Diluting a system that contains metal complexes can sometimes cause surprises. This chapter 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 four-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 pHs. Anti-Buffering TOPOS, a downloadable Excel workbook in a supplemental file, 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.https://scholarworks.umt.edu/topos/1005/thumbnail.jp
