The Determination of Lead in Gasoline by Atomic Absorption Spectrometry M. F. M. Coleman Wellington Polytechnic, Wellington, New Zealand
In the analytical and inorganic section ut the New Zealand Certificate in (:hemistr.v tht: s t u ~ l t ! nare ~ ~intnrlucrii to atomic absorption spectrometry and the topic is further developed in subsequent years of the course. In the first year the students carry out three experiments: the determination of sodium in potassium chloride by both AAS and flame photometry aeainst hoth oure sodium chloride standards and standards containing ah ionization buffer (potassium chloride), the determination of copper in steel using a standard additions method, and the determination of the lead content of gasoline. The third experiment always generates high interest because of the controversial status of lead in gasoline. For the determination of lead in gasoline atomic absorption methods are widely practiced. These methods use nonaqueous standards and sample dilution with solvents such as isooctane or methyl isohutyl ket0ne.1,~As they involve the aspiration of a combustible solution into the flame, they are hazardous and require a high degree of skill. The method described here avoids the aspiration of organic solvents; i t involves, instead, the extraction of the lead into an aqueous solvent using iodine monochloride reagent3 In addition to this solvent extraction procedure, the experiment is used to illustrate two further features: the use of wavelengths other than the most sensitive wavelength (permitted by the high concentrations of lead) and the effects of flame stoichiometry and burner height upon the absorbance. Extraction of Lead from Gasoline It is advisahle to carry out the extractim step in H separate room and in a fume hood to prevent inhalation of vapors. Dry pipets must he used for the organic solvents, and pipetting b; mouth is forbidden, Into a separating funnel (100ml), pipet 5 ml of gasoline, 25 ml of cyclohexane,isooctane, or a hydrocarbon of similar volatility and 10 ml of iodine monochloride reagent. (The volume of gasoline may be varied depending on the lead content.)Stopper the funnel and shake thoroughly for 3 min, frequently opening the tap to release any pressure inside the funnel. Allow to stand until the layers have sepRobinson, J. W., Anal. Chim. Acta, 24, 451 (1961). Kashiki, M.. Yamazoe, S.. and Oshima. S., Anal. Chim. Acta, 53, 95 119711 i''~tomicAbsorption Spectrophotometry;' Pye Unicam Ltd., England, 1975.
arated, then transfer the lower aqueous layer to a 100 ml volumetric flask. Add a further 10 ml of water to the funnel and shake for 1 min. Again, run off th? I ~ , ~ ~ . r a q u layer e o u ~inm the wlurnetric flask, then make the contrnti up tu r h t mark. A t h n k withmr dasdme la prepared identically Experimental Details The instruments used are a single-heam Varian 12M)and a douhle-beam IL 457. Hollow cathde lamps supplied by hoth Varian and Philips are operated at 5 mamp. The hazards and precautions associated with the use of acetylene cylinders must he emphasized before the students are permitted to use the instrument. The orocedures for the innition and extinction of the flame must be k n w n . The instrummud mom muat br wrll wntilatrd and have an turartor cu remg.ve thc ettluent from rlte burner. Reagents A 500-ppm lead standard is made by dissolving the appropriate quantity of lead nitrate in water and further diluting. Working standards are prepared to cover the range from 2-50 ppm. To make the iodine monochloride reagent, add 112 ml of 25% potassium iodide solution carefullv to 112 ml of concentrated hvdrochloric acid (specify gravity l.i8), and cool the solution to laom temperature. To the solution add slowly, with stirring, 18.75gof potassium iodate. A clear hrawn-orange solution usually results. It is further diluted to 250 ml with water. This solution has been found to he stable over at least 3 mos. Results and Discussion Flame Stoichiometry and Burner Height The students are shown how to adjust the fuel to obtain each of the three flame types, oxidizing (acetylene lean), stoichiometric, and reducing (acetylene rich)designated 0, S, and R respectively in Figure 1. A colored photograph of each flame is situated near the instrument. and the students refer to this later when adiustine the fuel. u s k g one of the lead standard solutions, they prep& d o t s of hurner heieht versus absorbance for the three flame Hnd hurner types. k e x t they select the optimum flame and hurner height for all subseauent measurements. The stoichinmetric name is trsu:illy selrcted. Although the reducmg flame hassliahtlv areater sensitivitv, it also hasa hirher noise level.
Volume 62
Number 3 March 1985
261
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F i w 2. Abswbance versus concentration (ppm) at 217.0 and 283.3 nm
H
Figure 1. Absorbance versus burner height (mm).
Calibration Plot The calibration plots are prepared at two wavelengths, 217.0 and 283.3 nm, the former being twice as sensitive as the latter (Fig. 2).3 The plot at 217.0 nm is clearly nonlinear, which is especially useful since students mistakenly assume that all absorbance-versus-concentration plots are, linear. T o encounter a nonlinear plot at an early stage 1s helpful if the teacher then stresses that the calibration plot must he the line of best fit and is not necessarily a straight line.
262
Journal of Chemical Education
The calibration plot a t 283.0 nm is almost invariably a straight line, and the contrast between the two wavelengths is easilv The results for the lead content of the easoline ~" seen. ~ ~ ~ obtained at the two wavelengths are usually within 2'for an individual student. while the variation between students is within 5%. Finally the students calculate the AAS sensitivity (concentration in DDm eivine an absorbance of 0.004) from the absorbance of theAs&nZardof lowest concentration, They can then compare these results with reported values." ~
