In the Classroom
Chromium Pollution: An Experiment Adapted for Freshman Engineering Students
W
Penny Seymour Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, ON M5S 3E5, Canada;
[email protected] Chemistry experiments for first-year students usually concentrate on basic chemical principles but don’t always demonstrate the application of those principles. Engineering students, especially, are concerned that the material they learn should be applicable to their intended careers. This experiment is an adaptation of one published in this Journal several years ago (1). It has three main goals: to help engineering students learn to adapt their existing knowledge to a new problem; to include a “design component” in the laboratory (not always easy to achieve under safety constraints); and to provide an opportunity to monitor students’ written communication skills. Problem Scenario (Adapted from Herrmann [ 1]) An unacceptable concentration of Cr(VI) has been detected downstream from several industries that utilize chromium in their processes or products: a steelworks, a municipal power plant, a metal plating company, a tannery, a specialty chemicals manufacturer, and a paint company. The student, as a consulting engineer, has a contract to analyze water samples collected near the various industrial sites, to determine the apparent source of the chromium contamination. Typical clients include government departments and environmental monitoring agencies. (The identity of the client is changed periodically to discourage “recycling” of reports.) Since the river in question forms part of an international boundary, both American (EPA) and Canadian (federal and Ontario provincial) standards are relevant in this scenario. It is made clear to the student that this analysis is preliminary to a more detailed investigation, which would be carried out after the apparent source has been identified. In a real-life situation, the client would not instruct the consultant on how to fulfill the contract. Thus, the contract stipulates that the “consultant” will develop an appropriate method of analysis. Hypothetical water samples from the industrial sites (Fig. 1) are provided and the student is told that
Cover Me Paint Co. Shiny Plating ABC Steel 1
3 2
St. Clair City
Municipal Steam Generating Plant
6 4
5
Chemicals Skinned Alive To Go Tannery
7 200 µg/L Cr(VI)
Figure 1. Industrial sites on the St. Clair River near St. Clair City.
the concentration of Cr(VI) in samples taken upstream from this area is below the detection limit. The detailed experimental protocol given by Herrmann is replaced by a set of experimental conditions and a list of reagents provided: Since the molar absorptivity for Cr2O72{ itself is too low for accurate determination in the parts-per-million concentration range, the analysis for chromium will be based on the absorbance of the diphenylcarbazide (DPC) complex of Cr(VI), measured at 540 nm. The following reagents are provided: a solution of Cr(VI) (as K2Cr2O7) in 0.18 M H2SO4; [Cr(VI)] ≈ 250 mg/L. diphenylcarbazide, 2.50 g/L in acetone (freshly prepared) sulfuric acid, 0.18 M in water and 3 M in water The experimental conditions are as follows: all solutions are 0.18 M in H2SO 4 all solutions are 0.125 g/L in DPC Assume that the maximum concentration of Cr(VI) in the unknown water samples will be less than 1.5 mg/L, and prepare standard solutions to bracket the expected range of concentrations of chromium in the prepared water samples. Allow 5 minutes for color development of solutions.
It is assumed that the student has had an introduction to the use of a visible spectrophotometer and to the preparation of precise solutions. The appropriate analytic procedure consists of preparing a series of standard Cr(VI) solutions, adjusting the water samples to the same experimental conditions as the standard solutions, and measuring the absorbances of the standard solutions and water samples. (Student procedures are checked by the lab staff to ensure that they are workable, since the students have only one 3-hour lab period for the analysis.) A typical complete set of student data is given in the table. Note that, since each contaminated water sample is diluted with acid and complexing agent, the absorbance of the treated sample solution will reflect the reduction in concentration. Report The report is submitted in the form of a letter with accompanying appendices, addressed to the client. The letter should outline the problem, describe in general how the contamination was traced to that firm’s site (referring to the appendices), and offer some suggestions for remedial action. Since the “consultant” is not familiar with the operations of any of the firms shown, specific detailed solutions cannot be proposed at this point. However, the student is expected to recognize that while dilution might solve the immediate legal problem of excessive chromium in the effluent, it does not deal with the underlying issue of discharging a toxic substance into the environment. Therefore, we look for suggestions that modifications to the company’s operations might lead to
JChemEd.chem.wisc.edu • Vol. 76 No. 7 July 1999 • Journal of Chemical Education
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In the Classroom Typical Student Data for Report Standard Solutions: Standard solutions were prepared from a solution of Cr(VI), 250.2 mg/L in 0.18 M H2SO4, which had been diluted to12.51 mg/L Cr(VI). Freshly prepared diphenylcarbazide (DPC) was 2.50 g/L in acetone. All standard solutions were made up to a total volume of 100.0 mL with 0.18 M H2SO4.
The equation of the regression line for these data is: Abs = (0.351 L/mg) [Cr(VI)] Solution #
mL Cr(VI)
mL DPC
[Cr(VI)]/mg L{1
Absorbance 1
Absorbance 2
Blank (B)
0
5
0.0
0.000
0.000
1
2
5
0.25
0.088
0.088
2
4
5
0.5
0.176
0.177
3
6
5
0.75
0.263
0.262
4
8
5
1.0
0.351
0.352
5
10
5
1.25
0.439
0.438
Sample Solutions: Sample solutions were prepared by adding 5.0 mL DPC and 6.0 mL 3 M H2SO4 to 89.0 mL of the water from each sampling site. [Cr(VI)] values for the sample solutions were calculated from the regression line for the standard data above, and were corrected for dilution of samples by dividing by 0.89. (The corrected values are given in µg/L to conform with the units in the guidelines.)
Site
Abs. 1
Abs. 2
Mean Abs.
[Cr(VI)]/mg L{1
Corr. [Cr(VI)]
ABC Steel
0.192
0.189
0.1905
0.543
610 µg/L
St. Clair Steam Plant
0.161
0.161
0.161
0.459
515 µg/L
Shiny Plating
0.153
0.153
0.153
0.436
490 µg/L
Skinned Alive
0.108
0.107
0.1075
0.306
344 µg/L
Chemicals To Go
0.121
0.121
0.121
0.345
387 µg/L
Cover Me Paint
0.094
0.106
0.100
0.285
320 µg/L
All absorbances (standards and samples) were measured at a wavelength of 540 nm.
elimination of the toxic substance from the effluent stream as well as a decrease in the cost of raw materials. References 2–7 discuss chemical methods of dealing with metal wastes, as well as methods of process modification leading to reduced production of metal wastes. Both the content of the report and its presentation, including written communication skills, are evaluated. Note W The student handout for this experiment is available on JCE Online at http://jchemed.chem.wisc.edu/Jour nal/issues/1999/Jul/ abs927.html.
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Literature Cited 1. Herrmann, M. J. Chem. Educ. 1994, 71, 323. 2. Dhawale, S. W. J. Chem. Educ. 1993, 70, 395. 3. Beszedits, S. Chromium in Natural and Human Environments; Nriagu, J. O.; Nieboer, E., Eds; Advances in Environmental Science and Technology 20; Wiley-Interscience: New York, 1988. 4. Chen, J. M.; Hao, O. J. J. Chem. Technol. Biotechnol. 1997, 69, 70. 5. Singh, V. K.; Tiwari, P. N. J. Chem. Technol. Biotechnol. 1997, 69, 376. 6. Tiravanti, G.; Petruzzelli, D.; Passino, R. Water Sci. Technol. 1997, 36, 197. 7. Patterson, R. R.; Fendorf, S.; Fendorf, M. Environ. Sci. Technol. 1997, 31, 2039.
Journal of Chemical Education • Vol. 76 No. 7 July 1999 • JChemEd.chem.wisc.edu
