Transfer Gibbs energy

The voltammetric information given here suggests that the transfer of an objective cation from Wl to LM can be achieved under a smaller membrane potential when an anion for which the Gibbs transfer energy at the LM/W2 interface is smaller is added into W2. In the case of the above-mentioned membrane system, the transfer of K+ from Wl to LM in the presence of 0.01 M MgBr2 in W2 is expected to be attained even at the membrane potential 0.19 V (which corresponds to the Gibbs energy of transfer of 18.3... 

Gibbs transfer energy of an ion i from phase a to p AG g Gibbs energy for ion-solvent interaction in phase a A log P partition coefficient difference between two solvent systems A 0 Galvani potential difference between a and p phases Ag(pi/2 half-wave potential... 

This coefficient has various names (medium effect, solvation activity coefficient, etc.) the name recommended by the responsible IUPAC commission is the transfer activity coefficient. In this book the effect of solvation in various solvents will be expressed exclusively in terms of standard Gibbs transfer energies. 

In electrolytes, the ionic and the overall Gibbs transfer energies must be distinguished. These quantities are defined in the usual manner. For example, for the most usual type of electrolyte AB— A 4- B,... 

As separation of the standard Gibbs transfer energy of the electrolyte as a whole into the individual contributions for the anion and the cation can, of... 

The values of the standard Gibbs transfer energies for H+ then determine the solvent affinity for protons. 

Table 3.6 Standard Gibbs transfer energies and standard electric potential differences of transfer... 

The membrane phase m is a solution of hydrophobic anion Ax (ion-exchanger ion) and cation Bx+ in an organic solvent that is immiscible with water. Solution 1 (the test aqueous solution) contains the salt of cation Bx+ with the hydrophilic anion A2. The Gibbs transfer energy of anions Ax and A2 is such that transport of these anions into the second phase is negligible. Solution 2 (the internal solution of the ion-selective electrode) contains the salt of cation B with anion A2 (or some other similar hydrophilic anion). The reference electrodes are identical and the liquid junction potentials A0L(1) and A0L(2) will be neglected. 

The standard Gibbs transfer energy can only be found thermodynamically, for example, by using distribution equilibria, both for nonelectrolytes and for electrolytes as a whole. The distribution coefficient for substance X between phases a and /3 is given by the equation... 

The most important applications of Cu ISEs are in the direct determination of Cu " in water [169, 372,410], complexometric titration of various metal ions using Cu " as an indicator [30, 143,269, 385] and complexometric titrations of Cu " [409]. This ISE has also been used in the determination of the equilibrium activity of Cu in various Cu complexes in order to determine the stability constants (see [46, 285, 317, 318,427, 445]), in the determination of the solubility of poorly soluble salts [122] and in the determination of the standard Gibbs transfer energies [58]. It can also be used in concentrated electrolytes [170]. 

Fig. 7.1. Dependence of difference between standard Gibbs transfer energies on the logarithm of the selectivity coefficient for an ISE based on 2-nitro-p-cymene. (After Scholler and Simon [185].)...

The required Gibbs transfer energies, AGj) jj, can be obtained from activity coefficient measurements according to Eq. (5-8a),... 

Observed standard molar Gibbs transfer energies 4G , at 25°C for Cs ion, based on the TATB assumption (see Table 4). 

Further inspection of the agreement of the SL model with observed Gibbs transfer energies suggests the involvement of underlying donor-acceptor factors. Previously, it was found for chloride ion transfer into various... 

In the preceding section, factors that were theoretically connected with ion-transfer processes included ion radius, solvent dielectric constant, and solvent molar volume. Failure to account totally for the standard molar Gibbs transfer energies led to the suggestion that donor-acceptor properties of the solvent could be used to augment the electrostatic approach. In this section, we briefly describe statistical approaches that have been employed to elucidate the major factors governing ion transfer. The approaches that will be treated here rely on linear solvation energy relationships of the form... 

According to Parker [6] the standard Gibbs energy for transfer of the tetraphenyl-arsonium ion (TPAs ) and of the tetraphenylborate ion (TPB"") are equal for any pair of solvents. The standard Gibbs energy for transfer of the TP As and TPB ions can be determined from the distribution coefficients between any pair of immiscible solvents. If the distribution coefficient for the TP As A salt is found for any arbitrary ion A , then its standard Gibbs transfer energy is... 

By means of this approach we can define the electrical potential difference between the phases w and o, in spite of the fact that they are chemically different. We can also Kst the standard Gibbs transfer energies of individual ions as well as the standard electrical potential differences between the phases concerned for these ions in the same way as, for example, the hydrogen scale for standard electrode potential is composed. 

Besides determination of data of theoretical interest such as standard Gibbs transfer energies, distribution coefficients and stability constants, voltammetry at ITIES has found the following applications ... 

Figure 5. Variation of the Gibbs energy of activation for tetralkylammonium transfer (obtained from Fig. 6 of Ref. 5) as a function of the Gibbs transfer energy calculated from the values oi k and kl (given in Table I of Ref. 5) using the relation A(j = RT n k lkl).

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