0.5 micron, requiring less expensive equipment and offering more foolproof operation. Yet, in few cases*.g., metal oxides-this technique could not be utilized successfully because of the large bulk density correction factors. When 100% of the particles are below 1 micron, the technique utilizing electron microscopy is much more practical, requiring only a fraction of the time per determination needed for centrifugal sedimentation. Obviously, the particular centrifugal sedimentation technique utilized in this work cannot be used for materials with particles below 0.1 micron because of the excessive time
required per determination and inaccuracies due t o Brownian motion. ACKNOWLEDGMENT
The authors thank J. R. Condray and W. W. Morgenthaler for running some of the experiments. LITERATURE CITED
(1) h e s , D. P., Irani, R. R., Callis, C. F., J.Phys. Chem. 63,531(1959). (2) Heiss, J. F.,Coull, J., J. Chem. Eng. Progr. 48, 133 (1952). (3) Irani, R. R., ANAL. CHEM.32, 1162 119601. (4) Irani, R. R.,Cereal Sci. Today 6, 35 (1961).
(5) Irani, R. R., J. Phys. Chem. 63, 1603 (1959). (6) Irani, R. R., Ames, D. P., ASTM Bull., in press. . (7) Irani, R. R., Callis, C. F., ANAL. CHEM.31,2026(1959). (8) Irani, R.R., Fong, W. S., Cereal Chem. 38,67 (1961). (9) Marshall, C. E., Proc. Roy. SOC. (London),Ser. A 126,427(1930). (10) Michaels, A. I., Weaver, T. L., Nelson, R. C., ASTM Bull. No. 247, 140 (1960). (11) Young, J. Z.,Roberts, F., Nature 167.231 (1951). (12) Whitby, K.'T.,Heating Piping Air Conditioning 61, 33 (1955); 61, 449 (1955). RECEIVEDfor review January 30, 1961. Accepted June 5, 1961.
Determination of Mixed Lead Alkyls in Gasoline by Combined Gas Chromatographic and Spectrophotometric Tech niques W. W. PARKER, G. Z. SMITH, and R. L. HUDSON Ethyl Corp., Baton Rouge, l a . ,An accurate and simple method for the determination of mixed (methylethyl) lead alkyls in gasoline is described. This rapid method utilizes gas chromatography to separate the lead alkyls and a dithizone lead procedure to measure them.
N
o
SIMPLE METHOD has been published for the determination of mixed (methyl-ethyl) lead alkyls in gasoline. The elution characteristics of gasoline mixtures prevent the use of normal chromatographic techniques. However, chromatography may be combined with a spectrophotometric analysis to effect a rapid and accurate determination. The lead alkyls are separated by a chromatographic column, individually collected in iodine scrubbers, and measured by a dithizone spectrophotometric lead analysis procedure (1, 3, 4) . A simple mixture of T M L (tetramethyllead) and T E L (tetraethyllead) may be analyzed in approximately 25 minutes with an accuracy of *2%. This is accomplished by a direct dithizone analysis of the sample for total lead, followed by similar analysis of the eluted TML. The T E L is then calculated by the difference between the total lead and the lead present as TML. While the method is primarily designed for the determination of mixed TML-TEL in gasoline, it can easily be adapted to the analysis of more complex mixtures containing in addition MeJPbEt (trimethylethyllead), Mer
1 170
ANALYTICAL CHEMISTRY
PbEtl (dimethyldiethyllead), and MePbEta (triethylmethyllead). This mixture in gasoline requires five scrubbers and can be analyzed in 11/* hours. Elution times for each of the five components are shown in Figure 1. There may be temperature, gas flow rate, or packing variations that could vary the elution times using apparently similar columns. The only critical time involved is the 3.75-minute time for exchanging the T M L and Me3PbEt scrubbers. This time can be easily determined for any column from its relationship to retention time of toluene: Mid-point time between TML and MesPbEt = toluene retention time X 1.30
The precision of the method was calculated at three lead concentrations. The standard deviation a t 1 pg. is A0.49 pg. At 4 pg. it is +0.43 pg. and a t 50 pg. it is *0.66 pg.
