3-D Topo Surface Visualization of Metal Ion Anti-buffering: An Unexpected Behavior in Metal–Ligand Complexation Systems

Abstract

Diluting a system with metal complexes can sometimes cause free metal ions to increase in concentration. This paper describes “metal ion anti-buffering”, a situation in which free metal ion concentrations rapidly increase as system dilution drives dissociation. It only occurs under excess free ligand conditions when a solution is dominated by higher stoichiometry complexes. The Law of Mass Action is used to provide a mathematical justification for the phenomenon. A Cu2+-ethylenediamine mixture exhibits this phenomenon when excess free ethylenediamine (en) is present. For example, it occurs when diluting a solution containing a 4-fold excess of en over Cu2+. As this mixture is diluted by a factor of ∼5600, the modeled free Cu2+ concentration shows a ∼ 470-fold increase. Taken together, this is 2.5 million times higher than dilution of the system would yield in other circumstances. Included are experimental data confirming anti-buffering in the Cu2+-en system. Many other metal–ligand systems can display this behavior. Four additional examples are illustrated, including an amino acid under physiological pH. Anti-buffering TOPOS, a free downloadable Excel workbook, allows readers to simulate this behavior for many metal–ligand systems. A PowerPoint lecture and teaching materials are also provided, suitable for inclusion in upper division and graduate courses in analytical chemistry, biochemistry, and geochemistry.