Solvents, amphiprotic dielectric constants

If the dielectric constant of an amphiprotic solvent is small, protolytic reactions are complicated by the formation of ion pairs. Acetic acid is often given as an example (denoted here as AcOH, with a relative dielectric constant of 6.2). In this solvent, a dissolved strong acid, perchloric acid, is completely dissociated but the ions produced partly form ion pairs, so that the concentration of solvated protons AcOH2+ and perchlorate anions is smaller than would correspond to a strong acid (their concentrations correspond to an acid with a pK A of about 4.85). A weak acid in acetic acid medium, for example HC1, is even less dissociated than would correspond to its dissociation constant in the absence of ion-pair formation. The equilibrium... 

From the practical point of view, nonaqueous solvents can be described broadly as acidic, basic, or neutral. For carrying out titrations, properties such as dielectric constant, melting point, boiling point, and (for amphiprotic solvents. Section 4-3) autoprotolysis constant are important (Table 6-1). 

Water, being both a proton donor and a proton acceptor, is said to be amphiprotic. It has a considerable dipole moment and a very high dielectric constant so that solvation of the ions, together with the dielectric effect (p. 121), discourages their recombination. These characteristics are responsible for the simple form taken by the ionic theory before the study of non-aqueous solvents revealed the more complex relationship between concentration and degree of ionisation. 

The extent of the ionization step depends on the relative strength of the conjugate acid-conjugate base pairs. The amphiprotic properties of the solvent have an essential effect on the equilibrium constant of this reaction step. The extent of the dissociation step is influenced by the polarity of the solvent, increasing with the dielectric constant of the solvent. In water, all products of acid-base reactions of moderate to low concentrations are essentially completely dissociated into solvated ions (Pecsok et al., 1976). The dissociation step is suppressed by addition or substitution with cosolvents of lower polarity, e.g., alcohols in aqueous formulations. The ion-pair aggregates may have absorption spectra different from the dissociated species. Thus, the amphiprotic properties and polarity (expressed as the dielectric constant) of the solvent are essential for the acid-base equilibrium of the drug and thus the absorption spectrum of the compound. This subject is further discussed in Section 14.2.3. 

Water can by no means be regarded as a typical solvent. It stands out as a consequence of its high polarity, high relative permittivity (dielectric constant), amphiprotic nature, and the physical and chemical properties stemming from these [Ho 72, Lu 74]. Hence, a deeper understanding of the equilibria occurring in aqueous solution also requires the investigation of non-aqueous solutions [Ad 67, Be 81, Gu 68, La 66, Lu 74, Pa 68, Pa 72, Ro 59]. 

Barbosa J, Bosch E, Cortina JL, and Roses M (1992) Ionic equilibria in amphiprotic solvents of low dielectric constants. Titration curves and buffer capacity of bases in anhydrous acetic acid. Analytica Chimica Acta 256 211-220. 

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