226 ELECTROMOTIVE FORCE. chap.
electromotive forces between copper and dilate copper sul- phate, and between copper and concentrated copper sulphate.
In order to explain a formula of this sort, Nemst intro- duced the following conception, which was afterwards further developed by Ostwald {18).
Suppose we have a substance, e,g. sugar, in contact with a liquid, eg, water, the solid dissolves until a saturated solution is formed. This process corresponds exactly with the vaporisation of a liquid, which goes on until the vapour .space is saturated and the vapour possesses a certain pressure — its maximum pressure at the particular temperature.
On account of this analogy the osmotic (partial) pressure exerted by the saturated solution of sugar is termed the solution pressure, or solution tension, of the sugar at the particular temperature (according to van't Hoflfs law).
Now, if we consider the metals — for instance, zinc in sulphuric acid — we see that they do not pass into solution unchanged, but that they strive to dissolve as ions. It seems natural to suppose that the metal passes into solution until the concentration of the ions, and with it the osmotic pi*essure, has reached a certain value, which pressure is termed the electrolytic solution pressure.
We shall denote this pressure by F, Let us suppose that a gram-ion (65 grams) of zinc passes into solution in the form of ions, and in the solution the zinc ions have the osmotic pressure p ; this process can be conducted reversibly by dis- solving the zinc at constant pressure P, whereby no work is done (just as when water evaporates into a vacuum), and then by expanding the zinc ions from pressure P to pressure p, whereby the work done is —
f /• p "
The total work done is therefore —
RTla^-. P
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