Activities of Electrolyte Components

For non-electrolytes, we saw that the next step was to define a standard state such that f° was the fugacity of the solute in an ideal one molal solution. Another way of saying this is that f° is the (Henry s Law) constant of proportionality for /ab cx toab 

Using this same line of thought for electrolytes, we choose a standard state such that f° is the constant of proportionality for 

An electrolyte is a compound that splits up into charged particles when dissolved in water. For example, halite, NaCl, splits up into Na and Cr particles (ions) in solution. What would you expect to be the consequences of this for activity and fugacity To examine this let s consider the compound AB which splits up into particles A and B in solution. But before doing that, we should think about what we expect the relationship to be. 

In other words, f° is the (Henry s law) constant of proportionality for a a,., or / a m in very dilute solutions. So we expect fugacity to be directly proportional to concentration in dilute solutions. 

Now consider electrolyte AB, which dissociates into particles A and B. For the moment it doesn t matter whether they are electrically charged or not. In general, there will be an equilibrium constant for the reaction AB° = A -f B, and we can write an equilibrium constant for this (dispensing with activity coefficients for the moment, which just complicate but do not change the point we are getting at). 

So we find that f cxm for a binary electrolyte like HCl or NaCl. In dilute solutions then, 

If we went through the same procedure for an electrolyte which dissociates into three ions (e.g., Na2S04) we would find / a m, and for four ions (e.g., AICI3) we would find / a m, and so on, so that for example, 

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Most of the methods we have described so far give the activity of the solvent. Often the activity of the solute is of equal or greater importance. This is especially true of electrolyte solutions where the activity of the ionic solute is of primary interest, and in Chapter 9, we will describe methods that employ electrochemical cells to obtain ionic activities directly. We will conclude this chapter with a discussion of methods based on the Gibbs-Duhem equation that allow one to calculate activities of one component if the activities of the other are known as a function of composition. 

Thus, the deviation in the behaviour of electrolyte solutions from the ideal depends on the composition of the solution, and the activity of the components is a function of their mole fractions. For practical reasons, the form of this function has been defined in the simplest way possible ... 

Thermodynamic methods also measure the activity coefficient of the solvent (it should be recalled that the activity coefficient of the solvent is directly related to the osmotic coefficient—Eq. 1.1.19). As the activities of the components of a solution are related by the Gibbs-Duhem equation, the measured activity coefficient of the solvent can readily be used to calculate the activity coefficient of a dissolved electrolyte. 

Although potential measurements are used primarily to determine activities of electrolytes, such measurements can also be used to obtain information on activities of nonelectrolytes. In particular, the activities of components of alloys, which are solid solutions, can be calculated from the potentials of cells such as the following for lead amalgam ... 

Beyond any doubt, the electrode/electrolyte interfaces constitute the foundations for the state-of-the-art lithium ion chemistry and naturally have become the most active research topic during the past decade. However, the characterization of the key attributes of the corresponding surface chemistries proved rather difficult, and significant controversy has been generated. The elusive nature of these interfaces is believed to arise from the sensitivity of the major chemical compounds that originated from the decomposition of electrolyte components. 

Hence, the presence of trace impurities, which either pre-exist in pristine electrode and bulk electrolyte or are introduced during the handling of the sample, could profoundly affect the spectroscopic images obtained after or during certain electrochemical experiments. This complication due to the impurities is especially serious when ex situ analytic means were employed, with moisture as the main perpetrator. For cathode/electrolyte interfaces, an additional complication comes from the structural degradation of the active mass, especially when over-delithiation occurs, wherein the decomposition of electrolyte components is so closely entangled with the phase transition of the active mass that differentiation is impossible. In such cases, caution should always be exercised when interpreting the conclusions presented. 

The first step in the MSM is to measure the 7 M Xt of a KCl solution whose concentration is adjusted to he identical to the I of the electrolyte solution for which the activity coefficients are desired. For seawater a KCl solution of Ia 0.7 would he used. One begins with KCl because the K+ and Cl ions interact almost exclusively in an electrostatic manner, without formation of interfering ion pairs and other complexes. Next one assumes that 7 m X6 measured for any solution is the geometric mean of the individual activities of the component ions. The general formula for the geometric mean is... 

By applying equation 3.47 and the principle that electrolyte activities can be expressed as the product of activities of the component ions, equation 3.48 can be reexpressed in a form independent of the nonadsorbing Cl anion ... 

