SODIUM HYDROXIDE BY ELECTROLYSIS 185PREPARATION 17
SODIUM HYDROXIDE BY ELECTROLYSIS OF SODIUM CHLORIDE
SOLUTION. FARADAY'S LAW AND THE CURRENT EFFICIENCY
Another widely used commercial method for the production of
sodium hydroxide lies in the electrolysis of sodium chloride solu-
tion — a method which is economical when the chlorine simul-
taneously produced can also be utilized.
In this preparation we shall construct a laboratory cell embody-
ing some of the features of the commercial cell and determine the
ratio of the actual yield of sodium hydroxide to that theoretically
obtainable from the current. We shall use the entire product in
making the determination and, therefore, there will be no prepa-
ration to be preserved.
The section on electrochemical reactions and Faraday's law,
pages 123-126, should be carefully reviewed before proceeding
with this preparation.
Coulometer. A cell consisting of copper electrodes immersed
in a copper sulphate solution furnishes one of the most reliable and
exact methods of measuring the amount of electricity flowing
through the circuit into which it is inserted. We shall measure
the decrease of weight of the copper anode, rather than the in-
crease of weight of the copper cathode, because some of the copper
deposit on the cathode may not be firmly attached, and may
powder off in the process of washing, drying, and weighing. It
should be remembered that since the Cu++ ion has two charges,
the equivalent weight of copper is one-half the atomic weight,
and 1 faraday will dissolve 31.8 grams rather than 63.6 grams from
the anode.
In the commercial electrolysis of brine, carbon anodes and iron
cathodes are employed. The anode reaction is mainly the dis-
charge of chlorine ions:
2Cr->Cl 2 + 2O
but to a small extent hydroxyl ions are discharged, liberat-
ing oxygen:
2OIT -> H 2 0 + iO 2 + 20and a part of this oxygen attacks the electrode, producing CO2.
The cathode reaction consists almost exclusively in the dis-