Tetraethyllead in Gasoline
Rapid Polarographic Determination of Tetraethyllead in Gasoline KENT A. HANSEN, THOMAS D. PARKS', AND LOUIS LYKKEN Shell Decelopment Company, Emerycille, Calif.
A rapid, direct polarographic method for determination of tetraethyllead in gasoline is described. The sample is dissolved and the tetraethyllead is decomposed in anhydrous Cellosolve containing hydrogen chloride, and the lead ions in the resulting solution are determined directly by the polarograph. The method is applicable to tetraethyllead in gasolines in the range of 0.5 to 8 ml. per gallon, and is generally accurate to within *3% of the lead content in freshly prepared blends. Aged gasolines containing high concentrations of unsaturates and peroxides may give appreciable errors. A single determination requires about 30 minutes; but a series of 5 determinations requires only 1 hour.
I
S RECENT years, there has been a sustained interest in
finding rapid and accurate methods for the determination of tetraethyllead in gasoline. Lykken, Treseder, Tuemmler, and Zahn ( 5 ) recently reviewed the existing chemical methods and noted that a comnlon disadvantage of all the methods was the amount of time required per analysis; they recommended two new direct evaporation procedures which gave quantitative analysis of lead in all types of gasolines but which required 3 or 4 hours per sample, depending on the type of base stock. Perhaps the most rapid method for the determination of tetraetliyllead in gasoline is the x-ray absorption method of Sullivan and Friedman ( 8 ) , which uses a Geiger counter to measure the x-ray absorption of a leaded gasoline sample and which gives precise and accurate results, even by unskilled workers, in an elapsed time of 5 minutes per sample. One disadvantage of the x-ray for plant control is the expensive equipment required. The Widmaier iodometric method (6, 9) is simple and rapid, requiring only 10 to 15 minutes per sample, but it is not sufficiently accurate for many applications. The authors' experience indicates that the Widmaier method gives slightly high results for saturated stocks and up to 10% too high values for unsaturated stocks, even when the pretreatment with 70% sulfuric acid is used as Tecomniended b y Widmaier (9) and Xewman, Philip, and Jensen (6). Frediani arid Bass ( 3 )described a polarographic method for analyzing the lead solution obtained by extracting the gasoline samples with hydrochloric acid ( 1 ). Although their method permits a derrense in time per analysis in comparison with chemical methods, they pointed out that a direct determination witliout previous acid extraction would result in saving much additional time in gasoline analysis. The recently published method of Borup and Levin ( 2 ) is essentially a simplification and refinement of this extraction method. This paper describes a rapid method for the direct polarographic determination of lead in a nonaqueous solution of gasoline, eliminating the extraction step. The leaded gasoline is added to a solution of hydrochloric arid in anhydrous Cellosolve (ethylene glycol monoethyl ether) with which i t is miscible ( 7 ) . The tetraethyllead is decomposed by heating the mixture and without further treatment the resulting solution is analyzed polarographically for lead. APPARATUS
Polarograph. A Sargent Model XX polarograph and a unitized dropping mercury electrode and cell assembly (4) proI
present address, Stanford Research Institute, Stanford, Calif.
vided with temperature control and operated a t 25" were used in all analyses.
* 0.5" C.
