Approximation of weak stability constants of paramagnetic complexes by spin-lattice relaxation measurements of water protons

Abstract

The literature was briefly reviewed concernling the determination of stability constants of metal ions with various complexones using water proton relaxation methods. Experimentally, weak stability constants (Kstab) of ethylenediaminetetraacetic acid (EDTA) and citric acid complexes with several transition and lanthanide ions were obtained at constant pHvia measurements of spin-lattice relaxation times (T1) of water protons. The results were in general agreement with those determined earlier by various methods. In cases where more than one kind of complex was formed, the experimentalKstab reflected an average value of all species. Although this method is limited to measurements of weak complexes, it provides experimentally a simple approach with satisfactory results where neither great accuracy nor knowledge of equilibria and other parameters are required. It is believed that this method could be more widely applied for screening new contrast agents for MRI, and for exploration by MRI of weak low molecular weight complexes derived from biological sources.