Terrance B. Tripp' and J e a n n e 1. N a d e a u University of Moine at Presque Isle Presque Isle, Moine 04769
Roadside Salt Pollution and the Absorption-Emission Spectra of Sodium
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A general chemistry experiment
There are relatively few experiments relating closely t o the concept of energy levels in atoms which can be performed by freshmen, making it difficult to schedule suitable laboratories when atomic structure is being studied in the general chemistry course. In recent years, the atomic absorption spectrophotometer has become readily available and provides a rapid and relatively simple method for elemental analysis. We are using this instrument in the general chemistry laboratory t o illustrate the ahsorption-emission snectra of atoms bv analvzine soil samoles " for sodium. T h e experiment also stimulates a n awareness of the environmental problems associated with the accumulation of common salt along roadsides resulting from ice removal operations.
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Procedure T h e laboratory session begins with a discussion of the principles of atomic absorption and flame emission spectrophotometry with particular emphasis on the relationship between the absorption-emission spectra of atoms and electronic energy levels. T h e care and use of volumetric flasks and pipets are reviewed. The students, working in groups of two, prepared standard solutions containing 2 and 4 pgfml sodium by proper quantitative dilution of a 40-rgfml stock solution. In general, students were left alone to decide the proper procedure for preparing the volumetric ware; for example, when to rinse the pipet with distilled water and when ta use the solution to be pipetted. Any errors in dilution or washing become obvious when a calibration curve is prepared and the necessity of preparing new standards assures that a lesson has been learned. Samples of roadside soil were collected at 1-m intervals back from the highway pavement up to 20 m. These samples were mixed, dried at 125"C, crushed, and sieved to 60 mesh. Each student team selected a sample from this collection and weighed out a 1.00-g portion for analysis. The soluble sodium was extracted from the sample in the following way. The soil was placed in a test tube, 10.0 ml of 1.0 M ammonium acetate was added and mixed with the soil by shaking for 2 min. After centrifugation, the liquid layer was decanted and placed in a 1CQ-ml volumetric flask. The soil was then washed three times with distilled water using the above procedure. These washings were placed in the volumetric flask and the solution was diluted to the mark. With supervision, each group of students calibrated the spectrophotometer (Perkin Elmer Model 103) using their standards and measured the sodium concentration of their soil extract solution. These data were used to construct a calibration curve and evaluate the soluble sodium concentration of the soil sample. The results on soil samples taken at 20 m from the roadside provided an indication of the normal background extractable sodium. During the course of the laboratory period, the students were required to examine the emission spectra of hydrogen, sodium, 'To whom correspondence should be addressed. Huling, E. E., and Hollocher, T. C., Science, 176, 288 (1972); Stroffe, M. A,, (Editor), "Understanding Environmental Pollution." C. V. Mosby Company, St. Louis, 1971, p. 270. 130
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Journal of Chemical Education
and mercury displayed with small spectroscopes in the laboratoThey were asked to describe and explain their observations in the laboratory report and suggest why the sodium spectral line at 5830A was used for the analysis. ry.
Results Figure 1 summarizes student results on soil samples from the side of U. S. 1 one mile south of Presque Isle, Maine taken in t h e spring of 1973. The accumulation of salt along roadsides in the northern United States is well documentedZ and these student results are consistent with this trend a s well a s with results from a n earlier study a t UMPI by upper level students. In a rural area like northern Maine, most domestic water supplies are wells and the contamination of wells by the leachine of roadside salt is very possible. ~ o n s e q u e n t f ythis experiment not only illustrates the relationship between absorption-emission spectra and the electronic energy levels in- atoms h u t it presents t o the students a n example of a direct threat to their environment and well-being. We have experienced no difficulty in performing this experiment with a class of 24 students during one 3-hr laboratory session. T h e students enjoy the challenge of a quantitative experiment which uses modern instrumentation and allows t h e m t o obtain results which are relevant.
DISTANCE FROM ROADSIDE METERS Figure 1. Profile of sodium concentration in roadside soil samples taken onemilesouth of Presque Isle. Maineon U. S. No. 1, April. 1973.
