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Castner-Kellner process

    The ''Castner-Kellner process'' is a method of electrolysis on an aqueous alkali chloride solution (usually sodium chloride solution) to produce the corresponding alkali hydroxide

    Process details

    Castner-Kellner aparatus
    The aparatus shown is divided into two types of cells separated by slate walls The first type shown on the right and left of the diagram uses an electrolyte of of sodium chloride solution a graphite anode (A) and a mercury cathode (M) The other type of cell shown in the center of the diagram uses an electrolyte of sodium hydroxide solution a mercury anode (M) and an iron cathode (D) Note that the mercury electrode is shared between the two cells This is achieved by having the walls separating the cells dip below the level of the electrolytes but still allow the mercury to flow beneath them
    The reaction at anode (A) is:
    2Cl → Cl2 + 2e

    The chlorine gas that results vents at the top of the outside cells where it is collected as a byproduct of the process The reaction at the mercury cathode in the outer cells is
    2Na+ + 2e → 2Na (amalgam)

    The sodium metal formed by this reaction dissolves in the mercury to form an amalgam The mercury conducts the current from the outside cells to the inside cell In addition a rocking mechanism (B shown by fulcrum on the left and rotating eccentric on the right) agitates the mercury to transport the dissolved sodium metal from the outside cells to the inside cell
    The anode reaction in the center cell takes place at the interface between the mercury and the sodium hydroxide solution
    2Na + 2e → 2Na+

    Finally at the iron cathode (D) of the center cell the reaction is
    2H2O + 2e → 2OH + H2

    The net effect is that the concentration of sodium chloride in the outside cells decreases and the concentration of sodium hydroxide in the center cell increases As the process commences some sodium hydroxide solution is withdrawn from center cell as output product and is replaced with water Sodium chloride is added to the outside cells to replace what has been electrolized