Cyclodextrin assisted nanophase determination of alkaloid salts.

The poor water solubility of the free base and the high dissociation constant (K(a)) hinder mainly the assay of alkaloid salts. We have elaborated an environment friendly method that can be carried out in aqueous media. The stability difference of the cyclodextrin (CD) complexes of free and protonated bases were used for this purpose. The base is included into the hydrophobic cavity of the CD (which serves as an apolar solvent phase on molecular level) and its solubility in water is increased. Since the base forms more stable inclusion complex than its protonated species, the pK(a) is decreased and the potentiometric titration is promoted by this way, too. Six different hydrohalide alkaloid salts have been investigated and the most appropriate CDs were chosen (depending on the size of the molecules and/or substituents). The results of the assays agree well with those obtained by the direct nonaqueous titrations. The stability constants of the inclusion complexes have been also computed.