Laboratory Experiment pubs.acs.org/jchemeduc
Determination of Wear Metals in Used Motor Oil by Flame Atomic Absorption Spectroscopy Julie A. Palkendo,* Jessica Kovach, and Thomas A. Betts Department of Physical Sciences, Kutztown University, Kutztown, Pennsylvania 19530, United States S Supporting Information *
ABSTRACT: For decades, railways, trucking companies, and commercial airlines have utilized oil analysis as a diagnostic tool to prevent engine-component failures. This interesting application was developed into an undergraduate lab experiment to introduce chemistry and environmental science majors in their second to fourth year to metal analysis using flame atomic absorption spectroscopy with a non-aqueous sample matrix. Copper, lead, iron, chromium, and silver were quantified by the method of standard additions or by external calibration curves with matrix matching. Students in both environmental analysis and analytical chemistry courses used their data to assess the health of a vehicle’s engine.
KEYWORDS: Second Year Undergraduate, Upper-Division Undergraduate, Analytical Chemistry, Environmental Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Applications of Chemistry, Atomic Spectroscopy, Metals, Quantitative Analysis Table 1. Wear Metals in Engine Oil2
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s the internal components of an engine spin, whir, and grind, motor oil is there to lubricate, cool, prevent corrosion, and disperse dirt. Eventually, the stress of the engine environment and the buildup of contaminants reduce the effectiveness of the motor oil, and regular oil changes are recommended. Evidence regarding engine wear is contained in the used oil, and many transportation companies (trucking, airline, rail, etc.) invest in extracting this information by analyzing this oil. Impending engine-component failures can be predicted by higher than normal wear-metal concentrations in the used lubricating oil. For example, high levels of chromium may indicate excessive wear of piston rings, whereas increases in copper and silver concentrations could point to worn bearings.1 Metals that are commonly assessed in used oil along with associated engine components2 are highlighted in Table 1. Typically, used oil from a given engine is analyzed at each oil change to establish a history of “normal” concentrations of wear metals for that particular engine. However, in this lab experiment students rely on the analysis of a single sample compared to acceptable metal concentrations, which are based on statistical analyses of many different engines.3 This laboratory exercise provides a novel and engaging application of atomic spectroscopy, especially for undergraduates who may be car enthusiasts. In addition, motor oil is an interesting and environmentally relevant matrix that chemistry or environmental science majors may encounter in future careers. A sample of used motor oil can be acquired from any vehicle or oil recycling center, and the concentrations of wear metals can be easily quantified with straightforward sample preparation and a basic atomic absorption instrument. © XXXX American Chemical Society and Division of Chemical Education, Inc.
Element Fe Cr Cu Pb Al Ag Sn Sia Naa a
Possible Problem with Engine Component Cylinder liner, valve and gear train, oil pump, rust in system Piston ring wear Bearings and bushing wear Bearing corrosion Piston and piston thrust bearing wear Bearing wear Bearing wear Intake of dirt or sand from a leaking air intake system Coolant leak
Acceptable Level/ppm 100−200 10−30 10−50 40−100 10−30 2−5 10−30 10−30 varies
Not wear metals, but indicate contamination from external sources.
The matrix is complex enough to warrant matrix matching of external standards (to maintain similar viscosity) or the method of standard additions. Although flame atomic absorption (AA) spectroscopy has been used extensively to analyze wear metals in oil samples4−6 and was used in this work, methods are available for those who have access to inductively coupled plasma-optical emission spectroscopy (ICP-OES).6−8 This situation can provide an excellent opportunity for students to appreciate the simultaneous, multielement advantage of ICPOES over AA.9
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Laboratory Experiment
Figure 1. Student generated standard addition curves for copper and lead.
accounting for dilution, and reported results to the class. The data analysis and interpretation took approximately 1.5 h for all student pairs to complete. Students individually prepared written lab reports and used the compiled class data to predict engine problems in the Ford F-150 truck.
