Hittorf method

As dissociated ions are always solvated, electrolysis entails transport of solvate with the ions. This phenomenon has been known for a long time in 1900, Nernst determined the transport of water in, for example, sulfuric acid [447]. Washburn quantified it in the so-called Washburn number [440], which simply expresses the net number of moles of solvent carried by the electrolyte. 

The method is based on the addition of an inert substance remaining stationary its concentration after electrolysis gives information about possible dilution or increase of concentration and therefore errors in concentration determination. But the work of Washburn has come under criticism, because the used reference systems raffinose and sucrose are polar and consequently no longer stationary [436], 

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Of all the techniques, it is those of Group 1 that are likely to give the most realistic data, simply because they measure transport of charged species only. They are not the easiest experimental techniques to perform on polymeric systems and this probably explains why so few studies have been undertaken. The experimental difficulties associated with the Tubandt-Hittorf method are in maintaining nonadherent thin-film compartments. One way is to use crosslinked films [79], while an alternative has been to use a redesigned Hittorf cell [80]. Although very succesful experimentally, the latter has analytical problems. Likewise, emf measurements can be performed with relative ease [81, 82] it is the necessary determination of activity coefficients that is difficult. 

For obtaining internal or external mobilities, the corresponding transport numbers are usually measured. There are several methods for determining transport numbers in molten salts that is, the Kleimn method (countercurrent electromigration method or column method), the Hittorf method (disk method), the zone electromigration method (layer method), the emf method, and the moving boundary method. These are described in a comprehensive review. ... 

There is difficulty in defining the absolute mobilities of the constituent ions in a molten salt, since it does not contain fixed particles that could serve as a coordinate reference. Experimental means for measuring external transport numbers or external mobilities are scarce, although the zone electromigration method (layer method) and the improved Hittorf method may be used. In addition, external mobilities in molten salts cannot be easily calculated, even from molecular dynamics simulation. 

The methods for determination of transport numbers include the Hittorf method and the concentration cell method (p. 121). 

The Hittorf method is based on measuring the concentration changes at the anode and cathode during electrolysis. These changes can be found by a sensitive analytical method, e.g. conductometrically for a suitable cell... 

Hittorf method phys chem A procedure for determining transference numbers in which one measures changes in the composition of the solution near the cathode and near the anode of an electrolytic cell, due to passage of a known amount of electricity. hi-dorf, meth-od ... 

Transport numbers can be measured by different methods. The small mono- and divalent inorganic ions have been used to demonstrate skin permselectivity, and have been determined from membrane potential measurements, or by the Hittorf method [10,25,77,79]. The latter method has been frequently used for drugs alternatively, the transport number can be estimated from the slope of a plot of drug flux as a function of current intensity (Figure 14.4) [18,66]. 

The quantity [ zlLkf izfLJ is the transference number of the fcth ion as determined by the Hittorf method and, therefore, Equation (12.105) may be written as... 

The transport number of an ion varies with the ionic constitution of the solution, and is another way of expressing conductivities or mobilities. There are two important methods for measuring transport numbers the Hittorf method and the moving boundary method5. 

Fig. 2.6. The Hittorf method for determining transport numbers. In the diagram the passage of a current I for time t is shown. It is assumed that t+ +1 = 1. The electrolytic cell is divided into three compartments.

Ideally, to determine limiting ionic conductances in a solvent, precise transport number data are required for an unassociated salt for which the value of A0 is known accurately. Several attempts now have been made to determine transport values, by the Hittorf method, for ions in NMA using a variety of salts85,86,13S. A summary of values obtained for Xqk+ in NMA at 40 °C is provided in Table V-3. 

Hittorf Method Experimental Procedure.—In Hittorf s original determination of transference numbers short, wide electrolysis tubes were used in order to reduce the electrical resistance, and porous partitions were inserted to prevent mixing by diffusion and convection. These partitions are liable to affect the results and so their use has been avoided in recent work, and other precautions have been taken to minimize errors due to mixing. Many types of apparatus have been devised for the determination of transference numbers by the Hittorf method. One form, which was favored by earlier investigators and is still widely used for ordinary laboratory purposes, consists of an H-shaped tube, as shown... 

