STUDY OF PH-DEPENDENT DRUGS SOLUBILITY IN WATER*

The solubilities of five sparingly soluble drug-compounds in water were mea-sured at constant temperatures (298.2 and 310.2 K) by the classical saturation shake-flask method. All substances presented in this work are derivatives of anthranilic acid: flufenamic acid (FLU), mefenamic acid (MEF) niflumic acid (NIF), diclofenac sodium (DIC) and meclofenamic sodium (MEC). All of them have anti-inflammatory action. Since the aqueous solubility of the ionized drug is significantly higher than the unionized, the experimental conditions that affect equilibrium solubility values such as composition of aqueous buffer were examined. The Henderson-Hasselbalch (HH) relationship was used to predict the pH-dependent solubility profiles of chosen drugs at two temperatures. For this purpose the pKa values of the investigated drugs were determined using the Bates-Schwarzenbach spectrophotometric method at a temperature of 310.2 K. At temperature of 298.2 K these values were reported previously. Similar values of pKa were obtained from the solubility measurements