REAGENTS
Cellosolve-Hydrogen Chloride Electrolyte. Prepare by passing anhydrous hydrogen chloride into Cellosolve until the solution is approximately 1 N , as determined by diluting a portion with water and t i m t i n g with standard base. Tetraethyllead Standards. Add sufficient tetraethyllead fluid to lead-free gasoline to make solutions containing 0.5, 1.0, 4.0, and 8.0 ml. of tetraethyllead per gallon. Determine the exact concentration by A.S.T.M. D 526 ( 1 ) or method of equal accuracy (5). Nitrogen. Tank nitrogen containing less than 0.5% oxygen. PROCEDURE
By means of a pipet or buret, measure exactly 10.0 ml. of Cellosolve-hydrogen chloride electrolyte and 3.0 ml. of leaded gasoline into a 25-ml. borosilicate glass volumetric flask. Heat the flask on a steam bath for 15 to 20 minutes, cool to room temperature, and transfer a portion of the solution to the polarographic cell. Adjust the temperature of the cell to 25" C. Purge the cell by passing.nitrogen through the liquid in the cell for 5 minutes a t a rate of 150 ml. per minute. Adjust the mercury pressure so that a drop time of 4 seconds per drop is obtained with 0.4 volt applied to the electrodes. Make a polarogram covering the range of 0.2 to 0.5 volt. Measure the height of the polarographic wave and from a calibration curve determine the concentration of tetraethyllead ip the sample. EXPERIMENTAL
In order to develop a method that would be applicable to the analysis of any gasoline, it was considered necessary to base the determination on the reduction of the lead from the tetraethyllead compound and not from any other reducible substance in the Ethyl fluid. Thus an electrolyte was desired which would not only dissolve and decompose the tetraethyllead but also would permit the direct polarographic determination of the resulting lead ion in the same medium without further treatment. It was found that a 1 -V solution of hydrochloric acid in Cellosolve was miscible with about 20% of its volume of leaded gasoline, and that it gave a diffusion current for lead ion after heating a short time to decompose the tetraethyllead. Later, it was found that a better reagent with greater solubility for gasoline was obtained by passing anhydrous hydrogen chloride gas into Cellosolve until it was about 1 N as determined by titration. This electrolyte was miscible in all proportions with gasoline. No apparent decomposition took place when a leaded gasoline was mixed with the Cellosolve-hydrogen chloride solution at room temperature. 1232
1233
V O L U M E 2 2 , NO. 10, O C T O B E R 1 9 5 0 However, experiments showed that heating a t steam temperature for 15 minutes was sufficient to decompose the tetraethyllead completely and to make the lead available for polarographic analysis. Consistent, reproducible results were obtained when the heating time was maintained between 15 and 20 minutes, but low wave heights were found with longer heating, due possibly to precipitation of lead chloride. Using a heating time of 15 to 20 minutes, the elapsed time for one sample was about 30 minutes, but a group of five samples was completed in 1 hour. The heating was carried out in 25-ml. volumetric flask to give a degree of refluuing, thus minimizing volume changes due to evaporation and making further dilution to volume unnecessary. h number of leaded gasoline samples having a variety of hydrocarbon types were analyzed by this method and a t the same time by h.S.T.11. D 526 (1). The results, summarized in Table I, agreed i n most cases within 3y0,with a precision of better than 1%. Figure 1 depicts a typical polarogram and shows the method used to measure the wave height. Calculations were minimized by preparing the calibration curve directly in milliliters of tetraethyllead per gallon. A straight-line calibration was obtained. While the most accurate concentration range is from 0.5 to 8.0 ml. per gallon, samples containing as little as 0.1 ml. per gallon were estimated with an accuracy of 5%.
samples. Such a procedure would be extremely rapid and would be particularly useful in control or blending work where smiples of the added batch of fluid are available for making a calibration curve. However, i t would have very limited application in the analysis of unknown samples.
