Standard Gibbs energies of ion transfer

A relative scale of the standard Gibbs energies of ion transfer or the standard ion transfer potentials can be established based on partition and solubility measurements. The partition eqnilibrium of the electrolyte can be characterized by a measnrable parameter, the partition coefficient P x-... 

The standard Gibbs energy of electrolyte transfer is then obtained as the difference AG° x ° = AG° ° - AG° x. To estabfish the absolute scale of the standard Gibbs energies of ion transfer or ion transfer potentials, an extrathermodynamic hypothesis must be introduced. For example, for the salt tetraphenylarsonium tetraphenyl-borate (TPAs TPB ) it is assumed that the standard Gibbs energies of transfer of its ions are equal. 

Table 2.1. Values o f standard Gibbs energies of ion transfer from water to nitrobenzene in electron volts. From P. Vanysek, Thesis, J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Prague (1982).

Gibbs energy of ion and dipole transfer — The standard -> Gibbs energy of ion transfer (see also -> ion transfer at liquid-liquid interfaces) can be represented as the difference of two -> solvation energies A Gf = A acGA -... 

Cyclic voltammetry has been used mainly for the determination of the standard ion-transfer potential Aq (or the standard Gibbs energy of ion transfer A ttx °), and e ion diffusion coefficient. The Figure shows an example of the cyclic voltammogram for the Cs+ ion-transfer reaction at ITIES in the electrochemical cell... 

Standard Gibbs energies of ion transfer are reported for various ions in Tables 20.10 and 20.11. Values are given for the two most common solvents used with water to form immiscible electrolyte boundaries, namely nitrobenzene and 1,2-dichloroethane. 

Table 1. Standard Gibbs energies of ion transfer and corresponding standard Galvani potentials in the water-nitrobenzene systems...

Updated lists of the values of standard Gibbs energies of transfer for various individual ions are given in Professor Hubert H. Girault s website at the EPFL of Switzerland (36). More valnes can be found in a data survey of Gibbs energy of ion transfer for 57 different solvents pubhshed by lUPAC (37). Table 17.3.1 lists some data for W/NB and W/DCE systems. 

TABLE 32.2 Standard Gibbs Energy of Transfer and Standard Ion Transfer Potentials for Ion Transfer Between Water and Nitrobenzene Derived from Partition Measurements... 

This dependence is fundamental for electrochemistry, but its key role for liquid-liquid interfaces was first recognized by Koryta [1-5,35]. The standard transfer energy of an ion from the aqueous phase to the nonaqueous phase, AGf J, denoted in abbreviated form by the symbol A"G is the difference of standard chemical potential of standard chemical potentials of the ions, i.e., of the standard Gibbs energies of solvation in both phases. 

TABLE 4 Standard Gibbs Energies of Transfer of Ions from NB to W and Their Charge-Independent and Charge-Dependent Components at 25°C... 

It is important to notice that the standard Gibbs energy of transfer refers to the transfer from pure w to pure organic o. It is therefore different from the Gibbs energy of partition, which refers to the transfer between mutually saturated solvents. Nevertheless, in the case of solvents of low miscibility such as water-DCE or water-nitrobenzene, the transferred ion is practically not hydrated by water present in the organic phase, so that... 

Although the inner potential difference is not measurable in principle, it would be useful to have at least good estimates. We can see from Eq. (12.3) that this problem is equivalent to determining the difference in the chemical potential of individual ions. If we knew the standard Gibbs energies of transfer of the ions ... 

Standard Gibbs energy of the ion transfer Electric current, A... 

It is apparent that the deviation from Nemstian behaviour depends on the activity of the determinand and anion B in the studied solution. It decreases with increasing magnitude of the sum of the standard Gibbs energies of transfer of ions J and B " from water into the membrane phase. The effect of the interfering anion is suppressed by increasing the concentration of the ion-exchanger ion in the membrane. 

The use of ISEs in non-aqueous media(for a survey see [125,128]) is limited to electrodes with solid or glassy membranes. Even here there are further limitations connected with membrane material dissolution as a result of complexation by the solvent and damage to the membrane matrix or to the cement between the membrane and the electrode body. Silver halide electrodes have been used in methanol, ethanol, n-propanol, /so-propanol and other aliphatic alcohols, dimethylformamide, acetic acid and mixtures with water [40, 81, 121, 128]. The slope of the ISE potential dependence on the logarithm of the activity decreases with decreasing dielectric constant of the medium. With the fluoride ISE, the theoretical slope was found in ethanol-water mixtures [95] and in dimethylsulphoxide [23], and with PbS ISE in alcohols, their mixtures with water, dioxan and dimethylsulphoxide [134]. The standard Gibbs energies for the transfer of ions from water into these media were also determined [27, 30] using ISEs in non-aqueous media. 

In contrast to ISEs with neutral ion carriers in the membrane, not even qualitative rules have been formulated for the solvent effect on the behaviour of ISEs with ion-exchanger ions in a liquid membrane. A basic condition for the ion-exchanger ions is that they be strongly hydrophobic. It must hold for the standard Gibbs energy of transfer of the ion-exchanger ion X and the deter-minand Y that... 

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