Direct-current conductivity, ionic

From the thermodielectric profiles shown in Figures 4.44 through 4.47, it is eveident that the present effect did not give rise to a peak. This can be clarified with the help of the relationship between the direct current conductivity, in the case of ionic transport, and temperature [15,114] ... 

The conductivity of an ionic conductor can be assessed by direct current (dc) or alternating current (ac) methods. Direct current methods give the resistance R and the capacitance C. The corresponding physical quantity when ac is applied is the impedance, Z, which is the total opposition to the flow of the current. The unit of impedance is the ohm (fl). The impedance is a function of the frequency of the applied current and is sometimes written Z(to) to emphasize this point. Impedance is expressed as a complex quantity ... 

Conductance of a solution is a measure of its ionic composition. When potentials are applied to a pair of electrodes, electrical charge can be carried through solutions by the ions and redox processes at the electrode surfaces. Direct currents will result in concentration polarization at the electrodes and may result in a significant change in the composition of the solution if allowed to exist for a significant amount of time. Conductance measurements are therefore made using alternating currents to avoid the polarization effects and reduce the effect of redox processes if they are reversible. 

Polar Cell Systems for Membrane Transport Studies Direct current electrical measurement in epithelia steady-state and transient analysis, 171, 607 impedance analysis in tight epithelia, 171, 628 electrical impedance analysis of leaky epithelia theory, techniques, and leak artifact problems, 171, 642 patch-clamp experiments in epithelia activation by hormones or neurotransmitters, 171, 663 ionic permeation mechanisms in epithelia biionic potentials, dilution potentials, conductances, and streaming potentials, 171, 678 use of ionophores in epithelia characterizing membrane properties, 171, 715 cultures as epithelial models porous-bottom culture dishes for studying transport and differentiation, 171, 736 volume regulation in epithelia experimental approaches, 171, 744 scanning electrode localization of transport pathways in epithelial tissues, 171, 792. 

We wish only to remind readers that there are three main methods of electrochemical re-vealment conductivity, direct current (d.c.) amperometry, and integrated amperometry (pulsed amperometry is a form of integrated amperometry). In revealment by conductivity, the analytes, in ionic form, move under the effect of an electric field created inside the cell. The conductivity of the solution is proportional to the mobility of the ions in solution. Since the mobile phase is itself an electrolytical solution, in order to increase the signal/noise ratio and the response of the detector, it is very useful to have access to an ion suppressor before the revealment cell. By means of ionic exchange membranes, the suppressor replaces the counterions respectively with H+ or OH , allowing only an aqueous solution of the analytes under analysis to flow into the detector. 

For a number of years the existence of a porous or polar pathway through the stratum comeum, in parallel with the lipoidal pathway, has been hypothesized. Although there has been some criticism of this concept, it is our belief that the root of the lack of a common consensus among scientists in the field can be attributed largely to the limited number of systematic studies in the literature that directly address the issue of the diffusion of polar and ionic permeants across skin. Based upon recent studies that have focused upon this aspect of transdermal diffusion, the existence of a porous permeation pathway through HEM is clear (Hatanaka et al., 1993, 1994 Peck et al., 1993, 1994, 1995). At this point, we have made no attempt to correlate the findings from our studies with specific structural properties of the HEM. In some cases, authors have implicated shunt routes such as hair follicles and sweat ducts to account for permeation data not consistent with the concept of lipoidal membrane permeation (Cornwell and Barry, 1993 Scheuplein and Blank, 1971). Under ionto-phoretic conditions, such shunt routes have been shown to contribute to current conduction (Cullander and Guy, 1991 Scott et al., 1993). When efforts have been made to estimate the effective Rp of skin samples under iontophoretic conditions (Ruddy and Hadzija, 1992), osmotic conditions (Hatanaka et al.,... 

The ionic conductivity of amorphous materials can be obtained not only from the conventional direct current measurement but also from the dielectric loss in the dielectric measurement. The ionic conductivity from the dielectric measurement is based on the following concept ... 

Electrical transients (12) can also be used to evaluate particle mobilities in special circumstances. Charged particles, their counterions and other excess ions present in the suspending fluid contribute to the electrical current. When the concentration of excess ions is very low compared to the concentration of counterions, it is sometimes possible to determine the current contributed by particles versus that contributed by ions. Ionic concentrations define the extent of double layers in colloids. Transient and AC conductivities can be related most directly to the ionic concentrations and mobilities. But, again, the measurements in low conductivity fluids have to be performed in planar cells with narrow electrode spacings in order to ensure well defined electric fields. 

Figure 8. The ionic current (in milliamperes per centimeter squared) across a voltage-gated channel as a function of time (in milliseconds) for different gating currents. The fast gating curve has a conductance that is 15 times greater than the slow gating, and this difference leads to a change in the direction of the ionic current. (Reproduced with permission from reference 12. Copyright 1984.)...

The ionic conductivity is due to both cation and anion species. The cationic transference number = (o / o. ). The anionic transference number x = 1 - x. Different ionic and electronic contributions to the total conductivity can be measured by an electrolysis experiment with the use of selective blocking electrodes which is for blocking all the conducting ion species but the desired one [16, 22].This is a direct current (d.c.) experiment. 

The size of electroconductivity compressed samples MoClj j(C3(, jHgQ j), measured at a direct current at a room temperature-( 1.3 3.3) 10 Ohm -cm is in a range of values for a trans-polyacetylene and characterizes a composite as weak dielectric or the semiconductor. The positioned size of conductivity of samples at an alternating current tr = (3.1 4.7)-10 Ohm cm can answer presence of ionic (proton) conductivity that can be connected with presence of mobile atoms of hydrogen at structure of polymer. 

Electrochemistry involves the contact between different materials which conduct electricity. The two terminals in the electrochemical system linked to the external control device must be electronic conducting materials if the electric parameter is to be controlled, for instance using a direct current (DC) power supply. This system must also include at least one ionic conducting material. To illustrate, an electronic n/p junction... 

Planar bilayer membranes are characterized by their electrical response since the insulating bilayer membrane and the two conducting ionic solutions are electrically equivalent to a capacitor with the membrane as the dielectric. The current through a capacitor is directly proportional to the rate of change of the voltage on the capacitor, i = C dV/dt. The capacitance, in turn, is related to the thickness of the membrane and its dielectric constant. The membrane capacitance is determined by applying a ramp potential with a constant dV/dt across the membrane to give a constant current that can be converted to the mem-... 

We have successfully paoduced the N5-type glass-ceramic conductors by bias crystallization of the glasses with the composition NaiosYassPasSi yOs in an electric field. The microstructure and the conduction properties were dependent on the current direction in the process of crystallization. The cross sections which are parallel and perpendicular to the electric field direction showed the ionic conductivities of 0.0923 and 0.132 mS/ cm at 300°C, respectively. The microstructure and the electric conductivity of the glass-ceramics perpendicular to the electric field direction were significantly different from those in p>arallel. 

A solution will conduct electricity if it contains mobile ions. This idea can be used to explain the results of electrolysis. When a direct current is passed through an ionic solution electrolysis occurs and ions are removed from the solution. Electrolysis can be avoided by using an alternating voltage. 

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