APPROXIMATION OF WEAK STABILITY-CONSTANTS OF PARAMAGNETIC-COMPLEXES BY SPIN-LATTICE RELAXATION MEASUREMENTS OF WATER PROTONS

The literature was briefly reviewed concerning the determination of st ability constants of metal ions with various complexones using water p roton relaxation methods. Experimentally, weak stability constants (K( stab)) of ethylenediaminetetraacetic acid (EDTA) and citric acid compl exes with several transition and lanthanide ions were obtained at cons tant pH via measurements of spin-lattice relaxation times (T1) of wate r protons. The results were in general agreement with those determined earlier by various methods. In cases where more than one kind of comp lex was formed, the experimental K(stab) reflected an average value of all species. Although this method is limited to measurements of weak complexes, it provides experimentally a simple approach with satisfact ory results where neither great accuracy nor knowledge of equilibria a nd other parameters are required. It is believed that this method coul d be more widely applied for screening new contrast agents for MRI, an d for exploration by MRI of weak low molecular weight complexes derive d from biological sources.