Dissociation constant macroscopic

In common with all protonic dissociation, this reaction pathway assumes that all k species of the same charge (k=j + i +1) are equivalent and takes advantage of the assumed independence of the macroscopic dissociation constants Kh K2,.Kj+i. Moreover, this dis-... 

To calculate microscopic constants from stepwise constants and tautomeric ratios, consider Eq. 6-76 in which [HP]a and [HP]B are the concentrations of the two tautomers and kj is the first stoichiometric or macroscopic dissociation constant for the diprotonated species H2P. 

The four microscopic dissociation constants were determined using 0.1 ionic strength carbonate/ bicarbonate buffers with the values pki = 9 74, pk2= 10.56, pk 2 = 11 17 and pk2i = 10.35. From these the macroscopic dissociation constants were determined from the relations ... 

The experimentally determined dissociation constants for the various stages of dissociation of polyprotic and zwitterionic dmgs are referred to as macroscopic values. However, it is not always easy to assign macroscopic dissociation constants to the ionisation of specific groups of the molecule, particularly when the pK values are close together, as discussed in section 3.5.4. The diprotic drug morphine has macroscopic plC, values of 8.3 and 9.5, arising from ionisation of amino and phenolic... 

Ross and Riley (11) determined the macroscopic dissociation constants for lomefloxacin using UV spectrophotometric measurements at 266 nm. Takacs-Novak et al. used UV spectrophotometry to determine octanol/water partition coefficients for lomefloxacin and related quinolones and related them to their protonation equilibria (18). 

Many glycosidases exhibit such a pH-rate profile and the temptation to equate Ka with the dissociation constant of the acid-catalytic group and A b with the dissociation constant of the base or nucleophile frequently proves irresistible. Even in an Alberty-Bloomfield system, however, the equation should be not made unthinkingly. and A b are macroscopic dissociation constants, which are related to the four microscopic dissociation constants of the two-carboxylate system (Figure 5.13) by eqns (5.38) and (5.39) (since Figure 5.13... 

Yu XQ, Britten NJ and Davidson GWR, Solubilities and apparent macroscopic dissociation constants of ciprofloxacin and norfloxacin, Pharm. Res., 9,101, S-219 (1992). NB From alternative solubility method, pKg = 6.45 pKa2 = 8.69. 

Potentiometric titration of phenylephrine vs NaOH (Figure 4-8) yields the macroscopic dissociation constants while the microscopic dissociation constants are accessible from simultaneous observation of potentiometric and C H -NMR-controlled titrations (Figure 4-9 and Figure 4-10). TTie microscopic dissociation species defined in Table 4-1 are involved in the microscopic dissociation equilibrium shown in Figure 4-6. 

Figure 4-8 Titration curve pH = f(r) phenylephrine + HNO3 vs NaOH macroscopic dissociation constants pAii = 9.17 and p. 2 = 10.22.

The amino acids are ampholytes, having both acid and basic properties. Their protolytic properties are best described by values (or pAg values), macroscopic dissociation constants for the acid groups involved. In Table II pA values are given for all the protein amino acids. The pAg values listed... 

Amino acid Macroscopic dissociation constants, group involved Charge at pH ... 

The macroscopic dissociation constants are those determined directly from titration curves and can be used directly for calculation of the charge of the molecules at various pH values. They do not reveal which protons and acid groups are involved in the different ionization steps. For the simple monoamino monocarboxylic acids it is well known that the first pK value corresponds to that of the COOH group, the second to that of the NHJ group in agreement with the top part of the following scheme ... 

For cysteine the first pK value can be assigned to the COOH group. The two remaining pXg values corresponding to macroscopic dissociation constants represent the following equilibria ... 

The problem of interpreting macroscopic dissociation constants in terms of which groups in the molecules actually dissociate does not only occur when the pX values are close to each other. Thus, for 2,3-diaminopropionic acid with pXa and pXg values 6.78 and 9.53 the following set of pX values corresponding to microscopic dissociation constants have been determined pXg, 7.07 pXg, 7.10 pX, 9.25 pX a, 9.21. Thus, the ratio between (XXIII) and (XXIV) is 0.9, indicating equal amounts of the two species (Sayer and Rabenstein, 1976). 

Figure 7.1 shows the pK s of some dicarboxylic acids along with the pK s of some related monocarboxylic acids. As noted earlier, complications can arise when a compound contains groups that have dissociations with close pK values [93,173,434]. In such a case one must consider the equilibria shown in Figure 7.2 for dissociation of an original acid RH Ht [173], In this scheme K, Kg, Kc, and are so-called microscopic equilibrium constants and is the equilibrium constant for tautomerization between RH and RH. . The experimentally measured (macroscopic) dissociation constants Kj and K2 pertain to the following equilibria ... 

The tetracyclines have three macroscopic dissociation constants and thus their ionic behaviour is a complex function of pH. In the pH range of the solubilization... 

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