Page:Outlines of Physical Chemistry - 1899.djvu/274

 256 OUTLINES OF PHYSICAL CHEMISTRY

electrolytes the experimental confirmation of an equation

Theory op Isohydric Solutions

The following question has been attacked by Arrh&nius : What conditions must be fulfilled in order that no distur- bance of the equilibrium take place when two solutions are mixed ? The most interesting case is that of the mixture of two solutions one of which at least is only imperfectly dissociated, and both of which contain a common species of

ionised molecule ; for example, two acid solutions containing

+ —

basic water HOH, two base solutions containing HOH, or

solutions of salts derived from the same acid or from the same base. Experiment and theory both show that the equilibrium of dissociation only persists after mixing, if the common ionised molecules have the same concentration in the origi- nal solutions. Such solutions are said to be isohydric.

The theoretical deduction of this law offers no difficulty. Suppose we have a solution of acetic acid characterised by the equation of equilibrium

and a solution of sodium acetate, for which

cw = Wy 2.

In order that these two solutions may be isohydric, the

ionised molecules C 2 H 3 2 H must have the same concen- tration in both solutions ; consequently, u x =w lt After mixing 1 litre of the first solution with 4 litres of the

second, the concentration of the molecules C 2 H 3 2 H will

��1 The constants k are in harmony with the relative strength of the corresponding acids. But they are evidently not at all propor- tional to this strength. Besides, this new measure of affinity suffers by being applicable only to a certain category of substances, especially to binary electrolytes of moderate conductivity (organic acids, Ac).

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