In the mercury cathode to keep apart the products of the electrolysis of brine chlorine is simply produced in a different container from the hydrogen and sodium hydroxide. Figure 3 (3) the mercury cathode cell
brine fed from the end box is electrolysed at the anode to produce the chlorine gas. The sodium ion generated reacts with the mercury to form sodium amalgam (an alloy of mercury and sodium), which flows out of the end box to a vertical cylindrical tank. About 0.25% to 0.5% sodium amalgam is produced in the cell. The sodium amalgam reacts with water in the decomposer (the second container), packed with graphite particles and produces caustic soda and hydrogen. Hydrogen, saturated with water vapours, exits from the top along with the mercury vapours. The caustic soda then flows out of the decomposer as 50% caustic.
In the membrane cell, the membrane allows cations such as Na+(aq) to pass through it, but not anions, like Cl (aq) or water molecules.
Figure 4 (2) the membrane cell
The hydroxide ions cannot pass through the membrane and so the concentration of sodium hydroxide on the cathode side of the membrane increases. Also the chlorine ions also can’t pass through the membrane, the sodium hydroxide solution is uncontaminated with salt and has a high purity. The membranes used are copolymers of tetrafluoroethene etc. consisting of chains of carbon atoms with fluorine atoms attached and side chains of acid groups.
Chlorine is produced by the of sodium chloride solution,” brine." Thus, when sodium chloride is dissolved in water, it into sodium and chloride The chloride ions are at the to form chlorine gas and water are at the to form hydroxyl anions and hydrogen gas. The sodium in the solution and the hydroxyl ions produced at the cathode constitute the components of sodium hydroxide formed during the electrolysis of sodium chloride. Chlorine happens to be very abundant in the sea due to it being more reactive and therefore brine is a valuable source for the economy side of the process of extraction. Three basic cell technologies generate chlorine at the anode, and hydrogen along with sodium hydroxide in the cathode compartment (or in a separate reactor for mercury cells). Separation is achieved in a diaphragm cell by a and in a membrane cell by an
In the membrane process there are no environmentally harmful substances such as mercury or asbestos involved, however chlorine is a carcinogenic chemical, which contributes to the depletion of the ozone. Also the power needs are low to drive apart the products of the process compared with the mercury cell, which consume large amounts of electrical energy.
Iodine is classified as a rare element due it being less reactive and very expensive to extract. There aren’t many ores containing iodine. It is obtained primarily from extracting seaweed, hence, the high cost.
In the brine extraction after purification and acidification the iodine is reacted with chlorine in a redox reaction because the chlorine is a powerful oxidising agent. The iodine vaporises to be passed into the absorbing tower where it is reduced by the sulphur dioxide, later to be filtrated to recover the purified crystals because it’s greater than it’s solubility.
In the extraction of nitrate ores redox reaction is used again to extract the iodine. Iodate (V) solution is reduced to iodide ions further iodate (V) liberates iodine. The solid iodine is then extracted using kerosene under heat. The kerosene evaporates and melts the iodine because iodine has a higher melting and boiling point. Nevertheless the cost of extracting iodine is high due the high demands for energy and other resources involved.
B Courtois discovered iodine in 1811AD. He treated the liquor obtained from the extraction of kelp (seaweed), which is now known to contain iodide salts, with sulphuric acid to produce a vapour with a violet colour.
Figure 5 (4) the reaction, which led to iodine being discovered
Iodine is 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. This led to a large scale burning of kelp way back in the 19th century. However it later became possible to import barilla or other cheap readily available raw materials superior to kelp. It was also shown that the yield of iodine could be maximised by drying seaweed under cover and by burning it in a kiln to eliminate loss of iodine. But later in Chile sodium nitrate (Chile saltpetre), which contains a rich source of iodine, was discovered and from this, extraction methods were developed further.
The uses of chlorine, sodium hydroxide, hydrogen and iodine in present-day industry
Word count 998
References
- Chemical Ideas, George Burton… et tal, Heinemann (2000), 9.1 page 205
- OCR AS Chemistry (Salters) open-book paper, 15 February 2002 (2852/01), Article 2, pages 6 and 8
-
Internet
- OCR AS Chemistry (Salters) open-book paper, 15 February 2002 (2852/01), Article 1, page 3
