Mobility, of an ion

Thus the actual mobility of an ion, in centimeters per second, is obtained by dividing the equivalent conductivity by the faraday. Some values of mobility are given in Table 3. 

In general, if at any two temperatures 1 and 1 the viscosity of a solvent has the values tji and ij2, and the mobility of an ion has the values Mi and m9 the quantity Utrit/iiiVi would be equal to unity for any ion whose behavior is in accordance with (41). Taking T2 to denote a temperature greater than 7 i, if we write... 

The ionic mobilities also depend on the solvent. In 1905-1906, Paul Walden, Lev Pisarzhevsky, and others established the rule according to which the product of limiting mobility of an ion and viscosity q of the solution is approximately constant ... 

When applied to the motion of ions in a crystal, the term drift applies to motion of ions under the influence of an electric field. Although movement of electrons in conduction bands determines conductivity in metals, in ionic compounds it is the motion of ions that determines the electrical condu-ctivity. There are no free or mobile electrons in ionic crystals. The mobility of an ion, ji, is defined as the velocity of the ion in an electric field of unit strength. Intuitively, it seems that the mobility of the ion in a crystal should be related to the diffusion coefficient. This is, in fact, the case, and the relationship is... 

The electrophoretic mobility of an ion is inversely related to the ionic strength of the buffer rather than to its molar concentration. The ionic strength (ytt) of a buffer is half the sum of the product of the molar concentration and the valency squared for all the ions present in the solution. The factor of a half is necessary because only half of the total ions present in the buffer carry an opposite charge to the colloid and are capable of modifying its charge ... 

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

The ionic mobility " of an ion can be described according to the following equation ... 

On the other hand, the mobility of an ion is known to vary inversely with its solvation radius rt according to the Stokes—Einstein relation ... 

Thus the mobility of an ion can be influenced by its pFa value the more it is ionised the greater its mobility and its molecular shape in solution. Since its degree of ionisation may have a bearing on its shape in solution, it can be seen that the behaviour of analytes in solution has the potential to be complex. For many drugs... 

Here q is the Bjerrum-distance ( -efe /2DkT), the Boltzmann Constance, to the mechanical mobility of an ion Kd(E=0) the equilibrium constant in the absence of electric field and D and dielectric constant of the medium. From these expressions we see that the shift in dissociation constant upon the application of an electric field is given by ... 

The ionic mobility of the simpler ion of a complex salt is usually known, and the other more complex ion will have a mobility, because, as W. Ostwald has shown, the mobility of an ion decreases as the number of constituent atoms increases. For example, dinitrotetramminecobalt chloride, [Co(NH3) 4(N02)2]C1, furnishes two ions, the mobility of the chlorine ion is comparatively higb, that of the other low. 

The charge mobility of an ion represents the speed that acquires the ion per unit of electric field. The electric migration current corresponding to the ionic movement of a single kind of charge is equal to the flux of charge, i.e., to the rate at which the charge cross any plane normal to the flow (see Eq. 1.141) [56]... 

The mobility of an ion in air is its velocity under an electric field. The mobility and Brownian diffusivity of ions determine their rate of attachment to condensation nuclei and to surfaces. 

Separation by electrophoresis is based on the differences in the analytes electrophoretic mobility. The electrophoretic mobility of an ion is proportional to the electric force that the ion experiences and inversely proportional to its frictional drag through the medium. Electrophoretic mobility can be expressed as... 

A larger mobility of an ion results at the same electric field in a greater drift velocity. Typical velocities are around 6 x 10-8 m2 s-1V-1. Only values for protons... 

Molar ionic conductivity — This quantity, first introduced by -> Kohlrausch, is defined by A = Zi Fui (SI unit Sm2 mol-1), where Zj and 14 are the charge number and -> ionic mobility of an ion, respectively. The molar -> conductivity of an electrolyte M +X (denoted by A) is given by A = u+X+ + i/ A, where A+ and A are the molar ionic conductivities of the cation and anion. The A value of an ion at infinite dilution (denoted by A°°) is specific to the ion. For alkali metal ions and halide ions, their A values in water decrease in the orders K+ > Na+ > Li+ and Br- > Cl- > F-. These orders are in conflict with those expected from the crystal ionic radii, because the smaller ions are more highly hydrated, so that the -> hydrated ions become larger and thus less mobile. Based on Stokes law, the radius of a hydrated ion... 

This relation shows that, owing to the Stokes viscous force, the conventional mobility of an ion depends on the charge and radius of the solvated ion and the viscosity of the medium. The mobility given by Eq. (4.183) is often called the Stokes mobility. It will be seen later that the Stokes mobility is a highly simplified expression for mobility, and ion-ion interaction effects introduce a concentration dependence that is not seen inEq. (4.183). 

This indicates a cooperative motion of the lithium ions. If the concept is adopted that a lithium ion always moves as a member of a group of n lithium ions, and that the momentary mobility of an ion is the mean of the eigen mobilities of the momentary members of the group it belongs to, the mean isotopic lithium mobilities in a mixture will be... 

A first approach to take into account the solvent s effect on the absolute mobility of an ion was made by Walden. It is based on the Stokes law of frictional resistance. Walden s rule states that the product of absolute mobility and solvent viscosity is constant. It is clear that the serious limitation of this model is that it does not consider specific solvation effects, because it is based on the sphere-in-continuum model. However, it delivers an appropriate explanation for the fact that, within a given solvent, the mobility depends on temperature to the same extent as the viscosity (in water, for example, the mobility increases by about 2.5% per degree Kelvin). The mobilities do not deviate too... 

The mobility of an ion is related to the velocity v and the electric field in which an ion is moving. The corresponding electric force, accelerates the ion until the fric-... 

In connection with Lewis s suggestion regarding a possible increase in the mobility of an ion as the concentration increases, it is woith while to draw attention to some staking results obtained many years ago by Arrhenius on the rate of diffusion of HC1 into NaCl solutions of different concentrations, in which it was shown that the more concentrated the salt solution the greater the diffusion coefficient of the HC1 into the salt solution This is illustrated by the following data A solution 1 04 molar with respect to HC1 and at the same time o r molar with respect to NaCl diffused into a column of liquid o r molar with respect to NaCl The diffusion coefficient of the acid, t e the hydrogen ion, was 2 50... 

The case of the hydrogen ion is particularly important because of its great mobility Even in this case Bates considers that the best evidence indicates that its mobility is constant1 [If the mobility of an ion, not its transport number, were shown to be independent of concentration the case would be fairly well made out in favour of the conductivity method of determining ionisation]... 

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