Mobility electrochemical

The quotient is called the electrochemical mobility and is tabulated along with ion mobilities. It is important to pay attention to the units because of possible confusion. Values of /, are given in Table 2-2. Raising the temperature usually increases ion mobility, while increasing the concentration reduces the conductivity due to interactions ... 

Moreover, it is worth noting that the negative ion spectrum of myoglobin at pH 3 shows a better signal-to-noise ratio than the same spectrum at pH 10 (Figure 1.27). This results from the fact that protons have a high electrochemical mobility, and is the first indication of the importance of the reduction process, when negative ions are analysed, that occurs at the probe tip. 

In electrochemical literature, however, mobilities of ions are not usually expressed in the absolute form defined in Eq. (4.149). Instead, they are more normally recorded as the drift velocities in unit electric field (1 V cm ) and will be referred to here as conventional (electrochemical) mobilities with the symbol... 

The use of Stokes law also permits the derivation of a very simple relation between the viscosity of a medium and the conventional electrochemical mobility conv Starting from the earlier derived equation (4.177)... 

The equivalent conductivity, however, has been related to the conventional electrochemical mobilities" and u of the current-carrying ions by the following expression... 

When there is an electrical potential gradient, including diffusion potential, the flux of z, Ji(e), is proportional to the gradient of the electrical potential, (dW/dx), the concentration, C and valence, zi of ion i and its electrochemical mobility w ... 

The basis of ionic conduction is the mobility of ions [3]. In liquid electrolytes it is the consequence of a three-dimensional random movement of ions. The characteristic of the random walk is that the mean distance traveled by the ion is zero, but the mean square distance is proportional to time. Because of this movement, the concentration of ions is uniform throughout the volume of the electrolyte in the absence of an electric field. Under the influence of a certain force, e.g., in an electric field, the ions acquire a nonrandom component of velocity in the direction of the force. The velocity developed under unit applied force is called the absolute mobility of the ion. The conventional, or electrochemical, mobility is the velocity of ions in a unit electric field. The relationship between the absolute and conventional mobility is... 

The electrochemical mobility unit is therefore mol s kg or mol m s J . Thanks to the link between molar flux density and local velocity described in section 4.1.1.1, one ends up with the same proportionality link between flux and force (thermodynamics of linear irreversible processes) as seen previously ... 

The common expression for current density does not involve electrochemical mobility but rather ... 

The balance at the microscopic level between the friction force and a unit external force (namely with a modulus equal to 1 N) applied to an ion i which is considered to be a sphere with an radius r,, results in a movement with a steady-state velocity. The velocity modulus, once divided by the Avogadro constant, is equal to the electrochemical mobility ... 

The concept of fast or slow couples is therefore independent of the potential applied, since it is intrinsic to the system. However it does depend on other experimental parameters through the mass transport rate constant. The latter parameter is in fact a function of the quantities specific to the mass transport of the species in question (diffusion coefficient or electrochemical mobility), but it also depends on other characteristics in the system which vary according to each type of experiment, as illustrated in the examples below. 

How then is the fall in the conductivity at the critical concentration to be explained According to Hartley there are here two effects which operate against one another. The association of the fatty acid anions into a micelle would — when nothing else happened — undoubtedly lead to an increase of the conductivity (Me Bain effect). Indeed the single fatty acid ions have a fairly large mass in proportion to their charge and thus — in view of Stokes law — a small electrochemical mobility. In the association of these anions into a spherical micelle the charge increases much more rapidly than the radius of the particle. 

The average drift velocity hi per unit force may be replaced by the electrochemical mobility w,- (i. e. by the average drift velocity per unit electric field strength). Then, through the relationship bi/ui = IjzieQ = Nq/ziF, the modified Nernst-Einstein relation is obtained as ... 

The final bracketed expression in eq. (9-15) is obtained from the equilibrium condition of eq. (9-14), and from eq. (5-17) for and. The constant contains the ionic partial conductivity, the electrochemical mobility of the electrons, and the equilibrium constant of eq. (9-14). 

In a simple hydrodynamic approach, transport parameters such as the ionic conductivity <7 , the diffusion coefficient D , and the electrochemical mobility Ui of ionic/atomic species i in Uquid/glassy systems are linked to the dynamic viscosity t] by the Stokes-Einstein equation ... 

The second major assumption of this approach is to assume that the electrochemical mobility is constant throughout the interfacial region, in other words, that the viscous drag is the same in the two solvents, or that the diffusion coefficients are equal in the adjacent phases. Doing so, we have... 

Besides this, let us now consider another mass-transport mechanism. The velocity developed by a charged particle under unit applied force is called absolute mobility of the ion the conventional or electrochemical mobility is the velocity of ions in a unit external electrical field, they are related themselves by ... 

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