Mobility, ionic

This is like Worked Problem 11.13 where the electrolyte is unsymmetrical. 

The ionic mobility, transport number and ionic molar conductivity are all linked. The velocity with which an ion migrates is proportional to the drop in potential over the region in which the ion migrates. Put otherwise, the velocity of migration is proportional to the electric field. 

In conductance studies the conductance cell is connected to a source of alternating current. In one half-cycle the negative pole of the source of the current is connected to the negative electrode of the conductance cell which thus acts a cathode, and the positive pole of the source is connected to the positive electrode of the conductance ceU which acts as an anode. In the subsequent half-cycle the situation will be reversed. But for the following, the discussion will be restricted to what happens in one half-cycle. The field is taken to be aligned in a direction from the positive electrode to the negative and this is indicated by +—The cations will migrate under the influence of the field in the direction of the field, and the anions will move in the opposite direction. 

The velocities of migration, v+ for the cations, and v for the anions are different. Since these velocities are proportional to the field, X, then  

The time taken for cations to reach the cathode on the right, is T+ =— (11.110) 

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We know from equation A2.4.32 and equation A2.4.34 that the limiting ionic conductivities are directly proportional to the limiting ionic mobilities in fact... 

From equation A2.4.38 we can, finally, deduce Walden s rule, which states that the product of the ionic mobility at infinite dilution and the viscosity of the pure solvent is a constant. In fact... 

Migration is the movement of ions due to a potential gradient. In an electrochemical cell the external electric field at the electrode/solution interface due to the drop in electrical potential between the two phases exerts an electrostatic force on the charged species present in the interfacial region, thus inducing movement of ions to or from the electrode. The magnitude is proportional to the concentration of the ion, the electric field and the ionic mobility. 

In general, ionic mobilities are inversely proportional to gas density. Ionic velocities in the usual electrostatic precipitator are on the order of 30.5 m/s (100 ft/s). 

The ionic mobility is the average velocity imparted to the species under the action of a unit force (per mole), i is the stream velocity, cm/s. In the present case, the electrical force is given by the product of the electric field V in V/cm and the charge per mole, where S" is the Faraday constant in C/g equivalent and Z is the valence of the ith species. Multiplication of this force by the mobihty and the concentration C [(g mol)/cm ] yields the contribution of migration to the flux of the ith species. 

Fully hydrated potassium ion coordinates about 10-11 molecules of water, whereas sodium coordinates about 16-17 molecules [115]. The ionic mobility of potassium is about 50% greater than that of sodium. In simple terms, this means that more of the water in a potassium-catalyzed resin will be available as free water for viscosity reduction and that movement of water from a glue line into the wood will have less effect in moving the adhesive off of the glue line with it. 

Electrical properties of liquids and solids are sometimes crucially influenced by H bonding. The ionic mobility and conductance of H30 and OH in aqueous solutions are substantially greater than those of other univalent ions due to a proton-switch mechanism in the H-bonded associated solvent, water. For example, at 25°C the conductance of H3O+ and OH are 350 and 192ohm cm mol , whereas for other (viscosity-controlled) ions the values fall... 

D2O and the tritium analogue T2O (p. 41). The high bp is notable (cf. H2S, etc.) as is the temperature of maximum density and its marked dependence on the isotopic composition of water. The high dielectric constant and measurable ionic dissociation equilibrium are also unusual and important properties. The ionic mobilities of [H30] and [OH] in water are abnormally high (350 X 10 " and 192 x 10 cms per V cm... 

LQZwischen, adv. meantime, lod, n. iodine. For compounds see Jod-. lonen-art, /. kind of ion. -austausch, m. ion exchange, -beweglichkeit, /. ionic mobility. 

Shortly after the formulation of the Debye-Huckel theory, a survey of the data on ionic mobilities from this point of view was made, extrapolating the values to infinite dilution.1 Table 4 gives values of Cl for atomic and molecular ions for 7 = 0°C and T2 = 18°C. 

Further wc may notice that there is a striking resemblance to Fig. 28 in Chapter 4, where the temperature coefficient of the ionic mobility was plotted against the mobility itself. This resemblance is more interesting when it is recalled that the experimental values plotted in Fig. 28 are obtained for each species of positive and each species of negative ion separately and do not contain any arbitrary factor (like the assignment... 

It is clear, however, that, from the state of affairs envisaged by Gibson, we can obtain quite a different view of the ionic mobility of Li+ and Na+ —a view in good agreement with the concepts of order-disorder already... 

Conductivities k of electrolytes are related to molar conductivities A, ion conductivities A, and ionic mobilities w(- by Eq. (57)... 

The temperature is low so that ionic mobility on the electrode surfaces is negligible, i.e. there is no spillover. 

The temperature is now increased to the point that the ionic mobility on the electrode surfaces is high, so that now there is ion spillover. 

The frequency-dependent spectroscopic capabilities of SPFM are ideally suited for studies of ion solvation and mobility on surfaces. This is because the characteristic time of processes involving ionic motion in liquids ranges from seconds (or more) to fractions of a millisecond. Ions at the surface of materials are natural nucleation sites for adsorbed water. Solvation increases ionic mobility, and this is reflected in their response to the electric field around the tip of the SPFM. The schematic drawing in Figure 29 illustrates the situation in which positive ions accumulate under a negatively biased tip. If the polarity is reversed, the positive ions will diffuse away while negative ions will accumulate under the tip. Mass transport of ions takes place over distances of a few tip radii or a few times the tip-surface distance. 

We have performed ionic mobility studies on mica and in alkali halide surfaces. Here we shall describe some results obtained on mica with different surface ions. Alkali halides will be discussed in detail in the next section. 

Holz, M Lucas, O Muller, C, NMR in the Presence of an Electric Current, Simultaneous Measurements of Ionic Mobilities, Transference Numbers, and Self-Diffusion Coefficients Using an NMR Pulsed-Gradient Experiment, Journal of Magnetic Resonance 58, 294, 1984. Hooper, HH Baker, JP Blanch, HW Prausnitz, JM, Swelling Equilibria for Positively Ionized Polyacrylamide Hydrogels, Macromolecules 23, 1096, 1990. 

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