Titrations Based on Complexation Reactions
The earliest titrimetric applications involving metal–ligand complexation were the determinations of cyanide and chloride using, respectively, Ag+ and Hg2+ as titrants. Both methods were developed by Justus Liebig (1803–1873) in the 1850s. The use of a monodentate ligand, such as Cl– and CN–, however, limited the utility of com- plexation titrations to those metals that formed only a single stable complex, such as Ag(CN)2– and HgCl2. Other potential metal–ligand complexes, such as CdI42–, were not analytically useful because the stepwise formation of a series of metal– ligand complexes (CdI+, CdI2, CdI –, and CdI 2–) resulted in a poorly defined end point.
The utility of complexation titrations improved following the introduction by Schwarzenbach, in 1945, of aminocarboxylic acids as multidentate ligands capable of forming stable 1:1 complexes with metal ions. The most widely used of these new ligands was ethylenediaminetetraacetic acid, EDTA, which forms strong 1:1 complexes with many metal ions. The first use of EDTA as a titrant occurred in 1946, when Schwarzenbach introduced metallochromic dyes as visual indicators for signaling the end point of a complexa- tion titration.
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