Page:Text-book of Electrochemistry.djvu/264

 deposited at the platinum electrode, and we then really have the element —

Zn I ZnS04 | HaS04 | Ha on Pt.

The current is weakened because the hydrogen bubbles diminish the conductivity, but this we may neglect. It is further weakened on account of the deposited hydrogen, which possesses a higher solution pressure than the platinum. This solution pressure is, moreover, proportional to the pressure of the evolved hydrogen ; it may easily be imagined that if this pressure is sufficiently great, the solution pressure of the zinc would not exceed that of the hydrogen, and the current would stop. This would take place only at an enormously high pressure, and it cannot be realised. The quantity of hydrogen in the neighbourhood of the electrode can be diminished by addition of an oxidising volts), nitric acid (Grove's, Bunsen's element, ^ = 1*9 volts), manganese dioxide (Leclanch6*s element, -& = 1'48 volts), etc. The greater the intensity with which the oxidising agent

reduces the pressure of the hydrogen at the platinum, the

p greater does log — become, where P is the solution pressure

of the zinc, and p that of the hydrogen gas, and the higher is the electromotive force of the element. These elements may, therefore, be regarded as a kind of oxidation elements. If the current strength becomes too great it may happen that the oxidising agent does not diffuse sufficiently quickly to the platinum in Grove's element, or to the carbon in Bunsen's element, to allow of complete depolarisation. This is i)ar- ticularly the case when manganese dioxide is used as depolariser, for in this case the separated hydrogen must dififuse to the oxide in order to be oxidised. Consequently, too much current must not be drawn from these elements if it be required that the electromotive force is not to sink too greatly. A small diminution of the electromotive force always takes place, because the oxidising agent gradually

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