Page:Text-book of Electrochemistry.djvu/154

 ascertain experimentally the values of m and (1 — m) it ia only necessary U> divide the liquid column into two portions at P after the current has passed (e.ff. by slipping into the trough a -weU-fitting glass plate), and then by chemical analysis to find by how much the quantity of potassium in the part £P has increased and by how much the amount of chlorine has increased in the part AP. As the oiigisal composition of the solution is kuown, it is sufficient to analyse the liquid in one part (AP or BP) after the current has passed. In these experiments appreciable changes in concentration take place at the electrodes, and, besides, there ia frequently an evolution of gas or deposition of long crystals (dendritic) which fall off and so stir up the liquid. The disturbances caused thereby can be avoided by various devices. The apparatus shown in Fig. 35, devised by Hopfgartner (6) from HittorTs model, gives good results in determining the changes of concentration.

The vessel B fits into the neck of a thin-walled flask, A, which is provided with a tnbu- lus, (?. disconnected with the wide tube -D by means of the u-tube C By raising or lowering the plug F the vessel B may be opened or closed. The side tube K of the u- . tube is closed by means of a rubber tube and clip. The anode is intro- | duced through a rubber stopper j in E ia used as cathode. ■!

Hittorf (7) was the first to in- vest^te the miration of the ions and prove that the anions and cations have different migration * • i^;:; -' velocities. For the chlorine ion of f,(,, 33,

a very dilute solution of potassium

chloride the value of u generally accepted is 0503, and consequently for the potassium ion 0'497.

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