Acid base permittivity effect

The classification of solvents has been dealt with in various books on non-aque-ous solvents [25, 26]. In the classification of solvents, it is usual to use some solvent properties as criteria. In order to discuss solvent effects on chemical reactions, it is convenient to use relative permittivities and acid-base properties as the criteria. 

Ionic equilibria in solvents of low relative permittivity have been described in the literature and equations for calculating titration curves have also been proposed. The equations involve the partial dissociation of the acids, bases, and salts in these media, and the effect of the activity coefficients. The equations can be applied to pH and titration curve computation in solvents such as t-butyl and isopropyl alcohols, ethylene diamine, pyridine, or tetrahydrofuran. 

Here, A denotes an add of type HA-, HA or BH+, and z and q denote the charge and the radius, respectively, of species i. The influence of permittivity on p/C, depends on the charges, radii and the charge locations of the add and its conjugate base. Table 3.3 shows the pKa values of some acids and add-base indicators in water, methanol and ethanol [3], The solvent effects on pK l are smaller for BH+-type adds than for HA- or HA-type acids. For the BH+-type acids, zA=l and zB=0 in Eq. (3.16), and the influence of solvent permittivity is expeded to be small. 

Eq. (4-10) can be used only for solvents of equal acid and base strength, because only the effect of the solvent relative permittivity on the degree of ionization is considered. Under these conditions, Eq. (4-10) predicts that the logarithm of the ionization constant of HA should be inversely proportional to the relative permittivity of the solvent in which HA is dissolved. However, one has to take into account the fact that the relative permittivities near solute ions can differ considerably because of the effect of dielectric saturation, which hinders the precise calculation of electrostatic interactions. Because of these restrictions, Eq. (4-10) can be expected to yield only semiquantitative results. Nevertheless, it allows us to predict qualitatively how the charge type of an acid affects the ionization constant in solvents of different relative permittivities. 

In capillary electrophoresis (CE), several criteria can be applied to classify solvents [e.g., for practical purposes based on the solution ability for analytes, on ultraviolet (UV) absorbance (for suitability to the UV detector), toxicity, etc.]. Another parameter could be the viscosity of the solvent, a property that influences the mobilities of analytes and that of the electro-osmotic flow (EOF) and restricts handling of the background electrolyte (BGE). For more fundamental reasons, the dielectric constant (the relative permittivity) is a well-recognized parameter for classification. It was initially considered to interpret the change of ionization constants of acids and bases according to Born s approach. This approach has lost importance in this respect because it is based on too simple assumptions limited to electrostatic interactions. Indeed, a more appropriate concept reflects solvation effects, the ability for H-bonding, or the acido-base property of the solvent. 

Globular proteins are soluble in polar solvents such as water and in aqueous solutions of acids and bases. Structural fibrous proteins are insoluble in water. Prolamins also dissolve in less polar solvents such as ethanol. In addition to the protein structure, solubility depends on the relative permittivity of the solvent, the pH value of the solution (the minimum solubility is at, or in the vicinity of, the isoelectric point), its ionic strength (a low concentration of salt increases the solubility by the salting-in effect, higher concentrations ofsalt reduces the solubility by the salting-out effect see Section 7.6.3.2.3), temperature and other factors. 

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