Experimental methods of determining transference number

The molar conductivity of an electrolyte is the more generally useful quantity since the Kohlrausch law allows its limiting value to be resolved into those of its constituent ions. Comparison between different electrolytes with a common ion therefore allows the determination of an unknown molar conductivity. However, the quantity typically measured is the overall electrolytic conductivity. A way to apportion the conductivity (and hence mobility) to the individual ions of the electrolyte is required. Equation (20.1.2-11) shows that resolution of the molar conductivity into the terms arising from its constituent ions is possible if the transference number of the ion is found. Although this property and the methods developed to measure it may seem rather arcane, it has been of fundamental importance in the understanding of the conductivity and diffusion potentials developed within electrolyte solutions. Experimentally, a number of ways of measuring transference numbers have been developed these are summarised below. 

One important point to make is that the quantity accessed experimentally, in general, is the net transference number for a given species regardless of its exact speciation (6). Eor complex ions, if the Hittorf method (see below) is used to measure the transport of chloropalladate ions for example, by analysis of the amount of Pd accumulated on an electrode surface, one has no way of distinguishing between the transfer of [PdCy and [PdClj] , both of which occur in solution. The existence of rapidly occurring equilibria, which will typically interconvert on time-scales much shorter than the measurement time-scale, means that the net transference of anionic palladium species of whatever form is 

The classical methods of experimental transference number determination can be divided into three general groups. The first (the Hittorf method) is essentially an analytical approach, which relates changes in cell composition to the transference numbers of the electrolyte solution. The second group of methods relates the motion of the boundary separating zones of different composition to the transference numbers. The final approach relates the cell potential, which arises from the diffusion potential, to the transference number. Each of these methods is summarised, in turn, below. 

Cathodic Compart- ment Anodic Compart- ment 

This approach requires an observable boundary between the front of differing composition and the rest of the cell. The front is established by gravitational stratification between 

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