In the Laboratory
Complexometric Titration of Zinc An Analytical Chemistry Laboratory Experiment
Downloaded via MIAMI UNIV on June 26, 2018 at 13:32:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S. G. Novick Department of Chemistry, 135 Hofstra University, Hempstead, NY 11549-1350 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., Zn2+, Fe3+ , Cu2+, Ni2+, Pb 2+, Al3+) (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-Eeze™ cold lozenges are a product designed to reduce the duration and severity of the common cold through the actions of ionic zinc in the mouth, throat and nasal cavity (4–8). Therefore, the zinc ions must be quantitatively released from the cold lozenges into saliva. Analytical studies have shown that 93% of the zinc ions are released into saliva (9). However, EDTA is a much stronger complexing agent that any other ingredient in the lozenges, and can be used to quantitatively determine zinc content. The cold lozenges are designed to be slowly dissolved in the mouth where the pH of saliva is on average 5.5. For this reason, the titrations are performed at that pH. The cold lozenges also contain gluconate, glycine, and a hard candy base. The theoretical amount of zinc per lozenge is 11.5 mg or 14.5 mg, depending on the type of lozenge. Amounts of zinc per lozenge are printed on the packages.
3. Prepare the EDTA solution by weighing 0.93 g of “dry” Na2 EDTA·2H2O into a 250-mL volumetric flask and diluting to the mark. Deduce the actual molarity assuming that the EDTA is pure. 4. Perform each titration as follows: Dissolve one cold lozenge in 50.0 mL of buffer; gentle heating may be necessary. Cool to room temperature. (Optional: measure the pH of the resultant solution.) Prepare a blank solution to determine the endpoint color. Add a few drops of Xylenol Orange solution and titrate with EDTA solution to the endpoint. Use one titration to determine the endpoint and carefully complete three additional titrations. Results Results from analyses of lozenges by this method have yielded consistent data. For the 14.5-mg lozenges, 108 analyses had an average of 14.33 mg zinc with a standard deviation of 0.74, and 119 analyses of the 11.5-mg lozenges had an average of 11.58 mg zinc with a standard deviation of 0.62. This is well within the FDA guideline of 90–120% of the theoretical amount. Conclusion This experiment is straightforward and easy to perform, giving students experience in buffer preparation and EDTA titration. The endpoint is crisp with a visible change of color. Students are given the opportunity to analyze a marketed product in the same way as is used by the manufacturers. Students can consider themselves the “quality control” chemists for the manufacture of these cold lozenges. They can compare the average milligrams of zinc to the expected amount and comment on the consistency of manufacture. Acknowledgments I wish to acknowledge John C. Godfrey and NancyJane Godfrey for analytical results. Notes
Experimental Procedure
1. Order product directly by calling 1-800/505-COLD. 2. Xylenol Orange is available from Aldrich Chemical Co., product # 22,785-4. The author has found that solutions made with pure Xylenol Orange perform better than those prepared from a prepackaged mixture of Xylenol Orange and NaCl (diluent).
Reagents
Literature Cited
1. Cold-Eeze™ cold lozenges (Quigley Corporation, Doylestown, PA)1 2. pH 5.5 acetate buffer 3. Xylenol Orange indicator (0.1% solution in DI water)2 4. 0.01 M Na 2EDTA solution
Procedure 1. Dry Na2 EDTA·2H2O for about one hour in the oven; cool in a desiccator. 2. Prepare the pH 5.5 buffer by combining 1.35 g of glacial acetic acid (C AUTION!) and 10.25 g of sodium acetate (or 17.01 g of sodium acetate trihydrate) in a 250-mL volumetric flask and diluting to the mark. Adjust pH to 5.5 with either 6 M NaOH or glacial acetic acid.
1. Day, R. A., Jr.; Underwood, A. L. In Quantitative Analysis, 6th ed.; Prentice Hall: Englewood Cliffs, NJ, 1991; pp 623–624. 2. Harris, D. C. In Quantitative Chemical Analysis; 4th ed.; W. H. Freeman: New York, 1995; pp 793–794. 3. Harris, D. C. In Quantitative Chemical Analysis; 4th ed.; W. H. Freeman: New York, 1995; p 323. 4. Al-Nakib, W.; Higgins, P. C.; Barrow, I.; Batstone, G.; Tyrell, D. A. J. Antimicrob. Chemother. 1987, 20, 893–901. 5. Godfrey, J. C. Antimicrob. Agents Chemother. 1988, 32, 605–606. 6. Godfrey, J. C.; Conant-Sloane, B.; Turco, J. H.; Mercer, N.; Godfrey, N. J.; Smith, D. S. In ICAAC; Chicago, 1991; Abs. No. 1381. 7. Godfrey, J. C.; Conant-Sloane, B.; Smith, D. S.; Turco, J. H.; Mercer, N.; Godfrey, N. J. J. Int. Med. Res. 1992, 20, 234–246. 8. Godfrey, J. C.; Godfrey, N. J.; Novick, S. G. Alternative Therapies 1996, 2(6), 63–72. 9. Zarembo, J. E.; Godfrey, J. C.; Godfrey, N. J. J. Pharm. Sci. 1992, 81, 128–130.
Vol. 74 No. 12 December 1997 • Journal of Chemical Education
1463
