Cationic acid, ionization constant

The acid ionization constant (pKa) of the aqua complex ion determines whether or not the ion would form complexes with a protein. This depends greatly on the pH of the medium. Since the ionization constant of low charge is 12.6, the ion would form a stable complex only with negatively charged protein in alkaline media. It cannot bind to cationic proteins because it does not share electrons to form a covalent bond. This consideration explains why the activity of Ca2+-activated protease is optimum in the alkaline pH range. Thus a decrease in its activity at acidic pH values may partly be due to a change in the electronic state of Ca2+ (Asghar and Bhatti, 1987 Barbut and Mittal, 1985). 

Acid ionization constant. The equilibrium constant for the acid ionization. (15.5) Actinide series. Elements that have incompletely filled 5/ subshells or readily give rise to cations that have incompletely filled 5/subshells. (7.9) Activated complex. The species temporarily formed by the reactant molecules as a result of the collision before they form the product. (13.4) Activation energy. The rninimum amount of energy required to initiate a chemical reaction. (13.4)... 

Ans. Since both anion and cation will hydrolyze, it is necessary to compare basic and acidic ionization constants in order to determine the extent of hydrolysis. The ammonium ion will produce H30 , and monohydrogen phosphate will produce OH in water. 

Tetramethylammonium 1,1,2,3,3-pentacyanopropenide is useful for preparation of pentacyanopropenide salts of other metal and quaternary ammonium cations by metathesis.8 The free acid, which may be obtained by use of an ion-exchange resin,3 has an ionization constant comparable to that of a strong mineral acid (pK < —8.5 the anion is not detectably protonated in 12M sulfuric acid).4... 

The effect of various surfactants, the cationics-eetyl trimethyl ammonium bromide (CTAB), and cetyl pyridinium chloride (CPC), the anionic-sodium lauryl sulfate (SLS), and the nonionic-polysorbate 80 (Tween 80), on the solubility and ionization constants of some sparingly soluble weak acids of pharmaceutical interest was studied (Gerakis et al., 1993). Benzoic acid (and its 3-methyl-, 3-nitro-, and 4-tert-butyl-derivatives), acetylsalicylic acid, naproxen, and iopanoic acid were chosen as model drugs. The cationics, CTAB and CPC, were found to considerably increase th< ionization constant of the weak acidS Ka ranged from-0.21 to-3.57), while the anionic, SLS, showed a negligible effect and the nonionic, Tween 80, generally decreased the ionization constants Solubility of the acids increased in aqueous micellar and in acidiLed micellar solutions. 

The acid dissociation constant (pKa) of an organic compound is useful in assessing its environmental fate. The pKa value can be used to define the degree of ionization of a compound at a given pH and the potential for sorption to surfaces by cation exchange. The extent to which a compound is sorbed can have a significant effect on its bioavailability, transport, photolysis, and biodegradability. 

Recently, Weber and co-workers reported on the application of dialysis to the determination of the complexing capacity of fulvic acid for a series of metal cations (19, 20). In the present paper we report a further elaboration on the dialysis technique to allow simultaneous determination of complexing capacity for the humic acid sample, and interaction constants for the metal ion - humic acid complexes, as well as the stoichiometry of the latter. The technique furthermore opens up a possibility for an indirect determination of apparent ionization constants for the humic acids. 

Note that nothing has been stated about the charge of the acid HA, which can be a neutral molecule HA or a cation BH such as the ammonium ion. The add cannot, however, be an anion HA , because in general such an anion is also a weak base and so two acid-base systems in addition to water are involved. For a cationic add BH the ionization constants of the acid and of the conjugate base are related. Thus the ionization constant of BH as an acid is... 

The ionization constant for the cationic acid is thus KJKi, the classical value for the hydrolysis constant of the cation (conjugate acid) of a weak base. Thus for a solution containing ammonium ions the acid-base reaction can be written... 

Qualitatively the maximum ionization constant occurs when the tendency toward increased proton transfer with rising temperature has been just counterbalanced by the decreasing tendency toward charge separation. For cationic acids there is no net separation of charged species, with the result that the ionization constant increases regularly with increasing temperature (see NH4, Figure 3-3). 

Table 1 Ionization Constants of Neutral and Cation Acids of type HA and HB respectively, in Water, Amphiprotic, and Dipolar Aprotic Solvents...

THF has a low dielectric constant compared to DMSO and CHjCN and therefore disfavors the ionization of neutral acids HA compared to cationic acids HB. This explains why separate lines are found for pKoi (HA) and pK F(HB ) when plotted against pK OMso qj- pK weCN Values of pKot(HA) and pK (HB+) determined in other low dielectric constant solvents like CHjClj and DCE should also have two separate correlations of this type because of ion-pairing effects. 

That acid reduces the charge on the pectin molecule has been proved by electrophoretic analysis SS) and is to be expected from the ionization constant of the acid groups in pectin. Near pH 2 the pectin shows practically zero charge, and near this pH the strongest pectin gels are formed in the absence of polyvalent cations (11). 

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