8/18/2019 Complexometric Titration of Zinc

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Vol. 74 No. 12 December 1997 • Journal of Chemical Education 1463

In the Laboratory

Complexometric Titration of Zinc

An Analytical Chemistry Laboratory Experiment

S. G. Novick

Department of Chemistry, 135 Hofstra University, Hempstead, NY 11549-1350

Complexometric titrations with EDTA have tradition-al ly been performed in undergraduat e ana ly t ical chemistrycourses to determine t he calcium or magn esium cont ent ofwa ter. These t i trat ions ar e performed at a basic pH, w hereth e forma tion consta nt s of Ca -ED TA and Mg-ED TA com-plexes are high. These types of problems are well-treatedin the analy t ical chemistry textbooks (1, 2 ) . In contrast ,tr eat ment of meta l ions w hose ED TA complexes occur sig-nif icantly at l ow  pH (e.g., Zn 2+ , Fe3+ , C u2+ , Ni2+ , P b 2+ , Al 3+ )(3 ) is sparse. An incorrect conclusion ca n be r eached by t hes tudent tha t practical ED TA tit ra tions a re only performedat high pH. In addition, widening the window of possiblemetal ions for complexometric titration affords the possi-bi l i ty of analyzing real world products , such as the coldlozenges discuss ed below.

Discussion

Zinc content in a sample can be determined quant i ta-tively by complexometr ic titra tion wit h E DTA at pH 5.5.The effective forma tion const a nt of the Zn-ED TA complexis ≥ 106 above pH 4 (3 ). Xylenol Orange is used as an indi-cator; it is yellow when it is free and red when complexedwith zinc. A competition is set up between EDTA and theindicator. When all of the zinc ions have been complexedwi th ED TA, only the f ree ind ica t or remains , c a us ing achange in color from pink to yellow.

Cold-Eeze™ cold lozenges ar e a product designed toreduce the dura t ion and sever i ty o f the common coldthrough the actions of ionic zinc in the mouth, throat and

nasa l cavi ty (4–8 ). Therefore, th e zinc ions mu st be qua nti-ta tively released from the cold lozenges into sa liva. Analyticalstudies have shown that 93%of the zinc ions are releasedinto sal iva (9 ). How ever, ED TA is a much st ronger complex-ing agent t hat any other ingredient in the lozenges, and canbe used to quantitatively determine zinc content. The coldlozenges are designed to be slowly dissolved in the mouthwhere the pH of saliva is on average 5.5. For this reason,the titrations are performed at that pH. The cold lozengesa lso conta in gluconat e, glycine, an d a ha rd candy ba se. Thetheoretical amount of zinc per lozenge is 11.5 mg or 14.5mg, depending on the type of lozenge. Amounts of zinc perlozenge are print ed on th e packages.

Experimental Procedure

Reagents 

1. Cold-Eeze™ cold lozenges (Quigley Corpora t ion,

Doylestown , PA)1

2. p H 5. 5 a c et a t e b u ff er

3. Xylenol Orange indicator (0.1%solution in DI w ater)2

4. 0.01 M Na 2ED TA solut ion

Procedure 

1. Dry Na 2EDTA·2H 2O for about one hour in th e oven;cool in a desiccator.

2. Prepare the pH 5.5 buffer by combining 1.35 g ofglacial a cetic acid (CAUTION!) and 10.25 g of sodium a ceta te(or 17.01 g of sodium a ceta te t rihydr at e) in a 250-mL volu-metric f lask an d diluting to the ma rk. Adjust pH to 5.5 wit h

either 6 M NaOH or glacial acetic acid.

3. Pr epare t he ED TA solution by w eighing 0.93 g of“dry” Na 2EDTA·2H2O into a 250-mL volumetric f lask anddiluting to the ma rk. Deduce the ac tual molari ty a ssumingtha t t he ED TA is pure.

4. Perform each titration as follows: Dissolve one coldlozenge in 50.0 mL of buffer; gentle heat ing ma y be neces-sa ry. Cool to room tempera tur e. (Optional: mea sure th e pHof the resul tan t so lu t ion . ) Prepare a b lank solu t ion todetermine the endpoint color. Add a few drops of XylenolOra nge solution an d titr at e with E DTA solution to the end-point. U se one titra tion to determine the endpoint and car e-fully complete three additional titrations.

Results

Results from an a lyses of lozenges by this method ha veyielded consistent data . For the 14.5-mg lozenges, 108ana lyses had a n a verage of 14.33 mg zinc with a s tan darddeviation of 0.74, and 119 analyses of the 11.5-mg lozengeshad an average of 11.58 mg zinc with a s ta ndard deviat ionof 0.62. This is well within the FDA guideline of 90–120%of the theoretical amount.

Conclusion

This experiment is stra ightforward an d easy to perform,giving students experience in buffer prepar at ion a nd E DTAtit ra tion. The endpoint is crisp wit h a visible cha nge of color.Students are given the opportunity to analyze a marketedproduct in the same way as is used by the manufacturers.St udents can consider themselves the “qualit y cont rol” chem-ists for t he ma nufa cture of these cold lozenges. They can com-pare t he avera ge milligrams of zinc to the expected amountan d comment on the consistency of ma nufacture.

Acknowledgments

I wish to acknowledge J ohn C. Godfrey and Nan cyJ aneGodfrey for a naly t ical results .

Notes

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 withpure Xylenol Orange perform better than those prepared from aprepackaged mixture of Xylenol Orange and NaCl (diluent).

Literature Cited

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cer, N.; Godfrey, N. J . J. I nt. M ed. Res. 1992, 20 , 234–246.8. Godfrey, J . C. ; Godfrey, N. J . ; Novick, S . G . Alt ern ati ve Th erapi es 

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