Effect of diffusion layer pH and solubility on the dissolution rate of pharmaceutical bases and their hydrochloride salts. I: Phenazopyridine.

The pH-solubility profile of phenazopyridine as determined by the addition of HCl or NaOH solutions to its aqueous suspension was identical to that of its hydrochloride salt except during phase transition from base to salt. With the addition of HCl to a suspension of the base, the pH dropped to a certain point and then remained constant until a supersaturated solution was formed. Only after a high supersaturation did precipitation of the hydrochloride salt occur. The solubility of the salt decreased at low pH due to a common ion effect. Unlike solubility profiles, the pH-intrinsic dissolution rate profiles of the base and its salt differed greatly. At low pH, the dissolution rate of the hydrochloride salt decreased with an increase in HCl concentration, whereas the dissolution rate of the base increased. The self-buffering action of the base and the increase in solubility, leading to a supersaturation of the diffusion layer was responsible for the increase in its dissolution rate with a lowering of the pH of the medium. Good conformity with the Noyes-Whitney equation was demonstrated when the solubility values under pH conditions such that the diffusion layer thickness approaches zero (Cs,h = 0) were used rather than solubilities under pH conditions of the bulk media (Cs). Supersaturation of the dissolution medium was observed during dissolution of the hydrochloride salt at pH 7.