186 ALKALI AND ALKALINE EARTH METALScharge of H+ ions furnished by the progressive ionization of water:
H,O-»H+ + OH~
O+H+->±H 2
The 0H~ ions, which thus accumulate in the cathode compartment,
are balanced by the Na+ ions, which are brought up by the electro-
lytic conduction. The commercial sodium hydroxide is obtained
by evaporating the cathode solution to a high concentration of
NaOH, in which NaCl is insoluble. The crystals of NaCl are
removed in centrifugal filters, and the purified solution is evap-
orated until molten NaOH is left. This is poured into molds and
allowed to solidify. If the total sodium hydroxide could be re-
covered, 40 grams would be realized for each faraday. It is the
object of this experiment to determine what percentage of this
ideal yield can be obtained in a simple cell.
Side Reactions Which Decrease the Yield. The products in the
anode and cathode compartments will react if they are allowed to
mix.
Cl, + 2NaOH -> NaOCl + NaCl + H 2 O
Commercial cells are so designed as to reduce this mixing to a
minimum. In our experiment we employ a loose plug of cotton
wool to prevent the mixing of the solutions. But even if con-
vection currents are eliminated completely, the unfortunate con-
dition holds that the electrolytic conduction carries OH~ as well
as Cl~ ions from the cathode through the cotton wool plug into
the anode compartment, where the OH~ ions react with the dis-
solved chlorine.
Cl 2 + 2OH- -» CIO" + Cl" + H 2 O
(This is identical to the above reaction, except that it is here
written ionically.)
Power Efficiency. Since the cost of electricity is based on the
energy consumed rather than the amount of electricity, the cur-
rent efficiency which we are determining is not a measure of the
cost efficiency. The energy consumed is proportional to the
product of the coulombs and the volts. The minimum voltage
to discharge CI2 from a normal NaCl solution, and H 2 from a
normal NaOH solution, in a cell is 2.17 volts. A somewhat higher
voltage must be applied to make a sufficient current flow through
the cell. The cell we use in this experiment, however, has a high
internal resistance, and more energy will be expended in over-