Weakly acidic or basic drugs and their salts

If the ionisation of a weak acid is represented as described above, we may express an equilibrium constant as follows:

Assuming the activity coefficients approach unity in dilute solution, the activities may be replaced by concentrations:

Ka is variously referred to as the ionisation constant, dissociation constant, or acidity constant for the weak acid. The negative logarithm of Ka is referred to as pKa, just as the negative logarithm of the hydrogen ion concentration is called the pH. Thus

Similarly, the dissociation constant or basicity constant for a weak base is

The pKa and pKb values provide a convenient means of comparing the strengths of weak acids and bases. The lower the pKa, the stronger the acid; the lower the pKb, the stronger is the base. The pKa values of a series of drugs are given in Table 3.6. pKa and pKb values of conjugate acid-base pairs are linked

Box 3.8 The degree of ionisation of weak acids and bases

Weak acids

Taking logarithms of the expression for the dissociation constant of a weak acid (equation 3.63)

-log Ka = -log [H3O+] - log which can be rearranged to [A-]

Equation (3.70) may itself be rearranged to facilitate the direct determination of the molar percentage ionisation as follows:

Therefore, percentage ionisation = percentage ionisation =

Weak bases

An analogous series of equations for the percentage ionisation of a weak base may be derived as follows. Taking logarithms of equation (3.65) and rearranging gives

Rearranging to facilitate calculation of the percentage ionisation leads to percentage ionisation =

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