Activity unsymmetric electrolytes

So far we have considered only symmetrical 1 1 electrolytes such as HC1, K.CI, or MgS04. For unsymmetrical electrolytes, the limiting law takes a different form, and different relationships between activity, molality and activity coefficient are obtained. For example, for the 2 1 electrolyte, Na SO,, the dissociation reaction is... 

Section 8.22 and Worked Problems 8.13 and 8.14 derive the expressions for the mean activity coefficient exphcitly for specific electrolytes. This is preferable to simply fitting into a generalised expression which is often quoted for unsymmetrical electrolytes as it makes quite clear exactly what the thermodynamic argument actually is. The generalised expressions are ... 

The difficulties engendered by a hypothetical liquid standard state can be eliminated by the use of unsymmetrically normalized activity coefficients. These have been used for many years in other areas of solution thermodynamics (e.g., for solutions of electrolytes or polymers in liquid solvents) but they have only recently been employed in high-pressure vapor-liquid equilibria (P7). 

The local compostion model is developed as a symmetric model, based on pure solvent and hypothetical pure completely-dissociated liquid electrolyte. This model is then normalized by infinite dilution activity coefficients in order to obtain an unsymmetric local composition model. Finally the unsymmetric Debye-Huckel and local composition expressions are added to yield the excess Gibbs energy expression proposed in this study. 

A second type of ternary electrolyte systems is solvent -supercritical molecular solute - salt systems. The concentration of supercritical molecular solutes in these systems is generally very low. Therefore, the salting out effects are essentially effects of the presence of salts on the unsymmetric activity coefficient of molecular solutes at infinite dilution. The interaction parameters for NaCl-C02 binary pair and KCI-CO2 binary pair are shown in Table 8. Water-electrolyte binary parameters were obtained from Table 1. Water-carbon dioxide binary parameters were correlated assuming dissociation of carbon dioxide in water is negligible. It is interesting to note that the Setschenow equation fits only approximately these two systems (Yasunishi and Yoshida, (24)). 

Unsymmetrical Salts. As an example of unsymmetrical salts, let us consider a salt such as BaCl2, which dissociates into one cation and two anions. By analogy with the case of a uni-univalent electrolyte, we can define the ion activities by the expression... 

Klotz (1964) points out that these definitions of activity for unsymmetrical salts imply new and rather strange standard states for these electrolytes. If we insist on having, for example,... 

Further, since ions are nonvolatile, we must use unsymmetrically normalized activity coefficients the fugacity of a pure volatile electrolyte liquid which is not ionized doesn t tell us anything that would be useful for a dilute aqueous solution where the solute is, at least in part, in ionic form. 

There are many, even recent, examples in the literature of p-type symmetric supercapacitors operating in aqueous [34,37] and in organic electrolytes [34,35,38-41] in which one p-dopable pol)nner, such as polyamline, polypyrrole or poly-3,4-ethylenedioxythiopene, is the active material on both electrodes, and p-type unsymmetric supercapacitors [35,38,41] based on two p-dopable conventional polymers (such as polypyrrole, polythiophene, poly-3-methylthio-phene and polyanUine) selected by virtue of the difference in potential ranges over which they become p-doped, although their performance is not competitive with that of the DLSs on the market. 

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