Limiting ionic conductivities

We know from equation A2.4.32 and equation A2.4.34 that the limiting ionic conductivities are directly proportional to the limiting ionic mobilities in fact... 

Table 3. Limiting ionic conductivities of cations in selected organic solvents All values measured at 25 °C unless otherwise indicated...

The specific conductivity of the solution, K (Q-cm)1, can be found in reference handbooks or estimated from limiting ionic conductances. 

Solid-Electrolyte Hydrogen Sensor. Most of solid gas sensors so far developed need high temperature operation because of limited ionic conductivities when the electrolyte is near room temperature. If solid electrolytes with sufficiently large ionic conductivities are available, unique gas sensors operative near room temperature can be fabricated. An example is the following proton conductor hydrogen sensor proposed by our group (10, 11). 

We now explore whether the pattern of reactivity predicted by the Marcus theory is found for methyl transfer reactions in water. We use equation (29) to calculate values of G from the experimental data where, from (27), G = j(JGlx + AG Y). The values of G should then be made up of a contribution from the symmetrical reaction for the nucleophile X and for the leaving group Y. We then examine whether the values of G 29) calculated for the cross reactions from (29) agree with the values of G(27) calculated from (27) using a set of values for the symmetrical reactions. The problem is similar to the proof of Kohlrausch s law of limiting ionic conductances. 

TABLE 7.7 Limiting Ionic Conductivities of Ions in Selected Solvents"... 

Measurement of limiting ionic conductances and their variation with solvent can in principle offer information about the solvation... 

Passivity — An active metal is one that undergoes oxidation (-> corrosion) when exposed to electrolyte containing an oxidant such as O2 or H+, common examples being iron, aluminum, and their alloys. The metal becomes passive (i.e., exhibits passivity) if it resists corrosion under conditions in which the bare metal should react significantly. This behavior is due to the formation of an oxide or hydroxide film of limited ionic conductivity (a passive film) that separates the metal from the corrosive environment. Such films often form spontaneously from the metal itself and from components of the environment (e.g., oxygen or water) or may be formed by an anodization process in which the anodic current is supplied by a power supply (see -> passivation). For example, A1 forms a passive oxide film by the reaction... 

Transpassivity — Certain metals exhibit the property of -> passivity, whereby the metal resists - corrosion under conditions in which it should react significantly, usually due to the formation of an oxide or hydroxide film of limited ionic conductivity (a passive film) that separates the metal from the corrosive environment. An-... 

Ideally, to determine limiting ionic conductances in a solvent, precise transport number data are required for an unassociated salt for which the value of A0 is known accurately. Several attempts now have been made to determine transport values, by the Hittorf method, for ions in NMA using a variety of salts85,86,13S. A summary of values obtained for Xqk+ in NMA at 40 °C is provided in Table V-3. 

Several interesting trends are apparent on examining the data in Table V-4. For simple, singly-charged cations the limiting ionic conductances follow the order ... 

This order suggests that the smaller alkali metal cations interact quite strongly with the NMA solvent so that their effective Stokes radii are larger than those of Rb+ or Cs+. Similarly the values of Xq for the halide ions follow the order Cl- < Br- < I-. It is also clear that there is no abnormally large limiting ionic conductance for the hydrogen ion in NMA solution. 

The values for the limiting ionic conductances in NMA at 40 °C in this table are self-consistent and are based on XoK+ equal to 8.54 x 10-4 ohm-1 m2 eq-1. The values have been calculated by considering all available conductance work for salts of each ion at 40 °C. No attempt has been made, however, to weight the results according to the precision of the experimental conductance work. All limiting conductance data determined from conductance data for salts with association constants greater than 103 also were not included. For ions where five or more salts have been studied, the values probably are accurate to better than 1% exclusive of any errors in Xj)K+. 

Table 2.10 Limiting ionic conductances in water at 298 K ((A/cm2) (V/cm) (g equiv/cm3))...

For a weakly ionized substance, A varies much more markedly with concentration because the degree of ionization a varies strongly with concentration. The equivalent conductance, however, must approach a constant finite value at infinite dilution, Ag, which again corresponds to the sum of the limiting ionic conductances. It is usually impractical... 

Fig. 17. Apparent standard rate constant k% vs. the limiting ionic conductance in water for the transfer of (1) Pr4N +, (2) EtPrsN, (3) Et3PrN+, (4) Et4N4, (5) MejBuN+, (6) EtjMeN-", (7) MejPrN", (8) choline, (9) EtjMejN, (10) EtMe3N, (11) Me4N, and (12) Me3NH across the water-nitrobenzene interface. Vertical bars indicate the 95% confidence intervals. (After [42]).

These several assumptions do not lead to the same conclusions. For example, transfer activity coefficients obtained by the tetraphenylarsonium tetraphenyl borate assumption differ in water and polar aprotic solvents by up to 3 log units from those based on the ferrocene assumption. From data compiled by Kratochvil and Yeager on limiting ionic conductivities in many organic solvents, it is clear that no reference salt can serve for a valid comparison of all solvents. For example, the tetraphenylarsonium and tetraphenyl borate ions have limiting conductivities of 55.8 and 58.3 in acetonitrile. Krishnan and Friedman concluded that the solvation enthalpy of... 

Table 6.4 Limiting Ionic Conductances for Monovalent Cations in Various Solvents at 25°C...

Using the limiting ionic conductances recorded in table 6.4 for the alkali metal ions in acetonitrile, estimate their diffusion coefficients and Stokes radii. 

Determine the acidity constant for this acid and its limiting ionic conductivity. 

Separations of anions are based on differences in electrophoretic flow. Inorganic ions are generally smaller and therefore more mobile than organic ions. The electrophoretic mobilities of inorganic ions are an inverse function of their hydrated ionic radii. Electrophoretic mobility is also affected by the charge on an ion and by the solvent medium. Tables of limiting ionic conductance are a convenient source for estimating electric mobilities of ions. 

This is in stark contrast to the situation where hmiting ionic conductivities are concerned (see Sections 11.11 to 11.13). Here limiting molar conductivities can be split up into individual limiting ionic conductivities for the cation and the anion, so that a table of these can be constmcted, e.g. ... 

It is used for calculating the limiting molar conductivity, JsP, of an electrolyte from tabulated individual limiting ionic conductivities, X°, i.e. for very, very low concentrations. Under such conditions, the law can handle calculations of predicted limiting molar conductivities for both strong and weak electrolytes. 

Values of limiting ionic conductances for some ionic species at 298 K are included in Table 1.4. If conductance values at other temperatures are needed, an approximate correction factor is 77334 iw, where (rw is the viscosity of water at T, in cP. 

Anions are usually less strongly hydrated, as indicated above, and from equation A2.4.38 this would suggest that increasing the charge on the anion should lead unequivocally to an increase in mobility and hence to an increase in limiting ionic conductivity. An inspection of table A2.4.2 shows this to be borne out to some extent by the limited data... 

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