Ionization constant definition

The values of Hn and E are zero for water, by virtue of the constants 1.74 and 2.60. In these definitions, pKa refers to the acid ionization constant of the conjugate acid of the nucleophile, and E° to the standard electrode potential for the two-electron half-reaction ... 

C). The value of a may be determined from its definition in terms of the ionization constant if the appropriate benzoic acid derivative has been measured. Such a values may then be used to determine p values for other reactions, and these p values in turn lead to the possibility of determining new o values. Equation 7.4.24 implies that a plot of log KA or log kA versus oA should be linear for a given series of reactions involving the same reactive groups. Extensive experimental evidence attests to this relation. Table 7.2 contains some values that have been reported in the literature. Extensive tabulations... 

In this equation, pK° is the pK of the unsubstituted molecule, o refers to the constant assigned to a respective substituent and p is a constant assigned to the particular ionizable group. The approach was later refined by Taft et al extending the theory and equation to aliphatic and other systems. The principles and methodologies of a classic experimental pK determination are well known. Ionization constants for an n-protic weak acid may be represented by the following generic definitions ... 

The influence of solvents on the ionization equilibrium is related to their electrostatic and their solvation properties. The value of the ionization constant of an analyte is closely determined, in practice, by the pH scale in the particular solvent. It is clear that it is most desirable to have a universal scale which is able to describe acidity (and basicity) in a way that is generally valid for all solvents. It is, in principle, not the definition of an acidity scale in theory which complicates the problem it is the difficulty of approximating the measured values in practice to the specifications of the definition. The pH scale, as is common in water, is applicable only to some organic solvents (i.e., mainly those for which the solvated proton activity is compatible with the Brpnsted theory of acidity). The applicability of an analog to the pH scale in water decreases with decreasing relative permittivity of the solvents and with their increasing aprotic character. 

The foregoing example illustrates how equilibrium constants for overall cell reactions can be determined electrochemically. Although the example dealt with redox equilibrium, related procedures can be used to measure the solubility product constants of sparingly soluble ionic compounds or the ionization constants of weak acids and bases. Suppose that the solubility product constant of AgCl is to be determined by means of an electrochemical cell. One half-cell contains solid AgCl and Ag metal in equilibrium with a known concentration of CP (aq) (established with 0.00100 M NaCl, for example) so that an unknown but definite concentration of Kg aq) is present. A silver electrode is used so that the half-cell reaction involved is either the reduction of Ag (aq) or the oxidation of Ag. This is, in effect, an Ag" Ag half-cell whose potential is to be determined. The second half-cell can be any whose potential is accurately known, and its choice is a matter of convenience. In the following example, the second half-cell is a standard H30" H2 half-cell. 

We should recall that the thermodynamic definition of K is in terms of activities. In dilute solutions, the activity of the (nearly) pure H2O is essentially 1. The activity of each dissolved species is numerically equal to its molar concentration. Thus the ionization constant of a weak acid, K, does not include a term for the concentration of water. 

It follows from Eq. (9) that when a macromolecule is in the shape of a globe, the intrinsic ionization constant for a definite type of ion-exchangeable sites is changed in the presence of a charge so that ... 

It must be remarked, however, that the curves shown in Fig. 3 are hardly typical. Two definite points of inflection are observed for a dibasic acid only if the two ionization constants are of quite different orders of magnitude, Auerbach and Smolczyk7 have shown, theoretically, that the two points of inflection will not appear unless the ratio Ki/E2 is greater than 16. However, it is safe to say that the ratio must be considerably greater than that if the inflection points are to be obtained experimentally with any accuracy. Also, if the two ionization constants are not quite different the values of the constants determined by the potentiometric titration method described above may be somewhat in error, as is shown by the authors just mentioned and by Simms. ... 

By setting SQ = [HA] for acids or [B] for bases and recalling our definition of the ionization constant, it follows that for weak acids with only one ionization centre, the change in solubility with respect to pH is given by... 

In a second series of papers, Derick goes farther and studies the polarity of elements and radicals measured in terms of a logarithmic function of the ionization constant. This is evidently the direction in which advance is to be expected, a study of the action of various groups on the equilibria of certain reactions in terms of affinity changes. The following definitions are given by Derick ... 

In general, enzymes exhibit maximum catalytic activity at a definite pH. This optimum pH is generally in the vicinity of pH 7 ( 1), although exceptions are well known. The dependence of the kinetic parameters of the fumarase reaction on pH is illustrated in Figs. 9-2 and 9-3. Several different possible effects of pH on the reaction must be distinguished. In the first place, many substrates may have ionizable groups and only one of the ionized forms of the substrate may be acted upon by the enzyme. Since substrate ionization constants can be determined quite easily and precisely,... 

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