Abnormal Transport Numbers. — It is evident that the transport number must lie between and 1, for otherwise the positive ion would be travelling against the current or the negative ion with the current, and this is inconceivable. Nevertheless, Hittorf found for the transport number of iodine in a 4*8 per cent solution of cadmium iodide in- alcohol the value 2*1, and in a 3 per cent, solution the value 1*3. At a very high dilution the value would probably sink below 1, i,e. would lose its abnormality.
Hittorf explained this peculiar phenomenon as follows : He assumed that cadmium iodide forms complex molecules
perhaps of the formula Cdsle, which form the ions Cdsle and
+ +
Cd. For the sake of simplicity let us imagine that the
+ + cation Gd remains at rest, and that only the anion Cd^Io
passes through the solution, in the direction opposite to that
of the (positive) current. For every quantity of electricity grams Cd + 762 grams I) must pass a cross section of the solution. Instead of 2 equivalents of iodine, which if iodine alone migrated would be sufficient to transport the same quantity of electricity, an amount three times as large must pass through the cross section. Consequently, if the transport number of the iodine in the former case were 1, it would in the second case be 3. Now, as the cation also migrates with a certain velocity, the transport number obtained for the anion will be less than 3. However, it is obvious that we have only to make the assumption of the existence of a particular molecular complex in order to be able to explain in this way any transport number. In the example quoted, if the transport number of the iodine is 3, that of the cadmium must be — 2, since the sum must be equal to 1.
Cadmium iodide in concentrated solution behaves more anomalously than in dilute solution, and it must therefore be assumed that in concentrated solution there are more
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