Page:Outlines of Physical Chemistry - 1899.djvu/224

 206 OUTLINES OF PHYSICAL CHEMISTRY we find the speed of migration of hydrogen 285*8. For bases, Kohlrausch has found at 18°C. :

Dilution KOH NaOH

The conductivity of potash diminished by the coefficient for potassium gives the value 154*3 for hydroxyl.

The relations are not quite so simple for acids and bases containing multivalent ions. Furthermore, there aie some more or less difficultly electrolysable compounds whose conductivity is even lower than that of the hydrogen or the hydroxyl which they contain.

��i

��Dilution

�iH^O,

�iH^O.

�C a H 4 O a

�NH«0H

�1 litre.

�100 litres.

�1,000 „

��In order to reconcile this seemingly anomalous state of things, Arrh&nius, Ostwald, and several other distinguished theorists assume that when an electrolysable compound (acid, base, or salt) is dissolved a dissociation into fkee ions takes place. For good conductors this dissociation takes place readily, and at a dilution of 1,000 or 2,000 litres it may be assumed to be complete. Sufficiently dilute equivalent solutions must then contain the same number of free ions, and consequently the conductivity of these solutions depends only on the speed of the ions. Indeed, Faraday's law teaches us that the electric charge on any ion whatever is a constant quantity.

For bad conductors (phosphoric acid, acetic acid, ammonia) the dissociation is only partial and does not take place so readily. As the free ions are not very numerous, the conductivity will be much diminished com- pared with what it would be if the dissociation were complete.

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