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Analytical chemistry is generally introduced into the college chemistry curriculum via a quantitative analysis (quant) course. The quant course has generally been limited to "wet chemical" procedures, usually followed by an advanced course focusing on instrumental methods of analysis. Volumetric-analysis procedures are among the oldest of the quantitative methods still in use, although complexometric titrations are one of the most recently developed of these methods. The most significant advance in formulating practical complexometric experiments as we know them today was the introduction of ethylenediaminetetraacetic acid (EDTA) as a hexadentate titrant by Schwarzenbach in 1945 (1). EDTA proved to be quite valuable because it formed stable, water-soluble complexes having a 1:1 stoichiometry with most metal ions. The most popular complexometric procedure in use today uses EDTA for the titration of calcium and magnesium either in limestone or as part of a water-hardness measurement.
The Titration
Napthylazo Dye as Indicator
The original indicator used for the calcium and magnesium determination was Eriochrome Black T, first reported by Schwarzenbach and Biedermann in 1948 (2). While Eriochrome Black T still sees occasional use, it has become more common to use calmagite (1-(1-hydroxy-4-methyl-2-phenylazo)-2-naphthol-4-sulfonic acid), a similar napthylazo dye molecule developed by Lindstrom and Diehl (3). The major advantage of calmagite over Eriochrome Black T is its greater stability in aqueous solution; also, the calmagite color change is considered to be sharper at the endpoint.
Calmagite (Fig. 1)(Figure 1 omitted) is a triprotic acid that is predominantly present in its blue monoprotic form within the basic pH range used for this experiment (usually about 10, but 9-12 is acceptable) (4). When the monoprotic form of calmagite complexes with magnesium, a wine red is generated. Because the calmagite is a much weaker ligand for calcium and magnesium than is EDTA, near the equivalence point of the titration these metal ions are taken from the calmagite by the EDTA. This produces the blue, uncomplexed calmagite that signals the endpoint for the titration.
This endpoint has proven to be particularly difficult for those suffering from red-green color blindness. Color blindness is not an isolated instance; approximately 8% of males and 0.03% of females suffer from some form of color blindness (5). We routinely screen the quantitative analysis students for color blindness (6); we found...