Transference number Hittorf

To date there have been few reliable measurements of Hittorf transference numbers in solid polymer electrolytes because of experimental difficulties in applying the technique. Leveque, Le Nest, Gandini and Cheradame (1983) have, however, applied it to highly cross-linked networks where cells could be formed using a series of non-adherent thin... 

Equation (6.171) shows the direct relation between the electrical conductance of the solution and the phenomenological coefficient. Similar relations are obtained by measuring the fraction of the total current that is carried by each ion, also under the conditions V/li, = 0. This fraction is called the Hittorf transference number (t,) and is expressed by... 

The Hittorf transference number may be defined as the number of equivalents of a given ion constituent which, on passage of one faraday of electricity, cross a plane fixed with respect to the solvent, usually, of course, water. In a determination by the moving boundary method the position of a boundary is fixed with respect to the graduations of the tube. Hence, in order to obtain a value of a transference number comparable with that found by the Hittorf method, the motion of the water with respect to the tube must be computed. 

The observed volume through which a boundary sweeps can be seen from this typical example to be subject to a correction of AV. The Hittorf transference number is thus obtained from the equation... 

If, for instance, the positive ions carry more water into a solution surrounding a cathode than the negative ions carry out of it, the effect of the resulting movement of water will be a dilution of the solution around the cathode, and the measured Hittorf transference number of the cation will be smaller than would be the case if the ions were unhydrated, and moved at the same relative velocities. Similarly if the negative ions carry more water away from the cathode than the positive ions carry to it the effect on the Hittorf number will be in the opposite direction. 

Transference numbers obtained by a method which is uninfluenced by the movement of water of hydration have been called true 41 transference numbers. The first attempt to obtain such numbers was made by Nernst and associates.42 Successful measurements in this field have been carried out by Buchbock43 and much more extensively by Washburn.44 The procedure employed was essentially that of a Hittorf measurement. However, a second solute (usually a carbohydrate, such as sucrose or raffinose) is added to the aqueous solution, and, instead of referring the changes of salt concentration to the water, as in the computations for Hittorf transference numbers, the changes, both of salt and of water, are referred to the added solute. The apparatus used by Washburn has already been described. It is evident that if the added "reference substance is uninfluenced by the passage of the... 

This term is somewhat unfortunate since it implies that the Hittorf transference numbers are false. The latter are, however, the values used in thermodynamic relations, and are measures of a perfectly definite, though somewhat complex, process. The term true in this connection is, however, of too general usage to make a change advisable. 

Here NaCl (ft) represents a solution of sodium chloride, the chemical potential of the solute of which is and fan is the number of equivalents per faraday transferred from the higher to the lower concentration during the reversible operation of the cell. It will be noted that the operation of cell (12) is, differentially and reversibly, the opposite of the Hittorf method for determining the Hittorf transference number fan as described in Chapter 4. If now a second cell is made as follows... 

Table I. Comparison of Transference Numbers Obtained from the Effect of Gravity on EMF with the Hittorf Transference Numbers...

Wilhelm Hittorf Transference number determination first method of studying complexes in solution... 

H. Strehlow and H.-M. Koepp, Z. Elektrochem., 62, 373 (1958). Experiments were also carried out with various acetonitrile-water mixtures. Hittorf transference numbers of 0.25 m ZnCL in 25, 50 and 75 mol % CH3CN-H2O at 25°C were determined by H. Schneider and H. Strehlow, Ber. Bunsenges. Phys. Chem., 69, 674 (1965)... 

R. Gopal and O. N. Bhatnagar, J. Phys. Chem., 70, 3007 (1966). These workers have also reported Hittorf transference numbers for KBr in A-methylpropion-amide (/. Phys. Chem., 70, 4070 (1966) their values for KCl in formamide (J. Phys. Chem., 68, 3892 (1964) differ by ca. 0.01 from those in ref. 12... 

Similar phenomena such as diffusion potential and thermal diffusion potential in systems where ion transport is involved are also of considerable interest. Coupling of flow of ions relative to solvent is involved in the development of diffusion potential, while in the case of thermal diffusion potential, coupling of flow of ions and energy flow is involved. In such situations, the effective transference number as compared to Hittorf transference number is affected. Interesting experimental results have been reported in the context of galvanic cells (thermo-cells), in which the two electrodes are not at the same temperature where results have been interpreted in terms of thermodynamics of irreversible processes [3]. 

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