Hittorf experiment

If the amount of mass transferred is analyzed, further conclusions on the carrier type (ionic/electronic) can be drawn according to Faraday s law, simply because of the fact that the pure stationary electronic flux will not cause mass displacements. If also the direction of mass transport is analyzed, e.g., by recording the position variation of the boundaries, cationic and anionic conductivities may be distinguished (Tubandt-Hittorf experiments). 

If after the termination of a Hittorf experiment the concentration changes are referred to a fixed number of total moles of solvent molecules, the reference velocity is given by... 

A is the concentration change of species 2 in the cathode compartment during a Hittorf experiment in the mean molar velocity reference system. As can be shown using Eqs. (82), (92) and (97), the following relations also hold... 

Fig. 27.4. A Hittorf experiment on soKd electrolytes as proposed by Tubant. A silver salt is used as an example. The electrolyte is divided into three parts and placed between silver electrodes. Full lines, size before electrolysis. Broken lines, size after electrolysis. Cross hatched area, gain during electrolysis. Only silver ion conduction is assumed in (a) while additional anion conduction is assumed in (b).

In a d.c. experiment on monolithium salts of a-zirconium phosphate by Skou et al a difference between the a.c. and d.c. conductivities developed during the experiment and the d.c. conductivity ended one order of magnitude below the a.c. conductivity. The difference did not relax more than 10% within a period of 4 h after the d.c. load was removed. This was taken as an indication of simultaneous proton and lithium ion conduction and was proved by the Hittorf experiment described earlier. 

Conductance measurements Conductometric titrations Hittorf experiments Diffusion coefficients Dielectric constants... 

When Hittorf (1869) [and later Goldstein (1876)] carried out experiments with a discharge tube and thereby saw the formation of a shadow by a metal cross in this tube, he drew from this, precisely like Newton and his contemporaries, the obvious conclusion that he must have been dealing with a corpuscular radiation to these particles the name electrons was later given by Stoney (1891). 

If after termination of a Hittorf transference experiment the changes in concentration in the electrode compartments are referred to a fixed weight of solvent, the reference velocity co = v is equal to... 

During a Hittorf transference experiment in a mixed solvent electrolyte solution, the concentration of the electrolyte as well as the composition of the solvent changes in the electrode compartments. The determination of the solvent transport requires detailed analysis of the electrode compartment. This has been done using refractive index or density measurements electrolysis is limited, the... 

Fig. 4.80. The principle of Hittorf s experiment. (After E. A. Moelwhyn-Hughes, 1968)...

This compact formula for computing results of Hittorf transference experiments is due to Washburn.3... 

In a similar way Hittorf 38 determined the compositions of other complex salts, such as KdFe(CN)fl and NaaPtCl. Reychler30 has found that transference experiments on anunoniacal solutions of AgN03 and of CuSO lead to the formulas Ag(NH3)2 and Cu(NH3)4++ for these ammonio complex cations. 

Transport experiments on potassinm chromate were made by J. F. Daniell and W. A. Miller, J. W. Hittorh R. Lenz, 0. Masson, B. D. Steele, R. B. Denison, and A. Charpentier F. Kohlrausch calcidated 72 for the transport number of CiO from the conductivity data of calcium chromate and M. S. Sherrill, 76-7 from the data of ammonium chromate and 40 for the HCrO/don. W. Hittorf, E. H. Riesenfeld, and A. Charpentier made observations with potassium dichromate and C. W. D. Whetham found the velocity of migration of the Cr207-ion to be 0-00047 cm. per sec. for a potential difference of one volt and the transport number to be 91. 

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)... 

From experiments with dilute solutions of strong electrolytes, Kohlrausch developed a relationship that allowed the maximum (or "zero concentration ) value of the conductance, Aq, to be estimated from measurements made at finite concentrations. He also showed that Aq is the sum of the conductances of the anion and the cation. The additional information needed to find the separate ionic conductances from a knowledge of Aq is the ratio of the mobilities of the two ionic species. The faster-moving ion will have the higher ionic conductance. In the period 1853 to 1859 Johann Hittorf (1824-191 ) published the results of his experiments. These involved the analysis, after electrolysis, of the solutions around one or both of the electrodes. The "transference numbers thus obtained are related to the required ratio. 

Hittorf was not actually the first to have carried out this type of experiment. Daniell and Miller (8.), 8 years earlier, had electrolysed and analysed several electrolyte solutions but their results were vitiated by large experimental uncertainties (2.). Hittorf himself took considerable care in his research. On reading his papers one is struck by the precautions he took to avoid disturbing influences for instance, he checked that his results were independent of the current used. His theoretical analysis, too, is very clear and his calculations are described in great detail. Nevertheless, his papers aroused strong opposition from several quarters. Wiedemann could not understand how an electric current (sic) could produce a different effect on the cation and anion of a given electrolyte (J[) or how transference numbers could differ in water and in ethanol (10). Another... 

Transference numbers have formed one of the cornerstones in our understanding of electrolyte solutions. Hittorf s discovery in 1853 that transference numbers depended on the ion, the co-ion, and the solvent proved that each ion in a given solvent possesses its own individual mobility. Even today ionic mobilities must be determined by a combination of transference and conductance experiments for we still cannot predict their values accurately from first principles The importance of ionic mobilities can hardly be overemphasized since they are the only properties of individual ions that can be unambiguously measured (either directly or via trace diffusion coefficients). They therefore provide unique insight into ion-solvent interactions. Hittorf s later transference experiments also revealed the existence and composition of a variety of complex ions in solution. His approach has been followed in more recent structural investigations, for instance in studying the complex ions present in aluminium plating solutions (7A,). 

Some empirical assumptions have been made to avoid the difficult and time-consuming experiments (Hittorf or moving boundary method) to determine transference numbers. 

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