Tubandt method

The pulsed field gradient nuclear magnetic resonance (PFG NMR) technique is experimentally distinct from the radiotracer technique but the principle is very similar. A fraction of the nuclei of each constituent is labelled by flipping their spins and monitoring the diffusion of these species. Both techniques are influenced by the presence of neutral associates unlike the Hittorf/Tubandt method. 

See also - Tubandt method for the determination of transport numbers in - solid electrolytes. 

Transport numbers can be measured by several methods. The application of the Hittorf cell (-> Hittorf transport method), that was introduced in 1853, is still the most frequently used technique for the determination of the transport number [iv]. The moving boundary method, analogous to that used by -> Tiselius to measure -> electrophoretic mobilities is also used to measure transport numbers [v]. See also -> Tubandt method. 

Tubandt was a pioneer of - solid state electrochemistry. He introduced a methodology to determine the - transport numbers of ions in -> solid electrolytes [i], which is now referred to as -> Tubandt method. Together with his co-workers he performed seminal studies of conductivities and transport numbers of solid electrolytes, e.g., of silver, lead, and copper halides, and silver sulfide. He showed for the first time that the entire dark current of silver bromide is transported by silver ions, and also that slightly below the melting point silver iodide has a higher conductivity than the melt. 

The accuracy and precision of the Tubandt method is limited by the difficulty in separating each pellet from its neighbor (they sinter), by possible evaporation losses, and by errors due to weighing. The measurement of the open circuit voltage of an electrolyte (scale) under a.chemical potential gradient is limited by the contacts to the scale and yield an average value unless special experiments are carried out to obtain the cell voltage as a function of chemical potential [1]. 

The Tubandt method is a direct approach to the measurement of transport number according to Faraday s law. It is particularly applicable to network polymers," which can be arranged as a stack of discs placed in electrical series between two electrodes made of the same metal as the cation. For each Faraday of charge passed, the disc next to the positive electrode gains t /xm moles of salt which can be determined by weighing (or some other analysis) provided the disc can be separated from the stack at the end of the experiment. 

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. 

Fig. 32.—Tubandt s method for measurement of transport numbers of ions in a-Agl.

Diffusion constants in ionic lattices are in many instances calculated from the conductivities. The actual transfer in the ionic lattices may be demonstrated by the method of Tubandt(39) who employed cells such as... 

The second method is the measurement of the transport number, for which the experimental technique was developed by Tubandt and his co-workers (39). It was early discovered (67,68) that Faraday s laws were valid for the salts barium chloride and silver chloride, and thus tliat the current carriers are ions. Tubandt and his school carried these investigations much further by pressing salt cylinders together between metal electrodes and electrolysing the system. By weighing the cylinders and electrodes before and after electrolysis the amounts of material transported were estimated directly. However, it was shown that in many instances threads of metal formed stretching from anode to cathode, so that conduction soon became metallic. a-AgI did not behave in this manner, and it was sufficient to coat the electrodes with a protective layer of this salt to suppress the formation of metal threads. With a cell arranged as below ... 

The classical method of determining the transport numbers in electrolytes is by the validity of Faraday s law. Tubandt... 

Mixed Conductors, Determination of Electronic and Ionic Conductivity (Transport Numbers), Fig. 4 Tubandt s method for the detemnination of transport numbers for solid electrolytes and mixed conductors... 

Determination of the Transport Number with Tubandt s Method... 

Measttring the lorric Conductivity by the Short-Circuiting Method Simrrltaneorrs Measrrrement of the Electrorric and Ionic Conductivities Measrtrement of the lorric Corrductivity in Solid Electrolytes The Tubandt or Hittorf Method... 

The empirical phase of the development of solid electrolyte fuel cells was overcome only after many general advances in research on solids. These included development of X-ray structure analysis, new knowledge on the ion conduction of solids from the measurements of transport numbers by Tubandt (first detection of unipolar conduction by anions), the establishment of the theory of disorder in solids by Frenkel, Schottky, Wagner and Jost, and the development of isotope methods for the investigation of diffusion processes in solids. 

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