DETERMINATION OF COMPLEX STABILITIES WITH NEARLY INSOLUBLE HOST MOLECULES .1. SPECTROPHOTOMETRIC DETECTION

The determination of stability constants by measuring the increase in solubility of a nearly insoluble host molecule due to complex formatio n with a soluble guest is discussed. If the host molecule absorbs in t he UV-visible region, spectrophotometric measurements are easily perfo rmed to obtain the stability constants. The solubilities of the crown ether dibenzo-18-crown-6 and of the cryptand (222BB) in aqueous soluti ons and the molar absorptivities of these ligands and their complexes with alkali and alkaline earth metal cations were determined. The incr ease in solubility of these ligands due to complex formation with cati ons was used to calculate the stabilities of the complexes formed. Eve n under the assumption that neither the molar absorptivities nor the s olubilities are known, the calculated stability constants agree very w ell with those obtained using the measured values of the molar absorpt ivities and solubilities. The accuracy of the stability constants even increases with decreasing solubility of the ligands.