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Chlorine is mainly extracted from sea water (brine) which contains many different ions, some of which are sodium ions and chloride ions . Brine is electrolysed and the chlorine ions are liberated' to chlorine atoms. Although when electrolysing sodium chloride, it is sometimes necessary to keep the anode and cathode reactions apart as they can react, causing the products to become impure. This can be done by separating the two electrodes with a barrier. .
The mercury cathode cell keeps apart the products of the electrolysis of sodium chloride solution simply by having the chlorine produced in a different container from the hydrogen and sodium chloride. The first container is called the electrolyser and contains suspended titanium anodes coated with a metal oxide. The cathode is a layer of mercury flowing in the same direction as the sodium chloride solution along the sloping base of the container. .
2Cl‾ (aq) Ã Cl2 (g) + 2e‾.
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Diagram 2: Mercury Cell - Electrolyser .
At the mercury cathode sodium ions are discharged instead of hydrogen ions in the following manner:.
Na+ (aq) + e‾ Ã Na (l).
The now discharged sodium then dissolves in the mercury to form a liquid amalgam which is then run into the second stage of the reaction; the decomposer. Water is pumped into the tank containing graphite blocks suspended on the mercury. Sodium is then oxidised and the ions pass into the solution as shown below:.
Na/Hg (l) Ã Na+ (aq) + Hg (l) + e‾.
Hydrogen gas and hydroxide ions are formed when water is electrolysed and the hydroxide ions reacts with the sodium ions present in the water:.
2H2O (l) + 2e‾ Ã H2 (g) + 2OH‾ (aq).
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Diagram 3: Mercury Cell - Decomposer .
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The ion exchange membrane cell however uses a different technique. A membrane separates the anode and cathode. This membrane only allows cations like sodium ions to pass through but not anions like chlorine ions.