Sandra L. Watt, Theresa M. Martlno Margaret A. Chamberlin and Patty H. Laswickl Clarion State College Clarion. Pennsylvania 16214
A Simple Titrimetric Determination of Lead in Gasoline
"Getting the lead out" has become almost a household expression today. In spite of the availability of unleaded gasolines, leaded fuels are still prevalent; the rate of decline in leaded gasolines appears to be much less than had been anticioated. In a previous publication in this Journal, the problem of oollution from lead comwunds emitted in automobile exhaust k a s discussed and the read content of various brands of gasoline as determined bv X-rav fluorimetrv was reoorted.' An instrumental method utilizing atomic absorptibn spectroohotometrv has also been oublished.3 Besides these instru'Author t o whom correspondence should be addressed. 'Bills, J. M., Brier, K. S., Danko, L. G., Kristine, F. J., Turco, S. J., Zimmerman, K. S., Divelbiss, P. M., and Taekett, S. L., J. CHEM. EDUC., 49,715 (1972). :'Trent, Dorothy J., Atomic Absorption Newsletter, 4, 348 (1965). 'Siniramed, C., and Renzanigo, F., Riu. Combust., 19, 351 119fi51~ ,. ...,.
WMS,R., and Campbell, K., J. Inst. Petrol., 53,89 (1967). 6Comments are made concerning the atomic absorption method under "Results and Discussion." 'Free xylenol orange is lemon-yellow a t the p H employed, whereas lead-xylenol orange is violet. "A buffer concentration of 0.1 M was found to have inadeouate
mental methods, the literature contains several articles based on rather time-nmsuming and elaborate wet techniques.'..' The purpose of the prpsent work is to bring the determi. nation of lead in gasoline well within the reach of chemistrv majors, non-science majors, or even high school students, who we feel, should he made strongly aware of the amount and ultimate effects of this additive in the gasoline they may use. Realizing that many colleges-and certainly most high schools-do not have access to sophisticated atomic absorptionfior X-rav instruments. we have devised a reoroducible wet technique which may he carried out quantkatively or semi-quantitatively in approximately three hours. The technique has been designed to he performed entirely in one container. I t involves the use of easilv oroduced chlorine to break the lead-carbon bonds of the tekaethyl or tetramethyl lead additives. Excess chlorine is reduced, the aqueous layer adjusted to a pH at which Pb-EDTA is stable but interfering ions such as Ca2+are not, and the lead present titrated with EDTA (ethylenediaminetetraacetic acid) using xylenol orange as i n d i ~ a t o r . ~
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Experimental Procedure Standardization of EDTA A 0.001 M disodium EDTA solution is titrated into 2.00 ml of lead nitrate solution ( 4 . 6 g Pbll) to which has beenadded 1.0 mlof a0.4
prepare the buffer. 9pH control is critical. Below pH-3 the lead-xylenol orange complex does not form. Above pH-6 xylenol orange itself begins to exhibit a violet tinge. "The amount of indicator used should be as small as possible, since otherwise the delicate violet due to the lead-xylenol orange complex is swamped by the yellow of the indicator itself a t the p H employed.
M sodium acetate-acetic acid bufferd (adjusted to p H 4.7I9aand 3 drops of xylenol orange solution (0.1 gin 100 ml ~ f w a t e r ) . ' ~ T hsoe lution is titrated from a violet color to lemon-yellow. Although the transition to the final pure yellow color from an intermediate orange is delicate, it is quite sharp and can be discerned by employing a white background and carrying out the titration in a test tube.
Quantitative Method The fallowing direetions are given t o the students
The Lead Content of Various GarolinesGb 1Clarion.Pa.. March 19761 Average g Pblgal gasoline Student Valuer
Gasoline Sample
(3115 and 3/26]
valuer
(411 I
MK (31231
JW
DS
DS
JW
ND
(3127)
(31271
(31291
(411 1
(411)
" T h e latest 14/11 valuer are generally higher, no doubt due t o frequent opening o f the Containers and resultant evaporation b ~ u r n b e r sgiven in parentneser are average deviations.
