Page:Outlines of Physical Chemistry - 1899.djvu/222

 204 bUTJJNES OF PHYSICAL CHEMISTKY

row shows sis decomposed moleoulea and eight undecom- posed, five of which are to the left of a & and three to Hie right. The number of black ions to the left has increased by four (from seven to eleven), whilst to the right the number of white iona has increased by two. The ratio f expresses

the RELATIVE RATEB OF MIGRATION which we presupposed

If now we observe that during the electrolysis of a litres) the concentration diminishes quasi- uniformly at both electrodes, then we must conclude that the I and the CI ions migrate at the same rate. If the molecular

1 In order to experimentally determine this speed- ratio, it is find what concentration change takes place at one of the electrodes. An example will make the method clear. Suppose that s solution of silver nitrate which has been subjected to electrolysis for a certain time has deposited 1*2391 grams of metallic silver, and that a certain volume of the solution taken from just beside the cathode now contains 12*5533 grams of silver, whereas before the electrolysis this same volume contained 13*1426 grama. The solution at the cathode has therefore lost 0*5893 gram of metal and has sent in the direction of the anode an equivalent quan- tity of the NO, ion. At the same time 1*2391 -0-5893 = 0-6698 gram oi silver has migrated from the solution at the anode towards the cathode. The immigration of the silver is therefore to the emigration of is to 0*468. These numbers express the relative rates of migration of the ions of silver nitrate in a fairly strong solution (in dilute solution the NO, ion migrates faster than the Ag ton).

An apparatus more or less like that

shown in fig. 50 may be used for making

this determination. To find the change

g _ go which takes place in the solution near the

electrode a, a measured volume of the

. withdrawn through the side tube and analysed. For

the original papers of Hittorf, Nernst, Loeb, and others.

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