Question

In a Castner-Kellner cell for the manufacture of NaOH (caustic soda) _______ acts as a cathode as well as an anode.
[A] Graphite.
[B] Hg.
[C] Cu.
[D] Zn

Answer 1

Hint: The element which is used here has a shiny silver appearance. It is used here due to its property of forming amalgam which results in the deposition of the metal in form of amalgam and a higher discharge of ${{H}^{+}}$ ions during the course of electrolysis.

Complete step by step answer:
The Castner – Kellner process is a method of electrolysis of an alkali chloride solution in aqueous medium and it produces the corresponding alkali hydroxide at the end of electrolysis.
In this process, the apparatus has two kinds of cells.
In the first cell, we use sodium chloride solution as our electrolyte with a graphite anode and a mercury cathode. This cell surrounds the second cell from both sides.
In the second cell, we use electrolyte of sodium hydroxide solution and a mercury anode and iron cathode. There is a wall which separates both the cells but mercury is allowed to flow beneath them. This is the central cell.

The half-cell reaction at cathode includes formation of chlorine gas which is obtained as a by-product. The reaction is
     \[C{{l}^{-}}\to \dfrac{1}{2}C{{l}_{2}}+{{e}^{-}}\]
At the mercury cathode, sodium ion formed from the above reaction forms sodium metal which reacts with mercury to form an amalgam. The reaction is-
     \[N{{a}^{+}}+{{e}^{-}}\to Na(amalgam)\]

At the central cell, the anode reaction takes place at the interface of mercury and sodium hydroxide solution.
     \[2Na(amalgam)\to 2N{{a}^{+}}+{{e}^{-}}\]
And at the iron cathode of the central cell, the reaction is-
     \[2{{H}_{2}}O+2{{e}^{-}}\to 2O{{H}^{-}}+{{H}_{2}}\]

As we can see from the above discussion that mercury is used as both cathode and anode here.
So, the correct answer is “Option B”.

Note: Through this process, we can manufacture sodium hydroxide. The concentration of sodium chloride in the first cell decreases and simultaneously, the sodium hydroxide concentration at the central cell increases. As the electrolysis moves forward, we take out some sodium hydroxide from the central cell and replace it with water and we add sodium chloride to the outside cells to continue the process.