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 thi...
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