Page:Text-book of Electrochemistry.djvu/160

 IX. MIGRATION OF IONS IN MIXED SOLUTIONS. i+>

On the contrary, it is fonnd that in the two groups of atoms, Li, Na, and K on the one hand, and Ca, Sr, and Ba on the other, the heavier ions are the more mobila And again, the ions CI, Br, and I, vhich have very different masses, have almost the same mobilities.

From the table given below it can be seen that addition on to an atomic group exerts more influence on the smaller ions than on the lai^er. Were this not the case, then ions consisting of a large number of atoms would have the mobility 0, or even an impossible negative value.

��Fonnio acid Acetic acid. . Propionic acid. Butyric acid. Valerianic acid Caproic acid , Snocinnric acid Phthalouric aci'l Fhthalanilic acid

��Similar regularities were found by Bredig (/-5) froiu his results with positive ions.

Migration of Ions in Mixed Solutions. — Before leaving Hittorf'a work, an investigation which he made on the migration of ions in mixed solutions of potassium chloride and potassium iodide must be mentioned. When the current passes through this mixture only iodine appears at the anode, and the question arises whether the chlorine ions take any part in the conduction or not. According to our present views, of course, the answer is self-e^'ident. Every ion must, on account of the chaise which it carries, be set in motion when it is in an electrical field of varying potential. At the time when Hittorf carried out his in- vestigation (1853-1859) the matter was not so clear, for the conducting molecules were then supposed to be joined together, and a large share in the current conduction was attributed to the water (solvent). Hittorf found that the

�� �