Acid-ionization Constant Ka

Acetic acid, HC2H302, which is represented as HA, has an acid ionization constant Ka of 1.74 x 10 5. 

PKa a logarithm of acid ionization constant (Ka) pKg Equivalent to -logioKe. where by convention Kg is the equilibrium dissociation constant of an antagonist. Numerically, identical to pAz for simple equilibrium competition at a single site, The term apparent pKe may be used when there is no evidence whether or not there is competition, but there is assumed to be (e.g. when calculating pKe from a single concentration of antagonist). PKC protein kinase C. 

The rccison for the increased activity of protic acid-Lewis acid systems is ascribed to the enhancement of the protic acid acidity by the Lewis acid, (Table 2) 31). Whereas 100% HF has a value of —10.2, the acid system of HF +7% BF3 has an Ho value of —16.8, six orders of magnitude greater. The Hammett acidity function. Ho, is evaluated by the use of indicators, B, which are very weak bases and for which the acid ionization constants, Ka, of the conjugate acids, BH+, have been determined (usually spectrophotometrically), Eq. (9) ... 

Substitute the expressions for the equilibrium concentrations (from step 3) into the expression for the acid ionization constant (Ka)-... 

The acidity of hydrofluoric acid solutions vary with concentration owing to hydrogen-bond interactions of the fluoride ion. Dilute solutions are weakly acidic with an acid ionization constant Ka = 6.6 x 10 " (or pKa = 3.18), in contrast to corresponding solutions of the other hydrogen halides which are strong acids. Concentrated solutions of hydrogen fluoride are much more strongly acid than implied by this value, as shown by measurements of the Hammett acidity function Ho (or effective pH ). For 100%, HF has an Ho, estimated to be between -10.2 and -11, which is comparable to the value -12 for sulfuric acid. 

A weak acid ionizes only partially. The acid ionization constant, Ka, is... 

Hydroxyl radicals. The acid ionization constant of the short-lived HO transient is difficult to determine by conventional methods but an estimate can be made because HO, but not its conjugate base, O -, oxidizes ferrocyanide ions HO + Fe(CN) — OH- + Fe(CN)g . Use the following kinetic data26 for the apparent second-order rate constant as a function of pH to estimate Ka for the acid dissociation equilibrium HO + H20 =... 

Ka is also widely called the acid ionization constant or acid dissociation constant. 

The earliest successful parameterization of electrical effects is due to Hammett . Though Burkhardt reported the existence of QSRR two years before Hammett, he did not develop a general relationship. Hammett defined the and Op constants using the ionization constants Ka,x of 3- and 4-substituted benzoic acids in water at 25 °C as the reference set and hydrogen as the reference substituent Kh) to which all others are compared. For hydrogen, the values of the and Op constants were defined as zero (equation 3) ... 

The value of Kapp is pH dependent. It is seen in Chapter 5 (equation 5.8) that equation 3.80 is the equation for the ionization of an acid of ionization constant Ka. At high pH where [H+] < KA or K A, appOrighpH) = K. At low pH where [H+] Ka oiKa, A appdowpH) = KKa/K a. Applying the rules for cyclic equilibria above, rapp(highpH) = % The values of Km at extremes of pH should be obvious from examination of scheme 3. 

It is usual to discuss the ionization of a base B in terms of its conjugate acid BH+ in order to use the same set of equations for both acids and bases. The ionization constant Ka is defined by... 

Suppose that one of the kinetic quantities, such as fccat, depends on the enzyme being in the acidic form. Then the rate will depend upon the ionization constant Ka and the pH according to... 

The acid-ionization constant of H3NCH2C02H is 4.5 X 10 3 (pKa — 2.34, Equation 25-1), which is about 25 times greater than Ka for ethanoic acid. (Section 18-2). This is expected because of the electron-attracting... 

The compound can only be a carboxylic acid no other class containing only carbon, hydrogen, and oxygen is more acidic. A reasonable choice is HC=CCOzH C-2 is sp-hybridized and therefore rather electron-withdrawing and acid-strengthening. This is borne out by its measured ionization constant Ka, which is 1.4 X 10 2 (pATa 1.8). 

The acid-dissociation constant, Ka, is the equilibrium constant for the ionization of a weak acid to a hydrogen ion and its conjugate base ... 

Relative quantitative strengths of acids and bases are given either by their ionization constants, Ka and Kb, or by their pKa and pKh values as defined by ... 

