Determination of lron by Atomic Absorption in Vitamin-Mineral Tablets Earlier articles in THISJOURNAL have described the colorimetric determination of iron in vitamin tablets1 and the use of atomic absorption for determining iron in fmdstuffs.z~3We have developed an experiment for our quantitative analysis laboratory in which iron in multivitamin tablets is determined by atomic absorption. This technique is particularly useful because of its sensitivity, selectivity, and the relatively simple sample preparation. The students were particularly enthusiastic about the experiment because i t gave them an opportunity to analyze a "real" sample (which they might use themselves) and to compare their values to those given by the manufacturers. The students were given a handout describing the theory and experimental procedure. Questions were dealt with in a short lecture~discussionbefore starting the experiment.
Experimental In our laboratory, absorbance measurement- were made on a Varian Model 1100 Atomic Absorption Spectrophotometer at 386.0 nm with a slit width of 0.2 nm. A multi-element hollow cathode lamp was used, and measurements with a hydrogen continuum lamp showed no interference due to nun-atomic absorption. Standard solutions. Wash analvtical erade iron wire with distilled water and acetone to remove dust and erease. Drv to prevent splashing. After the wire is dissolved, transfer the solution quantitatively to a 1-1 volumetric flask and dilute to the mark with distilled water to give a stock solution containing 1000 pprn (mgll) iron. (Note: If 1-1flasks are not available, the quantities mav be scaled down.) Then Dreosre at least four standard solutions bv oioettine volumes ofthe stock solution - aoorooriate .. . into 100-ml volumetric flasks and diluting to the mark. These solution; should covers convenient range of concentrations (25-200 ppm) suitable for the analysis of the vitamin tablet. Calculations based on estimates of iron in the tablets should be done before coming tolab. Dissolution o f uitomin tablets. Determine the gross weight of the vitamin tablet on an analytical balance. Digest the tablet by heating in 50 ml of 3 M nitric acid in a 250-mL Erlenmeyer flask. Allow the mixture to boil gently fur 15 min after the tablet has dispersed to ensure complete digestion of any interferingorganic material. (Citric acid is daimed to decrease the observed absorbance by up to 50%)' Filter the mixture directly into a 250-mL volumetric flask and dilute to the mark with distilled water. Measure of iron concentration. A blank solution is prepared by following the above procedure for sample preparation without addition of a tablet to the nitric acid. The blank is then used to zero the instrument. Usually a standard calibration curve of absorbance versus concentration is prepared and the concentration of unknowns determined by interpolation. However, a feature of the Model 1100 also allowed us to use a more direct method. Using the "concentration mode" of the instrument and "high" (200 pprn) and "low" (50 ppm) standard solutions, the readout was calibrated directly in ppm. The concentration of the vitamin tablet solution was then read (3 readings) and the absolute amount of iron calculated. (Be sure not to neglect any dilution factors.) Results of Vitamin Tablet Analyses
Brand
Fe Content (on Label) mgltab
Fe Contents @hisExptJ rngltab
D 8 C Kelp Schiff Org Iron Thompson Nuplex One-A-Day Plus Iron Geritol
37 25 15 18 50
37.6 i 1.6 23.5 f 1.8 1 5 0 f 1.2 18.1 0.3 49.8 5 0.5
+
Average value of 3 to 5 determinationswith standard deviation.
The data show that eocd agreement was obtained with the values eiven bv the manufacturers. The values of the standard niques learned in the laboratory are indeed relevant to everyday life. Atkins, R. C., J. CHEM.EDUC.,52,550 (1975). Laswick. P. H.. J. CHEM. EDUC..50.132 (1973). Paschal, D. C., J. CHEM. ~ O U ~ . , 5 3 , ' l f (1976). i5 Roos, J. T. H., and Price, W. J., Spectrochim. A d o . 26B, 279 (1971). J o i n t Sciences Department Claremont Colleges Claremont, CA 91711
444
Journal of Chemical Education
Robert P. Pinnell Andrew W. Zanella
