Acidity constant variations

Solvent variation can gready affect the acidity of hydantoins. Although two different standard states are employed for the piC scale and therefore care must be exercised when comparing absolute acidity constants measured in water and other solvents like dimethyl sulfoxide (DMSO), the huge difference in piC values, eg, 9.0 in water and 15.0 in DMSO (12) in the case of hydantoin itself, indicates that water provides a better stabilization for the hydantoin anion and hence an increased acidity when compared to DMSO. 

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

The Dissociation Constant of Nitric Add. Alodcrately Weak Acids. The Variation of J with Temperature. Proton Transfers between Solute Particles. A Proton Transfer in Methanol Solution. Proton Transfers with a Negative Value for. / . The Hydrolysis of Salts. Molecules with Symmetry. Substituted Ammonium Ions. Deuteron Transfers in D2(). The Dissociation of Molecular Ions. 

Hogeveen138 measured apparent acidity constants of substituted /i-phenylthio-, /J-phenylsulfinyl- and / -phenylsulfonyl-acrylic acids (cis and trans) in 50% v/v ethanol. The p values for transmission through SCH=CH, SOCH=CH and S02CH=CH were 0.531, 0.389 and 0.320 respectively for the cis acids and 0.652, 0.282 and 0.331 for the trans acids. These results were discussed in considerable detail and compared with those of pertinent related systems. Little importance was attached to the small differences between p values for cis/trans isomers, and the relative transmissions were taken as the mean p values 0.59,0.34 and 0.33. The superior transmission of SCH=CH was attributed to greater polarizability. The values for pKJtrans) — pKJcis) of isomeric acids were also discussed. For the sulfonyl acids this is almost constant at 0.1 unit for the sulfinyl adds there is some variation about a mean value of 0.26 unit, and for the thio adds there is also some variation about a mean value of — 0.15 unit. The differing behavior of the three systems in this respect was explained in terms of hypothetical conformations and electrostatic interactions therein. 

Estimate the variation of surface charge of a hematite suspension (same charac-teristics as that used in Example 7.2) to which various concentrations of a ligand H2U (that forms bidentate surface complexes with the Fe(III) surface groups, FelT such a ligand could be oxalate, phtalate, salicylate or serve as a simplified model for a humic acid we assume acidity constants and surface complex formation constants representative for such ligands. The problem is essentially the same as that discussed in Example 5.1. We recalculate here for pH = 6.5. 

The acidity constants for the surface protolysis reactions provide a measure of the extent to which the reaction will proceed. For the iron oxides the values of the pJC,s usually fall between 5 and 10, but there is considerable variation in the values for dif... 

Hence, the Pi ligand parameter reflects, in an overall way, the combined a- and Tt -electronic properties of the coordination M—L bond. It is noteworthy to mention that it relates to the variation of the free-energy difference of the redox processes (consider the known expression AG = —nFE, in which n is the number of electrons transferred and F is the Faraday constant). It has analogies with the Hammett Up constant [11, 12] defined as og[Kx/Kh), that is, log Kx - log K, in which Kx and ATh are the acidic constants of the p-substituted benzoic acid HOOCCg H4X-4 and of benzoic acid itself, respectively [13] (consider also the known relationship AG = —RT nK). 

Acidity constants (p-Kjjj) of cyclopentanone and cyclohexanone enols have been determined by the halogen-titration method from the variations of the enol + enolate sum as a function of pH (Bell and Smith, 1966). However, since the values of the keto-enol equilibrium constants are questionable, these pflra-values (11.8 and 11.3, respectively) are doubtful as well, although they are in fair agreement with those expected. 

E. Bosch, C. Rafols, and M. Roses, Variation of acidity constants and pH values of some organic acids in water-2-propanol mixtures with solvent composition. Effect of preferential solvation, A a/. Chim. Acta 302 (1995), 109-119. 

Polar fishes, freezing resistance of, 195 12-Propanediol-water mixture dielectric constant of, 92 Protein fractionation at subzero temperatures, 77 -189 acid-base equilibria, 100-122 applications, 146-185 column chromatography, 140-141 density and viscosity changes, 82-85 dielectric constant variations, 85-99 general principles, 135-140 isoelectric focusing, 141-144 methods, 140-146 physicochemical data, 78-134 protein dissociation, 129-134 protein titration, 116-122 solubility of salts and solutes, 122-129... 

It should be mentioned also that solubilization in cationic micelles produces an equalization effect on the acidity constant and dissociation constants. For the compound studied the variation range of pN and dissociation... 

The efficiency of the regulation depends on the ability of the surface to respond, through charge variation, to the pH variations near the surface (pH ). This ability is itself proportional to the difference ApK = pK — pK between the acidity constants of the surface sites (Figures 8.2 and 8.3). A small ApK increases the... 

Although the proportion of nitric acid present as nitronium ions does not change between 90% and 100% sulphuric acid, the rate constants for nitration of most compounds decrease over this rai e. Fig. 2.1 illustrates the variation with acidity of the second-order rate constants of the nitration of a series of compounds of widely differing reactivities. Table 2.4 lists the results for nitration in 95% and 100% acid of a selection of less completely investigated compounds. 

The variation of the observed second-order rate constant with acidity. 

In this experiment the method of continuous variations is used to determine the stoichiometry and equilibrium constant for the organic complex of 3-aminopyridine with picric acid in CHCI3, and the inorganic complex of Fe +with salicylic acid. 

This experiment describes the use of FIA for determining the stoichiometry of the Fe +-o-phenanthroline complex using the method of continuous variations and the mole-ratio method. Directions are also provided for determining the stoichiometry of the oxidation of ascorbic acid by dichromate and for determining the rate constant for the reaction at different pH levels and different concentration ratios of the reactants. 

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