Conductivity of electrolytes

Table 8.35 Equivalent Conductivities of Electrolytes in Aqueous Solutions at...

The term equivalent conductance A is often used to describe the conductivity of electrolytes. It is defined as the conductivity of a cube of solution having a cross-section of one square centimeter and containing one equivalent of dissolved electrolyte. 

The search for a suitable electrolyte requires comprehensive studies. It is necessary to measure the conductivities of electrolytes with various solvents, solvent mixtures, and anions over the accessible concentration range of the salts, and to cover a sufficiently large temperature range and the whole composition range of the binary (or ternary) solvent mixture. Figure 11 shows, as an example, the conductivity plot of LiAsF6/GBL as a function of temperature and molality. 

A,. limiting molar conductivity of electrolyte in presence of ligand III.7 ... 

The huge literature on the electronic conductivity of dry conducting polymer samples will not be considered here because it has limited relevance to their electrochemistry. On the other hand, in situ methods, in which the polymer is immersed in an electrolyte solution under potential control, provide valuable insights into electron transport during electrochemical processes. It should be noted that in situ and dry conductivities of conducting polymers are not directly comparable, since concentration polarization can reduce the conductivity of electrolyte-wetted films considerably.139 Thus in situ conductivities reported for polypyrrole,140,141 poly thiophene,37 and poly aniline37 are orders of magnitude lower than dry conductivities.15... 

Barthel, J. Temperature Dependence of Conductance of Electrolytes in Nonaqueous Solutions 13... 

A combination of continuum transport theory and the Poisson distribution of solution charges has been popular in interpreting transport of ions or conductivity of electrolytes. Assuming zero gradient in pressure and concentration of other species, the flux of an ion depends on the concentration gradient, the electrical potential gradient, and a convection... 

These terms are no longer recommended. Instead, we consider the molar conductivities of electrolytes and ions as defined above and where necessary indicate the electrolyte units to which the concentrations refer for example, A(MgCl2) or A( MgCl2), A(Ca"+) or A( Ca +). We evidently have A( MgCl2) = -lAIMgCy. 

In the classical theory of conductivity of electrolyte solutions, independent ionic migration is assumed. However, in real solutions the mobilities Uj and molar conductivities Xj of the individual ions depend on the total solution concentration, a situation which, for instance, is reflected in Kohhausch s square-root law. The values of said quantities also depend on the identities of the other ions. All these observations point to an influence of ion-ion interaction on the migration of the ions in solution. 

It is not usual to talk about the resistance of electrolytes, but rather about their conductance. The specific conductance (K) of an electrolyte is defined as the reciprocal of the resistance of a part of the electrolyte, 1 cm in length and 1 cm2 in cross-sectional area. It depends only on the ions present and, therefore it varies with their concentration. To take the effect of concentration into account, a function called the equivalent conductance, A, is defined. This is more commonly (and conveniently) used than the specific conductance to compare quantitatively the conductivities of electrolytes. The equivalent conductance A is the conductance of that volume of the electrolyte which contains one gram equivalent of the ions taking part in the electrolysis and which is held between parallel electrodes 1 cm apart (units ohm-1 cm4). If V cubic centimeters is the volume of the solution containing one gram equivalent, then the value of L will be 1 cm and the value of A will be V square centimeters, so that... 

A special branch of the theory of strong electrolytes deals with the dependence of the electrical conductivity of electrolytes on concentration (see Section 2.4.3). For very low concentrations, Kohlrausch found empirically that... 

Ohm.cm2 (of visible surface area) or even less. Some visible ways to lower the electrode resistivity are (a) fabricating thin electrodes, (b) increasing the conductivity of electrolyte and electrode material, (c) matching the electrode porosity with the size of species in the electrolyte in order to facilitate the ion transport along the pores, and (d) assembling bipolar devices. 

It is possible however to analyze mathematically well defined models which we hope will give a correct approximation to real physical systems. In this section, we shall be concerned with the simplest case the zeroth-order conductance of electrolytes in an infinitely dilute solution. We shall describe this situation by assuming that the ions—which are so far from each other that their mutual interaction may be completely neglected—have a very large mass with respect to the solvent molecules we are then confronted with a typical Brownian motion problem. 

We have now established the complete equivalence between the two methods of sections (A) and (C). However one question remains unanswered it is concerned with the applicability of the present approach to the conductance of electrolytes. 

In the previous section, we verified that the screened Debye potential (314) does indeed give a satisfactory approximation for the collective interaction in the conductance of electrolytes. However, we have not yet considered the effect of the interactions with the solvent. 

In the four preceding sections, we have developed various approximations for the relaxation term in the limiting law for the conductance of electrolytes, starting from the generalized transport equation (111). 

We can recognize four main periods in the history of the study of aqueous solutions. Each period starts with one or more basic discoveries or advances in theoretical understanding. The first period, from about 1800 to 1890, was triggered by the discovery of the electrolysis of water followed by the investigation of other electrolysis reactions and electrochemical cells. Developments during this period are associated with names such as Davy, Faraday, Gay-Lussac, Hittorf, Ostwald, and Kohlrausch. The distinction between electrolytes and nonelectrolytes was made, the laws of electrolysis were quantitatively formulated, the electrical conductivity of electrolyte solutions was studied, and the concept of independent ions in solutions was proposed. 

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