Since the 1960s, lab experiments utilizing atomic spectroscopy have been proposed for the undergraduate chemistry curriculum10 and continue to be a staple at many colleges and universities. Most recently, AA experiments have been published to determine the extent of iron corrosion,11 iron and calcium in food items,12 various metals in mushrooms,13 and silver in leachate from food storage containers.14 In the mid-1980s, several educators included AA analysis of lead in gasoline15,16 by extracting the lead into an aqueous matrix. Alternatively, the approach described here involves dilution of an oil sample with a miscible organic solvent prior to AA analysis. These experiments require analysis of 1−3 wear metals in used motor oil to evaluate the “health” of an engine. The methods of standard additions and external calibration curves with matrix matching were employed in two different chemistry courses, environmental analysis and advanced analytical chemistry, with significant positive student feedback. The environmental analysis course is an introductory analytical course specifically designed for second- through fourth-year environmental science majors that are primarily focused in biology. The advanced analytical chemistry course comprises predominately second- and third-year chemistry majors.
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Advanced Analytical Chemistry
In this course, the motor oil analysis was presented as a projectbased lab option where one pair of students in each of two lab sections worked for two 3-h lab periods. Students were provided minimal procedural instructions (see the Supporting Information); however, they were asked to construct an external calibration curve using CONOSTAN 75 cSt Blank Oil (SCP Science), a 12-element organometallic standard, and solvent. The blank oil was used to maintain a viscosity similar to that of the used oil sample. The students provided the used oil samples. These advanced students were also instructed to quantify at least three wear metals of interest in their used motor oil samples. Students used instrument manuals and available literature to develop their procedures and interpret their results.
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HAZARDS Some components of used motor oil are known to be toxic, carcinogenic, and skin irritants. Therefore, nitrile gloves should be used to minimize contact between used motor oil and skin. To control flammable m-xylene vapors during the sample preparation steps, milligram-resolution balances with draft shields tall enough to fit sample vials were placed in fume hoods. Flammable solvents and flame atomic spectroscopy can be a hazardous combination, and it is important to follow all recommended safety practices from the spectrometer’s manufacturer.17 The drain tube from the nebulizer and waste container must be composed of materials that are resistant to organic solvents such as nitrile rubber and glass, respectively. All samples should remain sealed before analysis and again immediately after analysis. To minimize solvent evaporation near the spectrometer flame, vial caps can be replaced with caps that have a small hole (2−3 mm) drilled in them, through which to feed the aspirator tubing. After data collection, the flame should be turned off according to the manufacturer’s instructions. After the burner head cools, the liquid trap should be emptied, and the solvent disposed of properly. The waste container should be emptied after each laboratory period. Cleaning the liquid trap and spray chamber after each lab period is also recommended. Disposal of waste oil and solvent should comply with all regulations.
EXPERIMENTAL OVERVIEW
Environmental Analysis
Two lab sections of approximately 14 students each worked in pairs to analyze a sample of used 10W-30 motor oil from a 1995 Ford F-150 truck. Workstations were set up in fume hoods and included a shielded, milligram-resolution balance, a 100 μg/g working standard of a 12-element organometallic standard (SPEX CertiPrep), m-xylene (Fisher Scientific), a sample of the used motor oil, and disposable glass and plastic pipets. Students prepared four samples for the method of standard additions that each contained 2.0 g of the used motor oil, enough organometallic standard to achieve metal concentrations ranging from 0−8 ppm, and enough m-xylene to bring the total mass of the sample to 10 g. The samples were then sonicated for approximately 5 min. Sample preparation took students approximately 45 min. Each student pair selected a wear metal to analyze and with the help of the instructor, adjusted the operating conditions of a Varian FS220 AA spectrometer (see the Supporting Information). The spectrometer was operated with an air to acetylene flow ratio of 13.50 to 2.00 L/min, and multielement hollow cathode lamps were utilized. Students immediately plotted their absorbance data versus spiked-metal concentration, determined the original concentration of selected wear metals after B
dx.doi.org/10.1021/ed4004832 | J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 2. Advanced analytical student generated external calibration curves with matrix matching for copper and lead. These standards and samples were prepared in a 3:1 ratio of oil to solvent by weight.
