Page:Text-book of Electrochemistry.djvu/245

 CHAPTER XIV.

Potential Difference between Two Bodies.

Electrical Double-layer. — When a zinc plate is immersed in a solution of zinc sulphate, it tends to send more ions into the solution, provided that the osmotic pressure p of the zinc ions is smaller than the electrolytic solution pressure P of the metal.

The solution becomes positively charged by these positive zinc ions, and the zinc plate, which was formerly neutral, takes on a negative charge. At the surface of separation a highly charged double-layer is formed, corresponding with a Franklin condenser one side of which consists of the negatively charged zinc and the other of the positively charged ions in the zinc sulphate solution.

On the other hand, if we have a metal whose electrolytic solution pressure, P, is smaller than the osmotic pressure p of the corresponding cations in the salt solution, say copper in copper sulphate solution, some of the positive ions are deposited on the metal, which thereby becomes positively charged, whilst the solution becomes negatively charged. The two parts of the Franklin condenser are then the positive metal and the solution which, on account of the excess of negative ions, is negatively charged. This sort of charged contact surface has been termed by Helmholtz an dectrical double-layer.

In the first case, the smaller the osmotic pressure of the zinc ions in the solution, the more ions must go into solution when this is in contact with the metal, and the stronger will

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