Table 1. Tetraethyllead Results for Various Types of Gasolines by Rapid Polarographic Method Gasoline Sample Motor Motor, low sulfur l l o t o r , loa. Sillfur
0.84
.i\.. 0 . 86 ? J o t o r , high sulfur
.i\.iation I
Several solvents were tested in an effort to find a medium in which leaded gasoline could be examined polarographically. Gasoline was found to be insufficiently soluble for this purpose in the following solvents mixed with the required amount of concentrated hydrochloric acid: ethyl and isopropyl alcchols, methyl or ethyl Carbitol (diethylene glycol monomethyl or monoethyl ether), benzene-isopropyl alcohol mixture, and rnet,hyl Cellosolve (ethylene glycol monomethyl ether). Gasoline was found to be very soluble in dioxane and in ethyl Cellosolve, but the latter was preferred for polarographic work because of the rapid formation of peroxide in dioxane ( 7 ) . Samples of a leaded gasoline were dissolved in 0.1 S solution of tetra-n-butyl ammonium iodide in Cellosolve and, without any further treatment, were examined polarographically over the voltage range of 0.0 to -2.0 volt,s. Waves were obtained a t an applied potential of - 1.30 and - 1.70 volts, but these were found to be due to ethylene bromide and ethylene chloride, respectively, and not to lead. Because ethylene dibromide is a component of both motor and aviation tetraethyllead fluids, it could possibly be used in an indirect indication of the tetraethyllead present in certain gasoline
1.68 4,.51 4.40
Aviation I1
..\viation I11
DISCUSSION
TEL Content, MI. per Gal. A.S.T.11. D 526 Polarographic ' Difference 0 , ,i4 0 ,.54 0.56 0.58 . i v , 0.5,i 0 . d6 0.01 1 , .il 1.52 1.32 1.jl .A\-. 1 . 3 1 1 ,.52 0.01 0,86 0.89 0.81
.I\iation I V
Special aviation Sperial aviation ..\viation
.4v. 4..50 .3.83 3.89 3.88 .Av. 3.86 :i ,532 3.84 -4v. 3 . 8 3 4.01 4.01 4.04 .4v. 4 . 0 2 3.07 J .03 .4v. 5 05 8.67 8.65 .Ax.. 8 . 6 6 :3,44
3.45 .\I-, 3 . 4 3
Thermally cracked Catalytically cracked
1.12 2.13 .iv. 2.13 2.28 2.27 A\.. 2 . 2 8
0.86 0 . 83 1.69
0.01 0.01
4.61 4.61
4.61 4.02 1.13 4.07 :3.92 3.92 3.92
0.11
0.21 0.09
4.02 4.00
4.01
0.01
5.0 (5.0)
0.05
8.40 8.40 8.40
0.26
x,57 3 , .7i 3.57 1.17 2.17 2.17 2.12 2.10 2.11
0 12
0.04
0.17
In the analysis of aged gasoline stocks containing high concentrations of unsaturated hydrocarbons and/or high concentrations of organic peroxides errors as large as 10% have been obtained. S o satisfactory explanation for this error was found. A portion of the error, however, can be attributed to the poorer polarographic reduction waves obtained when the unsaturated hydrocarbons or peroxides are present. Some knowledge of the type of sample being analyzed should be available before the results of the polarographic method can be properly evaluated. For freshly prepared gasoline stocks, these difficulties are generally avoided and an accuracy within *3% can be obtained. LITERATURE CITED
I
I
-0.35
I
- 0.40
I
-0.45
APPLIED POTENTIAL , IN VOLTS (VS. THE MERCURY POOL)
Figure 1. Typical Polarogram of Lead Ion in CellosolveHydrogen Chloride Electrolyte Sensitivity, 0.036 @. per ml. Temperature, 25' C. Tetraethyllead concentration, 2.1 ml. per gallon
( 1 ) Am. SOC. Testing Materials, "Standards on Petroleum Products and Lubricants," A.S.T.M. Designation D 526-42, October 1947. (2) Borup, R., and Levin, H., - 4 n ~SOC.Tmting MateriaIs Proc.. 47, 1010 (1947). (3) Frediani, H. A,, and Bass, L. A , , Oil Gas J . , 38,No. 20, 51 (1940). (4) Lykken, L., Pompeo, D. J., and Weaver, J. R., IND. ENG.CHEIr., ANAL.ED.,17, 724-8 (1945). (5) Lykken, L., Treeeder, R., Tuemmler. F. D., and Zahn. V., Zbid., 17,353-60 (1945).
(6) Newman, L., Philip, J. F., and .Jensen, A. R., ANAL.C H E \ f . , 19, 451-3 (1947). (7) Parks, T. D., and Hansen, I(.A , , I h i d . , 22, 1268 (1950). (8) Sullivan, M. V., and Friedman, H., IND.ENG.CHEM.,ANAL.ED., 18, 3 0 4 4 (1946). (9) Widmaier, O., Luftfahrt-Forsch., 20, 181 (1943). RECEIVED April 17, 1950.
