Dissociation constants, effect

But a solution of carbon dioxide in water behaves as a very weak acid since the effective dissociation constant K is given by ... 

Electron withdrawing effects of the cx-chlorine give chloroacetic acid a higher dissociation constant than that of acetic acid. 

The equihbrium constant of this reaction is 5.4 x 10 at 25°C, ie, iodine hydrolyzes to a much smaller extent than do the other halogens (49). The species concentrations are highly pH dependent at pH = 5, about 99% is present as elemental at pH = 7, the and HIO species are present in almost equal concentrations and at pH = 8, only 12% is present as and 88% as HIO. The dissociation constant for HIO is ca 2.3 x 10 and the pH has tittle effect on the lO ion formation. At higher pH values, the HIO converts to iodate ion. This latter species has been shown to possess no disinfection activity. An aqueous solution containing iodate, iodide, and a free iodine or triodide ion has a pH of about 7. A thorough discussion of the kinetics of iodine hydrolysis is available (49). 

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). 

First Carbonation. The process stream OH is raised to 3.0 with carbon dioxide. Juice is recycled either internally or in a separate vessel to provide seed for calcium carbonate growth. Retention time is 15—20 min at 80—85°C. OH of the juice purification process streams is more descriptive than pH for two reasons first, all of the important solution chemistry depends on reactions of the hydroxyl ion rather than of the hydrogen ion and second, the nature of the C0 2 U20-Ca " equiUbria results in a OH which is independent of the temperature of the solution. AH of the temperature effects on the dissociation constant of water are reflected by the pH. 

Calcium ion enters the system not ordy in the form of water hardness but also in the form of calcium salts contained in the sod. Other heavy-metal ions such as aluminum and ferric iron may also be present in the sod, and must be removed by an appropriate budder to achieve good sod removal. Effective budders for cotton washing are those for which the calcium dissociation constant, expressed as or —logif -, is >4 and preferably >7 (33). 

The protonation equilibria for nine hydroxamic acids in solutions have been studied pH-potentiometrically via a modified Irving and Rossotti technique. The dissociation constants (p/fa values) of hydroxamic acids and the thermodynamic functions (AG°, AH°, AS°, and 5) for the successive and overall protonation processes of hydroxamic acids have been derived at different temperatures in water and in three different mixtures of water and dioxane (the mole fractions of dioxane were 0.083, 0.174, and 0.33). Titrations were also carried out in water ionic strengths of (0.15, 0.20, and 0.25) mol dm NaNOg, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent (dioxane), temperature, and ionic strength on the protonation processes of hydroxamic acids is presented and discussed to determine the factors which control these processes. 

The influence of NH., and CO, on the chromatographic behaviour of benzoic acid and its derivatives (o-, m-, p-hydroxybenzoic, nitrobenzoic, aminobenzoic, chlorobenzoic acids) was studied. The work was carried out by means of upgoing TLC on Sorbfil plates. Isopropanol- and ethyl acetate-containing water-organic eluents were used as mobile phases in the absence or presence of gaseous modifiers in the MP. The novel modification of TLC has been found to separate benzoic acids with different values of their dissociation constants more effectively than water-organic mobile phases. 

As might be expected intuitively, there is a relationship between the effectiveness of general acid catalysts and the acid strength of a proton donor as measured by its acic dissociation constant K. This relationship is expressed by the following equation, which is known as the Brensted catalysis law ... 

Figure 4.10 is plot of the Bronsted relationship for hydrolysis of an enol ether. The plot shows that the effectiveness of the various carboxylic acids as catalysts is related to their dissociation constants. In this particular case, the constant a is 0.79 ... 

A very interesting steric effect is shown by the data in Table 7-12 on the rate of acid-catalyzed esterification of aliphatic carboxylic acids. The dissociation constants of these acids are all of the order 1(T, the small variations presumably being caused by minor differences in polar effects. The variations in esterification rates for these acids are quite large, however, so that polar effects are not responsible. Steric effects are, therefore, implicated indeed, this argument and these data were used to obtain the Es steric constants. Newman has drawn attention to the conformational role of the acyl group in limiting access to the carboxyl carbon. He represents maximum steric hindrance to attack as arising from a coiled conformation, shown for M-butyric acid in 5. 

The dissociation constants are thermodynamic constants, independent of ionic strength. Equation (8-33), which was derived from (8-30), is, therefore, identical in its form, and its salt effect, with Eq. (8-31). Therefore, salt effects cannot be used to distinguish between Eqs. (8-30) and (8-31). Another way to express this is that if kinetically equivalent forms can be written, it is not possible to determine, on the... 

The affinities of the two states for substrate, S, are characterized by the respective dissociation constants, Kn and Kj- The model supposes that Kj- Kn. That is, the affinity of Rq for S is much greater than the affinity of Tq for S. Let us choose the extreme where Kn/Kj- = 0 (that is, Kj- is infinitely greater than Kjp). In effect, we are picking conditions where S binds only to R. (If Kj-is infinite, T does not bind S.)... 

The determination of pi a values is probably the most generally useful method for the investigation of tautomerism. This method was first employed in the heterocyclic field in the early 1950 s by Tucker and Irvin and by Angyal and Angyal. There are two empirical dissociation constants, and K2, for the conjugate acid (HXH+) of a tautomeric compound. Constants Kt and K2 are, in effect, a summation of the true dissociation constants Ka, Kb, Kc, and Kd of the individual tautomeric forms (see scheme 43, where XH and HX are tautomers) and the tautomeric constant, Kt] these constants are related by the following equations ... 

Recently, Eq. (11) was extensively studied by Dewar and Grisdale, who synthesized several substituted 1 -naphthoic acids and determined the dissociation constants in mixed aqueous solvents in connection with a study on the mechanisms of transmission of the inductive effect. 

If a methyl group replaces a hydrogen atom on the carbon of the C==N bond across which addition of water occurs, a considerable reduction in the extent of water addition is observed. Conversely, the existence of such a blocking effect can be used as a provisional indication of the site at which addition of water occurs, while the spectrum and acid dissociation constant of the methyl derivative provide a useful indication of the corresponding properties of the anhydrous parent substance. Examples of the effect of such a methyl group on equilibria are given in Table IV. 

Kaumann, A. J., and Marano, M. (1982). On equilibrium dissociation constants for complexes of drag receptor subtypes Selective and nonselective interactions of partial agonists with two P-adrenoceptor subtypes mediating positive chronotropic effects of (-) isoprenaline in kitten atria. Nannyn Schmiedebeberg s Arch. Pharmacol. 219 216—221. 

Black, J. W., Feff, P, Shankley, N. P., and Wood, J. (1985). An operational model of pharmacological agonism The effect of E/[A] curve shape on agonist dissociation constant estimation. Br. J. Pharmacol. 84 561-571. 

Cheng-Prusoff correction, published by Cheng and Prusoff (Biochem. Pharmacol. 22, 3099-3108, 1973), this method is used to derive the equilibrium dissociation constant of a ligand-receptor (or enzyme) pair from the experimentally observed ICso (concentration that produces 50% reduction in effect) for that molecule see Eqn 4.11. 

Uncompetitive antagonism, form of inhibition (originally defined for enzyme kinetics) in which both the maximal asymptotic value of the response and the equilibrium dissociation constant of the activator (i.e., agonist) are reduced by the antagonist. This differs from noncompetitive antagonism where the affinity of the receptor for the activating drug is not altered. Uncompetitive effects can occur due to allosteric modulation of receptor activity by an allosteric modulator (see Chapter 6.4). 

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