Ion mobility conduction

Conductometric Analysis Solutions of elec trolytes in ionizing solvents (e.g., water) conduct current when an electrical potential is applied across electrodes immersed in the solution. Conductance is a function of ion concentration, ionic charge, and ion mobility. Conductance measurements are ideally suited tor measurement of the concentration of a single strong elec trolyte in dilute solutions. At higher concentrations, conduc tance becomes a complex, nonlinear func tion of concentration requiring suitable calibration for quantitative measurements. 

The quotient is called the electrochemical mobility and is tabulated along with ion mobilities. It is important to pay attention to the units because of possible confusion. Values of /, are given in Table 2-2. Raising the temperature usually increases ion mobility, while increasing the concentration reduces the conductivity due to interactions ... 

Table 2-2 Ion mobilities 1. in S cm mol for calculating specific conductivity with Eq. (2-12) between 10 and 25°C, conductivity increases between 2 and 3% per °C...

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

The discussion of molecules and molecular ions will be continued in Sec. 29. Here we shall begin the detailed examination of solutes that are completely dissociated into ions. The conductivity of aqueous solutions of such solutes has been accurately measured at concentrations as low as 0.00003 mole per liter. Even at these concentrations the motions of the positive and negative ions are not quite independent of each other. Owing to the electrostatic forces between the ions, the mobility of each ion is slightly less than it would be in a still more dilute solution. For example, an aqueous solution of KC1 at 25°, at a concentration of 3.2576 X 10 6 mole per liter, was found to have an equivalent con-... 

The above methods measure ion transport rates as ionic conductivities. By varying the parameters of the experiment, it is often possible to indirectly identify the mobile ion(s),173 and in some cases to estimate individual ion mobilities or diffusion coefficients.144 Because of the uncertainty in identifying and quantifying mobile ions in this way, EQCM studies that provide the (net) mass change accompanying an electrochemical process36 have played an important complementary role. 

As a result of this, the conductivity of the solution falls and continues to fall with each subsequent addition of alkali until the end-point is attained. On addition of a little sodium hydroxide after the neutralization point, there will be a small concentration of OH ions and conductivity will once again rise, this being the result of OTT ions having the second greatest mobility. The point corresponding to the minimum conductivity represents, therefore, the end-point of titration. 

Electrophoretic separations occur in electrolytes. The type, composition, pH, concentration, viscosity, and temperature of the electrolytes are all crucial parameters for separation optimization. The composition of the electrolyte determines its conductivity, buffer capacity, and ion mobility and also affects the physical nature of a fused silica surface. The general requirements for good electrolytes are listed in Table 1. Due to the complex effects of the type, concentration, and pH of the separation media buffer, conditions should be optimized for each particular separation problem. 

Sometimes in the literature the equivalent ionic conductivity at infinite dilution is erroneously termed ion mobility however, eqn. 2.17 clearly shows the interesting linear relationship between both properties with the faraday as a factor. 

In order to provide more insight into transference numbers, ionic conductivities and ion mobilities, some data collected by Maclnnes2 are given in Table 2.1 and 2.2 the data for A0 were taken from the Handbook of Chemistry and Physics, 61st ed. all measurements were made at 25° C in aqueous solutions. 

EQUIVALENT IONIC CONDUCTIVITIES AND ION MOBILITIES AT INFINITE DILUTION IN AQUEOUS SOLUTIONS AT 25° C... 

So far, the data mentioned were measured at 25° C as is usual in electrochemical practice. However, it should not be forgotten that the ion mobilities increase considerably with temperature (see the Smithsonian table of equivalent conductivities as different temperatures in the Handbook of Chemistry and Physics, 61st ed.), although with the same trends for the various ions therefore, the change in transference numbers remains small and shows a tendency to approach a value of 0.5 at higher temperatures. 

Methods of Measurement. The positive and negative small-ion concentrations, the electrical conductivity, and ion mobilities were measured with a Gerdien-type instrument. From analyses of current-versus-voltage curves obtained with the apparatus operated at constant volume flow, values for the ion density, the conductivity, and the mean mobility of the ions can be determined (Wilkening and Romero, 1981). 

Table 2 lists limiting equivalent conductance and association constant values for a number of 1 1 electrolytes in the solvents of Table 1, and Table 3 gives single ion mobility values. The data include results that appear to have sufficient precision to give meaningful values when treated by the Fuoss-On-sager conductance equation. In a few cases data of somewhat lower precision have been included to indicate the magnitude of the association constants, which can often be determined with fair accuracy from such data. 

To minimize the effects of viscosity for purposes of comparing data between solvents, plots areoften made using the product of the ion mobility and the viscosity (Walden product) in place of mobility alone. A plot of the Walden product against the reciprocal of the crystallographic radii for several solvents is shown in Fig. 6. Arbitrary curves have been drawn to indicate general trends. Values in solvents for which precise transference numbers and conductance data are available, such as acetonitrile and nitromethane, give smooth curves. 