Although the Hittorf method is simple in principle, accurate results are difficult to obtain it is almost impossible to avoid a certain amount of mixing as the result of diffusion, convection and vibration. Further, the concentration changes are relatively small and any attempt to increase them, by prolonged electrolysis or large currents, results in an enhancement of the sources of error just mentioned. In recent years, therefore, the Hittorf method for the determination of transference numbers has been largely displaced by the moving boundary method, to be described later. 

True and Apparent Transference Numbers.—The fundamental assumption of the Hittorf method for evaluating transference numbers from concentration changes is that the water remains stationary. There is ample evidence, however, that ions are solvated in solution and hence they carry water molecules with them in their migration through the electrolyte this will result in concentration changes which affect the measured or apparent transference number. Suppose that each cation and anion has associated with it and w- molecules of water, respectively let T+ and be the true transference numbers, i.e., the actual fraction of current carried by cations and anions, respectively. For the passage of one faraday of electricity the cations will carry w T+ moles of water in one direction and the anions will transport W-T- moles in the opposite direction there will consequently be a resultant transfer of... 

If the net amount of water (x) transported were known, it would thus be possible to evaluate the true and apparent transference numbers from the results obtained by the Hittorf method. 

It may be noted that the values obtained by the moving boundary method, like those given by the Hittorf method, are the so-called apparent transference numbers (p. 114), because the transport of water by the ions will affect the volume through which the boundary moves. It is the practice, however, to record observed transference numbers without applying any correction, since much uncertainty is attached to the determination of the transport of water during the passage of current. Further, in connection with the study of certain types of voltaic cell, it is the apparent" rather than the true" transference number that is involved (cf. p. 202). 

Influence of Temperature on Transference Numbers.—The extent of the variation of transference numbers with temperature will be evident from the data for the cations of a number of chlorides at a concentration of 0.01 N recorded in Table XXX these figures were obtained by the Hittorf method and, although they may be less accurate than those in Table XXIX, they are consistent among themselves. The transference... 

Structure breaking and the Achilles heel in the Hittorf method. 

Much use of transference numbers has been made in the development of electrochemistry. The chief methods for their determination are (a) the Hittorf method, (6) die moving boundary method and (c) the electromotive force method. Of these the first two will be considered in this chapter. 

Transference Measurements by the Hittorf Method. The early transference measurements by the Hittorf method have been summarized by McBain, and by Noyes and Falk.5 In general this early work contained enough sources of error to make its use uncertain in studying the properties of salt solutions. However, recent researches by Jones... 

In spite of its simplicity accurate results are very difficult to get with the Hittorf method. The main difficulties are, first, the necessity for avoiding mixing of the electrode and middle portions during an electrolysis, which may take from sixteen to twenty-four hours, and secondly, the need for extremely accurate analyses of the solutions, since the method depends essentially on small differences between large quantities. For these reasons the more recent accurate data on transference numbers have been obtained, in greatest part, by means of the more complicated, but more speedily and accurately carried out, method of moving boundaries, which will be next described. 

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. 

Although we have assumed that the moving boundary and the Hittorf methods, when correctly used, yield the same values of transference numbers, there has been no adequate test of this assumption until recently. The only measurements of transference numbers by the Hittorf method available for this comparison, in which modern technique has... 

Table V. Cation Transference Numbers at 25° of Aqueous Solutions of Electrolytes Determined by the Hittorf Method, Compared with the Results of Moving Boundary Determinations...

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... 

Solution Molality of solution m Emf, volts per centimeter height, X10 ——Cation Transference Number. Gravity Hittorf method method Reference... 

There are two main methods for determining transport numbers, both of which were developed early in the study of conductance. They are the Hittorf method and the moving boundary method. The emf method has been described in Section 9.21. 

This method for determining transport numbers is the preferred method since it is inherently more accurate than the Hittorf method. Even more important is the fact that the transport numbers can be found over a range of concentrations. This, in turn, means that ionic mobilities and individual ionic molar conductivities can also be found over a range of concentrations. 

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