Page:Text-book of Electrochemistry.djvu/257

 2^z POTENTIAL DIFFERENCE. chap.

are those obtained 48 hours after the element had been put together, the others were obtained immediately after the element had been constructed.

��ZnCu. MgCu.

NO,. . . 1000(100'0) 159-1(1797)

CI ... 90-0 180-4(l77-8)

��CdCu. ZoCd.

81-8(68-1)

79-6(75-9)

17-5(32-0)

��Very Small Ionic Concentrations.— Occasionally the salt is very difficultly soluble, and when double salts are formed. As an example of the former case, we may take the silver halides. Quite different values are obtained when these are used from those found when a salt solu- tion of finite concentration is employed. Wright and Thompson found the following values for the electromotive forces of elements constructed on the plan : silver | silver salt I zinc sulphate | zinc, when different silver salts were used : —

��Volts.

��Volts.

In this case the silver corresponds with the copper in a Daniell element. Consequently the electromotive force of the element is the greater the higher the concentration of silver ions in the neighbourhood of the silver. For the three comparatively easily soluble silver salts,, sulphate, nitrate, and acetate, the electromotive force is almost the same, but for the difficultly soluble chloride, bromide, and iodide it is decidedly lower.

In other experiments the solution contained, besides the silver halide, other chlorides, bromides, or iodides, which depress the solubility of the silver salt. As a consequence of this, it was found that the electromotive forces were appreciably smaller, the smallest being obtained with tTie

�� �