Salt is found in large quantities on the earth and it can be extracted fairly easily. As sodium ions are positively charged they have a tendency not to attract to attract electrons as much as water molecules. This makes it easier to transfer the electrons onto the water molecules. Its necessary to keep the products formed at each electrode separate because they will react and explode. There are 3 types of cells used to do this.
The first of which is a mercury cathode cell. In this cell the products are separated and formed in different containers. The cell has titanium anodes located above a mercury cathode, which flows along the bottom of the cell. Sodium chloride (NaCl), chlorine (Cl) is liberated at the anode and the sodium (Na+) dissolves into the mercury cathode to form an amalgam. This amalgam is passed into a separate reactor, where it reacts with water (H2O) to produce hydrogen.
(3) The electrolyser
In electrolysis cell uses a membrane cell that acts as a barrier to all the gas and the liquid flows and allows only the transport of charged sodium ions between compartments. Sodium ions can pass through in the hydrated form but the membrane is impermeable to free water molecules.
(4) The membrane cell
However this membrane cell was used until early 1970’s because of poor quality materials of the membrane (5)
There are 4 points, which a membrane cell should:
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Permeable to Na+(aq) ions
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Impermeable to Cl- (aq) ion
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Resist to chlorine and sodium hydroxide solution that is almost 9 mol dm-3
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Stable at approximately 90°C
Most modern methods of extracting chlorine and iodine involves either electrolysis to extract the chlorine or by redox reactions of brine to extract the iodine. This is a modern technique. Iodine can also be extracted from kelp, which is obtained by burning seaweed. Salts such as sodium chloride, potassium chloride, and potassium sulphate are removed from the kelp by washing with water. The residue is then heated and concentrated, the iodine is liberated Chlorine and iodine are both from group 7 in the periodic table, halogens, which means they have similar chemical properties. Iodine is a violet dense solid, which is insoluble in water, but it is soluble and when heated it gives a violet vapour that sublimes.
Chlorine is a highly toxic greenish yellow gas and it has a pungent odour and fumes in most air. Chlorine and Iodine arise from a dangerous nature. These process relay mainly on redox reactions, which can cause displacement reactions to occur. Where a chlorine molecules can take the place of iodine from the compound its in. They are the most reactive group and they tend to remove electrons from other elements, as they are oxidising agents. (6)
Iodine was discovered as part of a mistake of adding excess sulphuric acid when investigating the uses of seaweed (kelp) for the process of forming nitrate ions. When the kelp was boiled, sludge was collected at the bottom. To avoid corrosion, acid was added, too much sulphuric acid was added and this produced violet fumes and a dark metallic looking crystal was found, iodine, as shown in the equation.
(7)
This method of burning seaweed is the traditional method of extracting iodine. Nowadays iodine is extracted by a redox reaction from brine. After purification and acidification with sulphuric acid, the iodine is reacted with chlorine. Air is blown and the iodine vaporises and this passes into an absorbing tower where sulphur dioxide is added to reduce the iodine.
(7)
When the solution is then chlorinated again the purity of the iodine is about 99.5% and any residue is recycled.
Another method of extracting iodine is through nitrate ores. Nitrate ions are extremely powerful oxidising agents. The solution of iodide ions are mixed with tiny amounts of the original Iodate(V) solution to liberate the iodine.
(7)
The methods of extraction of iodine between the nineteenth and twentieth century have changed dramatically. These changes are mainly due to improvement in technology and the chance of discovery by accident. The accident has turned out to be a vital part of modern life.
As technology becomes more and more advanced the properties of elements and the way that they react will be discovered more intensely.
They are many uses of the products or electrolysis of sodium chloride. The pie charts below show the main ones.
The annual world production of chlorine is about 39 million tonnes per year.
The miscellaneous products are those of water treatment and insecticides.
41 million tonnes of the sodium hydroxide is produced every year.
The hydrogen is mainly used to make ammonia. 36 million tonnes of hydrogen is produced every year.
At the present day the annul production of iodine is about 13000 tonnes per year. This is fairly a small amount compared to the products of the electrolysis of sodium chloride.
Word count: 1000
References:
(1)- The half equations for redox reaction – Article 2 pg 6 (The essential chemical by Bill Fox, Chemistry Review, 1996, volume 6, number2)
(2)- The explanation of Redox Reactions - Chemical Ideas, Burton G, Holman J, Lazonby J, Pilling G, Waddington D, Heinemann (2000) 9.1 pg 204
(3)- Diagram of the mercury cathode cell – Article 2, pg 7, fig1
(4)- Diagram of membrane cell – Article 2, pg 8, fig 3
(5)- Specification of membrane cell – notes from course
(6)- Halogens - Chemical Ideas, Burton G, Holman J, Lazonby J, Pilling G, Waddington D, Heinemann (2000) 11.4 pg 258
(7)- The equations of extraction of iodine – Article 1 pg 5 (Adapted from articles by Margaret Ferguson, Chemistry Review, 2000, Vol 10, Num 1 and P A Swain, School Science Review, 1994, 75 (273)
Chemical Storylines, Burton G, Holman J, Lazonby J, Pilling G, Waddington D, Heinemann (2000) pg 52
http://library.thinkquest.org/C004970/redox/electrolysis
http://www.ucc.ie/ucc/depts/chem/dolchem/html/elem/elem053
