In the Laboratory
W
Mineral Analysis of Whole Grain Total Cereal Paul Hooker Physical Science Department, Westminster College, Salt Lake City, UT 84105;
[email protected] Experiments we have implemented into the chemistry curriculum at Westminster College that deal with the analysis of everyday materials familiar to students have proven to be the most popular laboratory exercises. More importantly, they are excellent learning opportunities as the analyses typically involve complex mixtures as opposed to specially prepared standard materials. The quantitative analysis of elemental iron in Whole Grain Total Cereal using visible spectroscopy as described in this article is suitable for a general chemistry course for science or nonscience majors. The more extensive mineral analysis of the cereal, specifically for the elements iron, calcium, and zinc, is suitable for an instrumental or quantitative analysis chemistry course. Total Cereal was selected as the food of choice as it is a familiar consumer product and claims to contain 100% of the daily value of several vitamins and minerals.1 A 30 g serving is therefore expected to contain 18 mg of Fe, 15 mg of Zn, and 1000 mg of Ca (1). The actual quantities of these elements required in the diet of an individual is very dependent on the person’s age, sex, and so forth. For example, the dietary reference intake (DRI) for Fe is 18 mg兾day for premenopausal women but only 8 mg兾day for men and postmenopausal women (2). This quantity increases to 27 mg兾day for pregnant women. The DRI’s and UL’s (tolerable upper intake limit below which there is likely no risk of adverse affects) for Fe, Ca, and Zn are summarized in Table 1 (2, 3). Wheat is a natural source of Fe, but to ensure 18 mg of Fe per serving, supplemental Fe must be added to the flakes. The Fe is added as small Fe filings, that is, in its elemental reduced form, and there is enough Fe present in the cereal to make the flakes magnetic. A flake of cereal floating on water is clearly attracted to a strong neodymium magnet, a well-known and popular demonstration (4). It is the magnetic property of the Fe that enables it to be easily separated from the complex cereal mixture and quantitatively determined in a typical laboratory session. In more advanced chemistry courses a cereal sample is heated in a furnace and rendered to an acid soluble ash before analysis. The elemental Fe can be magnetically removed prior to the ashing procedure so that the supplemental Fe can be deter-
mined separately from the naturally occurring Fe from the wheat. Fortification of flour and cereals has been practiced in the U.S. since 1941; cereal products are typically supplemented with either reduced Fe or iron(III) phosphate (5). Folic acid is often added to these food products to enhance the bioavailability and absorption of the Fe, as well as to prevent neural tube defects in utero. Anemia is a common consequence of Fe deficiency in the diet, especially for pregnant and lactating women. However, there can also be consequences of too much dietary Fe intake (6). Hemochromatosis is a genetic disease affecting up to 1-in-400 individuals of Northern European descent; in the course of this disease the increase of intestinal absorption of Fe can lead to severe organ damage (7). In an informal experiment as part of this study, 20 students in a general chemistry class were given a packet of Total Cereal to take home and pour their “normal” serving size at breakfast. It was found that their typical serving size was nearly three times the 30 g specified on the packet.1 This result is corroborated by earlier studies where an actual serving size taken was 75 ± 6 g for adult males and 66 ± 4 g for adult women. (5). Clearly, by eating Total Cereal, a person would likely exceed the DRI for Fe in one meal, if not the UL. However, it is unlikely that the relatively large iron particles present in the cereal would be completely absorbed in the digestive tract. The link between an adequate calcium intake and the prevention of osteoporosis is now well established. The few reports of calcium carbonate overdosing indicate that it is not the excess calcium intake that causes damage, but the basic carbonate anion (8). For Zn, the UL is set at 40 mg兾day not because a dietary excess would cause severe poisoning, but because there is evidence that an excess of Zn may inhibit the absorption of Cu (9). Several excellent articles concerning food analysis have appeared in this Journal (10–15). This article adds to the list of analytical experiments dealing with foods and
Table 1. Dietary Reference Intake (DRI) and Tolerable Upper Intake Level (UL) for Fe, Ca, and Zn
• Unlike earlier published procedures (14), the methodology presented extracts the Fe from the flakes before digestion resulting in cleaner solutions. These solutions are more amenable to analysis by complex formation and visible spectroscopy.
Element Iro n Calcium Zinc
DRI/(mg/day) Men
Women
UL/ (mg/day)
8–11
8–27
45
1000–1300
1000–1300
2500
11
8
40
www.JCE.DivCHED.org
•
• Provides a simple methodology for a three-hour laboratory procedure where the results lend themselves to relevant discussion and student engagement. It allows for student exploration and experimentation during the laboratory period as the extraction methodology is refined.
• Provides a methodology for a more in-depth mineral analysis suitable for advanced analytical chemistry courses.
Vol. 82 No. 8 August 2005
•
Journal of Chemical Education
1223
In the Laboratory
General Chemistry Laboratory Exercise
Results and Discussion
Cereal, 10 g, is ground to a fine powder using a mortar and pestle. The powder is transferred to a glass beaker and enough water added to make a mobile slurry. To extract the iron filings the slurry is stirred magnetically with a Teflon coated stirrer bar for approximately 10 minutes. The stirrer bar and collected Fe are transferred to a test tube and covered with concentrated HCl (1–2 mL) to dissolve the Fe. The resulting solution is quantitatively transferred to a volumetric flask and diluted to 100.0 mL. The concentration of Fe2+ can be determined spectrophotometrically at 522 nm by forming the red 2,2´-dipyridyl Fe2+ complex and comparing to a standard calibration curve constructed from 1, 2, 4, 5, and 8 ppm standard Fe2+ solutions (16).
