The properties of electrolyte solutions

It will already be clear that the properties of the electrolyte solution within the electrolysis cell contribute to its characteristics. Most obviously the conductivity determines the cell resiscance but, in addition, the properties of the solvent and the electrolyte determine their interaction with the electroactive species and thereby influence the electrode reactions. An extensive discussion of the physical chemistry of electrolyte solutions is outside the scope of this book and this section only seeks to emphasize the most important features. 

The conductivity of an electrolyte solution is a key property. It is easily measured with an ax. device and its detailed study is responsible for much of our knowledge of electrolyte solutions. The electrolytic conductivity k is calculated from the resistance of the solution between two electrodes, area A and separation S  

Some illustrative conductivities are shown in Table 1A The table reports values for electrode materials, aqueous solutions and some lithium salts in various solvents. Of course, electrode materials are partly selected because of their high conductivity. However, some values arc reported here to note the variation of conductivity between the highly conducting metals, Oi and AI, and some other metals, particularly Hg, and the relatively poorly conducting graphite. Also, the large diflerence in conductivity between electrode materials and electrolyte solutions and between the aqueous and non-aqeuous solutions should be noted. 

Non-aqueous solvents are unable to solvate ions to the same extent as water and, hence, there is commonly incomplete ionization. When comparing solvents , two factors should be considered (1) the ability of the solvent to interact with ions (2)i the viscosity, since it determines the ease with which ions may move through the solution. It is, for example, the second factor which causes the low conductivity of propylene carbonate solutions despite its high value of dielectric 

I mol dm LiCI in HjO 1 moldm LiCIO in HjO 0.66 mol dm LiCIO in propylene carbonate 

Further advances in the analysis of conductivity data are usually made by defining a molar conductivity  

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In the last two decades experimental evidence has been gathered showing that the intrinsic properties of the electrolytes determine both bulk properties of the solution and the reactivity of the solutes at the electrodes. Examples covering various aspects of this field are given in Ref. [16]. Intrinsic properties may be described with the help of local structures caused by ion-ion, ion-solvent, and solvent-solvent interactions. An efficient description of the properties of electrolyte solutions up to salt concentrations significantly larger than 1 mol kg 1 is based on the chemical model of electrolytes. 

The parameters of molar conductivity of the electrolyte, A = a/c,, and molar conductivity of ions, Xj = ZjFuj (units S cm /mol), are also used to describe the properties of electrolyte solutions (A is used only in the case of binary solutions). With Eq. (1.14), we can write for a binary solution... 

The second period, from 1890 to around 1920, was characterized by the idea of ionic dissociation and the equilibrium between neutral and ionic species. This model was used by Arrhenius to account for the concentration dependence of electrical conductivity and certain other properties of aqueous electrolytes. It was reinforced by the research of Van t Hoff on the colligative properties of solutions. However, the inability of ionic dissociation to explain quantitatively the properties of electrolyte solutions was soon recognized. 

ELECTRONEUTRALITY, If one describes the properties of electrolytic solutions in terms of ionic species, one has to take account of the fact that the concentrations of all species are not independent because the solution as a whole is neutral. 

Solutions are usually classified as nonelectrolyte or electrolyte depending upon whether one or more of the components dissociates in the mixture. The two types of solutions are often treated differently. In electrolyte solutions properties like the activity coefficients and the osmotic coefficients are emphasized, with the dilute solution standard state chosen for the solute.c With nonelectrolyte solutions we often choose a Raoult s law standard state for both components, and we are more interested in the changes in the thermodynamic properties with mixing, AmjxZ. In this chapter, we will restrict our discussion to nonelectrolyte mixtures and use the change AmjxZ to help us understand the nature of the interactions that are occurring in the mixture. In the next chapter, we will describe the properties of electrolyte solutions. 

The three appendices in this volume give selected sets of thermodynamic data (Appendix 5), review the statistical calculations covered in Principles and Applications (Appendix 6), and summarize the equations and parameters required to calculate the properties of electrolyte solutions, principally from Pitzer s equations (Appendix 7). 

