J. M. Bills, K. S. Brier. 1. G. Danko, F. J. Kristine, S. J. Turco, K. S. Zimmerman, P. M. Divelbiss, and 5. 1. Tackeft' Indiana University of Pennsylvonio Indiana, Pennsylvania 15701
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X-Ray Fluorescence Determination of Lead in Gasoline
concern is being expressed about the increasing air pollution by lead from the combustion of leaded fuels in motor vehicles (1-6). Medeiros gives a brief but excellent description of the role played by lead in motor fuels, and the subsequent pollution ( 6 ) . Lead.free fuels can be produced at a slight increase in cost, indeed some major oil companies are proceeding in this direction. Two such gasolines are sold in our area. "Non-leaded" gasolines have occasionally been rumored to contain lead. The validitv of such rumors can be readily checked. Both the qualitative and quantitative analysis of lead in gasoline can be easily and rapidly performed by X-ray fluorescence spectrometry. KO sample pretreatment is required, since the liquid sample can be measured directly as received. Birks and co-workers reported an accuracy comparable to the best conventional methods for the X-ray fluorescence detcrmination of lead and bromine in gasoline (7). Bromine, presentin ethyl fluid usually as ethylene dibromide, forms the volatile PbBrz inside the engine to prevent lead de~osits. The bromine X-rav I< s~ectrumoccurs in the same region as the lead X-ray L spectrum and so bromine can be determined at the same time. We propose this as an interesting and useful laboratory experiment for analytical courses, or for independent study. A knowledge of the lead levels in contemporary gasolines is useful to individuals concerned about lead pollution, and the principles involved in the experiment serve as good examples of general analytical techniques. The experiment requires the use of X-ray fluorescence instrumentation, which is available in most medium-sized to large schools.
t,ions containing pnre tetraethyllead in isooctane (2.91 g P h / g d ) , ethyl fluid in isooctane (2.93 g Ph/gal), and 5.77 X At ethylene dihromide in pet,roleum ether (4.10 g ethylene diwere prepared, A liquid cell was designed and constructed of Plexiglas, as shown in Figure 1. The cell dimensions, 11/1 in. in diameter and in. deep, were chosen to fit our existing rotating sample holder. The surface thickness of 0.010 in. allowed irradiation of t,he liquid sample with a minimum of X-my absorption. The cell filled by pipeting the sample through one of the holes ~ h i kair escaped through the other hole. Both holes were subsequently stoppered with small corks, and the holder was invertkd and placed in the X-ray sample compartment. For the experiment, the Picker solid stat,e X-ray generator was operated a t 50 KV and 30 mA. The X-radi~t.ion was dispersed by s. lithium fluoride cryst,al and detected with a Scintillation detector. A signal from the Picker Radiation Anelyser was recorded on a Servo-riter I1 recorder. The reaion from 26" to 35" 28 was scanned for each sample, which g w e both the Kg and K, peaks for bromine (26.7g0 and 29.96"), and the Lg and L, peaks for lead (28.25" and 34.08'). Radiation intensities were determined hv measurine the heiehts of the bromine K, peak and the lead "LO peak. l ~ o t hwere corrected for background radiation. The concentration of lead and bromine in gasoline is directly proportional lo the memuredX-ray intensities,
Results
Thc recorder trace for a typical gasoline sample (Boron Octron) is shown in Figure 2. For most
The Experiment Smnples of gasolines were collected from most service stations in the community surrounding our University. Standard solu-
To whom correspondence should be addressed.
Top Figure 1. sampler
View
Bottom View
Plexiglas sell for direct X-ray onolyris of liquid gasoline
Volume 49, Number 10, October 1972
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715
Comparison of the Relative Intensities of Lead LB and Bromine K, Peaks for Various Gasoline Sampler
Sample
-Lead (Lp)-Relative Intensity (mm) g/gal.
Ethylene dibromide standard 5.77 X 10-8 M Pb Std #I, TEL in isooctane Pb Std #2. Ethvl Fluid in isooctaie Esso Regular Big Plus (Low Pb) Extra Sunoco 220 200 Amoco Regular Super Premium (No Pb)
-
-
-Bromine Relative Intensity
(mm)
(GIg/d.
0 217.5 218.5 126.2 38.6 153.4
147.1 0
rhlf
Gulftane (Low Pb) 34
Texaco Firechief (Reg.) Sky Chief (Prem.) Boron Regdm Octron (Low Pb) Cetron (No Pb) Quaker State Sterling (Reg.) Super (Premium) Arco Regular Supreme Sinclair Dino Supreme Keystone Powerfuel (Reg.) Powerflight (Prem.) Thrifty Regular Premium Skat 95 (Regular) 101 (Premium) Red Head Regular Ethyl Super Premium
samples the selected recorder sensitivity was 3000 counts per second for a full scale deflection (250 mm). The relative intensities for our samples are given in the table. For convenience, the relative intensities were measured in units of mm. Good agreement was obtained for the two lead standards. However, we do not know the actual concentrations of lead in the commercial gasolines, so we cannot evaluate the accuracy of our results. The relative lead contents of the various brands and grades of gasoline are significant and interesting, even if the reported concentrations are in slight error. Conclusions (1) For a given brand of gasoline, the premium grades con-
tained more lead than the regular, indicating that the higher octane range is achieved by the addition of more tetraethyl lead.
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Journal o f Chemical Education
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2 8 Angle ( d e g r e e s ) Figure 2. Actual recorder hace for Boron Ostron gasoline. peaks mme from the W target in the X-ray tube.
The W
(2) The economy brands of gasoline tend to contain more l e d than do the well-known brands. Perhaps this is due to the use of less efficient cracking which results in a lower content of branched and aromatic hydroearhons, requiring a higher concentration of anti-knock additives. (3) The ratio of lead to bromine was found to be constant in all
(4) Two companies in our community Bdvertise a no-lead gasoline. They me AMOCO super premium, and Boron Cetron. True to their advertising, those gasolines were lead-free. The X-ray scans showed no trace of lead or bromine for either of the samples. Summary
The X-ray fluorescence determination of lead in gasoline is simple, straightforward, and reliable, and the analysis is rapid, since no sample treatment is required. This, coupled with the fact that lead pollution is an ever increasing menace, makes the experiment timely and desirable. Acknowledgment
Mr. David L. Ramsey of the Indiana University of Pennsylvania Physics Department constructed the cell. We thank Dr. C. H. Ehrhardt, of the American Oil Company, for his useful suggestions. Literature Cited L..Science.169,577 (19701. (21 Gnmn.13.F.. Chcn.Eng.Ncws.48 [471.52 (19701. (31 N ~ c r z n s o s 6. . P.. J. C ~ E M Eouc., . 3 1 , 560 (1054). (4) S c ~ v o rE. , A.,ANDL o c n ~ J,. K.,Enuiron.Sci. Tech.,4,324 (1970). (51 U.S. Public Health Service Serial Pub. No. 999-AP-12 (1965). (6) Mmernos, R. W.. ChrmirLry,44 I101. 7 (19711. (71 Bmns.L.s.,B n o o r s . E . J . . F n ~ e oH..ANDRoB.R. ~~~. M.. A n d Cham. (1) C ~ a wT. . I..A N D E m L , J .
22 1101, 1258 (1950).
