Visualization of Metal Ion Buffering via Three-Dimensional Topographic Surfaces (Topos) of Complexometric Titrations

Complexation TOPOS is a free software package to generate 3-D topographic surfaces ("topos") for metal-ligand complexometric titrations in aqueous media. It constructs surfaces by plotting computed equilibrium parameters above a composition grid with "volume of ligand added" as the x-axis and overall system dilution as the y-axis. The sample systems in this paper are restricted to EDTA as a ligand. Other chelating ligands that form exclusively 1:1 complexes could also be modeled with this software. The surfaces show the quality of the equivalence point break under various conditions. More importantly, they develop the phenomenon of metal ion buffering. They clearly distinguish the difference between "pseudo buffering" and "true buffering". They introduce terminology for two different forms of metal ion buffering: (1) 'mass action metal ion buffering under excess ligand conditions and (2) precipitate metal ion buffering when hydroxide precipitates are present under excess metal ion conditions. Systems modeled are EDTA titrations of Cu2+ and Mg2+ Supporting Information for this paper includes the Complexation TOPOS software, an Excel workbook that generates topos in under 20 seconds. Required inputs are the following: (1) a stability constant for the metal ligand complex, (2) acid dissociation constants for the ligand, (3) stability constants for hydroxy complexes of the metal, and (4) a Km, value and stoichiometry for hydroxide precipitates. Many constants are tabulated in a workbook tab. Also included are a PowerPoint lecture and teaching materials (for lecture, homework, and prelaboratory activities) that are suitable in general chemistry courses or third-year and graduate courses in analytical chemistry, biochemistry, and geochemistry.