iodide. The same remarks apply to the chlorides, bromides, and iodides of lead and mercury.
Double salts behave in quite the same way. In the element Cu | KCN | ZnS04 | Zn the cuiTent does not go in the usual direction from zinc, through the solution, to copper, but in the opposite direction. The reason for this is that the osmotic pressure of copper ions in potassium cyanide solution is exceedingly small. When copper dis- solves in this solution, the double salt K2(CN)4Cu is formed
with the ions 2K and Cu(CN)4, and only a trifling quantity
+ + -
of Cu(CN)2 is produced, which dissociates into Cu and 2(CN).
The osmotic pressure pi of the copper ions thus becomes so small that the expression log — counterbalances the
expression log •
Measurements of the electromotive force may be used to determine the solubility of difficultly soluble salts or the degree of dissociation of double salts. The alkalis, theii* sulphides, thiocyanates, ferrocyanides, and similar salts, behave, in aqueous solution, like potassium cyanide. If the electromotive force of the element, silver | potassium volts at 17° (T = 290°), it follows that—
114 = 00002 riog-.
Pi
Now, if the silver nitrate solution is 01-normal, log jt>i = —1, therefore p^ = 10" '"^, i,e. 108 grams of silver are contained in 10*^^ litres of potassium argentocyanide in the form of silver ions.
Since, in a Daniell element, the solution pressure of the zinc is very much greater than that of the copper, the zinc replaces the copper dissolved as ions, and we may rightly regard the Daniell element as a machine which is driven by osmotic pressure (really, solution pressure).
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