Hittorf s method

We express the altered concentration in terms of the adsorption excess. If all the adsorbed substance were contained to the extent of k gr. per cm.2 on a superficial layer of zero thickness and surface total mass present in the volume Y would be m = V + kto. The layer of altered concentration must, however, have a certain thickness. We will therefore imagine a plate 2 placed in front of the surface and parallel to it, and define the adsorption excess as the concentration in the included layer minus the concentration in the free liquid. That this result is independent of the arbitrarily chosen thickness is easily proved when we remember that the problem is exactly the same as that of finding the change of concentration around an electrode in the determination of the transport number of an ion by Hittorf s method. 

The second method, namely the moving boundary method, is based on the direct observation of migration of ions under the influence of an applied potential - unlike Hittorf s method in which the changes in concentration at the electrodes are measured. 

Fig. 2.10 Schematic design of a cell for the determination of transport numbers from measurements of the concentration decrease in electrode compartments (Hittorf s method)...

Unequal velocities of ions cause changes in concentrations in the proximity of electrodes. From these changes the transference numbers can be calculated provided the quantity of electricity passed through the electrolyte is known (Hittorf s method). 

In total, t+ mol HC1 have disappeared from the anode compartment and t mol HC1 from the cathode compartment. By quantitatively analyzing the solutions around the anode and cathode before and after the passage of a known amount of charge, it is, therefore, possible to determine the transport numbers. It is also clear that the principle of Hittorf s method, sketched above, can be extended without difficulty to... 

Hittorf s Method. This method of determining transport numbers was devised as long ago as 1901 and has been described in innumerable papers and many books. Nevertheless, it is not all that simple to understand and contains a number of assumptions not always stated. 

To calculate the transport numbers of potassium chloride from results obtained by Hittorf s method. 

Modifications of Hittorf s method of determining transport numbers were used by a large number of experimenters. The moving boundary method was first used by Oliver Lodge, who showed that the hydrogen ion moves with the velocity of 0 0026 cm./sec. under a potential gradient of i volt/cm. It was improved by Whetham. ... 

The ionic transport numbers for alkaline PVA-based SPE are very important because the high anionic transport number of SPE can limit the carbonation problems [56]. The anionic transport number (f) was measured by the dynamic Hittorf s method [57]. A test cell, as shown in Fig. 11, with two Pt electrodes was made for electrolysis and the electrolysis current was imposed by a power supply. All PVA-based polymer electrolyte membranes were located and fixed at two separated compartments with the same 1 M KOH solution. The reaction occurred at the Pt-cathode, producing O2, H2 and OH while consuming water and OH . The balance of OH ions in each compartment led to OH transport number after a fixed amount of charge was passed through the polymer membranes. 

The anionic transport numbers of alkaline PVA-based SPE at 25 °C measured at 20 mA cm by Hittorf s method are smnmarized in Table 7 in 1 M KOH, 1 M NaOH and 1 M LiOH solutions. It has been noted that the higher the applied current density, the higher is the value for anionic transport number. High alkali electrolyte concentration would also cause the system to be more complex and can influence the ion transport ability. Fig. 12 shows that the value of f decreases significantly while the alkali electrolyte concentration goes beyond 1 M. 

In useful variant of Hittorf s method, the solvent that is containing a small amount of (ideally) noninteracting molecules is analyzed as weU. This experiment suggested by Nemst yields specific solvation numbers for anions and cations and can also contribute to a better precision of transference numbers determined by Hittorf s method as cations and anions generally transport different number of solvent molecules. 

The number of precise methods to measure transference numbers in liquid aqueous electrolytes is quite acceptable [9-14]. Various methods are already used for more than a hundred years, such as the moving boundary, Hittorf s method, or the indirect determination of transference numbers by conductivity measurements. In contrast, accurate data for nonaqueous liquid electrolytes, especially with respect to hthium salts, are very rare. In hterature, the most often used methods are the potentiostatic polarization method and determination by NMR [21, 22]. Interestingly, the first was developed for solid electrolytes the latter is only valid for ideal solutions. To measure concentrated electrolyte... 

To determine transference numbers of liquid electrolytes, generally classical methods are used. These classical methods, meaning moving boundary [436], Hittorf s method, and combined data from emf [437], involve extensive experiments. 

In Hittorf s method a known quantity of electricity passes through the solution for a defined period. After electrolysis the cell is split into different sections that are separated and analyzed [438, 439]. To ensure that no mixing between the two electrode compartments has occurred, more than one middle compartment has to be incorporated into the cell, see Figure 17.17. The concentrations in the middle compartments must not change. 

The essential experimental circuit for Hittorf s method is shown in Fig. 

Hittorf s method provides an excellent demonstration of the nature of electrolysis processes. On the practical level, however, it has obvious drawbacks, not least of which is the usually low precision with which the small concentration changes may be determined. 

A dynamic method called Hittorf s method allows determining transport numbers under migration, a method comparable with a fuel cell under operation. The two compartments in the cell are separated by an ion exchange membrane and both compartments are filled with the same KOH solution (same concentration). A current is applied between the two electrodes on either side of the compartments. At the cathode, hydrogen and hydroxide are produced, while at the anode hydroxide is consumed and oxygen is produced. To maintain electroneutrality, the cations (K" ) and anions (OH ) present in the solution migrate through the membrane from one compartment to the other. Anionic and cationic transport numbers f and f ) can subsequently be determined [20,205,206] ... 

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