SOLUBILITY OF A WEAK ELECTROLYTE IN SALT SOLUTIONS. Calculation of the solubility of a volatile strong electrolyte, such as HCl, in aqueous salt solutions is straightforward. However, solubilities of weak electrolytes are more difficult to model accurately, since the dissolved speciation must frequently be determined in addition to the activity of the component of interest. Thus, in the case of NH3, the relevant ionic interactions involving NH4 and OH" must be known in addition to parameters for the interaction of dissolved salts with the neutral NH3 molecule. See, for example, the work of Maeda et al. (47) on the dissociation of NH3 in LiCl solutions. 

The approach most frequently used by geochemists over the past several decades for calculating activities of minor components in concentrated salt solutions was suggested by Helgeson (1969). This was an outgrowth of earlier work by other eminent chemists such as Scatchard and Hamed, summarized by Pitzer and Brewer (1961, pp. 326,578 and Appendix 4). The idea is to define a deviation function B ( B-dot ) as the difference between observed and predicted activity coefficients for an electrolyte such as NaCl. This was redefined by Helgeson as... 

An electrode which is reversible to electrons but irreversible to ions is a common situation in both aqueous and solid state electrochemistry. For determinations of ionic conductivity in electrolytes, this type of electrode has proved useful, because the concentrations of majority ionic species do not depend critically on the imposition of a well-defined thermodynamic activity of the electroactive neutral species. Measurements with two irreversible electrodes of a nonreactive metal are then permissible numerous examples are found in the solid-electrolyte literature. Minority electronic transport however, typically depends very strongly on the activity of neutral components, and care must be taken to utilize thermodynamically meaningful experiments to determine minority conductivities. Asymmetric cells using one reversible electrode and one irreversible electrode are then appropriate, but have actually been little explored using ac impedance methods. 

The minority electronic properties of solid electrolytes may vary considerably with changes in composition. It is therefore often necessary to study the minority charge carrier transport as a function of the activity of the components. 

TABLE 3. Variation of Half-Wave Potential Due to Effect of Electrolyte Concentration on Activity of Reaction Components... 

This paper presents a brief review of the literature of nickel-based cermet electrodes for application in solid oxide cells at temperature from 500 to 1000 °C. The applications may be fuel cells or electrolyser cells. Variables that are used for controlling the properties of Ni-cermet-electrodes are (1) Ni/electrolyte volume ratio, (2) additives, e.g. alloying of the Ni or infiltration of the composite with nanoparticles of other elements or compounds, (3) the chemical composition of the electrolyte component and (4) porosity and particle size distribution, which is mainly affected by raw materials morphology, application methods and production parameters such as milling and sintering possibly followed by infiltration of nanosized electrocatalytic active particles. The various electrode properties are deeply related to these parameters, but also much related to the atomic scale structure of the Ni-electrolyte interface, which in turn is affected by segregation of electrolyte components and impurities as well as poisons in the gas phase. 

Oxidation and chlorination of the catalyst are then performed to ensure complete carbon removal, restore the catalyst chloride to its proper level, and maintain full platinum dispersion on the catalyst surface. Typically, the catalyst is oxidized in sufficient oxygen at about 510°C for a period of six hours or more. Sufficient chloride is added, usually as an organic chloride, to restore the chloride content and acid function of the catalyst and to provide redispersion of any platinum agglomeration that may have occurred. The catalyst is then reduced to return the metal components to their active form. This reduction is accompHshed by using a flow of electrolytic hydrogen or recycle gas from another Platforming unit at 400 to 480°C for a period of one to two hours. 

In the deduction of the Law of Mass Action it was assumed that the effective concentrations or active masses of the components could be expressed by the stoichiometric concentrations. According to thermodynamics, this is not strictly true. The rigorous equilibrium equation for, say, a binary electrolyte ... 

Numerous research activities have focused on the improvement of the protective films and the suppression of solvent cointercalation. Beside ethylene carbonate, significant improvements have been achieved with other film-forming electrolyte components such as C02 [156, 169-177], N20 [170, 177], S02 [155, 169, 177-179], S/ [170, 177, 180, 181], ethyl propyl carbonate [182], ethyl methyl carbonate [183, 184], and other asymmetric alkyl methyl carbonates [185], vinylpropylene carbonate [186], ethylene sulfite [187], S,S-dialkyl dithiocarbonates [188], vinylene carbonate [189], and chloroethylene carbonate [190-194] (which evolves C02 during reduction [195]). In many cases the suppression of solvent co-intercalation is due to the fact that the electrolyte components form effective SEI films already at potential which are positive relative to the potentials of solvent co-intercalation. An excess of DMC or DEC in the electrolyte inhibits PC co-intercalation into graphite, too [183]. 

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