Acid dissociation constant determination

To evaluate the dissociation constants it would be necessary to measure the equDibrium constant for the reaction in Scheme 4. The dissociation of thiazolecarboxylic acids has been studied principally by Erlenmeyer et al. (47, 48). It seems that no systematic and reliable determination of the acidity dissociation constants have been realized until now. 

This relationship between and Kb simplifies the tabulation of acid and base dissociation constants. Acid dissociation constants for a variety of weak acids are listed in Appendix 3B. The corresponding values of Kb for their conjugate weak bases are determined using equation 6.14. 

The plT at which an acid-base indicator changes color is determined by its acid dissociation constant. For an indicator that is a monoprotic weak acid, ITIn, the following dissociation reaction occurs... 

In the second limiting situation the analyte is a weaker acid or base than the interferent. In this case the volume of titrant needed to reach the analyte s equivalence point is determined by the concentration of both the analyte and the interferent. To account for the contribution from the interferent, an equivalence point for the interferent must be present. Again, if the acid dissociation constants for the analyte and interferent are significantly different, the analyte s determination is possible. If, however, the acid dissociation constants are similar, only a single equivalence point is found, and the analyte s and interferent s contributions to the equivalence point volume cannot be separated. 

Directions are provided in this experiment for determining the dissociation constant for a weak acid. Potentiometric titration data are analyzed by a modified Gran plot. The experiment is carried out at a variety of ionic strengths and the thermodynamic dissociation constant determined by extrapolating to zero ionic strength. 

Using its titration curve, determine the acid dissociation constant for the weak acid in problem 6. 

The numerical values of the terms a and p are defined by specifying the ionization of benzoic acids as the standard reaction to which the reaction constant p = 1 is assigned. The substituent constant, a, can then be determined for a series of substituent groups by measurement of the acid dissociation constant of the substituted benzoic acids. The a values so defined are used in the correlation of other reaction series, and the p values of the reactions are thus determined. The relationship between Eqs. (4.12) and (4.14) is evident when the Hammett equation is expressed in terms of fiee energy. For the standard reaction, o%K/Kq = ap. Thus,... 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

The physical properties of the pyridopyrimidines closely resemble those of their nearest A-heteroeyclie neighbors the quinazolines and the pteridines. Thus, in common with the pteridines, the presence of groups capable of hydrogen-bonding markedly raises the melting point and lowers the solubility. - The acid dissociation constants (pif a values) and ultraviolet absorption spectra of all four parent pyridopyrimidines have been determined by Armarego in a comprehensive study of covalent hydration in these heterocyclic systems. The importance of these techniques in the study of covalent hydration, and... 

Apparent partition coefficient (logZ)) at an ionic strength of / = 0.02M, log P value of the neutral microspecies and the acidic dissociation constant of 5 was calculated (97ANC4143). The distribution coefficient of 5 was determined between 1-octanol and universal buffer in the pH range 3-10 at a... 

SPECTROPHOTOMETRIC DETERMINATION OF THE pAT VALUE OF AN INDICATOR (THE ACID DISSOCIATION CONSTANT OF METHYL RED)... 

Discussion. The dissociation of an acid-base indicator is well suited to spectrophotometric study the procedure involved will be illustrated by the determination of the acid dissociation constant of methyl red (MR). The acidic (HMR) and basic (MR-) forms of methyl red are shown below. 

Spectrophotometric determination of the pK value of an indicator (the acid dissociation constant of methyl red) 718... 

If the agent is an acid or a base its degree of ionization will depend on the pH. If its acid dissociation constant,is known, the degree of ionization at any pH may be calculated or determined by reference to published tables. 

It is worth mentioning that an attempt was made by Tsao and Willmarth to determine the acid dissociation constant of HO2. The reaction between hydrogen peroxide and peroxydisulphate was used for the generation of the HO2 radical. However, these experiments, like others where the HO2 radical is studied under steady-state conditions, could yield only a value of acidity constant multiplied by a coefficient consisting of a ratio of kinetic parameters. Unfortunately, in this case there are no independent data for the kinetic coefficient, and the value of cannot be evaluated. Considering the kinetic analogue of the titration curve it can be stated only that ionization of HO2 becomes important in the pH range from 4.5-6.5. The value of acidity constant of HO2 obtained by Czapski and Dorfman is (3.5 + 2.0)x 10 mole.l. . ... 

Acid dissociation constant Regression correlation coefficient Regression coefficient of determination Water solubility... 

Formazans behave as weak acids as well as weak bases. Salts of formazans have been isolated.26,334,335 The acid dissociation constants of some substituted formazans have been determined from their solution spectra.336... 

Tam, K. Y. Takacs-Novak, K., Multiwavelength spectrophotometric determination of acid dissociation constants. Part II. First derivative vs. target factor analysis, Pharm. Rese. 16, 374-381 (1999). 

Because the acid dissociation constants in solution are generally known, and also, often the solvation energies of the neutral AH have been determined by neutral gas-phase solution equilibria, equation 8 can be used to obtain the sum of the solvation energies AG°ol(A-) + AG°ol (H30+). When this approach is applied to a series of acids AH, values for the solvation energies AG°ol (A-), relative to the constant AG, (H30+), can be obtained. Such thermodynamic determinations of the... 

To conduct meaningful mechanistic and kinetic studies in alcohol media reliable and simple measurement and control of the solution jjpH is essential. Potentiometric titration is the method of choice for obtaining acid dissociation constants or metal ion complex stability constants and in favorable cases the speciation of mixtures of metal-ion-containing complexes in solution can be proposed.20 Titrations in non-aqueous solvents are not nearly as widely reported as those in aqueous media, particularly in cases with metal ions21 and determination of pH in a non-aqueous solvent referenced to that solvent is complicated due to the lack of a way to relate the electrode EMF readings to absolute jjpH (see footnote and ref. 6) so non-aqueous solvents are generally inconvenient to use22 for detailed studies of reaction mechanisms where pH control is required. 

The 13C chemical shift pH dependence for both the Mo(IV) and W(IV) systems is similar to that observed in Fig. 6 for the Re(V), with the exception that only one protonation step is observed in weak to mild basic solutions (there is a large difference in the pKal and pKa2 values for both the Mo(IV) and W(IV) systems, see Table II). The corresponding acid dissociation constants were similarly determined as in the case of Re(V) complexes. 

The line-broadening data as a function of pH, typically shown for the W(IV) in Figs. 13 and 14, incorporating the known pKa values (Table II), were fitted in 5 X 5 Kubo-Sack matrices describing the exchange based on the above schemes (6, 57). The experimentally determined chemical shift and linewidth data in the absence of exchange for the aqua oxo, hydroxo oxo, and dioxo species and the pH-dependent species distribution as calculated from the acid dissociation constants for the four systems were all introduced in the different matrices and the spectra were computer simulated. For each set of chosen rate con-... 

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