The Determination of Calcium in Dietary Supplement Tablets by lon-Exchange A Freshman Laboratory Experiment Mark L. Dletz' Indiana University of Pennsylvania, Indiana, PA 15705
Since their introduction in the 1930'5, ion-exchange resins have found widespread application throughout the field of chemistry. During this time, a number of experiments illustrating the various uses of these resins have appeared in THIS JOURNAL (1-19). Many of these experiments, however, are not suitable for use in an introductory-level chemistry laboratory course containing largely students who are not science maiors. For such students, an exercise that illustrates ion-exchange methodology with "real world" samples and simple equipment and techniques is desirable. With this in mind, we have devised a simple ion-exchange experiment in which the amount of calcium present in dietary supplement tablets is determined. In this paper, the experimental procedure is described and some typical student results for ieveral brands of tablets are presented. Experimental All reagents used were analytical grade. Distilled water was used in the preparation of all solutions.
Column Preparation The ion-exchange column is conveniently prepared in a 50-mL buret. First, a plug of glass wool is pressed into the buret. Then, an aqueous slurry of strong cation-exchanger is added to make a column-10 cm long. (We used Dowex 50W-EX, 20-50 mesh, exchange capacity 1.8meq/mL wet.) Three 25-mL aliquots of 3 M hydrochloric acid are then passed through the column to convert the resin to the H+ form. Finally, the column is rinsed with distilled water until free of chloride. (To enahle students to complete the experiment in a R-h lalnwator).period, we usually provide rhem with mlwnns containing t l w rrrin under distilled unter whah haw bem prwiuusly prepared by slud~nlar8i~l.inls.J Sample Preparation and Analysis A weighted calcium lactate2 tablet is ground to a fine powder using a mortar and pestle. An accurately weighed portion of this ground tablet (0.20-0.50 g) is then placed in a 150-mL beaker and dissolved in about 100 mL of hot (70-80°C) distilled water. The resulting solution is then passed through #1 Whatman filter paper to remove traces of undissolved hinders. The beaker and the filter paper are rinsed with hot water, and these rinsings are combined with the filtrate. This solution is allowed to cool to room temperature and then diluted to 250 mL with distilled water. A 25.0 mL aliquot of this solution is passed through the exchange column at a rate of -2-3 mL/min. This is fallowed by three 25.0-mL aliquots of distilled water eluted at a slightly faster rate (-5 mL/min). The eluate is collected in a 250-mL Erlenmeyer flask. When approximately 100 mL has been collected, it is titrated with a standard (-0.01 M) sodium hydroxide solution using phenalphthalein as an indicator. This procedure is then repeated for two additonal 25.0-mL portions of the sample solution. From the average volume of sodium hydroxide solution required to reach the endpoint, the amount of calcium present in the tablet is calculated.
Results Some typical student values for the calcium content of three different brands of tablets are shown in the table. Also shown are the corresponding nominal values of calcium content vrovided hv t h r manuiacturersand the resulrsoiKlYl'A titrations perfirmed by the author as a check of the students' results (20). As can be seen, there is reasonably good agreement among the values. Dlscussion This experiment has several features that make it attractive as an introduction to the use of ion-exchange resins. First, because of its simplicity, even students with relatively weak lahoratorv hackerounds can obtain satisfactory results. As with most ion-exchange experiments, rapid of the sample solution through the column is the most significant source of error. This can be remedied if the students are r e ~ e a t e d l vreminded that low flow rates are necessary to ensure that efficient exchange occurs. Next, the experiment provides students with an opportunity to acquire experience in the analysis of "real world" samples. Finally, it demonstrates the relevance of laboratory theory and techniques to everyday life. Acknowledgment The author wishes to express his appreciation to Millicent Firestone and Aueusta Svtv . . for their helpful comments and " suggestions. Llterature Cited (1) De1mas.M.: Ka1ck.P.; Gorrichon, J.;Gaset,A.J. C h e m . E d w . 1982.59.7W. (2) Anderson. J. L.; Gmhs, J.;Frick,D. J . Chrm. Educ 1980.57.521. 131 Coine1ius.R. J. C h e m E d u r . 1980.57.316. 141 Piosni.G. F. J . Cham. Educ. 1976.53.733. ( 5 ) Renganalhan. S.; Mehts. 6.J. J . Chsm. Educ. 1976.53,347. (6) Kouhek,E.J . Chem.Educ. 1976.53.254. (7) Bedetti, R.; Carunchio, V.: Tommetti, M. J. Chem.Educ. 1916.53.122. (8) Mumi. J. F.: Steams, R. L. J. C h e m Educ. 1975,52.750. (9) Olron.M.V.:Crawford,J.M. J. Chem.Educ. 1975.52.546. (101 Stock. J.T. J. Chrm.Edur. 1974.51,491. (111 Davies,M. B.;Lethhridge, J. W . J. C h s m E d u e . 1913.50.793. (12) Woyh, J. A. J . Cham.Edur. 1970,47,715. (13) Ruach. A. J. J. Chsm. Edur. 1969,46,591. (14) H8zel.F. J. J . C h e m E d u c . 1966,44636. (15) Mucci, J. F.: Hol1irtcr.C.; Mar8hsll.L. R. J. Chem.Edue. 1964,41,602. (16) Bryant, J.I.;Rosemvaarer,H.J. Chem. Edur. 1962.39.296. (17) Bunner, 0. D.; Jackson, R.: and Rogers, 0. C . J . Chem.Educ. 1962,39,37. (181 Vick,M.M.:Harrin,E.L.J . Chrm.Educ. 1961,38,414. (19) Mucci. J. F.:Spiegel,D.E.:Stea~ns.R.L. J.Chem.Educ. 1961.38.348. (20) skoog, n. A.; west,n. M. "Fundamentals of Analytical Chemisfry", 3rd ed. Rinehsrt, and Winston: New York. 1976:p 738.
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Present address: Department of Chemistry, University of Arizona, Tucson, AZ 85717. %Thelimited water solubility of calcium carbonate and phosphate makes su~olementscontaining these compounds unsuitable as samples.
Volume 63
Number 2
February 1986
177