1 1 60
PEAK
EXPERIMENTAL
Apparatus. A Perkin-Elmer Vapor Fractometer, Model 154, was used t o separate the lead alkyls. T h e column used t o effect the separation was a short 1-foot by '/d-inch column packed with 40% Nujol on 35-80 mesh Chromosorb. The helium flow rate was 100 cc. per minute and a chart speed of '/z inch per minute was used, The short column and a low temperature (70" C.) were used to minimize possible decomposition of the organolead compounds. The absorption cells used as gas scrubbers and the cell container of the Beckman DU spectrophotometer were modified as described by Henderson and Snyder (2). Reagents. Solution A. Dissolve 12.5 grams of ammonium citrate, 5 grams of potassium cyanide, and 30 grams of sodium sulfite in 200 ml. of distilled water. Dilute to 1 liter with concentrated ammonium hydroxide.
Pi
ELUTIW
NO COMPD.TIME. MIN I
TML
2 3
MeSPbEl
4
MezPbEt2 MePbEI,
5
TEL
375 40 11 20 45
50 TIME, MINUTES
Figure 1.
Chromatogram of a mixture of lead alkyls
Iodine solution, 0.2N in methanol. Dissolve 6.25 grams of iodine in 250 ml. of methanol. K I solution, 25%. Dissolve 82 grams of pQtassium iodide in 250 ml. of distilled water. Dithizone solution. Dissolve 60 mg. of diphenylthiocarbazone in 1 liter of chloroform. Procedure for TML-TEL Mixtures in Gasoline. T h e Vapor Fractometer is operated at a column temperature of 70' C. with a helium gas flow rate of 100 cc. per minute. Add 5 ml. of methanolic iodine solution to an absorption cell. Attach the cell to the vent line of the.Vapor Fractometer by means of a glass tube and rubber connector, so that the end of the glass tube protrudes into the iodine solution. Introduce a 100-p1. sample of gasoline t o the chromatograph. Collect the vent gases in the iodine scrubber for approximately 3.75 minutes. Remove the scrubber. Add 5 ml. of iodine solution to each of two additional absorption cells. T o one add 100 pl. of sample directly; use the other as a reagent blank. Add 3 ml. of K I solution to each tube and heat for 3 minutes in a bath of boiling water. Cool and add 50 ml. of distilled water. Add 35 ml. of Solution A and shake. Add 25 ml. of dithizone solution and shake the mixture vigorously for 1 minute. Measure the absorbance of the chloroform layer in each tube a t 520 mp in the spectrophotometer against air as a reference. Applying blank corrections, refer to a standard curve of absorbance us. micrograms of lead t o determine the amount of lead in the total sample and in the eluted (TML) sample. The difference represents the amount of lead as TEL. The procedure for a multiple mixture of the five methyl-ethyl lead alkyls in gssoline is the same, except that five samples are obtained in series as each component is eluted. Figure 1 shows the elution of a mixture of this fivecomponent system. DISCUSSION
The present method was designed to determine a simple mixture of T M G T E L in gasoline a t approximately the 3 ml. per gallon level. However, it was felt that the initial development should include other lead alkyls to broaden its applicability. As a consequence, the early work was based on the separation and analysis of a multiple mixture of methyl-ethyl lead alkyls in gasoline with the ultimate goal of adapting the procedure to the relatively simple TMLT E L mixture in gasoline. A mixture of TEL, TML, MePbEtr, MezPbEtz, and MeaPbEt was prepared. It was then analyzed, using the Vapor Fractometer to demonstrate the resolution and performance of the Nujol column. The percentage of each
component in the mixture was calculated using normal chromatographic area measurements. Since hydrocarbons in gasoline obscure the lead alkyl peaks and thus preclude a direct measurement, it was decided to collect the individual fractions as they are successively eluted and measure them by a dithizone method. The mixture of lead alkyls was diluted with toluene to a concentration of a p proximately 3 ml. of total lead alkyls per gallon. Toluene, unlike gasoline, permitted the eluted lead alkyls to be measured chromatographically without obscuring the recorder peaks. The retention time of the eluted lead alkyls was not affected by the presence of the large excess of toluene.
