpg] In diaphragm cells brine flows through the separator (made of asbestos/polymer modified asbestos composite) to the cathode compartment. In membrane cells the separator is an ion exchange membrane however mercury cells contain no diaphragm or membrane, instead the mercury itself acts as a separator. ...read more.


[image008.gif] Cl[2] + 2OH^- (ClO)^- + Cl^- + H[2]O Furthermore, the products formed can react explosively with each other. In the mercury cathode cell the mercury itself acts as the separator `by forming an alloy of sodium and mercury (sodium amalgam) which is subsequently reacted with water to form sodium hydroxide and hydrogen in a separate reactor.'^ (1) In the membrane cell separation of the anode and cathode products is achieved by the use of the ion exchange membrane, which is permeable only to either positive ions (cation exchange membrane) or to negative ions (anion exchange membrane) and thus the products are kept apart. Extracting Iodine *200 Iodine can be obtained by various methods. Presently, the majority of iodine used commercially is obtained from two major sources namely the Chilean saltpetres and the Japanese brines. However before the discovery of the Chilean saltpetres, iodine was obtained by burning kelp. This accidental finding by Bernard Courtois in 1811 and Jean Francois Coindet's link with goitre led to the mass production of iodine. During the 18^th and 19^th centuries kelp was dried and burnt and the ashes leached with water. ...read more.


The iodine is then poured into cold air and solidifies into droplets known as prill. The method for extracting iodine from brine is quite different than the aforementioned method. In this case the brine has to first be purified through acidification with sulphuric acid and then it has to be reacted with chlorine gas so that the iodide is oxidised to iodine. [image011.gif] 2I^-(aq) + Cl[2] (g) I[2] (aq) + 2Cl^-(aq) Since the iodine solution is dilute it has to be concentrated and this is achieved by blowing a stream of air into the solution, which causes the iodine to vaporise. The vapour passes into an absorbing tower, which contains acid. Sulphur dioxide is then added to reduce the iodine. [image011.gif] SO[2](g) + I[2](aq) + 2H[2]O(l) 2H^+(aq) + 2I^-(aq) + H[2]SO[4](aq) The resulting solution has a high concentration of iodide ions, which need to be chlorinated again so as to produce iodine. As a result of the chlorination `the concentration of the iodine formed is greater than its solubility' which causes the iodine crystals to separate. These crystals are recovered by filtration and the remaining solution is recycled to the absorber tower. The resulting iodine has a purity of 99.5%. Total Number of Words: 999 ...read more.

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