As already mentioned, these irreversible elements never possess an absolutely constant electromotive force; for the measurement of electromotive forces we must, therefore, use reversible, so-called normal, elements.
Normal Elements.— The first element which was designed to fulfil this purpose was the Daniell cell. It was, however, soon found that the electromotive force varied with the concentration of the solutions, and so standard solutions were adopted. The normal Daniell element consists of pure copper, copper sulphate solution of sp. gr. 1*195 at 18°, solution of pure sulphuric acid of sp. gr. 1*075 at 18°, and amalgamated pure zinc. Results obtained with this element give—
1 normal Daniell = 1176 [1 + 00002(^ - 18)] volts.
One disadvantage of this element is that the copper sulphate gradually diffuses to the zinc where copper is deposited, and so the element is spoiled. It must, therefore, be freshly set together immediately before use.
The only negative metal (according to Volta's designation) which by deposition on amalgamated zinc (the positive metal) does not change the electromotive force of this is mercury ; it unites with the amalgam at the surface of the zinc, and by dissolving a corresponding quantity of fresh zinc leaves the positive metal unaltered.
For this i^ason all the other normal elements contain mercury as negative metaL In order to diminish the diffusion as much as possible, the mercury is covered with an excess of a diflBcultly soluble mercurous salt, so that the mercury forms an unpolarisable electrode of the second order. An example of this kind of normal element is the Helmholtz calomel element (6'), in which the positive mercury pole is covered with a paste of mercurous chloride and 10 per cent, zinc chloride solution. The electromotive force of this element is —
1 normal Helmholtz = 1'074 [1 + 00001(^ - 20)] volts.
�� �
