When selecting a dissolution medium for drug release testing, it is essential to consider the pKa to ensure the drug is adequately soluble in the medium. For acidic drugs with low pKa, a slightly basic medium may enhance solubility, while for basic drugs with high pKa, a mildly acidic medium might be more appropriate.
For an acid, the lower the pKa relative to the starting pH of the buffer and the higher its So, the higher its propensity to lower the pH as it dissolves. For a base, the higher its pKa relative to the starting pH of the buffer and the higher its So, the higher its propensity to increase pH as it dissolves.