Barthel, J. J. Temperature dependence of the properties of electrolyte solutions I a semi-phenomenological approach to an electrolyte theory including short range forces. Ber. Bunsen Ges. Phys. Chem. 1979, 83, 252-257. 

The basic effects responsible for the properties of electrolyte solutions are ion solvation, ion 2issociation to ion pairs and higher ion aggregates with and without inclusion of solvent molecules. FTIR (Fourier transform infrared) and MW (microwave) spectra are a valuable source of information on ion-solvent and ion-ion interactions and yield factual knowledge on the structure and dynamics of electrolyte solutions. The efficiency of these methods is exemplified for solvation in aptotic and protic solvents, hydrophobic solvation, association to charged and neutral ion aggregates, and stability of ion pairs. 

In addition to the short-range interactions between species in all solutions, long-range electrostatic interactions are found in electrolyte solutions. The deviation from ideal solution behavior caused by these electrostatic forces is usually calculated by some variation of the Debye-Huckel theory or the mean spherical approximation (MSA). These theories do not include terms for the short-range attractive and repulsive forces in the mixtures and are therefore usually combined with activity coefficient models or equations of state in order to describe the properties of electrolyte solutions. 

In this section we consider the application of the concept of ion association to describe the properties of electrolyte solutions within the ion or McMillan-Mayer level approach. In this approach the effects of solvent molecules are taken into account by introducing the dielectric constant into Coulomb interaction law and by appropriately choosing the short-range part of ion-ion interactions. To simplify, we consider here the restrictive primitive model (RPM)... 

In this section we consider the possibility of applying the ion association concept to the description of the properties of electrolyte solutions in the ion-molecular or Born-Oppenheimer level approach. The simplest ion-molecular model for electrolyte solution can be represented by the mixture of charged hard spheres and hard spheres with embedded dipoles, the so-called ion-dipolar model. For simplification we consider that ions and solvent molecules are characterized by diameters R and Rs, correspondingly. The model is given by the pair potentials,... 

The above procedure for determining activity coefficients for solid solutes is often applied to electrolytes. This important class of solutes always behaves non-ideally. The properties of electrolyte solutions are considered in detail in chapter 3. 

In the present chapter, the properties of electrolyte solutions in water are discussed in detail. Initially the solvation of ions in infinitely dilute solutions is considered on the basis of the Born theory. Then, the Debye-Hiickel model for... 

The first term on the right-hand side of equation (3.8.10) gives the sum of the ionic charges in the solution, which must add to zero. From the second term, one defines an important quantity used in assessing the properties of electrolyte solutions, namely, the ionic strength, 7. The definition is... 

In conclusion, the MSA provides an excellent description of the properties of electrolyte solutions up to quite high concentrations. In dilute solutions, the most important feature of these systems is the influence of ion-ion interactions, which account for almost all of the departure from ideality. In this concentration region, the MSA theory does not differ significantly from the Debye-Hiickel model. As the ionic strength increases beyond 0.1 M, the finite size of all of the ions must be considered. This is done in the MSA on the basis of the hard-sphere contribution. Further improvement in the model comes from considering the presence of ion pairing and by using the actual dielectric permittivity of the solution rather than that of the pure solvent. 

Emphasis has been placed on the electrical aspects of interfaces in this chapter. This is especially important in examining the properties of electrolyte solutions at... 

In this chapter we discuss some of the properties of electrolyte solutions. In Sec. 12-1, the chemical potential and activity coefficient of an electrolyte are expressed in terms of the chemical potentials and activity coefficients of its constituent ions. In addition, the zeroth-order approximation to the form of the chemical potential is discussed and the solubility product rule is derived. In Sec. 12-2, deviations from ideality in strong-electrolyte solutions are discussed and the results of the Debye-Hiickel theory are presented. In Sec. 12-3, the thermodynamic treatment of weak-electrolyte solutions is given and use of strong-electrolyte and nonelectrolyte conventions is discussed. 

