RPLC determination of acid dissociation constants and quantitative estimation for sulfasalazine

Acid dissociation constant value (p K a ) is key parameter for predicting the extent of the ionization of a drug molecule at different pH. The effect of the mobile phase composition on the acid dissociation constant values was studied by measuring the p K a values at different acetonitrile concentrations, ranging from 25 to 35 % (v/v). p K a values and limiting retention factors of SSZ were calculated chromatographically using equations derived. From calculated p K a values, the aqueous p K a values of SSZ were calculated by various approaches. Moreover, the correlation established between retention factors and the pH of the water-acetonitrile mobile phase was used to determine the optimum separation condition. Optimized condition was also validated with respect to system suitability, linearity, precision, accuracy, limit of detection and limit of quantitation, respectively. A Gemini NX C18 column (250 × 4.6 mm I.D., 5 µm particles) was used for all the determinations.