Sulfadiazine is a sulfa drug. the solubility of sulfadiazine is 3.07x10-4 mol dm-3 (as acid form). the pk a of the acid form is 6.48 given a solution of sulfadiazine-sodium (salt) of 4x10-2 mole dm-3 concentration, calculate at what ph starts the precipitation of the acid form

At pH 8.59 the precipitation of acid will form.

Explanation:

The pH below which the drug will begin to precipitate can be calculated using the relation

pH =

where S = total saturation solubility of the drug

S° = solubility of the undissociated species

substituting the respective values in equation, we get:

pH = 8.59