Determination of ionic velocity

Laser Doppler Scattering for the Determination of Ionic Velocity Distributions in Channels and Membranes... 

Mobihties are important in themselves simply as a property of the hydrated ions. But, more importantly they enable determination of ionic radii to be made for the hydrated ions. This makes use of Stokes Law (see Section 12.4.2), which requires that for an ion of charge ze and effective radius, a, moving with a velocity, v, in a medium of viscosity, rj, under the influence of an external field, X ... 

In theory, there are no limits to the accessibility of electrokinetic data. However, there are a number of physical situations which limit the range of electrokinetic data which can be obtained in the above described experimental set-ups. The experimental techniques described above require visual determination of particle velocities and are typically limited to the range 3-100 pm/s. Additionally, according to equation (19.24), current and solution conductivity affect the particle mobility. In experimental practice, the limit for current in the cell is around 300 pA with the use of blank platinum electrodes. Under higher currents, electrolytic reactions at the electrodes result in electrode polarization, heating and subsequent formation of gas bubbles and thermal convection in the cell. Furthermore, solution conductivity measurements are not reliable below about 10 pS/cm. The above limitations restrict the typical range of solution ionic strengths at which one can work to O.l-lOOmM. 

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) ... 

Charge exchange is important all along the high-LET tracks. The effective ionic charge is determined by cross sections of electron capture and loss, which depend predominantly on the ionic velocity. Electron loss may be simply described by an ionization of the incident ion in its own reference frame due to the impact of medium electrons and nuclei. Following Bohr (1948), Mozumder et al. (1968) wrote the cross section for this process as1... 

The electrophoretic separation principle is based on the velocity differences of charged solute species moving in an applied electric field. The direction and velocity of that movement are determined by the sum of two vector components, the migration and the electroosmotic flow (EOF). The solute velocity v is represented as the product of the electric field strength E and the sum of ionic mobility uUm and EOF coefficient /a OF ... 

Luo and Andrade have reexamined [66] the potential of CEC by comparing the effect of the conclusions of the Rice-Whitehead theory [35] of doublelayer overlap on the determination of minimum dp with those which result from more recent treatments of the velocity profile in electroosmotic flow. They concluded that, for ionic strength <10 mM, the particle size can again be less than 1 pm, and that plate numbers up to 1 x 106 should be theoretically possible. An obstacle to the realization of such efficiencies in CEC is, however, the consequence of the recognition [6] by Giddings that there is no satisfactory mathemat-... 

The velocity of inversion of sucrose and that of the saponification of methyl acetate under the catalytic influence of hydrogen ions are proportional to the concentration of these ions in the solution, so that measurement of these velocities affords a means of estimating the ionic concentration.6 The velocity of inversion of sucrose is conveniently determined by the aid of the polarimeter, and that of the saponification... 

As the potential that is applied across the electrodes is increased, the ionic velocities increase. Thus, the detector signal is proportional to the applied potential. This potential can be held to a constant value or it can oscillate to a sinusoidal or pulsed (square) wave. Cell current is easily measured however, the cell conductance (or reciprocal resistance) is determined by knowing the potential to which the ions are reacting. This is not a trivial task. Ionic behavior can cause the effective potential that is applied to a cell to decrease as the potential is applied. Besides electrolytic resistance that is to be measured, Faradaic electrolysis impedance may occur at the cell electrodes resulting in a double layer capacitance. Formation of the double layer capacitance lowers the effective potential applied to the bulk electrolyte. 

Mobility and electrical conductivity are therefore determined by the same phenomenological coefficient Lpp as the diffusion, see (4.538). But the situation is much more complicated in such salt solutions because salt is composed from cations and anions and the mixture has at least three constituents. Moreover solutions are electroneutral with high precision and therefore measuring Lpp of unique ion say by diffusion is difficult (difference between diffusion velocities of ions causes e.g. diffusion potentials , etc. see [3, 4, 203]). In fact the (near) electroneutrahty of ionic solutions permits to use our theory here which neglect long-range electrical forces, cf Rem. 6. 

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