C0S CE S TR-ITI 0S c" H .IS C;E S I S T H E ELECT ROL 1'sIS O F COPPER SULPHXTE SOLUTIOSS F3Y C
11.
BESSETT . \ S D C
0
BROIVN
I t has been noticed' in the electrolysis of copper sulphate Solutions n ith a rota1 ing copper cathode, that the potential drop across the cell, with constant current, increases nith an increase in the speed of rotation In vien of the fact that a more efficient stirring of the solution is obtained by increasing the rotation, concentriition dif'ferences being thereby lessened, a drop in potential diference might be expected as random prediction Since thic, increase is rather large, and 5eemingly abnormal, i t \vas deemed advisable to attempt an explanation of this phenomenon -1s the object n-as a study of the Loltage relations only, no attention n as given the deposited copper except to note that it \vas pure, and that it completely covered the aluminum cathode upon v hich it was deposited .Idrawing of the cell is shonn in Fig I . which may be described as follows The revolving cathode C n a s made from a piece of aluminum pipe. I " outside diameter, dnd 3" lonq. This n as scren ed to a brass shank which TI as mounted in a hollow steel spindle revolk ing in two bearings The loii er end of the cathode mas closed n i t h a rubber stopper The current !vas led to the cathode through a wire, E. dipping into mercury placed in the hollon- steel spindle This cathode, mounted on an iron stand, well braced, dipped into a glass battery jar of about I j liters capacity This also held the anode D, of pure cast copper The cathode was rexwlved by a shunt n-ound D C motor, G, operating on the rro-volt circuit, with varying speeds obtained by the starting box H The current passing through the cell IT as taken from the same circuit, with a suitable resistance, J ) and an ammeter, A, in series with the electrodes It as a t first planned to measure the back electromotive force of the cell by breaking the line
' Soc,
Iicnnett Jour. Phys Chem.. 16,294
21, z j j ( 1 9 1 2 ) .
(1012).
Trans Am. Elcctrochcm.
,
circuit and instantaneously connecting a very sensitive millivoltmeter a c r o s the cell. For this purpose, a switch was designed which consisted of tn.0 double pole switches, with the blades of one removed, the switches being mounted front to front with the end terminals a few hundredths of an inch wider apart than the width of the blade. This gave the effect of moving the end terminals of a double throw snitch
Fig.
I
very close together, so that the time for throwing the switch over was negligible. Ey covering this arrangement with transformer oil the connection being the same as an ordinary double throw switch, the line circuit could be broken even with 40 amperes flon-iiig, and the cell connected with a delicate millivoltmeter, I,, almost instantaneously. Voltage drop across the cell mas measured with a voltmeter, S , which was
changed, whenever necessary, to ensure an appropriate scale or range for the required accuracy. It was first thought advisable to check the measurement of the increase in voltage drop, mentioned above. For this measurement the solution was the same as that used in the pre\.ious investigation, consisting of a I 2 percent solution of copper sulphate and I j percent sulphuric acid. The current density as well as the speed of rotation of the cathode was 1-aried. The results are given in Table I . TABLEI-ISCREASEIS POTENTIAL DIFFEKEXE Current density Kevolutions per
3 5
9
Aimpereq/sq. dm
14
0
0
2 j 0
minutc
Differcnce of potential, volts I000
2000 ~000
0
3%
(' 390
o S80 o 890
o 910 0 919
2
250
2
30" qx) 300
0 0
400
0
90C)
(1
950
2
4Cl00
4"'
o 910
i)
96j
2
jOOi )
0
1I O
0
I
000
2 +jC)
930
The increase n i t h high currents is partly due to the fact that the resistance to the flow of a large current is affected relatively more than that with lon- currents, by the air which is drawn down into the solution, the latter occurring a t high speed of rotation The most natural assumption to make in attempting an explanation of this phenomenon is, that it is due to an iIicrease of resistance This resistance may be a true one, or a virtual one, of the nature of a back electromotive force. Since the latter m-as the most easily handled, it was studied first This order was chosen also on account of the fact, as will be seen below, that the results obtained here would show whether or not the real resistance factor was the important one I n order to determine whether concentration differences were important, attempts were made to measure the back
376
C . If'. Bciiiiett aiid C'.
i1 .
Brown
electromotive force of the cell directly, usin? the double throw oil switch described above .I characteristic measurement may be given
'r \IILE
2
-
Hack clcctroinotiTc iorcc
C D
I+
amp
5q
dm
13ack E. SI. I: Yolt
Speed of rotati(~ii
R . P . SL.
0.01s
0, 0 2 0 0 0 2I
0 0 2 2 0 027
slight difference was obtained. but not the difference sought for, which from Table I is seen to be 0.090 volt. I t was decided that the time element in throwing the switch entered in here, so that concentration changes n-ere equalized by the rapid stirring before the meter could be connected across the cell. This method of attack \vas therefore not pursued further. If the voltage rise from 1000 to 5000 r . p. m , as shon.n in Table I , be plotted against current, and the curve be extrapolated to zero current, an approximation of the real increase will be obtained, which eliminates the effect of resistance changes in the solution. This is shown by a curve in Fig. 2 , obtained by plotting the values from Table I . I t may be seen that this curve extrapolates back to about 0.012 volt with zero current. This would indicate that there was a counter electromotive force in the cell itself without current flowing of the order of magnitude of I O to I j millivolts The next measurement to be made, therefore, was that of the voltage of the cell, copper acidified CuSO, solution rotating copper electrode, a t varying speeds of rotation of the electrode, t o ascertain if there was a voltage which could act as a back electromotive force. For this a delicate millivoltmeter was used. The stationary electrode was cast copper, while ~
Chaiiges
ill
C o F p e ~Sulphate Solutions
