Electrolyte solutions ionic conductivity

This book focuses on three types of nonaqueous systems—liquid electrolyte solutions, ionically conducting polymers, and molten salts—with emphasis on the more commonly used liquid systems. It provides a review of a variety... 

Ionic conductors arise whenever there are mobile ions present. In electrolyte solutions, such ions are nonually fonued by the dissolution of an ionic solid. Provided the dissolution leads to the complete separation of the ionic components to fonu essentially independent anions and cations, the electrolyte is tenued strong. By contrast, weak electrolytes, such as organic carboxylic acids, are present mainly in the undissociated fonu in solution, with the total ionic concentration orders of magnitude lower than the fonual concentration of the solute. Ionic conductivity will be treated in some detail below, but we initially concentrate on the equilibrium stmcture of liquids and ionic solutions. 

Many of the reactions that you will study occur in aqueous solution. Water is called the universal solvent, because it dissolves so many substances. It readily dissolves ionic compounds as well as polar covalent compounds, because of its polar nature. Ionic compounds that dissolve in water (dissociate) form electrolyte solutions, which conduct electrical current owing to the presence of ions. The ions can attract the polar water molecules and form a bound layer of water molecules around themselves. This process is called solvation. Refer to the Solutions and Periodicity chapter for an in-depth discussion of solvation. 

A solvent, in addition to permitting the ionic charges to separate and the electrolyte solution to conduct an electrical current, also solvates the discrete ions, by ion-dipole or ion-induced dipole interactions and by more direct interactions, such as hydrogen bonding to anions or electron-pair donation to cations. Lewis acidity and basicity of the solvents affect the latter. The redox properties of the ions at an electrode depend on their being solvated, and the solvation effects electrode potentials or polarographic half-wave potentials. 

Electrode kinetics can be regarded as a switch from electronic to ionic conductivity at the electrochemical interface [14]. The contact of two phases such as the electrode (in most cases an electronic conductor) and the electrolytic solution (ionic conductor) determines the structure and properties of the electrochemical interface. 

A galvanic cell consists of two electrodes, or metallic conductors, that make electrical contact with the contents of the cell, and an electrolyte, an ionically conducting medium, inside the cell. The electrolyte is typically an aqueous solution of an ionic compound, although advanced cells make use of all kinds of exotic materials (see Box 12.1). 

The ionic conductivity (A) is an important transport property of electrolyte solution. This conductivity depends on the concentration of electrolyte in a nontrivial way. The simplest expression for this dependence is given by the celebrated Debye-Huckel-Onsager law, which is given as follows ... 

Standard KCl Solutions for Calibrating Conductivity Cells Molar Conductivity of Aqueous HF, HCl, HBr, and HI Equivalent Conductivity of Electrolytes in Aqueous Solution Ionic Conductivity and Diffusion at Infinite Dilution Activity Coefficients of Acids, Bases, and Salts... 

Thus, let us consider an alloy of tantalum and molybdenum comprising a solid solution. We wish to determine the activity of the tantalum in that solution at 1300 K. For this, we use a solid electrolyte with ionic conduction, which is a solid solution of thorium oxide Th02, containing 6% mass of yttrium oxide (Y2O3). We construct the following cell ... 

A gel polymer electrolyte has been prepared by mixing a high-molecular-weight PMMA with clay followed by adding plasticizer (1 M LiC104 solution in EC/PC). The polymer matrix is embedded between the clay layers, leading to an increase in Tg. The solvent can permeate into the clay and does not separate from the clay in this gel polymer electrolyte. The ionic conductivity increases with an increasing amount of clay, up to a clay content of... 

Electrolyte Solution that conducts ionic current via ionic transport (ions of dissolved salts, mineral acids, or strong bases). 

At low currents, the rate of change of die electrode potential with current is associated with the limiting rate of electron transfer across the phase boundary between the electronically conducting electrode and the ionically conducting solution, and is temied the electron transfer overpotential. The electron transfer rate at a given overpotential has been found to depend on the nature of the species participating in the reaction, and the properties of the electrolyte and the electrode itself (such as, for example, the chemical nature of the metal). 

When two conducting phases come into contact with each other, a redistribution of charge occurs as a result of any electron energy level difference between the phases. If the two phases are metals, electrons flow from one metal to the other until the electron levels equiUbrate. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode—solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equiUbrium. 

Electrolyte a substance, liquid or solid, which conducts electrical current by movement of ions (not of electrons). In corrosion science, an electrolyte is usually a liquid solution of salts dissolved in a solvent, or a molten salt. The term also applies to polymers and ceramics which are ionically conductive. 

When an ionic solution contains neutral molecules, their presence may be inferred from the osmotic and thermodynamic properties of the solution. In addition there are two important effects that disclose the presence of neutral molecules (1) in many cases the absorption spectrum for visible or ultraviolet light is different for a neutral molecule in solution and for the ions into which it dissociates (2) historically, it has been mainly the electrical conductivity of solutions that has been studied to elucidate the relation between weak and strong electrolytes. For each ionic solution the conductivity problem may be stated as follows in this solution is it true that at any moment every ion responds to the applied field as a free ion, or must we say that a certain fraction of the solute fails to respond to the field as free ions, either because it consists of neutral undissociated molecules, or for some other reason ... 

An element of uncertainty is introduced into the e.m.f. measurement by the liquid junction potential which is established at the interface between the two solutions, one pertaining to the reference electrode and the other to the indicator electrode. This liquid junction potential can be largely eliminated, however, if one solution contains a high concentration of potassium chloride or of ammonium nitrate, electrolytes in which the ionic conductivities of the cation and the anion have very similar values. 

Recent developments of the chemical model of electrolyte solutions permit the extension of the validity range of transport equations up to high concentrations (c 1 mol L"1) and permit the representation of the conductivity maximum Knm in the framework of the mean spherical approximation (MSA) theory with the help of association constant KA and ionic distance parameter a, see Ref. [87] and the literature quoted there in. 

Figure 13. Schematic diagram of the measurement of the ionic conductivity of a conducting polymer membrane as a function of oxidation state (potential), (a) Pt electrodes (b) potentiostat (c) gold minigrid (d) polymer film (e) electrolyte solution (0 dc or ac resistance measurement.133 (Reprinted with permission from J. Am Chem Soc. 104, 6139-6140, 1982. Copyright 1982, American Chemical Society.)...

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