Alcohol-water mixtures dissociation constants

The dissociation constants were establidied potentiometrically. As on titration in an aqueous medium die base liberated from die salt would precipitate, the pXa was detemined in several alcohol-water mixtures (80/20, 70/30, etc.). The strai t line dirou die measuring (szc) points was obtained by the method of least squares using equidistant X values. Next the pKa value in water was determined by extrapolation. In the same way die experimental error was calculated."... 

Rafols C, Roses M and Bosch E, Dissociation constants of several nonsteroidal anti-inflammatory drugs in isopropyl alcohol /water mixtures. Anal. Chim. Acta, 350,249-255 (1997). NB Potentiometric method using i-PrOH/water mixtures in the range 10/90 to 90/10 v/v. Metrohm autotitrator careful electrode calibration. Activity corrections applied and extrapolated to zero [i-PrOH]. Alternative data handling procedure gave pKa = 4.64 0.04. 

Equation (23) has been found to fit rates of proton exchange in the acid dissociation of aliphatic ammonium ions in water, methanol, and t-butyl alcohol-water mixtures. When the kinetic data are combined with values obtained for the acid dissociation constant Kai= kjk-j, all three rate constants in equation 23 can be evaluated. [44]. Results obtained for k and k-a by this approach are in good agreement with relaxation spectrometry representative values of k are included in Table 3. 

In organic solvents the acidity functions H corresponding to hydrogen dissociation from neutral indicator acids were reported for solutions of alkali metal alkoxides in various alcohols (2), using nitroanilines (21), aminobenzenecarboxylic acids (22), or indols (23) as indicators. For addition reactions of methoxide and ethoxide ions to neutral indicator acids, acidity functions J (also denoted as Hr) based on use of nitrobenzenes (21) and a-cyanostilbenes (18) as indicators in methanol and dimethylsulfoxide-methanol and -ethanol mixtures were reported. Recently (24) the acidity function J- (denoted as Jm) was derived for methoxide ion solutions in methanol using substituted benzaldehydes as indicators. These scales involve arbitrary choice of water as the solvent for determination of the dissociation constant of the anchoring acid. 

The same investigators have measured the pH of mixtures of weak bases and their salts. The following table contains certain of their data. Because the ion product of water in alcoholic solutions is not known accurately, the hydrolysis constants (cf. page 14) will be reported instead of the dissociation constants. The hydrolysis constant is... 

Kinetic studies of aquation of [Fe(phen)3] and derivatives in binary aqueous media remain popular. A group additivity approach has been applied to aquation of [Fe(5N02phen)3] in aqueous alcohols (faster reaction) and formic and acetic acids (slower), to investigate its potential for mechanism diagnosis. Rate constants for dissociation of the parent complex increase tenfold on going from water to 100% dimethylformamide. Aquation rate constants and activation parameters have also been reported for the 5-nitro, 5-phenyl, and 4,7-diphenyl derivatives in water-dioxan mixtures. Both papers contain obscure discussions of solvolysis mechanisms in DMF-rich and dioxan-rich media. In the latter media it seems that ion pairs play a key role, as evidenced by activation entropies. The discussion of reactivities in terms of hydrophobicities of the complexes and their respective transition states represents a qualitative initial state-transition state analysis. An explicit analysis of this type has been published for the iron(II) complexes of the... 

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