Acid-base equilibria of diazepam and prazepam in montmorillonite suspensions.
The distribution of ionic species of diazepam and prazepam in aqueous and aqueous montmorillonite clay suspensions at several pH conditions (pH 1-12) was monitored spectrophotometrically. Measurements were performed at 284 and 365 nm for diazepam and 285 and 361 nm for prazepam. The interaction between the negative clay surface and protonated species of the drugs studied relative to the unchanged species is responsible for the apparent displacement of pK(a) values from 3.3 and 2.7 to 4.4 and 3.9 for diazepam and prazepam respectively. Changes in the partial molar free energy of the ionic species of both drugs (DeltaG(i)) as a result of interactions with montmorillonite suspensions was -1.47 and -1.72 for diazepam and prazepam respectively. The effect of an additional ionic solute i.e. sodium chloride was also studied. The recovered amounts of both drugs from five different concentrations of veegum at pH 2, 5 and 10 indicates the effect of drug-clay interactions in drug analysis.
Ismail Mohamed
A E
AE
Department of Pharmaceutical Analytical Chemistry, University of Assiut, Assiut, Egypt.