Sections 3.3.1 and 4.2.1 dealt with Bronsted acid/base equilibria in which the solvent itself is involved in the chemical reaction as either an acid or a base. This Section describes some examples of solvent effects on proton-transfer (PT) reactions in which the solvent does not intervene directly as a reaction partner. New interest in the investigation of such acid/base equilibria in non-aqueous solvents has been generated by the pioneering work of Barrow et al. [164]. He studied the acid/base reactions between carboxylic acids and amines in tetra- and trichloromethane. A more recent compilation of Bronsted acid/base equilibrium constants, determined in up to twelve dipolar aprotic solvents, demonstrates the appreciable solvent influence on acid ionization constants [264]. For example, the p.Ka value of benzoic acid varies from 4.2 in water, 11.0 in dimethyl sulfoxide, 12.3 in A,A-dimethylformamide, up to 20.7 in acetonitrile, that is by about 16 powers of ten [264]. 

FIGURE 2-12 The relation between dimerization constant K2) and ionization constant (Ka) for some carboxylic acids. [From Allen and Caldin, Quart. Revs. (London) 7, 255-78 (1953).]... 

Antimicrobial activity, within a pharmaceutical context, is generally found only in the organic acids. These are weak acids and will therefore dissociate incompletely to give the three entities HA, H+ and A- in solution. As the undissociated form, HA, is the active antimicrobial agent, the ionization constant, Ka, is important and the pKa of the acid must be considered, especially in formulation of the agent. 

For weak acids, the magnitude of is very small, and as a result the resulting H3O+ and A ions will be produced in small amounts. Under those conditions, both Yjj+ and y will be approximately equal to one, and then one can approximate the thermodynamic equilibrium constant, K, by the concentration-based ionization constant, Ka. 

Acid dissociation constant, acidity constant, or the acid ionization constant, (Kg), is a specific type of equilibrium constant that indicates the extent of dissociation of ions from an acid. The acidity of a substance and its Ka value are directly proportional. A larger Kg (smaller pKg) means a stronger acid. Using the values of Kg, the strength of acids can be determined easily. 

As the number of carbon atoms increases in an acid, ionization decreases, and hence the ionization constant (Ka) decreases. Because of the decrease in the concentration of hydrogen ions, acidic strength decreases as shown in Table 3. 

A weak acid ionizes only slightly in solution, perhaps only to a few percent, and in solution between the molecular acid is in equilibrium with its ions. There are thousands of known weak acids and five of the more common ones are listed below with the equation for their equilibrium in solution. The equilibrium constant for the ionization of weak acids is symbolized Ka, and is called the acid-ionization constant. 

The equilibrium ionization constants KA for selected p-substituted benzoic acids including benzoic acid, their pKA values, and Hammett s constants are collected in Table 2. The plots of KA x 10s and also pKA against a for these acids are shown in Fig. 4. The data in Table 2 and the plots in Fig. 4 demonstrate their linear relationships. As K,a increases, pK decreases, and a increases. 

Generally, drug substances capable of undergoing ionization yield more complex pH-rate profiles. For example, each ionic and nonionic form of the drug could be subject to hydronium ion, hydroxide ion, and water catalysis. When this happens, the expression for , s may contain more than three terms. For example, apparent degradation rate constants for a drug substance that is a weak base will depend on the ionization constant, Ka of the conjugate acid of the weak base and the concentration of hydronium ion and other species, as described by... 

Many substances are partially ionized in the aqueous layer as weak acids. This introduces a pH effect on the extraction. Consider, for example, the extraction of benzoic acid from an aqueous solution. Benzoic acid (HBz) is a weak acid in water with a particular ionization constant Ka (given by Equation 18.4). The distribution coefficient is given by... 

An acid in the Bronsted sense is defined as a species having a tendency to lose a proton. The acid strength or acidity of such an acid is defined in terms of the equilibrium 1 for the dissociation of the acid, most commonly in water. The dissociation, or ionization, constant Ka is defined by equation 2 where the subscripted a terms refer to the activities of these species. For some purposes activities may be replaced wholly or in part by concentration, and so we may also have equations 3 and 4. The (a) thermodynamic , (b) concentration or classical and (c) practical or mixed values, respectively Ka, Ka and Ka, become indistinguishable at zero ionic strength, and for many circumstances the differences between these constants are unimportant, often being less than experimental uncertainty. Provided due prudence is used in comparing results... 

---