Migration liquid junction

Faraday s law (p. 496) galvanostat (p. 464) glass electrode (p. 477) hanging mercury drop electrode (p. 509) hydrodynamic voltammetry (p. 513) indicator electrode (p. 462) ionophore (p. 482) ion-selective electrode (p. 475) liquid-based ion-selective electrode (p. 482) liquid junction potential (p. 470) mass transport (p. 511) mediator (p. 500) membrane potential (p. 475) migration (p. 512) nonfaradaic current (p. 512)... 

Planck s solution for the liquid junction potential [30, 31] is based on the assumption of stationary state transport, through diffusion and migration, and... 

For studies in aqueous solutions, the external reference electrode is often an Ag/AgCl/KCl electrode. Electrical contact with the solution is achieved using a disc-like membrane made of porous fritted glass. Because ions have a tendency to migrate across the membrane, a small potential Ej is generated by this liquid junction. This phenomenon can be minimised by inserting a saturated KC1 solution as a salt bridge. 

Ah already stated the liquid junction potential results from the different mobility of ions. Consequently no diffusion potential can result at the junction of the electrolyte solution the ions of which migrate with the same velocity. It is just this principle on which the salt bridge, filled by solutions of those salts the ions of which have approximately the same mobilities, is based (the equivalent conductivities of ions Kf and Cl- at infinite dilution at 25 °C are 73.5 and 70.3 respectively and the conductivities of ions NH+ and NOg are 73.4 and 71.4 respectively). Because ions of these salts have approximately the same tendency to transfer their charge to the more diluted solution during diffusion, practically no electric double layer is formed and thus no diffusion potential either. The effect of the salt bridge on t he suppression of the diffusion potential will be better, the more concentrated the salt solution is with which it is filled because the ions of the salt are considerably in excess at the solution boundary and carry, therefore, almost exclusively the eleotric current across this boundary. 

Under these conditions the current is carried in solution by ions migrating in the solutions and also through the narrow tube, or the porous pot, or through the salt bridge. Such cells do not operate reversibly in a rigorous manner because the processes occurring at the junctions will contribute to the thermodynamic quantities. Such cells are called cells with liquid junctions. 

A second example of this type of liquid junction is 0.1 M KCI/0.01 M KCl. This situation is completely analogous to that above, except that in this case the K and Cr ions migrate at nearly the same rate, with the chloride ion moving only about 4% faster. So a net negative charge is built up on the right side of the junction, but it will be relatively small. Thus, Ej will be negative and is equal to —1.0 mV. 

The diffusion potential results from a tendency of the protons in the inner part of the gel layer to diffuse Toward the dry membrane, which contains —SiO Na, and a tendency of the sodium ions in the dry membrane to diffuse to the hydrated layer. The ions migrate at a different rate, creating a type of liquid-junction potential. But a similar phenomenon occurs on the other side of the membrane, only in the opposite direction. These in effect cancel each other, and so the potential of the membrane is determined largely by the boundary potential. (Small differences in boundary potentials may occur due to differences in the glass across the membrane—these represent a part of the asymmetry potential.)... 

If the two electrode systems that compose a cell involve electrolytic solutions of different composition, there will be a potential difference across the boundary between the two solutions. This potential difference is called the liquid junction potential, or the diffusion potential. To illustrate how such a potential difference arises, consider two silver-silver chloride electrodes, one in contact with a concentrated HCl solution, activity = the other in contact with a dilute HCl solution, activity = Fig. 17.7(a). If the boundary between the two solutions is open, the and Cl ions in the more concentrated solution diffuse into the more dilute solution. The ion diffuses much more rapidly than does the Cl ion (Fig. 17.7b). As the ion begins to outdistance the Cl ion, an electrical double layer develops at the interface between the two solutions (Fig. 17.7c). The potential difference across the double layer produces an electrical field that slows the faster moving ion and speeds the slower moving ion. A steady state is established in which the two ions migrate at the same speed the ion that moved faster initially leads the march. 

Figure 12.2 An electrolytic cell. Iron dissolves on one side, and copper precipitates on the other. A porous liquid junction allows sulfate to migrate between the solutions. The cell reaction is identical to the reaction in Figure 12.1,...

Ions migrate from areas of high to low concentration by diffusion. As ions move into the porous junction of the reference electrode, they establish a voltage known as the "liquid junction potential" or "diffusion potential" (Eg). The more mobile ions accumulate in the junction faster and build up an excess charge that slows down the further accumulation of these ions. The potential Eg and consequently the pH reading shifts until an equilibrium is reached. The potential 5 for the standard KCl reference electrolyte is relatively small because the potassium (K ) and chloride (Cr) ions electrolyte have about the same mobility, which means they accumulate in the junction at about the same rate. However, a KCl electrolyte is not normally used in process fluids with compounds such as... 

The liquid junction potential calculated with this equation should not be used for very accurate applications. The conditions assumed in its derivation are never all fulfilled in practice. The main assumption, that the participating ions migrate exclusively according to the concentration gradients so that pure diffusion results, is seldom valid. This is true especially for the common reference electrode constructions in which convection of the electrolyte arises (this is desired for other reasons). In addition, in order to calculate the individual ion activities one needs the individual activity coefficients, and these are not accessible through experimental measurements (as shown in the Appendix). In practice, the analytical applications ofEMF measure-... 

Figure 19.2 Glass electrode for measuring pH. The concentration of H+ ions is accessible through the potential difference which appears between the glass electrode and the external reference electrode (here an Ag/AgCl electrode). Above, a cross-section of the membrane permeable to H+ ions. When an H+ ion forms a sUanol bond, a sodium ion moves into solution to preserve electroneutrality of the membrane. Two processes occur ion-exchange and diffusion. Below left, a cross-section of a combined electrode in a concentric arrangement, where the reference electrode surrounds the glass electrode. The junction permits the migration of ions since the liquids from each side should not mix. Prior to its use, the pH-meter is calibrated with a buffer solution of known pH.

The molecular origin of thermoset behavior lies in the formation of a polymeric network. Unlike thermoplastic materials, which consist of individual macromolecules that can move relative to each other over infinite distances in the melt, the polymeric chains in a thermosetting material are attached to each other and can therefore not move any more relative to each other over greater distances. They are bound together through so-called crosslinks and form an infinite, three-dimensional structure. A consequence of the presence of junctions between the polymeric chains, other than the inability to behave like a liquid and flow, is the inability of this material to dissolve in another medium such as a solvent. Solvent or plasticizer molecules can migrate into the network structure, to swell it, but this is limited to a maximum determined by the crosslink density. 

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