188 ALKALI AND ALKALINE EARTH METALSCottonFIG. 22Procedure: Set up the apparatus as shown in Fig. 22. A car-
bon anode and a spiral iron cathode are placed in the electrolytic
cell at the right. A plug of cotton wool is placed at the bend of
the U-tube; it is not packed very tightly, only enough to hinder
the bodily mixing of the solutions in the two arms. The electro-
lytic cell is filled with a
saturated solution of com-
mon salt. The metal con-
nection on the carbon pole
should not touch the solu-
tion.
The coulometer, or left-
hand U-tube, should con-
tain saturated copper sul-
phate solution, which is
specially prepared for this
experiment. The cathode
is made of copper; the
anode is a spiral of heavy
copper wire free of any
coating of oxide. Clean and dry the anode and weigh it.
Connect the two U-tubes " in series," as in the diagram; connect
the terminals through a 100-watt lamp with the direct-current
laboratory circuit. The current should be turned on or off by
unscrewing the lamp from the socket, never by pulling the plug.
If the plug is pulled, in re-inserting, the direction of flow of the
direct current might be reversed.
Before starting the experiment, the polarity of the terminal
posts must be determined by placing across them a strip of pink
litmus paper moistened with NaCl solution. The spot touching
the cathode turns blue.
Let an instructor approve your arrangement of apparatus.
Note the time that the current is turned on.
Allow the current to flow for about 20 minutes. During the
flow of the current, prepare two clean beakers, which should be set
side by side on the desk, near the apparatus; also have a wash
bottle at hand. As soon as the current is turned off, place the
iron electrode in one of the beakers; set the carbon pole aside.
Pour the solution out of the electrolytic cell, holding it so that the
cathode arm will empty into the beaker containing the iron cathode,