Ionic mobility, electrolyte solutions

Figure 7.30(b) shows an electrode with solid contact gel. The gel is sticky and serves both as contact electrolyte and for electrode fixation. For this electrode, EEA = EA. The electrolyte conductivity is rather low because the solution is a gel with low ionic mobility. Electrolyte series resistance may be the dominating factor of electrode/skin impedance at high frequencies. This may introduce problems for use (e.g., for impedance plethysmography around 50 kHz). With this electrode type the skin is not wetted. With such constructions it is... 

Ionic conductors arise whenever there are mobile ions present. In electrolyte solutions, such ions are nonually fonued by the dissolution of an ionic solid. Provided the dissolution leads to the complete separation of the ionic components to fonu essentially independent anions and cations, the electrolyte is tenued strong. By contrast, weak electrolytes, such as organic carboxylic acids, are present mainly in the undissociated fonu in solution, with the total ionic concentration orders of magnitude lower than the fonual concentration of the solute. Ionic conductivity will be treated in some detail below, but we initially concentrate on the equilibrium stmcture of liquids and ionic solutions. 

In the classical theory of conductivity of electrolyte solutions, independent ionic migration is assumed. However, in real solutions the mobilities Uj and molar conductivities Xj of the individual ions depend on the total solution concentration, a situation which, for instance, is reflected in Kohhausch s square-root law. The values of said quantities also depend on the identities of the other ions. All these observations point to an influence of ion-ion interaction on the migration of the ions in solution. 

Table 8.2 lists the conductivities, transport numbers and molar conductivities of the electrolyte A = olc, and ions Xj = t+A for a number of melts as weU as for 0.1 M KCl solution. Melt conductivities are high, but the ionic mobilities are much lower in ionic liquids than in aqueous solutions the high concentrations of the ions evidently give rise to difficulties in their mutual displacement. 

The electrical conduction in a solution, which is expressed in terms of the electric charge passing across a certain section of the solution per second, depends on (i) the number of ions in the solution (ii) the charge on each ion (which is a multiple of the electronic charge) and (iii) the velocity of the ions under the applied field. When equivalent conductances are considered at infinite dilution, the effects of the first and second factors become equal for all solutions. However, the velocities of the ions, which depend on their size and the viscosity of the solution, may be different. For each ion, the ionic conductance has a constant value at a fixed temperature and is the same no matter of which electrolytes it constitutes a part. It is expressed in ohnT1 cm-2 and is directly proportional to the mobilities or speeds of the ions. If for a uni-univalent electrolyte the ionic mobilities of the cations and anions are denoted, respectively, by U+ and U, the following relationships hold ... 

In MACE, the alteration of the ionic mobility as a factor of the tenside concentration in the background electrolyte solution is a measure of the strength of interaction, which may be evaluated graphically. In Fig. 1, a schematic representation of MEKC is given for the separation of micelle and EOF markers as well as drug solutes of different lipophilicity. If the substances are neutral, their position between the EOF marker and the micelle marker is given only by their lipophilicity, which controls their affinity to the micellar phase. This means that S3 in Fig. 1 has the lowest hydrophilicity. 

The value of the electrophoretic mobility can be calculated considering the migration of an ion in an electrolyte solution at inhnite dilution where no ionic interactions occur. Under the action of an electric held, the ion is accelerated by a force Pa, directed toward the appositively charged electrode, which is given by... 

At high field strengths a conductance Increase Is observed both In solution of strong and weak electrolytes. The phenomena were discovered by M. Wien (6- ) and are known as the first and the second Wien effect, respectively. The first Wien effect Is completely explained as an Increase In Ionic mobility which Is a consequency of the Inability of the fast moving Ions to build up an Ionic atmosphere (8). This mobility Increase may also be observed In solution of weak electrolytes but since the second Wien effect Is a much more pronounced effect we must Invoke another explanation, l.e. an Increase In free charge-carriers. The second Wien effect Is therefore a shift in Ionic equilibrium towards free ions upon the application of an electric field and is therefore also known as the Field Dissociation Effect (FDE). Only the smallness of the field dissociation effect safeguards the use of conductance techniques for the study of Ionization equilibria. 

Direct contact techniques do not work with electrolytic solutions. One cannot, for example, measure the resistance of salt water in a cell with an ordinary ohmmeter. This is due to the fact that the mobile charge carriers in an electrolytic solution are ions, not electrons. The conversion of ionic to electronic conduction at the electrode interface can only take place through an electrochemi-... 

Media containing excess electrolyte whose ionic mobility is limited, that is solutions at low temperature or rigid polymeric solutions. 

We wish only to remind readers that there are three main methods of electrochemical re-vealment conductivity, direct current (d.c.) amperometry, and integrated amperometry (pulsed amperometry is a form of integrated amperometry). In revealment by conductivity, the analytes, in ionic form, move under the effect of an electric field created inside the cell. The conductivity of the solution is proportional to the mobility of the ions in solution. Since the mobile phase is itself an electrolytical solution, in order to increase the signal/noise ratio and the response of the detector, it is very useful to have access to an ion suppressor before the revealment cell. By means of ionic exchange membranes, the suppressor replaces the counterions respectively with H+ or OH , allowing only an aqueous solution of the analytes under analysis to flow into the detector. 

C. Concentration Dependence of Ionic Mobility and Viscosity of Electrolyte Solutions... 

Combined with densities, molecular weights, and transference numbers (fractions of the current carried by the various ionic constituents), the conductivity yields the relative velocities of the ionic constituents under the influence of an electric field. The mobilities (velocity per unit electric field, cm2 s-1 V-1) depend on the size and charge of the ion, the ionic concentration, temperature, and solvent medium. In dilute aqueous solutions of dissociated electrolytes, ionic mobilities decrease slightly as the concentration increases. The equivalent conductance extrapolated to zero electrolyte concentration may be expressed as the sum of independent equivalent conductances of the constituent ions... 

GHz. A detailed study of the internal conductivity of erythrocytes revealed the intracellular ionic mobility to be identical with that of ions in dilute electrolyte solutions if appropriate allowance is made for internal friction with suspended macromolecules (5). Tissue conductivities near 100 or 200 MHz, sufficiently high that cell membranes do not affect tissue electrical properties, are comparable to the conductivity of blood and to somewhat similar protein suspensions in electrolytes of physiological strength. Hence, it appears that the mobility of ions in the tissue fluids is not noticeably different from their mobility in water. 

As can be seen, the measurement of the conductivity of an electrolyte solution is not species selective. Individual ionic conductivities can be calculated only if the conductivity (or mobility) of one ion is known this in the case of a simple salt solution containing one cation and one anion. If various ions are present, calculation is correspondingly more difficult. Additionally, individual ionic conductivities can vary with solution composition and concentration. 

We distinguish between concentration cells without and with transference. In the first type the solutions surrounding both electrodes arc not brought into direct contact, while in concentration cells with transference two solutions arc in direct contact. The name cell with transference originates from the fact that during flow of the current a simultaneous transfer of the electrolyte takes place owing to the different ionic mobility. In the case of cells without transference the direct transfer of the electrolyte from one solution to the other is prevented in this instance the transport of the electromotive active substance proceeds exclusively as a result of reactions taking place at the electrodes. 

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