Page:Text-book of Electrochemistry.djvu/272

 XV.

��LE BLANCS INVESTIGATIONS.

��According to theory, therefore, the strength of the polari- sation current must increase proportionally with an ex- ponential function of the electromotive force —

i = const, ef^

«

where c denotes a constant.

In reality the intensity of the polarisation current in- creases enormously quickly with the polarising electromotive force until visible gas evolution occurs, when, of course, / cannot further rise. It has, nevertheless, been found that even after this point has been reached, E increases with the strength i of the polarising current. This phenomenon may be due to a sort of supersaturation of hydrogen taking place in the water. (The same applies, of course, to the oxygen.) Jahn (l!2) assumes that this supersaturation is proportional to the current density, and obtains for visible electrolysis a formula analogous to the above one, only with different constants (c is greater than in the former case). As a matter of fact, it is observed that immediately after the beginning of the gas evolution there is a much greater increase of the current density for the same increase of the polarising electromotive force (H) than before.

Le Blanc's Investigations. — By means of a galva- nometer or capillary electrometer the value of E can be determined at the decomposition point.

Le Blanc (13) found, as required by theory, that the electromotive force of polarisation is independent of the nature of the electrolyte, provided that the substance separated at the platinum electrodes is the same, a condition which is fulfilled when the ordinary oxygen acids or bases are used. The values obtained by him for E were —

Volte. I

��Sodium hydroxide. Potassium hydroxide Ammonia .... Methylamine. . Diethylamine. . Tetramethylammonium liy-

��Volts.

1*61)

�� �