Solubility of quinoline in aqueous systems: Effect of pH and ionic strength

Abstract

Nitrogen-heterocyclic compounds are commonly found in gas waste and coal cleaning residue. Within this general compound class, quinoline is especially important because it has the potential to induce liver carcinoma and it also has a high solubility in water. The pH-solubility profile of quinoline was determined in citrate-phosphate buffer at different ionic strengths. The pKa was observed to change with ionic strength. The intrinsic solubility is always reported as a constant. However, in this study the intrinsic solubility was observed to vary with ionic strength. The solubility of quinoline is dependent on pH as well as the ionic strength. At pH values lower than the pKa (4.96), quinoline is in the protonated form (QH⁺), and enhanced solubility was observed. At pH values higher than the pKa, quinoline is in the neutral form (Q) which is the form that determines intrinsic solubility (So). So decreased with increasing ionic strength. This observation can be explained as a salting out effect. From the solubility data the apparent ionization constant, pKa, of quinoline was obtained for the different systems.