What is the pH below which sulfadiazine (pKa = 6.48) will begin to precipitate in an infusion fluid, when the initial molar concentration of sulfadiazine sodium is 4 x 10 2 mol dm 3 and the solubility of sulfadiazine is 3.07 x 10 4 mol dm 3?


The pH below which the drug will precipitate is calculated using equation (5.11):

Basic drugs

Basic drugs such as ranitidine are more soluble in acidic solutions, where the ionised form of the drug is predominant. If S0 is the solubility of an undissociated base, RNH2, the expression for the solubility (S) as a function of pH can be obtained as follows. In water:

RNH2 + H2O


That is

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