The series of simultaneous equilibria including Ki, Kq (or K ) and the autoprotolysis constant Ks limits the quantitative discussion of electrolyte solutions to simple cases. However, the appropriate choice of such cases will give valuable insight into the properties of electrolyte solutions, especially those of ionophores where the ionisation step need not be considered. 

Reliable data of organic solvents are needed for the control of purity as well as for the parameters in the equations expressing the concentration dependence of the properties of electrolyte solutions. Table A-I contains a selection of currently used solvents which are arranged in classes according to the principles given in Section II, cf. also Table I. Data are given for 25 °C if not indicated otherwise. 

Stabilization of cations and/or anions by the solvent molecules (solvation) is essential for the comprehension of the properties of electrolyte solutions. The mean number of solvating particles, n or m in Eiqs. (8), depends on the nature of the solvent and solute and is specific for every solution. 

Despite such limitations as the overlapping of solvent classes or possible interactions evading the unambiguous classification of a solvent, such classifications are useful for understanding the properties of electrolyte solutions and for rationalizing the choice of appropriate solvents and solvent mixtures for particular investigations. 

Nikolic ND, Brankovic G, Pavlovic MG, Popov KI (2008) The effect of hydrogen codeposition on the morphology of copper electrodeposits II. Correlation between the properties of electrolytic solutions and the quantity of evolved hydrogen. J Electroanal Chem 621 13-21... 

Most of the material in this chapter is quite general, and can be applied to any kind of solution, although most of our examples are for aqueous solutions. The properties of electrolyte solutions introduce complications, discussed in Chapter 15. The properties of real gaseous solutions are often handled by equations of state, the subject of Chapter 13, and those of solid solutions have some unique aspects, discussed in Chapter 14. 

Relationship between Adhesion and Certain Parameters Characterizing Properties of Electrolyte Solutions. In certain cases, particle adhesion may be related directly to parameters characterizing the properties of electrolyte solutions. 

H15. Young, T.F. L.A. Blatz, "The variation of the properties of electrolytic solutions with degrees of dissociation", Chem. Rev., v44 (1949)... 

Barthel J, Gerber R, Gores HI (1984) The temperature dependence of the properties of electrolyte solutions. VI. Triple ion formation in solvents of low permittivity exemplied by lithium tetra uoroborate solutions in dimethoxyethane. Ber Bunsenges 88 616-622... 

Barthel J, Stroedm U, Iberl L, Hauuner H (1982) The temperature dependence of the properties of electrolyte solutions. TV. Determination of cationic transference numbms in methanol, ethanol, propanol, and acetonitrile at various temperatures. Ber Bunsen-Ges 86 636-645... 

Computer modeling of electrochemical interfaces has become a well-established branch of interfacial electrochemistry. In recent years the interest shifted from studies of pure water near smooth model walls to increasingly realistic models of the metal phase (e. g., [1-11]), the properties of electrolyte solutions near the interface (e. g., [12-24]), and to free energy studies within the framework of the Marcus theory of electron transfer (e. g., [20, 25-27]), partial charge transfer [28] and ion transfer reactions [29]. 

There are many physical properties of solvents for electrolytes that have been compiled ([1-3] and elsewhere) but are of less interest in the context of the solvation of the ions and the properties of electrolyte solutions in the solvents. Such properties include the critical temperature, pressure, and density on the one hand, when the solvent is heated, and the glass transition temperature, when the solvent is cooled and forms a glass on the other hand. The glass is homogeneous and isotropic and has a viscosity >10 Pa s, but is not in internal equilibrium. 

The Pitzer model includes a modified Debye-Hiickel-like contribution and a virial term to take short range interactions into account. Only two parameters having physical meaning must be adjusted. Accurate results have been obtained for the properties of electrolyte solutions attaining molalities up to 6 moles/kg of solvent. Activity coefficients have been calculated from this model for solutions containing different salts. They have been correctly predicted for solutions of NaCl, KCl and CaCl2 (from [DEM 91]). 

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