The water voltameter gives a very good credence to the idea of polarization. When the potential difference across the voltameter is less than 1.7volts, the current through it falls to zero in a minute or less. The water voltameter itself can then deliver a current for a short time. Its positive terminal, as a source, is that which is its anode, and its negative terminal is that which is its cathode. While current is being sent through the water voltameter, the cathode becomes covered with hydrogen, and the anode with oxygen. When the water voltameter acts as a source of current it has, in effect, electrodes of oxygen and hydrogen and the current through the external circuit flows from the oxygen plate to the hydrogen. In the simple cell, when it is polarized, there is no oxygen plate. We can therefore assume that it tends to drive a current, through the external circuit, from zinc to copper; that is to say, it sets up an e.m.f opposing that of the cell with the copper plate clean.
Nernst’s Theory of the Voltaic Cell and Electric Potential
If a metal is in contact with a solution of one of its own salts, it is surrounded by its own ions. Whether the ions deposit themselves on the metal, or the metal goes into solution, depends partly on the particular metal concerned, and partly on the strength of the solution. The stronger the solution, the greater is its tendency to deposit ions on the metal. If the solution deposits ions, the metal comes to a positive potential with respect to it; if the metal goes into solution as ions, it becomes negative with respect to the solution. The potential difference between a metal and a solution can be measured and these measurements shows that copper in normal copper sulphate solution(1 g-equivalent weight per litre) becomes 0.08 volt positive with respect to the solution.
Zinc in normal zinc sulphate solution becomes 1.03 volts negative. Imagine a Daniell cell in which zinc sulphate replaces the sulphuric acid, and both solutions are normal. If we assume that the solutions themselves set up no appreciable potential difference at their interface, then we will get a potential distribution such as that in an open circuit that has no load. The difference in potential between the copper and zinc is very nearly equal to the e.m.f. of a Daniell cell: 1.11 volts compared with 1.08. We may attribute the difference to the fact that a Daniell cell in practice has sulphuric acid, not zinc sulphate solution, in contact with the zinc; also the solutions are not normal: the acid is usually 1 to 4 of water, and the copper sulphate is saturated.