QS Study

Abnormal Transference number

It is sometimes found that the transport numbers of some ions decrease rapidly with increase in concentration of the ions. Examples are the transport number of silver in ammoniacal solution, cadmium in cadmium iodide. This change in transport number is attributed to complex formation, whereby the metallic ion forms a part of a complex anion and travels to the anode. For cadmium this is represented as follows:

2Cdl2 ↔ Cd2+ + [Cdl4]2-

It is apparent that the transference number or transport number have to lie between and 1, for otherwise the positive ion would be travelling alongside the current or the negative ion with the current, and this is inconceivable. Nevertheless, Hittorf found for the transference number or 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.