A considerable volume of literature has accumulated on conductance measurements in mixtures of solvents. Ion mobilities and association constants have been measured over a range of bulk dielectric constants with the aim of correlating bulk solvent properties with mobilities, ion association, and ion size parameters. An example of a widely used solvent mixture is water and 1,4-dioxane, which are miscible over all concentrations, providing a dielectric constant range of 2 to 78. The data obtained in systems containing two or more solvents must be treated with circumspection, as one solvent may interact more strongly with a given species present in solution than the other, and the re-... 

With the more conductive liquids, the ion concentration becomes so great that ion concentration fluctuations on a statistical basis are likely to be small. However, charging can take place by three other mechanisms (1) mechanical disruption of any double layer of ions that may exist at the surface in times that are short compared with the relaxation time, with a predominance of the surface ions going to the portion of fluid coming from the surface (2) unequal ion mobility with the larger ions unable to return to the bulk of liquid as readily as the smaller and more mobile ones and (3) contaminating materials, such as dust or surfactants at the interfaces serving as ion carriers into one portion or the other of the ruptured liquid. 

In order to produce a large W factor, the concentration of the mobile ions should be high compared to the concentration of the electronic species. However, c, should not be very much larger than c in order to keep the transference number of the electrons close to 1. These requirements are somewhat contradictory, but may best be fulfilled if the mobility of the small number of electrons is very large compared to the mobility of the ions. The conductivity of the electrons may in this way be kept larger than the conductivity of the ions. In order to come up with a quantitative relationship, the transference number of the electrons is substituted by the product of the concentrations and the diffusivities or mobilities... 

It is convenient for the purposes of this chapter initially to make a number of simplifying assumptions about the nature of the electrolytes under discussion. For example, Ag4RbIs will be assumed to be an ionic conductor with negligible electronic conductivity and with only the silver ion mobile. Likewise Na-jS-Al203 will be assumed to be a substance in which the only mobile charge is Na". Another simplifying assumption of a different sort which we will make is that the interfaces when formed do not, in general, have a third phase, e.g. an oxide film between the... 

One major drawback of these sulfonate salts is their poor ion conductivity in nonaqueous solvents as compared with other salts. In fact, among all the salts listed in Table 3, LiTf affords the lowest conducting solution. This is believed to be caused by the combination of its low dissociation constant in low dielectric media and its moderate ion mobil-ityi29 3 compared with those of other salts. Serious ion pairing in LiTf-based electrolytes is expected, especially when solvents of low dielectric constant such as ethers are used. ... 

Ion conductivity has essentially become the quantity used as the field-trial standard for any prospective electrolytes, because it can be easily measured with simple instrumentation, and the results are highly accurate and reproducible. The methodology and the fundamental principles involved with the measurement have been summarized in a detailed review. On the other hand, no reliable method has been available so far for the exact determination of ion mobility (or a related property, diffusivity and ionization degree, especially in electrolyte solutions in the concentration ranges of practical interest. ... 

So far, very few attempts at improving ion conductivity have been realized via the salt approach, because the choice of anions suitable for lithium electrolyte solute is limited. Instead, solvent composition tailoring has been the main tool for manipulating electrolyte ion conductivity due to the availability of a vast number of candidate solvents. Considerable knowledge has been accumulated on the correlation between solvent properties and ion conductivity, and the most important are the two bulk properties of the solvents, dielectric constant e and viscosity rj, which determine the charge carrier number n and ion mobility (w ), respectively. 

Earlier, Gavach et al. studied the superselectivity of Nafion 125 sulfonate membranes in contact with aqueous NaCl solutions using the methods of zero-current membrane potential, electrolyte desorption kinetics into pure water, co-ion and counterion selfdiffusion fluxes, co-ion fluxes under a constant current, and membrane electrical conductance. Superselectivity refers to a condition where anion transport is very small relative to cation transport. The exclusion of the anions in these systems is much greater than that as predicted by simple Donnan equilibrium theory that involves the equality of chemical potentials of cations and anions across the membrane—electrolyte interface as well as the principle of electroneutrality. The results showed the importance of membrane swelling there is a loss of superselectivity, in that there is a decrease in the counterion/co-ion mobility, with greater swelling. 

The equipment for screening passengers and baggage is designed to identify trace amounts of specific known explosives. Analytic trace detection is conducted using mass spectrometry, gas chromatography, chemical luminescence, or ion mobility spectrometry. Ion mobility spectrometry is most commonly used. Novel explosive material wiU not be probably detected by these systems. Information on the equipment s technical performance is not publicly available because of security reasons, which inhibits an independent analysis of equipment s performance [160]. 

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