The Diaphragm Cell
The diaphragm is made of a porous mixture of asbestos and polymers. The solution can seep through it from the anode compartment into the cathode side. Notice that there is a higher level of liquid on the anode side. That makes sure that the flow of liquid is always from left to right - preventing any of the sodium hydroxide solution formed finding its way back to where chlorine is being produced.
Production of the chlorine
Chlorine is produced at the titanium anode according to the equation:
2Cl-(aq) - 2e- --> Cl2 (g)
It is contaminated with some oxygen because of the reaction:
4OH-(aq) - 4e- --> 2H2O (l) + O2 (g)
The chlorine is purified by liquifying it under pressure. The oxygen stays as a gas when it is compressed at ordinary temperatures.
Production of the hydrogen
The hydrogen is produced at the steel cathode:
2H+(aq) + 2e- -> H2(g)
Production of the sodium hydroxide
A dilute solution of sodium hydroxide solution is also produced at the cathode. It is highly contaminated with unchanged sodium chloride solution. The sodium hydroxide solution leaving the cell is concentrated by evaporation. During this process, most of the sodium chloride crystallises out as solid salt. The salt can be separated, dissolved in water, and passed through the cell again. Even after concentration, the sodium hydroxide will still contain a small percentage of sodium chloride.
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