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RESULTS AND DISCUSSION Figure 1 shows student-generated data in the environmental analysis lab using the method of standard additions for copper and lead determinations. The reasonable R2 values (>0.98) indicate that the students accurately prepared their solutions. This is important to note because the solutions were prepared on a weight/weight basis as opposed the more common weight/volume or volume/volume methods in undergraduate analytical laboratories. Only 5 out of 27 students executing this lab experiment reported that they had prepared solutions by mass more than once before. After extrapolating the linear regression to the x intercept and accounting for dilutions, students determined the concentrations of copper and lead to be 27 and 102 ppm, respectively. Iron, chromium, and silver were also analyzed. Iron concentrations ranged from 120 to 180 ppm, and chromium and silver were not detected. In the students’ assessment of the class results, they concluded that due to abnormally high lead concentrations a bearing or bushing may need to be replaced in the Ford F-150 engine. In the project-based version of this experiment, the advanced analytical students were challenged to think critically about how experimental design factors such as selection of the standard concentration range, multiple measurements, and sample preparation relate to the data quality parameters of linearity, reproducibility, and sensitivity. Figure 2 shows external calibration curves with matrix matching to analyze the used motor oil of a student’s 1998 Nissan Maxima. Correlation coefficients of >0.98 were obtained, and the concentrations of copper, lead, and chromium were 2.0, 7.8, and 2.7 ppm, respectively. The student was relieved to learn that the oil analysis did not indicate any problems with the Nissan’s engine components. Twenty-seven students in the environmental analysis course were surveyed after executing the experiment, and 24 ranked the experiment as either interesting or very interesting. Additionally, 20 students found the experience to be highly applicable to their environmental science major. Students commented that they did not know this type of analysis was performed and were intrigued to learn about the chemistry behind regular vehicle maintenance. Some students were disappointed that they did not get to analyze their own vehicle’s motor oil, and some suggested testing recycled oil and used motor oil after x, y, and z miles driven on their vehicles. Overall, the feedback confirmed that students were fully engaged in the experiment and understood the chemical relevance of the analysis.
ASSOCIATED CONTENT
S Supporting Information *
Student handouts for both courses; instructor notes. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Corresponding Author Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors wish to thank the students in the spring 2013 courses of CHM/ENV220 and CHM340 for their participation and valuable feedback.
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REFERENCES
(1) Fitch, J. C. Sourcebook for Used Oil Elements; Noria Corporation: Tulsa, OK, 2000. (2) Bob is the Oil Guy, What is Oil Analysis? http://www. bobistheoilguy.com/what-is-oil-analysis/ (accessed Dec 2013). (3) “Used-Oil Evaluation − Trend Analysis” EMAL-TT15-609, 2009; Exxon Mobil Corporation. (4) Burrows, J. A.; Heerdt, J. C.; Willis, J. B. Determination of Wear Metals in Used Lubricating Oils by Atomic Absorption Spectrometry. Anal. Chem. 1965, 37 (4), 579−582. (5) McKenzie, T. Atomic Absorption Spectrophotometry for the Analysis of Wear Metals in Oil Samples; Application Note AA-10; Varian Techtron Pty. Limited: Mulgrave, Victoria, Australia, 1981. (6) Vähäoja, P.; Välimäki, I.; Roppola, K.; Kuokkanen, T.; Lahdelma, S. Wear Metal Analysis of Oils. Crit. Rev. Anal. Chem. 2008, 38 (2), 67−83. (7) Fassel, V. A.; Peterson, C. A.; Abercrombie, F. N.; Kniseley, R. N. Simultaneous Determination of Wear Metals in Lubricating Oils by Inductively-Coupled Plasma Atomic Emission Spectroscopy. Anal. Chem. 1976, 48, 516−519. (8) Hilligoss, D. “Analysis of Wear Metals and Additive Package Elements in New and Used Oil Using the Optima 8300 ICP-OES with Flat Plate Plasma Technology”; Application Note 009951A_01; PerkinElmer, Inc.: Waltham, MA, 2011. (9) Vähäoja, P.; Välimäki, I.; Heino, K.; Perämäki, P.; Kuokkanen, T. Determination of Wear Metals in Lubricating Oils: A Comparison Study of ICP-OES and FAAS. Anal. Sci. 2005, 21 (11), 1365−1369. (10) Rechnitz, G. A. Simplified Atomic Absorption Spectrophotometer. J. Chem. Educ. 1962, 39 (9), 475−476. (11) Malel, E.; Shalev, D. E. Determining the Effect of Environmental Conditions on Iron Corrosion by Atomic Absorption. J. Chem. Educ. 2013, 90 (4), 490−494. C
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