A special type of ionic bond is when one of the ions is either H+ or OH—, resulting in an acid or a base, respectively. Water serves as a source and sink for hydrogen ions that are generated or consumed by the dissociation of acids and bases. In fact, the very ability of other acids in solution to act as acids may depend on their being dissolved in water. Put another way, the presence of water shifts the acid dissociation equilibrium. The prototypical acid-base reaction is
where HA is the undissociated acid and A— is its conjugate base. The equilibrium constant is called the acid dissociation constant, Ka (square brackets denote molar concentration):
Taking the logarithm of equation (3.2), substituting the relations pH = —log[H+] and pKa = — log[Ka], and rearranging, we obtain an expression for the fraction of undisso-ciated acid:
Thus, the relative proportion of HA and A— depends on the pH. At low pH most of the acid is undissociated. At high pH most is dissociated. The pKa is the pH at which the
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