26o OXIDATION AND REDUCTION ELEMENTS, chap.
poict, the platinum cathode becomes covered with a film of copper, and in an electromotive respect behaves like a copper plate. However, the deposited film must assume a certain (although very small) thickness before it acts quite like pure copper. Oberbeck {14) found that when the deposited film on the platinum electrodes is 2*7 millionths of a millimetre thick in the case of zinc, and 1*9 millionths of a millimetre in the case of cadmium, the same electromotive force is obtained as when the pure metals (zinc or cadmium) are used. This phenomenon is also termed polarisation, and we therefore say that the electromotive force of polarisation in the decomposition of copper sulphate is for the cathode the same as the electromotive force Cu | CUSO4.
Grove's Investigations. — Grove {15) immersed in a dilute solution of sulphuric acid two platinum plates covereil with films of different gases. Between the platinised platinum plates charged with different gases and a platinum electrode saturated with hydrogen, he obtained the following t<ensions in volts (the unit adopted by Grove = 2 volts) : —
Volt. Volt.
In the form used by Grove these elements are by no means reversible, otherwise he would have found the same potential difference as Smale {9) did for the combination oxygen | hydrogen at a palladium electrode, whilst he only obtained something less than half this value.
Cathodic and Anodic Polarisation. — In studying polarisation phenomena, the action of the cathode can be distinguished from that of the anode by comparing the potential of each electrode with that of a so-called normal electrode, generally mercury under calomel and a O*l-normal solution of potassium chloride, the polarisation vessel being connected with the normal electrode by means of a
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