Susan E. Stemporzewski, Richard A. Butler, and Eugene F. Barry1
Lowell Technological Institute Lowell, Mosrochusetts 01854
An Atomic Absorption Experiment for Environmental Chemistry
The analysis of heavy metals and their effects on our environment are subjects of timely interest and concern (1). Particulate matter emitted from automotive vehicles has received considerable attention because of the inherently toxic characteristics of lead salts (2-4). T h e applicat i o n of analytical techniques t o environmental problems in undergraduate programs serves to enhance student interest a n d maintain relevancv and vitalitv in the promam. In our Instrumental ~ n a l y s i scourse we introduce the undermaduate chemistry majors t o Atomic Absorption Spectroscopy (AAS) with-a fair hour experiment focusing on the determination of lead and zinc in "street;sweepings" hv AAS. Students collect several s a m ~ l e sfrom a location o: their own choice and prepare the sample for analysis, a s well a s the necessarv standard solutions for AAS analvses. In addition t o famiiiarizing the student with the popular technique of AAS (5J, the experiment induces the student to develop an analytical approach to problem solving. Another feature of this experiment is t h e stimulation of students towards research activity in environmental analytical chemistry. Experimental
Samples are collected by gathering particulate dehris in wax paper "sandwich bags" prior to the laboratory period. Sampling sites include major highways, intersections, parking lots, etc., in the Greater Lowell area. The specimens are later transferred to glass vials which have been previously cleaned with 1% (v/v) HCI and distilled water. At the beginning of the lab the samples are ground in a mortar and pestle to produce more uniformity. Three 0.100 g samples are then carefully weighed and transferred to clean, dry 50 ml Pyrex beakers. Ten milliliters of cane. nitric acid are added and the sample digested for 1hr on a steam bath. After digestion the liquor is decanted quantitatively into a 50-ml volumetric flask. The residue is washed several times with distilled water, the rinsings also being added to the flask, and the contents in the flasks diluted to the mark with distilled water in order that the solution may he directly aspirated into the flame of the AA unit. Stock solutions of Ph and Zn were made in the following manner: 1.598 g of lead nitrate was dissolved and diluted to 1 l with 1% (v/v) HNO, corresponding to 1000 p g/ml (ppm) of Pb; a 500 pg/ml solution was prepared by dissolving 0.500 g of zinc metal in a minimum volume of 1:l HCl and diluting to 1 l with 1% (v/v) HCI. Standards for the spectrophotometer were made by adding 10 ml of conc. nitric acid to a 50-ml volumetric flask, adding an appropriate aliquot of the stock solution for the required metal ion concentration and diluting to volume. The addition of the nitric acid serves a dual purpose. In addition to bringing the viscosity of the standard solutions up to that of the sample solutions, it provides an internal blank for any trace metals in the acid. The absorhances of the sample and of several standards (four or five) covering a wide range of concentrations are measured by AAS. After the determination of the approximate metal coneentration of the sample solution, e series of standard solutions, whose metal concentrations more closely bracket those of the sample solutions are prepared, aspirated, and their absorhances measured. The absorbance of the sample ialution isthen remeasured and its concentration determined by interpolation from the constructed Beer's law plot. We employ a Perkin Elmer Model 403 atomic absorption spec-
1
To whom correspondence should be addressed
332
/
Journal of Chemical Education
Concentrations of Lead and Zinc Found at Tvnical Samolina Locations
Sampling Site Home Garaee of Author (S.E.S.) Parking Lot of Bank, Dracut, Mass. Dracut High School Parking Lot Dracut Post Office B P Gas Station Dracut Texaco Gas Station Dracut Shell Gas Station Dracut Lowell Tech Gym Parking Lot Lowell Car Wash Package Store in
Lead Concentration (rg/d
Zinc Concentration (fig/&?)
305
90
615
120
Street Entrance t o Administrative Bldg. a t Lowell Tech Intersection i f V.F.W. Highway and Textile Ave., Lowell Supermarket Parking Lot, Lowell Rt. 495 Exit t o Lowell
trophotometer with a liquid aspiration system, a 4 in. single slot, flathead burner. an acetvlene-air flame. and the aooronriate .. . hollow cathode lamps. lns;rurnmrnl paraherern are estnhlished in accordance w i r h the recornmendarton oithe manufncrurer ti, Results and Discussions
Typical concentrations of lead and zinc found a t selected sampling locations are presented in the table. Several interesting observations can h e made by examining the data. T h e concentrations of both metals in the sweepings are so extremely high (as expected) that any inexpensive single beam instrument may he employed with success. Of course, the only requirements are that experimental conditions are optimized and the determination of metal concentrations are made along the linear portion of the working curve. If necessary, the latter is accomplished by appropriate dilutions. Also, it is noteworthy t h a t for two of the three gas stations sampled, identical lead and similar zinc concentrations were found. T h e actual concentrations of lead and zinc in the sweepings are not known, hut the data may be statistically treated for precision. The experiment can focus on the analysis of other heavy metals such a s cadmium, if the appropriate single- or multielement hollow cathode lamps are available. Moreover, soil samples may also he collected a t measured distances from the roadside in order to establish correlations of concentration with air transport of the metals. Finally, since lead pollution in particular is a very timely topic, we have found t h a t this experiment is much more meaningful than a straightforward Beer's law verification followed with a determination of a n unknown.
Literature Cited (1) ..Cleaning Our Envimnment-~h. chemical ~~~i~far Action:' Society, Washington, D. C., 1969, p. 54. (21 Ksmran. H., Envimn. S c i Technol.. 7.13),223(19731. (31 Ksmrsn. H.. Enuimn.Sci Technol.. 1W. 239(19731.
Amniean Chemical
(4) ~ i - h h ~ ,D. A,. ~ i ~ b nLt , F.. ~ a m b F. , w., and ~iebvlski.L. M..~ n (EM. . Chem.. 19.1131(19571. (5) Skaog, D. A,. and West, D. M.,"Fundamentals of Analytical Chemistry." 2nd. Ed.. Holt, Rinehanand Win%tm.Inc.. New Ymk. 1969. P.709. (6) .,Analytical Methods for Afomie Absorption Spedmphotometry." Perkin Elm" Carp., Norwalk. Conn., 1968.
Volume 51, Number 5, May 1974
/
333
