Ionizable components, dissociation constants

Dissociation constants of ionizable components can be determined using various methods such as potentiometric titrations [85] CE, NMR, [86] and UV spectrophotometric methods [87]. Potentiometric methods have been used in aqueous and hydro-organic systems however, these methods usually require a large quantity of pure compound and solubility could be a problem. Potentiometric methods are not selective because if the ionizable impurities in an impure sample of the analyte have a pK similar to that of the analyte, this could interfere with determining the titration endpoint. If the titration endpoint is confounded, then these may lead to erroneous values for the target analyte pKa. 

The a-amino group of amine component V and the imidazole of IV are suggested to be involved in the metal binding from the shift of their dissociation constants. That is, the pKa-values of the amino and imidazole groups in metal-free BLM A2 are 7.4 and 4.7 respectively. However, in copper-chelated BLM A2 there is no ionizable group with a pKa-value between 4 and 9. Furthermore, acylation of the a-amino... 

In chemistry and biochemistry, a dissociation constant is a specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex falls apart into its component molecules, or when a salt splits up into its component ions. The dissociation constant is often also denoted as Kj and is the inverse ofthe affinity constant. In the special case of salts, the dissociation constant can also be called ionization constant. 

Sometimes the expression dissociation constant is used for ionization constant. The latter term is more precise because many complexes, such as enzyme systems, dissociate into their components, and micelles into their monomers the relevant equilibria are expressed as dissociation constants similarly derived from the law of mass-action. Nevertheless, such constants are not ionization constants. 

The discussion in the previous sections concerning solvated species indicates that a complete knowledge of the chemical reactions that take place in a system is not necessary in order to apply thermodynamics to that system, provided that the assumptions made are applied consistently. The application of thermodynamics to sulfuric acid in aqueous solution affords another illustration of this fact. We choose the reference state of sulfuric acid to be the infinitely dilute solution. However, because we know that sulfuric acid is dissociated in aqueous solution, we must express the chemical potential in terms of the dissociation products rather than the component (Sect. 8.15). Either we can assume that the only solute species present are hydrogen ion and sulfate ion (we choose to designate the acid species as hydrogen rather than hydronium ion), or we can take into account the weak character of the bisulfate ion and assume that the species are hydrogen ion, bisulfate ion, and sulfate ion. With the first assumption, the effect of the weakness of the bisulfate ion is contained in the mean activity coefficient of the sulfuric acid, whereas with the second assumption, the ionization constant of the bisulfate ion is involved indirectly. 

Any quantitative measure of the acidity of organic acids or bases involves measuring the equilibrium concentrations of the various components in an acid-base equilibrium. The strength of an acid is then expressed by an equilibrium constant. The dissociation (ionization) of acetic acid in water is given by the following equation ... 

The important point with any of the above preparation methods is that the initial solution must be a weak acid or base, otherwise the starting acid or base would already be 100% dissociated or ionized. Both components of a conjugate acid-base pair must remain in the solution to be able to neutralize any added acid or base. Regardless of the method of buffer production, the nature of the acid-base buffer always remains the same — a constant pH is maintained following any additions of small amounts of acid or base. 

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