The average quantity of iron extracted magnetically from 10 g of ground cereal by 61 general chemistry students was 2.7 mg with a standard deviation of 1.1 mg. The expected quantity of Fe in this sample size is 6 mg. Those students, who reason that the magnetic stirring should continue until no more Fe is collected on a clean stirrer bar, and therefore repeatedly stir the slurry, are able to extract close to the theoretical quantity of Fe. The results obtained by two analytical chemistry students for the more complete mineral analysis of the cereal are summarized in Table 2. The values for the Fe recovered after the ashing process are typical for non-fortified whole wheat cereal, indicating the magnetic extraction procedure, if performed carefully, is very effective at removing the majority of the elemental Fe (6).
Quantitative Analysis Laboratory Exercise To quantitatively analyze for iron, calcium, and zinc in the cereal with appropriate analytical care, the following protocol can be used. The elemental Fe is extracted from several 2.0 g samples of ground cereal as described above and quantitated spectrophotometrically or by using atomic absorption spectroscopy. The remaining slurry is dried, charred using a Bunsen burner until smoking ceases, and then rendered to an ash in a furnace at 650 ⬚C. The ash is dissolved in a minimum of 6 M HNO3, diluted to 10.0 mL, and analyzed for Fe, Ca, and Zn using atomic absorption spectroscopy or another appropriate methodology. The results are summarized in Table 2.
Acknowledgments This work was supported by monies from the Gore endowment for science and mathematics at Westminster College. I also thank students Rebecca Welch and Amie Christenson for provision of their results. W
Supplemental Material
Instructions for the students, including prelab questions and lab report information, and notes for the instructor are available in this issue of JCE Online.
Hazards
Notes
Concentrated HCl and 6 M HNO3 are both strong and corrosive acids. HNO3 is also a powerful oxidizing agent. When handling these acids, care must be taken to avoid spills and the release of toxic and irritating vapors. 2,2´-Dipyridyl is harmful if ingested or inhaled, but presents no special hazards.
1. Total Cereal is manufactured by General Mills. The daily values assumed in this study are from a box of cereal purchased in June, 2001.
Table 2. The Results of Fe, Ca, and Zn Analysis for Samples of Total Cereal Recovered Fe (%) Sample 1 2
After MagnetiAshinga callya 94.1 09.8 89.3
13.2
Total Recovered (%) Fea
Ca b
Znc
104.0
105
091
102.5
105
095
3
92.8
10.4
103.2
097
075
4
93.5
10.7
104.2
104
081
5
94.7
11.1
105.8
102
105
6
85.9
14.1
100.0
106
099
a
Based on 18 mg per 30 g serving size.
b
Based on 1000 mg per serving size.
c
Based on 15 mg per serving size.
1224
Journal of Chemical Education
•
Literature Cited 1. U.S. Food and Drug Administration Center for Food Safety and Applied Nutrition, A Food Labeling Guide, September, 1994. http://www.cfsan.fda.gov/~dms/flg-7a.html (accessed Apr 2005). 2. Panel on Micronutrients, Subcommittees on Upper Reference Levels of Nutrients and of Interpretation and Use of Dietary Reference Intakes, and the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc; National Academies Press: Washington DC, 2002. http://books.nap.edu/books/ 0309072794/html/772.html (accessed Apr 2005). 3. A Report of the Subcommittees on Interpretation and Uses of Dietary Reference Intakes and Upper Reference Levels of Nutrients, and the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board. Dietary Reference Intakes: Applications in Dietary Assessment; National Academies Press: Washington
Vol. 82 No. 8 August 2005
•
www.JCE.DivCHED.org
In the Laboratory
4.
5. 6. 7.
8.
DC. http://books.nap.edu/books/0309071836/html/284.html (accessed Apr 2005). JCE Editorial Staff. J. Chem. Educ. 2004, 81, 1584A. Iron in Cereal. http://www.kingsford.org/khsWeb/rfs/elemsci/iron2.html (accessed Apr 2005). Iron in Cereal: Separation. http:// chemmovies.unl.edu/chemistry/beckerdemos/BD007.html (accessed Apr 2005). Whittaker, P.; Tufaro, P. R.; Rader, J. I. J. Am. Coll. Nutr. 2001, 20, 247–254. Senozan, N. M.; Christiano, M. P. J. Chem. Educ. 1997, 74, 1060. Edwards, C. Q.; Griffen, L. M.; Goldgar, D.; Drummond, C.; Skolnick, M. H.; Kushner, J. P. N. Engl. J. Med. 1988, 318, 1355–1362. Calcium. http://www.fao.org/docrep/004/y2809e/y2809e0h.htm# TopOfPage (accessed Apr 2005).
www.JCE.DivCHED.org
•
9. A Report of the Subcommittees on Interpretation and Uses of Dietary Reference Intakes and Upper Reference Levels of Nutrients, and the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board. Dietary Reference Intakes: Applications in Dietary Assessment; National Academies Press: Washington, DC, 2001. http://books.nap.edu/books/0309072794/html/442.html (accessed Apr 2005). 10. Sheffield, M-C.; Nahir, T. M. J. Chem. Educ. 2002, 79, 1345. 11. Orth, D. L. J. Chem. Educ. 2001, 78, 791. 12. Kosteca, K. S. J. Chem. Educ. 2000, 77, 1321. 13. Adams, P. E. J. Chem. Educ. 1995, 72, 649. 14. Laswick, P. H. J. Chem. Educ. 1973, 50, 132. 15. Bazzi, A.; Kreuz, B.; Fischer, J. J. Chem. Educ. 2004, 81, 1042. 16. Moss, M. L.; Mellon, M. G. Ind. Eng .Chem., Anal. Ed. 1942, 14, 862.
Vol. 82 No. 8 August 2005
•
Journal of Chemical Education
1225
