Measurement of the Conductivity of Copper Sulphate Solution

Abstract

Much of the confusion in copper sulphate solution resistivity measurement derives, it seems to me, from surface insulating films and/or electrolytic action at the electrodes. The behaviour of the Wayne Kerr bridge is particularly revealing in this respect, since if a measurement of resistivity is made with it of a liquid resistor which has been made up some time, a large value of the capacity is also read, while at the same time the resistance comes out too high. This is interpreted as the effect of partial capacity in series with a resistor. If a current of a few tens of milliamps per cm of electrode is passed for ten or twenty seconds, new electrode surfaces are formed in part and the resistance falls and the capacity reading in the bridge becomes very small, one or two units on the dial. In one case this treatment caused black flakes to come off the electrodes and float about in the solution. In other cases, copper is presumably laid down on the resistive film while sulphate ions are free to attack the film at the other electrode, volt-ohmmeter readings also become lower and agree with the bridge measurement, although care must be taken that a small cell voltage (because of the different ion concentrations at the electrodes) does not strongly influence the ohmmeter readings. This can be done by measuring the resistance both ways round. The act of reversing the clearing voltage a few times does not seem to interfere with clearing the insulating film and does not leave any significant cell voltage afterwards. Another way round a residual voltage is to short out the resistor for a few tens of seconds.

AFWL Energy Storage and Dissipation Notes Note 4 SSWA/JCM/667/67 22 July 66