Acid solution dissociation constant, medium

The titration of a weak base in an anhydrous medium has been recently reported by Tamele (9 ). The catalyst, suspended in benzene, was titrated with n-butylamine, using p-dimethylaminoazobenzene as indicator. This indicator does not color pure silica gel or pure alumina. A number of solid acids of known strength in aqueous solutions were titrated in the same system. It was found that this method detects acids whose dissociation constants are about 10 or more in aqueous... 

An inflection point in a pH-rate profile suggests a change in the nature of the reaction caused by a change in the pH of the medium. The usual reason for this behavior is an acid-base equilibrium of a reactant. Here we consider the simplest such system, in which the substrate is a monobasic acid (or monoacidic base). It is pertinent to consider the mathematical nature of the acid-base equilibrium. Let HS represent a weak acid. (The charge type is irrelevant.) The acid dissociation constant, = [H ][S ]/[HS], is taken to be appropriate to the conditions (temperature, ionic strength, solvent) of the kinetic experiments. The fractions of solute in the conjugate acid and base forms are given by... 

The potentiometric titration curves of gels, which relate the pH of the exterior solution to the degree of ionization of the gel, resemble the titration curves of monofunctional acids or bases. However, the dissociation constants differ, often by two orders of magnitude, from the expected value for the functional group, and the slope of the curves is not the usual one. Addition of neutral salt changes the picture markedly and brings the curves closer to expectation. In the case of weak or medium... 

Hammett acidity function chem An expression for the acidity of a medium, defined as ho = Kbh+ BH / B, where Kbh+ is the dissociation constant of the acid form of the Indicator, and BH+ and B are the concentrations of the protonated base and the unprotonated base respectively. ham at a sid ad e. faijk shan ) hand sugar refractometer analy chem Portable device to read refractive Indices of sugar solutions. Also known as protelnometer. hand shCig ar. re.frak tam-3d-3r1... 

Fine crystalline quartz or amorphous substances (gel, quartz glass, etc.) are used as one of initial components for the synthesis [19]. Depending on temperature, pressure, pH of the medium and the presence of salts, silica can exist in solution both as simple ions or molecules, and as more complicated polymer particles. Under normal conditions silica passes into solution in monomer form, as silicic acid Si(OH)4 at large pH, silicate ions SiOj - are formed. Monosilicic acid is a very weak acid however, at increased temperature its dissociation constant increases substantially. The amount of monomer form also increases with temperature. The dissolution of Si02 is due to hydration as well as to depolymerization. 

Attempts to verify the linear relationship between — log K and 1/D by means of a series of dioxane-water mixtures have brought to light considerable discrepancies. The addition of dioxane to water results in a much greater decrease in the dissociation constant than would be expected from the change in the dielectric constant of the medium. Since the organic acids studied are more soluble in dioxane than in water, it is probable that molecules of the former solvent are preferentially oriented about the acid anion the effective dielectric constant would then be less than in the bulk of the solution. It is thus p>0B8ible to... 

Aliphatic amines generally have base dissociation constants on the order of 10 and can thus be titrated directly with a solution of a strong acid. In contrast, aromatic amines such as aniline and its derivatives are usually too weak for titration in aqueous medium ( Ib 10 °). The same is true for cyclic amines with aromatic character, such as pyridine and its derivatives. Many saturated cyclic amines, such as piperidine, tend to resemble aliphatic amines in their acid/base behavior and thus can be titrated in aqueous media. 

Catalytic supercritical water oxidation is an important class of solid-catalyzed reaction that utilizes advantageous solution properties of supercritical water (dielectric constant, electrolytic conductance, dissociation constant, hydrogen bonding) as well as the superior transport properties of the supercritical medium (viscosity, heat capacity, diffusion coefficient, and density). The most commonly encountered oxidation reaction carried out in supercritical water is the oxidation of alcohols, acetic acid, ammonia, benzene, benzoic acid, butanol, chlorophenol, dichlorobenzene, phenol, 2-propanol (catalyzed by metal oxide catalysts such as CuO/ZnO, Ti02, MnOz, KMn04, V2O5, and Cr203), 2,4-dichlorophenol, methyl ethyl ketone, and pyridine (catalyzed by supported noble metal catalysts such as supported platinum). ... 

Many drngs are acidic or basic componnds, which are ionized to a certain degree in aqueous medium. Their degree of ionization depends on their dissociation constant (pA J and the pH of the solution, according to the Henderson-Hasselbach equation (Equations (31.2) and (31.3)). 

For example, in acetic acid (c = 6), perchloric acid is completely ionized, but the dissociation constant of the ion pair H3CC00H2C10J is only about 10" mol/L and HCIO4 can be classified as a weak acid in this medium. The ionization of acids is enhanced in basic solvents and diminished in acidic solvents, but the dissociation of the ion pair depends only on the dielectric constant of the solvent. Nevertheless, the acidity of weak acids is enhanced in basic solvents, and the basicity of weak bases is enhanced in acidic solvents. Examples are solutions of phenol in ethylenediamine ... 

Andraos, J., Chiang, Y, Kresge, A.J., PojarKefF, I.G., Schepp, N.P., and Wirz, J., The a-Cyano-a-Phenylacetic Acid Keto-Enol System. Flash Photolytic Generation of the Enol in Aqueous Solution and Determination of the Keto-Enol Equilibrium Constants and Acid Dissociation Constants Interrelating All Keto and Enol Forms in that Medium, /. Am. Chem. Soc., 116, 73, 1994. 

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