Redox Reaction
When brine first enters the ion exchange* cell membrane it consists of sodium cations and chloride anions. The ion exchange membrane is such that it only allows cations to pass through it but not anions and therefore Na+(aq) ions diffuse past the membrane whereas Cl-(aq) ions stay in the anode compartment. Here chloride ions in the circulating brine come into contact with the titanium anode coated with an electrocatalytic layer of mixed oxides (1) and are oxidized resulting in the production of chlorine gas.
2Na+(aq) + 2 Cl-(aq) 2Na+(aq)+ Cl2(g) + 2e-
Meanwhile sodium ions pass into circulating dilute hydroxide solution where water molecules come into contact with the steel cathode and are reduced to hydroxyl anions and hydrogen gas.
2Na+(aq) + 2H2O(l) + 2e- H2(g) + 2Na+(aq) + 2OH-(aq)
Both the equations, occurring at the anode and cathode, are redox reactions because at the anode the chloride ions are oxidized and therefore lose electrons (2) producing chlorine gas while at the cathode, hydrogen atoms in the water molecules are being reduced and therefore gain electrons (2) as can be seen by the equations above.
Anode & Cathode Products
It is essential that the products formed at the anode and cathode be kept apart and there are several reasons as to why this is necessary the main one being to ensure the generation of pure products. Also, ‘the hydroxide* ions formed at the cathode can react with any chlorine that remains dissolved in the brine’ (3).
Cl2 + 2OH- (ClO)- + Cl- + H2O
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
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 18th and 19th centuries kelp was dried and burnt and the ashes leached with water. Sodium sulphate and sodium chloride were removed* by crystallisation and the remaining solution was concentrated by evaporation. The final solution containing 30-100grams iodine/litre was treated with sulphuric acid to remove sulphite. Manganese dioxide and concentrated hydrochloric acid were added to release iodine.
2 NaI + MnO2 + 2 H2SO4 --> I2+ MnSO4 + Na2SO4 + 2 H2O
After the discovery of the Chilean saltpetres, kelp burning has stopped entirely the reason being that the huge deposits of sodium nitrate contained much more iodine therefore making it a more efficient source. Additionally, the disposal of the spent black kelp ash proved to be problematic.
Kelp burning on the Aran Islands.
Modern Methods of Extracting Iodine
In the saltpetres, iodine exists in the form of iodate (IO3-). To obtain iodine, the sodium nitrate ore has to be leached with water. The resulting solution is cooled so that the sodium nitrate crystallises. These crystals are then separated from the solution, which contains 2-4g dm-3 of iodine in the form of iodate (V). To reduce the iodate (V) ions to iodine a spray of the cooled solution has to be passed into sulphur dioxide.
IO3-(aq) + 3SO2(g) + 3H2O I-(aq) + 6H+(aq) + 3SO42-(aq)
To liberate the iodine, the resulting solution is mixed with a small amount of the original iodate (V) solution.
5I-(aq) + IO3- (aq) + 6H+(aq) 3I2(aq) + 3H2O(l)
The iodine, which separates as a solid, is removed with kerosene-a hydrocarbon solvent. ‘The iodine/kerosene suspension is passed into a reactor where it is* heated at a pressure above 1 atmosphere to about 125oC to evaporate the kerosene and to melt the iodine.’(4) 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.
2I-(aq) + Cl2 (g) I2 (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.
SO2(g) + I2(aq) + 2H2O(l) 2H+(aq) + 2I-(aq) + H2SO4(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
