Para Amino Benzoic Acid (PABA) is a weakly acidic drug. So, Spectrophotometric method is applicable for determination of dissociation constant for PABA. Spectrophotometrically dissociation constant (pKa) of the indicator acid, PABA in 5,10,20,30 and 40 wt. % 1-butanol – water mixtures at 24.5±0.5ºC were measured. The pure indicator acid and potassium salt of indicator acid show maxima at 360 nm and 400nm. The spectrophotometic data have been analyzed in term of an ion – pair intermediate model. The experimental values of ЄIn- potassium salt of indicator acid and the derived values of intercept of ЄIn- values using equation, shows a good consistency ,suggesting the ion-pair intermediate model is applicable in the present instance. The maximum drop in pka was found to occur around 20 wt. % 1-butanol- water mixtures. This has been explained as due to increase in solvation energy for anion of the acid by Londondispersion force mechanism. The drastic increase in pka values above 20 wt% 1-butanol has been assumed to be due to decrease in dielectric constant of the medium. The values of pKa and pH are especially important to understand solubility profile, absorption, distribution of drug and drug product. Obviously it can mention here that this research work will helpful so greatly in the field of dissolution study, pre-formulation study, formulation & different types of dosage form development and drug delivery of drugs are weakly acidic in nature.
