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Recall that titration is the quantitative measurement of an analyte in solution by reacting it completely with a standardized reagent. Complexes form in a fixed stoichiometry so a standard solution of a ligand can be used to titrate a metal ion in solution. Similarly, a standard solution of a metal ion can serve as the titrant for a species that acts as a ligand.
Ethylenediaminetetraacetic acid (EDTA) is a common chelate because it makes 6 bonds with metal ions to form 1:1 complexes with large formation constants. The fully protonated form of EDTA is:
CH2COOH CH2COOH \ / :N-CH2CH2-N: / \ CH2COOH CH2COOH
The two nitrogen atoms can donate their lone pairs to form two bonds and the four -OH groups can lose thier protons to form four more bonds to the metal.
Calmagite indicator has two -OH groups with acidic protons. The color of calmagite changes depending on whether or not these protons are present. At pH=10 one proton is present and the color of the indicator is blue. A calcium or magnesium ion can displace both protons to form a calmagite-metal complex, which has a red color. Ca2+ and Mg2+ can be titrated using EDTA as the titrant and calmagite indicator because the EDTA binds Ca2+ and Mg2+ more strongly than the indicator. At the endpoint, the EDTA will bind all of the metal, leaving the calmagite with no metal ions. A solution containing calmagite will turn from red (or purple very near the endpoint) to blue.
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