pH-dependency in diffusion of some weakly acidic drugs, warfarin, sulfaethidole, and barbital, into organic phases.

The diffusion of three weakly acidic drugs (warfarin, sulfaethidole, and barbital) in cyclohexane-buffer and n-octanol-buffer systems was studied by two different methods; shaking and model cell type of experimentation. At low pH values the drugs moved generally more readily into the organic phases than at higher pH. Moreover, when the shaking method was used the drugs moved readily to the n-octanol phase even at higher pH levels. Lecithin improved the diffusion at low more than at high pH levels. Thus, the results varied according to the organic phase selected and the method used. Poor correlation was found between some results of this study and previously reported results of in situ absorption from the rat gastrointestinal tract of the same drugs. The results do not support the assumption that phospholipids may have a role in the absorption of ionized moieties.