262 1 Journal of Chemical Education
11) Calculations: Knowing the concentrationof the lead standard used in Step 5, calculate the molarity of the EDTA used, to
1) Into a series of clean 6-in. test tubescarefully pipet in the hood
1.00 ml of gapoline." Select several different gasolines and run in triplicate. You may use the same pipet for the different samples if you clean the pipet with the next sample to he used each time and discard the gasoline rinsings in the hood. 2) Add 4 drops of 6 M nitric acid." 3) Add 1.0 ml of household bleach. Immediately stopper with a cork. Gently mix.':' 4) Put all tubes in a preheated water hath (usea hot plate; avoid direct flames) maintained at 92-9VC. Heat for 1.5 h. Be sure that the corks are on initially, but not so tightly that they pop; tighten firmly after the vigorous evolution of chlorine has suhsided. Be sure, also, that the water level is nogreater than half-way up the sides of the tuhes.14 5) While the tubes are heating, standardize the EDTA titrant: Into each of three clean €.-in. test tubes add 1.00 ml of P I P stsnrlard. 1.0 s . ml of oH 4.7 acetate buffer. and 2 3 d r o ~ (no more!, of xylenol mange solution. Mix well, and then titrate (ubinga 25-ml huret, or one with smaller capacity if available) with 0.001 bt KDTA to a lemon-yellwv.Proceed slowly near the end; employ a white background and mix the contents thoroughly between additions. Note how critical it is not to overshoot, since less than 4 ml of titrant are involved. 6) Withdraw the tubes from the water hath and uncork. In order to reduce all remaining chlorine, make aslurr).of ascorbicacid in w a u r ( 4 . 1 5 g per 10drappofwater)and add 2dn)pstoeach tub?. Gently mix. Practice hefore thinadditron until you are able to discharge an even suspension. Note that since ascorbic acid is easily oxidized by oxygen, this slurry must he made immediately prior to use. 7) Add 2-3 drops (no more) of xylenol orange solution to each tube. Mix. 8) Carefully, dropwise, with continual miring, add aqueous ammonia (-7 M) to each tube until the samples turn violet. Then add nitric acid (2-3 M) dropwise until an arange-yellow or yellow results. Finally add dropwise an aqueous solution of sodium acetate (made by dissolving -0.6 g of anhydrous compound in 10 ml water) until a definite violet color is ohtained.IsThis process should ensure that the pH is 6 5 . 9) Next add 1.0 ml of acetate buffer (0.4 M with respect toeach component) to each tube. The contents should become a more vivid violet if s pale violet was attained in Step 8. 10) Titrate carefully with the same EDTA solution used in Step 5 until a lemon-yellow endpoint appears. Thorough mixing between additions is essential. In some cases, a hmwn "scum" from the gasoline decomposition may he present, but the endpoint will still be sharp if you observe the color changes carefully. ~~~~
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three sienificant fieures. Then determine the -mams Phleallon " " of gasoline for each of your samples, having first averaged your triplicate volumes.
Semi-Quantitative Method We feel that the following procedure can be followed by a nonscience major or a high school student. The instructor can prepare and standardize the EDTA titrant and tell the students the volume of EDTA that is wuivalent to..sav. ,. 1 e. Pbleal . easoline for a 1-mleasoline sample. Using medicine droppers, the students can add 4 drops of 6 M nitric acid, 1 ml gasoline (utilizingacalilrated medicinedropper), and 20 drops of bleach. The fmal tmation with EDTA can also be accomplished with a calibrated dropper.
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Results and Discussion The table shows values of lead in various brands of gasoline from the Clarion, Pa. area obtained by our quantitative method. It should be noted that the amount of additive present in a given brand of gasoline varies from batch to batch; therefore, the values we obtained might well be different a t a later time. Results from two of the authors as well as values from several chemistry students are compiled. The titrimetric values obtained by the authors are results of a t least three titrations. whose averaee deviation in the number of milliliters of EDTA used to tiGate each brand ranged from f0.02 t o f0.12 ml. Volumes were usually between 1 and 3 ml of titrant. As a check on the method, the titration values from eight identical samples of a known concentration of tetramethyl lead in isooctane were averaged. Calculated results are quite acceptable: Label value, 3.94 g Pblgal; result from the present method, 4.04 f.O3g Pblgal. Hoping to further verify the accuracy of our procedure, we ran a series of diluted tetramethyl lead standards and diluted (in isooctane) gasoline sampleson an atomic absorption apparatus. Good agreement was obtained with the first six easolines which were run after the standards. but as the run was continued a slow decrease in the absorbance values of the samples and standards was observed. Thus the reliability of this instrumental technique seems to be time-dependent. It is not surprising to note that premium grades of gasoline contain more lead additive than the corresponding regular erades. Of interest is the sienificant variation in the amount of this additive present a&ng the brands of gasolines studied. Acknowledgment We thank the first-year chemistry students who initially nerformed this exneriment and contributed sueeestions for i t s improvement. This research was supported in p m by two prants from the Institute on Hunlun E c k . ~. m of Northwestern Pennsylvania, and was sponsored during the tenure of the grants by the Clarion County Chapter of the League of Women Voters. T h e authors gratefully acknowledge the assistance of these organizations.
--
-
VP
BE
DL
1411 I
1411 1
(411 1
DLS I411 )
JJW
1411 I
"The high toxicity of tetraethyl and tetramethyl lead cannot he overemphasized. Proper caution should he exercised in the handling of the gasolines;contact with the skin should be scrupulously avoided. Witrie acid is used rather than hydrochloric acid in order to avoid the addition of unnecessary chloride ion, since chloride complexes with lead (11). ':'Formation of the red-brown oxidizing agent PbOn was observed when the order of adding acid and bleach was reversed. '4If the water level is kept low, gentle refluxing nicely rinses the sides of the test tuhes as they are heing heated. 'Vhese concentrations of acid and bases have been designed to keep the volume of liquid in the tubes as small as possible; otherwise adequate mixing is difficult and overtitration may result. This situation did in fact occur when several students testing this experiment employed much more dilute reagents. Volume 54, Number 4, Aprll 1977 1 263
