pH-solubility profiles or organic carboxylic acids and their salts

The solubilities of naproxen, 7-methylsulfinyl-2-xanthonecarboxylic acid, 7-methylthio-2-xanthonecarboxylic acid, and their sodium, potassium, calcium, and magnesium salts were determined as a function of pH. The results on the solubility of naproxen and its salts were in excellent agreement with the theoretical profiles describing the relationship between pH values of the solutions and the dissociation constant of the acid. The solubilities of the two xanthonecarboxylic acids were higher at higher pH values than the values calculated when complete dissociation in solution was assumed. The influence of the salt species on the solubility of organic carboxylic acids, at and above pH values of complete ionization, cannot be predicted even qualitatively from equations used for alkali and alkaline earth metal salts.