Spectrophotometrically dissociation constant (pKa) of the indicator acid, 2,4-Dinitrophenol in 5,10,20,30 and 40 wt. % 1-propanol – water mixtures at 24.5±0.5ºC were determined. The experimental values of ÐIn- potassium salt of indicator acid and the derived values of intercept of ÐIn- values using Ion-pair intermediate model 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-propanol- water mixtures. This has been explained as due to increase in solvation energy for anion of the acid by London- dispersion force mechanism. The drastic increase in pka values above 20 wt% propanol has been assumed to be due to decrease in dielectric constant of the medium. This method is suitable for determination of dissociation constant for weak acid and this method will be helpful for determination of dissociation constant for drugs are weak acid in nature.