Page:Text-book of Electrochemistry.djvu/256

 solution pressure of zinc, is very much greater than Pi, that

of copper. The value of log — can, as a rule, be neglected.

It shows, however, that the electromotive force of the element will be the greater according as the value of p% is great and that of pi small. In spite of the dif&culties attending the experiments on account of the smallness of the potential differences, this has been directly proved. If a Daniell element contains solutions of zinc sulphate and copper sulphate of such concentrations that pi and p^ are equal, whilst in another case the solutions are of such concentration that pi = 1000^2, the difference in the electromotive forces is only —

r99xji)j;* 291 x log 1000 = 0*087 volt.

It can, therefore, be seen that quite large di£Perences of concentration exert only a comparatively small influence. When the zinc sulphate in a Daniell element is replaced by sulphuric acid, the potential difference must evidently become higher; this is due to the fact that in this case p^ is exceedingly small, particularly at the beginning.

From the formula given we may conclude that the potential difference in such an element depends mainly (almost exclusively) on the ratio of the solution pressures. Some exceptions will be later considered in detail.

This rule, gained by experience, has been confirmed by the following numbers found by Streintz (12), Nevertheless, varying numbers have been obtained for the same metals, the differences amounting in some cases to as much as 02 volt, and further investigation on this subject is required to clear up the cause of these peculiarities.

The following table gives the magnitudes of the electro- motive forces of some elements of the type of the Daniell cell, that is, with unpolarisable electrodes of the first order. The salts used in these elements, in normal solution, are indicated by their negative ions. The numbers in brackets

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