Table
( 5 0 4 samples)
Pb, Pg. As MedPb As Me8PbEt As M&PbEt As MePbEttAa EtrPb Total Pb, direct analysis Pb recovery,
%
Sample Sample Sample 2
1
3
11 61 99 62 28 261
13 61 102 67 25
268
13 61 100 63 30 267
266
266
266
98
101
100
II. Determination of TML-TEL in Gasoline (504. samples) Pb as Et,Pb Pb aa Me,Pb
Added, pg. Found, pg. Recovery, yo
Table 111.
Table I. Triplicate Analysis of a 5Component Mixture of Lead Alkyls in Gasoline
53.3, 60.0, 60.0 5 3 . 0 , 61.0, 60.0 9 9 . 4 , 101.7, 100.0
81.2, 81.2, 83.0, 79.0, 102.2, 97.2,
81.2 79.0 97.2
Determination of Normal Concentrations of TML-TEL in Gasoline ( 100-pl. samples)
Added, pg. Found, pg. Recovery, yo Added, pg. Found, pg. Recovery, %
5.6, 5.7, 102,
Pb aa M e z b 5.6, 5.6, 5 . 6 4.9, 87,
5.2, 91,
6.3 112
11.2, 11.2, 11.2, 11.2 11.2, 11.8, 11.0, 12.6 100, 105, 98, 112
The next step was to use deleaded gasoline for the dilution of the mixture and to measure the separated lead alkyls by the dithizone procedure. Some heavy ends in gasoline affected the elution time of T E L in the gasoline, so that a longer collection time was required for TEL. However, none of the other lead alkyls were affected appreciably. Table I shows the analysis of a typical sample of mixed lead alkyls in gasoline containing about 15 ml. per gallon. Finally, pure T M L and pure T E L were added t o gasoline so that a 50pl. sample would contain approximately 50 pg. of each calculated as lead. Table I1 shows the results of determining known amounts of TML-TEL in gasoline. Table I11 shows results on samples approximating the lead levels found in commercial leaded gasolines. To simplify and increase the rapidity of analysis, the final procedure was modified to omit the T E L elution step and determine T E L by difference.
Pb as Et,Pb 15.0, 15.0, 15.0, 1 5 . 0 17.2, 1 5 . 4 , 14.0, 16.0 114, 103, 93, 107 30. 2s; 93,
30. 25; 83,
30. 30; 100,
30 32 106
The retention time for T E L is approximately l l / z hours. By determining total lead directly and then TML, which elutes in 3.75 minutes, the whole analysis may be accomplished within 25 minutes. ACKNOWLEDGMENT
The kind help and expert advice of
S. R. Henderson on modifications of the dithizone lead analysis are deeply appreciated. LITERATURE CITED
(1) Clifford, P. A,, Wichmann, H. J., J. Assoc. Oflc. Agr. Chemists 19, 130 (1936). (2) Henderson, S. R., Snyder, L. J., ANAL.CHEM.31, 2113 (1959). (3) Snvder. L. J.. Ibid.. 19.684 (1947). (4) &$der; L. J:, Barnes,' W. R., Tokoa, J. V., Ibid., 20,772 (1948).
RECEIVED for review November 21, 1960. Accepted April 17, 1961. Southwest Regional Meeting, ACS, Oklahoma City, Okla., December 1960. VOL 33, NO. 9, AUGUST 1961
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