Acid-base equilibria percent dissociation

Words that can be used as topics in essays 5% rale buffer common ion effect equilibrium expression equivalence point Henderson-Hasselbalch equation heterogeneous equilibria homogeneous equilibria indicator ion product, P Ka Kb Kc Keq KP Ksp Kw law of mass action Le Chatelier s principle limiting reactant method of successive approximation net ionic equation percent dissociation pH P Ka P Kb pOH reaction quotient, Q reciprocal rule rule of multiple equilibria solubility spectator ions strong acid strong base van t Hoff equation weak acid weak base... 

A quantitative measure of the degree of dissociation is given by the equilibrium constant for the acid or base. The higher the equilibrium constant is, the greater the percent dissociation of the acid or base. Therefore, a higher equilibrium constant means a stronger acid or base. Equilibrium constants, K and K, are listed for several com-mon weak acids and bases in Table 13.4. 

The common-ion effect is the shift in the position of an equilibrium that occurs when a substance is added that provides more of an ion already involved in the equilibrium. An example is the decrease in percent dissociation of a weak acid on addition of its conjugate base. 

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... 

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 this situation, if the hydrogen ion concentration increases (pH becomes lower), the reaction will be driven to the left by mass action (to the original condition), and the proportion of the drug in the nonionized form will increase and, hence, the number of lipid-soluble molecules. For example, if the pICa of a weak acid is 5.0 and it is placed in a medium of pH 4.0, 90 percent will be in the unionized form. Therefore, weak acids are preferentially absorbed in a relatively acidic environment. For a weak base, the equilibrium dissociation constant can be expressed as follows ... 

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