The determination of complex stabilities with nearly insoluble host molecules. Complexation of barium(II) with substituted diaza-18-crown-6 ligands in aqueous and methanolic solutions

The complex formation between Ba2+ and different diaza-18-crown-6 ligands has been studied using a new spectrophotometric method. In the presence of undissolved solid ligand the concentration of the dissolved ligand increases in salt containing solutions due to complex formation. The change in the total concentration of the dissolved ligand can be monitored by UV-VIS spectrometry if the ligand contains absorbing groups. However, any other method to measure the variation of the ligand concentration in solution, e.g. the total organic carbon content, allows the calculation of the stability constants of the complexes formed. It is not essential for the determination of the stability constant to know the absolute concentration of the ligand in solution. The experimental method described offers some advantages compared with other experimental techniques. The main advantage is that only very small amounts of ligands are needed.