Ionic mobility, influence

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... 

The ionic mobilities Uj depend on the retarding factor 0 valid for a particular medium [Eq. (1.8)]. It is evident that this factor also influences the diffusion coefficients. To find the connection, we shall assume that the driving force of diffusion is the chemical potential gradient that is, in an ideal solution,... 

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) ... 

The ionic mobility of an ion is its speed of migration under the influence of a fixed potential difference. 

Ionic conductivity o, which quantifies the ion conduction ability, reflects the influence of these two aspects, that is, solvation/dissociation and the subsequent migration, in terms of the free ion number rii and the ionic mobility fii, ... 

Fig. 3 Electrophoretic ionic mobilities fj, of propranolol as influenced by BS concentration buffer x mM BS, 20 mM phosphate, pH = 7.4, detection 220 nm m— electrophoretic ionic mobility /r, GC—glycocholic acid, GDC—glycodeoxycholic acid, GCDC—glycochenodeoxycholic acid.

It will be noted that with the exception of the organic cations and anions the more mobile hydrogen and hydroxyl ions are most readily adsorbed, whilst in the case of the metallic ions the influence primarily of the valency of the ion and both the position of the metal in the eleotrolytic potential series as well as the ionic mobility is most marked, the higher the valency and the more noble the element the more readily it is adsorbed. 

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... 

It is also established that water is able to enter a vitreous phase and, acting as a glass network modifier , enhance ionic mobilities. There is no doubt that the nature of the grain boundary phase, coupled with the presence of water vapour, influences the diffusion of silver from electrodes and terminations in multilayer capacitors leading to eventual inter-electrode shorting and component failure [9]. 

In this way we eliminate errors resulting from the application of equation (III-26), which is based on the assumption that ionic mobility is independent on the concentration of the solution. Equation (III-27) may be regarded as correct, because we compare now conductances of a partly dissociated (real) solution and of a fully dissociated (hypothetical) solution at the same ion concentration, in which case the interionic forces have the same influence upon the velocity of ions. 

The quantity lOOOac is the concentration of A ions (and also of B ions) in mol m 9 = N e is the Faraday constant (96485.31 C equiv ), and C/, is the ionic mobility of charged species i. Note that the mobility is defined as the migration speed of an ion under the influence of unit potential gradient and hence has the units m s V. It is now convenient to define a new quantity, the equivalent conductance A, by... 

The ionic mobility, of a species, i, is the velocity, v, of a particle that moves under the influence of an electric field, E, of unit strength ... 

We have recently developed a gas-phase ion chromatography technique and applied it to carbon cluster cations " " and anions""". A pulse of mass-selected cluster ions is injected into a high-pressure drift cell filled with 2-5 torr of helium. The ionic mobilities of different isomeric structures depend on their different collision cross-sections with He, and the isomers are therefore separated while drifting through the cell, under the influence of a weak electric field. The absolute value of the ionic mobility for a given cluster together with computer simulations often allows unambiguous determination of the cluster... 

When the catalytic reaction is studied at temperatures where surface or bulk ionic mobility exists, it is necessary to consider also the influence that the reactants or the products may have upon the surface structure or even the bulk composition of the catalyst. Changes in the surface defect structure may, in particular, vary with a modification of the composition of the reaction mixture. Moreover, interactions between reactants, in the course of the catalytic reaction, may also alter the smface defect structure or the surface composition and, consequently, the energy spectrum of active sites. 

Although paper electrophoresis has been recommended for pre-testing separations before performing them on a preparative column, t.l.c. is more suitable for this purpose. Electrophoretic mobility is not strictly proportional to the extent of complex-formation it depends also on the bulk and shape of the molecule, which influence ionic mobility. In particular, bulky substituents retard the passage of the complex ion through the electrolyte. These effects have been discussed in detail.T.l.c. is also affected by the bulk of substituents but to a much lesser extent hence, it is more likely to predict the separations obtainable on a column. ... 

Step 1. Metals extracted during this step are those which are exchangeable and in the acid-soluble fraction. These includes weakly absorbed metals retained on the sediment surface by relatively weak electrostatic interaction, metals that can be released by ion-exchange processes and metals that can be coprecipitated with the carbonates present in many sediments. Changes in the ionic composition, influencing adsorption-desorption reactions, or lowering of pH, could cause mobilization of metals from such fractions. 

In the case of conductance studies, the ions move under the influence of the electric field. Because the field is uniform this means that the ions migrate at a constant velocity. This will become relevant in the section on ionic mobilities (Section 11.17), and when discussing the relaxation and electrophoretic effects in the theories of conductance (see Sections 12.1, 12.2 and 12.4). 

The velocity at which an ion moves under the influence of an external field is proportional to the field, with the constant of proportionality being the mobihty of the ion. Provided this strict simple proportionality remains for large fields and, provided the mobihty is known, the velocity of an ion can be calculated. 5 x 10 cm s V is a typical value for an ionic mobility. 

Buffered eluents are important for the separation of ionic solutes because they maintain a constant desired pH. Very often, only a defined and constant pH (therefore a buffer) will allow the desired selectivity and reproducibility. So far, so good. Sometimes, you adopt a method with 5 mM salt in the mobile phase, while the next method uses 100 mM or even 150 mM solutions. Sometimes you simply do not know, as in the following example Weigh out jr mg of salt X and add y ml of acid Y or base Z How critical is the ionic strength and how does the ionic. strength influence the separation and column lifetime ... 

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