Patty Hall Laswick Clarion State College Clarion, Pennsylvania 16214
The Iron Content of
As part of our one-semester physical science course for nonscience majors, we try to develop an awareness in our students of the contents and relative worth of various brands of bleaches, antacids, and other household products, once the mask of advertising has been removed. We let the students analyze these products themselves and give them access to the same quality of equipment used by our chemistry majors. By so doing, the students not only become more enlightened citizens and consumers (who may, we hope, develop the habit of reading labels) hut also have a better feeling for the tools and methods of present-day scientists. One experiment which generates considerable student interest is the determination of the amount of iron in commercial cereal products using an atomic absorption spectrophotometer. The students enjoy turning the knobs themselves, yet a class of twenty-four needs only about 2W30min with the instrument. They are given a pre-laboratory lecture on the theory of the instrument, and this helps them appreciate that the concept of electron energy levels is not merely a hothersome idea put in their way as an unpleasant and useless obstacle. Experimental Procedure
The students receive the following instructions Weigh 5.0 g (about % of a small package, or ?k of an ounce) of a cereal. Put it into a 250-ml beaker and add 50 ml of dilute hydrochloric acid (at least i M). Cover with a watch glass and place on a hot plate. Simmer gently for at least 30 min, to break down the cereal and bring the iron into soluble form. Let cool somewhat, stir, and filter a few milliliters into a graduated cylinder. (For the actual analysis, only a milliliter or two is used.) Take the filtered solution to the atomic absorption instrument and measure the % absorption due to the iron. While at the instrument, the class will collectively measure the absorption of several standard solutions, containing known amounts of iron. To find out how much actual iron the cereal samples contain, make a graph showing the absorption of the "standard" iron solutions as a function of their concentration (i.e., absorption versus mg Fell of solution). On this graph, locate the absorption readings for the cereal samples; read off the iron concentration in the cereal samples directly from the graph. Then convert to mg iron/ oz. (an average serving) of cereal. Compare with the other students' findings for other brands of cereal, and note the manufacturer'slisted figure on the box.
132 /Journal of Chemical Education
Results of Cereal Analysis
Brand Product 19 Raisin Bran 40% Bran Flakes Total Grape Nut Flakes Grape Nuts Shredded Wheat Frosty-0's Lucky Charms Wheat Honeys Cocoa Crispies Rice Krinkles Crispy Critters Super Sugar Crisp Sparkled Flakes
Milliprams iron . per ounce (student results)
Value listed on box (per ounce)
9 9 9 7
...
1.8 ...
1.1 1.7 0.6 0.8 0.8 0.5 0.4 0.4
1.0 0.9 0.7
..
0.3 0.3 0.4
1.2 1.3 0.7 0.6 0.7 0.4 0.3
One Droblem the students encounter in this ex~erimental procedure is a clogging of t h e burner slot of the atomic absorption instrument. The instructor therefore turns off the flame after every six samples or so, runs a sharp blade across the slot, and relights the acetylene. Aspirating distilled water into the flame after every sample also helps. The proportion of cereal to acid is designed to yield values falling on a standard curve, if standards to about 36 mg iron! are used. These are freshly made by dilutions from a stock bottle containing 1000 mg iron/liter. Results
The students construct a curve of percent absorption versus mg iron11 rather than converting to absorbances beforehand, Although the resulting plot is quite curved in the higher readings, the conversion might he confusing to the students and is unnecessary for the semi-quantitative accuracy obtained in this experiment. T h e cereals tested contained iron ranging from 10 to 0.3 mg/oz (100 to 3% of the minimum daily requirement). Typical student results are shown in the table. Though agreement is not close, the values do show a correct trend. The nutritional poorness of some products-not only in iron content but in vitamins and other minerals as well, as the students soon realize when they study the box listings-is impressed in their minds by this experiment. A mere lecture on the subject, we feel, could not accomplish nearly as much.
