Ionic mobility, definition

Salts such as silver chloride or lead sulfate which are ordinarily called insoluble do have a definite value of solubility in water. This value can be determined from conductance measurements of their saturated solutions. Since a very small amount of solute is present it must be completely dissociated into ions even in a saturated solution so that the equivalent conductivity, KV, is equal to the equivalent conductivity at infinite dilution which according to Kohlrausch s law is the sum of ionic conductances or ionic mobilities (ionic conductances are often referred to as ionic mobilities on account of the dependence of ionic conductances on the velocities at which ions migrate under the influence of an applied emf) ... 

Ionic mobilities generally have a temperature coefficient of around -i-2.5%/degree, but it is not exactly the same for all ions. Incidentally, water viscosity has a temperature coefficient of approximately -2.5%/degree. This sometimes leads to the erroneous conclusion that ionic mobility is, by definition, inversely proportional to liquid viscosity, which is an oversimplification, originating also from the following relationship ... 

The transport number has been defined in Section 9.1 as the fraction of the total current carried by a given ion. This is the definition most useful to the determination of transport numbers from emfs. In Chapter 11 the transport number is defined in terms of ionic mobilities, and/or individual molar ionic conductances (see Section 11.17), which are more directly linked to the methods described in that chapter. 

The problem of the thermodynamic activity highly organized and its specific properties are directly related to the making and the breaking of secondary bonds. When a balance sheet is drawn for the anionic and cationic contents of the cell, it is generally assumed that the inorganic ions have their full thermodynamic activity, as if they were in a dilute solution. This opinion stems from the consideration of osmotic equaUty between the cell interior and the extracellular space and from the determination of the ionic mobilities in the cytoplasm. 

This result is called the Nernst-Einstein relation it relates the diffusivity of the ion to the ionic mobility and is valid only at infinite dilution. It is clear from relations (3.1.108h) and (3.1.108i) and the definition (3.1.86) of diffusion coefficient D at infinite dilution that... 

One can now use the definition of ionic mobility uf to redefine the transport number ti in cases where there is no concentration gradient in the solution. Use equation (3.1.108g) to get... 

Now, following Ghowsi et al. (1990), we will briefly treat the migration velocity of the uncharged solute species i in terms of the electroosmotic mobility yig,p of the elec-troosmotic flow and the ionic mobility due to electrophoresis as in definition (6.3.10c), namely... 

I consider there to be a sharp distinction between the most polar form of a molecule and its ionically dissociated form. The reason for this is empirical An ion is defined as a species carrying a charge equal to an integral multiple of the electronic charge, and this definition implies that it will have a characteristic predictable electronic spectrum and, under suitable conditions, mobility in an electric field. There is so far no evidence which would compel one to abandon this definition, and I think it is important to distinguish clearly in this context between reaction intermediates (chain carriers, active species) of finite life-time, and transition states. 

The definition of perfectly mobile and inert components should also be valid for cases of ionic diffusion in rocks and where no fluid is involved (Korzhinskii, 1965), but this possibility does not concern us here since clay mineral equilibria are dominated by the presence of liquid or fluid aqueous solutions. 

Since the fraction of electrons and holes, although very small, depends on the (local) oxygen potential and since the mobility of the electronic defects is far larger than that of the ionic defects, the electronic conductivity may, by continuously changing the oxygen potential, eventually exceed the ionic conductivity. By definition, the transference number is t-loa = erion/(crion + crei)> which explicitly yields... 

Sometimes other variables must be investigated such as the pH and/or the ionic strength of the buffer in the mobile phase or the concentration of additives in the mobile phase such as for instance tensio-active substances in micellar chromatography. In such a case the first step in an optimization is to screen these factors and to identify the most important ones for the subsequent optimization. The screening (Section 6.4.2) leads to a definition of the experimental domain in which the optimum is probably situated. This is somewhat similar to the retention optimization step. It is followed by an optimization step (Sections 6.4 and 6.7), in which the most important variables are changed, often according to an experimental design. Similar methods are used in capillary zone electrophoresis. 

The authors state that while the above definition is used widely, other authors have defined tortuosity as 1/T, T, and l/T as these forms are frequently encountered in expressions for ionic conductivity and mobility through tortuous membranes. Experimental measurement of liquid membrane support tortuosity is described by Bateman et al. 

By definition, t on for an ionic conductor should be = 1, that is, c eiec cTjon atot- In these solids, the mobile carriers are the charged ionic defects, or, = c ef, where c ef represents the concentration of vacancies and/or interstitials. Replacing c j by Qef in Eq. (7.42) and comparing the resulting expression with Eq. (7.34), one sees immediately that... 

Vth equals Vqc of Eq. (22) since the electronic component in the combined electrolyte AgCl/LE vanishes (see Eq. (16a)). This demonstrates that the two definitions of the Nernst voltage are indeed equal when the membrane is a pure ionic conductor. This would not be so if electronic conduction would be present (as can be the case in SSE), as then V(h 7 l oc (Eq. 16a). Different values for the Nernst voltage defined as Voc rather than V(h would be obtained also when the LE contains variable charge-mobile ions, for example, Cu" " and Cu++, as then Voc as can be... 

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