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An Analytical Chemistry Laboratory Experiment
Complexometric titrations with EDTA have traditionally been performed in undergraduate analytical chemistry courses to determine the calcium or magnesium content of water. These titrations are performed at a basic pH, where the formation constants of Ca-EDTA and Mg-EDTA complexes are high. These types of problems are well-treated in the analytical chemistry textbooks (1, 2). In contrast, treatment of metal ions whose EDTA complexes occur significantly at low pH (e.g., Zn^sup 2+^, Fe^sup 3+^, Cu^sup 2+^, Ni^sup 2+^, Pb^sup 2+^, A1^sup 3+^) (3) is sparse. An incorrect conclusion can be reached by the student that practical EDTA titrations are only performed at high pH. In addition, widening the window of possible metal ions for complexometric titration affords the possibility of analyzing real world products, such as the cold lozenges discussed below.
Discussion
Zinc content in a sample can be determined quantitatively by complexometric titration with EDTA at pH 5.5. The effective formation constant of the Zn-EDTA complex is ? 106 above pH 4 (3). Xylenol Orange is used as an indicator; it is yellow when it is free and red when complexed with zinc. A competition is set up between EDTA and the indicator. When all of the zinc ions have been complexed with EDTA, only the free indicator remains, causing a change in color from pink to yellow.
Cold-EezeTM cold lozenges are a product designed to reduce the duration and severity of...