Ionic Distribution Diagrams

the separation limit between the anionic form in water and the organic solvent (c = c° ) is a horizontal straight line. As in the Pourbaix diagrams, the 

Finally, the line separating the neutral acid in water and the anion A in the organic phase is given by including the acidity constant in Eqnation 2.58 (Chapter 2) to give 

As in the Pbnrbaix diagrams, we obtain a delimiting line that depends on the pH as shown in Fignre 1.18. If the AH acid is lipophilic, we have to take into account the distribution of the acid in the organic phase  

This equation shows that, to extract an organic acid to an aqueous phase, one should work at a pH higher than that given by Equation 1.68. The separation limit between the two ionic forms is still the one given by the Nemst equation for the 

The drawing of an ionic partition diagram requires voltammetric measurements at different pH values with equilibrated solutions. This means that a new electrochemical cell has to be prepared for each pH. Different approaches have been proposed to make the measurement more automated. One approach is based 

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The concept of assisted ion transfer is, of course, applicable to proton-transfer reactions assisted by the presence of an acid or base, hydrophilic or lipophilic. As pioneered by Kontturi and Murtomaki [165], voltammetry at ITIES has proved to be an excellent method to measure the log values of protonated or deprotonated molecules. Indeed, for therapeutic molecules, the logF values, which are related the Gibbs energy of transfer as shown by Equations 1.11 and 1.12, provide an important physical parameter to assay the toxicity of a molecule. If a molecule is lipophilic, that is, logF >2, it is potentially toxic. In fact, with the concept of ionic distribution diagrams (vide infra) it is even possible to measure the logF values of the neutral associated bases. The application of voltammetry at ITIES to the study of therapeutic molecules has been one of the success stories of electrochemistry at liquid-liquid interfaces. The field has been reviewed over the years [166,167] and very recently by Gulaboski et al. [168]. 

Similar equations can be used to draw the ionic distribution diagram of a base as illustrated in Figure 1.18. To draw ionic distribution diagrams, one should measure by voltammetry the half-wave potentials for the different ion-transfer and assisted-ion-transfer reactions. 

The present authors studied the extraction of aromatic amines into ILs. As is seen from experimental data for [C4CiIm][PFg] (Figure 9.2), aniline, napthylamine, and o-toluidine are efficiently extracted from the alkaline aqueous solution. Thus, as in the case of phenols, neutral (molecular) forms of solutes were extracted. Another example of the same behavior is given by many polyfunctional compounds, for example, 8-hydroxyquinoline (Figure 9.3 presents a comparison of extraction pH-profile with the distribution diagram for ionic forms of the solute). 

Figure 9.3 Distribution diagram for the ionic forms of 8-hydroxyquinoline in comparison with pH-profile of its extraction into [C4CjIm][Pp5].

FIGURE 5.1 Distribution diagram showing the formation of vanadate and peroxovanadate species as a function of the concentration of hydrogen peroxide and of pH. Conditions for the simulation 2mmol/L total vanadate 0.1 tmol/L to 10 mmol/L total hydrogen peroxide 0.15 mol/L ionic strength with NaCl pH values, as indicated. The formation constants are from reference 11. 

Figure 6 Distribution diagrams of poly molybdate species in two different ionic media. The curves are labeled with p,q values as defined in equation (4) (With kind permissions of Klnwer Academic Publishers)...

Reymond, F. Steyaert, G. Carrupt, P.A. Testa, B. Girault, H. Ionic partition diagrams a potential-pH representation. J. Am. Chem. Soc. 1996,118, 11,951-11,957. Tsantili-Kakoulidou, A. Panderi, I. Csizmadia, F. Darvas, F. Prediction of distribution coefficient from structure, II. Validation of prologD, an expert system. J. Pharm. Sci. 1997, 86, 1173-1179. 

Liquid hydrogen chloride will dissolve many ionic compounds. Diagram how molecules of hydrogen chloride tend to distribute themselves about a negative ion and about a positive ion in such solutions. 

Figure 3 Distribution diagrams of polymolybdate species in two different ionic media (reproduced by permission of Kluwer Academic Press from Pettersson, L. Mol. Eng. 1993, 3, 29-42).

There is a report on the colombic force field of a polyelectrolyte gel based on the analysis of dielectric relaxation spectra. High electron density of a polymer ion forms an extremely strong coulombic field in its vicinity (see Fig. 5) [11]. This distribution diagram is obtained by the numerical calculation based on the Poisson-Boltzmann equation. An ionic polymer gel possesses a static potential well. The coimter ions that dissociated firom the polymer ions then gather arormd them and form a restricted phase. Unlike free ions, these restricted cormter ions show dielectricity. From dielectric relaxation spectra, the insight on the coulombic force field around the polymer ions and microscopic morphology of the gel can be obtained [12-15]. 

In qualitative terms, microscopic interactions are caused by differences in crystal chemical properties of trace element and carrier, such as ionic radius, formal charge, or polarizability. This type of reasoning led Onuma et al. (1968) to construct semilogarithmic plots of conventional mass distribution coefficients K of various trace elements in mineral/melt pairs against the ionic radius of the trace element in the appropriate coordination state with the ligands. An example of such diagrams is shown in figure 10.6. 

Figure 10.6 Onuma diagrams for crystal/melt trace element distributions. Ionic radii of Whittaker and Muntus (1970). (A) Augite/matrix distribution, data of Onuma et al.

FIGURE 5.2 Diagram showing the distribution of peroxovanadium species as a function of pH. Simulation conditions total vanadate, 2.0 mmol/L total hydrogen peroxide, 4.0 mmol/L ionic strength, 0.15 mol/L with NaCl pH range, 1 to 10. Formation constants are from reference 11. 

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