Acid–base equilibria and solubility of loratadine and desloratadine in water and micellar media

Acid–base equilibria in homogeneous and heterogeneous systems of two antihistaminics, loratadine and desloratadine were studied spectrophotometrically in Britton–Robinson’s buffer at 25 °C. Acidity constant of loratadine was found to be pKa 5.25 and those of desloratadine pKa1 4.41 and pKa2 9.97. The values of intrinsic solubilities of loratadine and desloratadine were 8.65 × 10−6 M and 3.82 × 10−4 M, respectively. Based on the pKa values and intrinsic solubilities, solubility curves of these two drugs as a function of pH were calculated. The effects of anionic, cationic and non-ionic surfactants applied in the concentration exceeding critical micelle concentration (cmc) on acid–base properties of loratadine and desloratadine, as well as on intrinsic solubility of loratadine were also examined. The results revealed a shift of pKa values in micellar media comparing to the values obtained in water. These shifts (ΔpKa) ranged from −2.24 to +1.24.