Acid-base equilibria percent ionization

The extent to which a weak acid ionizes depends on the initial concentration of the acid. The more dilute the solution, the greater the percent ionization (Figure 15.4). In qualitative terms, when an acid is diluted, the number of particles (nonionized acid molecules plus ions) per unit volume is reduced. According to Le Chateher s principle (see Section 14.5), to counteract this stress (that is, the dilution), the equilibrium shifts from nonionized acid to H+ and its conjugate base to produce more particles (ions). The dependence of percent ionization on initial concentration can be illustrated by the HF case discussed on page 608 ... 

The box below shows a system at equilibrium. It has a volume of 0.50 L and the symbol Q represents 0.10 mol of a weak acid, HB. The symbol O represents 0.10 mol of the conjugate base, B. Hydronium ions and water molecules are not shown. What is the percent ionization of the acid ... 

The equilibrium constant for the reaction of an acid with water is called the acid ionization constant (K ), equation (16.10), and that for the reaction of a base with water is called the base ionization constant (K, ), equation (16.13). Strong acids and strong bases have large ionization constants (Xa or Kb >i> 1) and are essentially completely ionized in water. Weak acids and weak bases have small ionization constants K or Xt, 1) and ionize to a limited extent in water. The extent to which acids or bases ionize in water is described in terms of either the degree of ionization (a) (equation 16.15) or the percent ionization (equation 16.16). For a weak acid or weak base, the degree of ionization increases with increasing dilution. 

This equation essentially describes the relationship between pH and the degree of ionization of weak acids and bases. When applied to drugs, the equation tells us that when pH equals the apparent equilibrium dissociation constant of the drug (pKJ, 50 percent of the drug will be in the unionized form and 50 percent will be in the ionized form (i.e., log[base/acid] = 0 and antilog of 0 = 1, or unity). Application of the Henderson-Hasselbalch equation can, therefore, allow one to mathematically determine the exact proportion of ionized and nonionized species of a drug in a particular body compartment if the pKa of the drug and the pH of the local environment are known. 

In other words, like many other lower-melting point salts, ionic liquids arising from protonation of amino acids and their ester derivatives may not be "ionic liquids in a strict sense. The situations where the neutral acid and base represent some tens of mol% of the ionic liquid should be described as liquid mixtures, containing the ionic liquid and neutral species, whereas the term ionic liquid should be used for situation in which the percent of ionization is higher than 99%. Unfortimatcly, values for equilibrium constants for such reactions are unknown under these (non-aqueous) conditions and they are not easy to measure, therefore this distinction may be made at the moment only on the basis of the data in aqueous solution. 

With weakly ionized resins the internal ionization, and therefore the number of actual exchange groups per unit of structure, is a function of the pH of the contacting solution. The capacity of the resin varies from very low to quite high, depending on the solution with which it is in equilibrium. The rates of neutralization are slow with half times reported from ten minutes to hours. Regeneration to the free acid or base form is pH dependent and may be essentially one hundred percent efficient. 

The add dissociates (ionizes) to only a shght extent The dissodation (ionization) equilibrium position lies far to the left Weak adds have relatively strong conjugate bases Percent dissociation of a weak acid... 

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