TETRAETHY L E A D IN GASOLINE A Symposium on Recent Developments in Instrumental Methods fot the Determination of Tetraethyllead in Gasoline, held by Research Division I11 on Elemental Analysis of A.S.T.M. Committee D-2 on Petroleum Products and Lubricants, February 21, 1950, Washington, D. C.
I
I
Rapid Determination of Tetraethyllead in Aviation Gasoline V. A. SMITH, W. E. DELANEY', W. J. TANCIG, AND J. C. BAILIE Standard Oil Company (Indiana), Whiting, Ind.
A new and rapid method for the determination of tetraethyllead in aviation gasoline is based on reaction of alcoholic silver nitrate with tetraethyllead to form metallic silver. The silver, present as a colloidal suspension, is determined by turbidimetric methods using a photoelectric colorimeter. The method is applicable to determination of the tetraethyllead content of aviation fuels and motor-fuel stocks sweetened with copper chloride but cannot be used with doctor-sweetened stocks because of interference by free sulfur or polysulfides.
T
H E determination of tetraethyllead (TEL) in gasoline has been the subject of numerous investigations. Chemical methods which have been developed for this determination (8)all require several hours elapsed time. Satisfactory methods involving polarographic (3, 6) or x-ray absorption (11 ) techniques have been described, but equipment for these methods is not generally available in control laboratories. A rapid method utilizing readily available equipment would therefore be desirable. The Russian worker, Bykhovskaya ( 4 ) , observed that tetraethyllead reacts with silver nitrate in alcohol solution to produce a dark-colored solution. The reaction was utilized to estimate small amounts of tetraethyllead in air by passing the sample of air through a tube of silica gel moistened with alcoholic silver nitrate and measuring the length of the darkened gel. This reaction appeared to provide the basis for a rapid method for the determination of tetraethyllead in gasolines. Preliminary experiments confirmed the reaction and provided the stimulus for further investigation; this resulted in the rapid turbidimetric method herein described. The reaction of silver nitrate with tetraethyllead involves the formation of ethyl silver ( 7 ) : (CJh)J'b
+
--f
+
(czHs)aPbNO~ C d U g
(1)
The latter is thermally unstable and decomposes to form metallic silver and free radicals. C&Ag +C&.
+ Ag
The silver liberated in alcoholic solution gives a yellow- to brownish-black coloration, the optical density of which can be determined by means of a photoelectric colorimeter. By comparison with a standard calibration curve, the tetraethyllead content of the fuel can be determined.
making up synthetic samples used in preparing the calibration curve, iso-octane (2,2,4trimethylpentane) was employed. Procedure. On the basis of the estimated tetraethyllead content, portions of the gasoline sample to be tested are accurately diluted with 95% ethyl alcohol to a tetraeth Head concentration between 0.10 and 0.50 ml. per gallon. k i t h a pipet 20.0 ml. of the diluted sample are then introduced into a 250-ml. glass-stop ered Erlenmeyer flask. From a graduated cylinder 100 ml. orsilver nitrate solution are added to the flask; a timer is started immediate] after the addition of the silver nitrate. The flask is stopperedl and shaken for a few seconds. The soiution is then transferred to a colorimeter cell, the colorimeter having first been adjusted to zero with the silver nitrate solution. Optical density is read at the end of the first, second, third, and fifth minutes indicated by the timer. The reaction is rapid and, in the absence of interfering substances, the drift of the colorimeter reading is usually slight. The reading for comparison with the calibration curve is best taken at the end of the second minute. The optical density observed is compared with a standard calibration curye, and the amount of tetraethyllead present is determined as milliliters per allon of ori 'nal gasoline. A complete determination can norm3Iy be ma% within 10 minutes. Preparation of Calibration Curve. For use with the Evelyn colorimeter, a calibration curve covering the range from 0.10 to 0.50 mi. of tetraethyllead per gallon has been found satisfactory and convenient. Since the tetraethyllead content of most gasolines lies between 1 and 5 ml. per gallon, this calibre tion range generally permits dilution of unknown samples by the pipetting of 10-ml. portions into 100-ml. volumetric flasks. Iso-octane containing 3.0 ml. of tetraethyllead per gallon was used as the basic standard. Into 100-ml. volumetric flasks there were transferred aliquots of 3, 5, 7, 10, 13, and 17 ml. of this standard, and these volumes were diluted to the mark with 95% ethyl alcohol. The resulting solutions contained 0.09, 0.15, 0.21, 0.30, 0.39, and 0.51 ml. of tetraethyllead per gallon, respectively. Portions (20 ml.) were treated with silver nitrate solution as described in the rocedure, and the optical densities were successively measurefin the colorimeter and plotted against the known tetraethyllead concentration. A typical calibration curve is shown in Figure 1.
EXPERIMENTAL
An Evelyn photoelectric colorimeter with a 520-mr filter was used in the developmental work on this method. Reagents employed were C.P. 95% ethyl alcohol and a saturated (approximately 5%) solution of C.P. silver nitrate in this alcohol. In 1 Present address, GriffinLaboratories,
Chicago,
Ill.
RESULTS AND DlSCUSSlON
Results of analyses of samples of isc-octane containing known amounts of tetraethyllead are shown in Table I. These samples were prepared and submitted to the analyst as unknowns. The extreme deviation was 0.10 ml. per gallon on one determination, and
1230
1231
V O L U M E 22, NO. 10, O C T O B E R 1 9 5 0 the average deviation for the eight samples was A0.026 nil. per gallon. Inasmuch as the results obtained were sufficiently accurate, the method was considered adequate for control work. There was no dye interference in the dyed gasolines a t the dilutions used. Results obtained with several aviation and motor fuels are shown in Table 11. For comparison, determinations by the standard A.S.T.M. method D 526 ( I ) are also shown. The ASS.T.M. samples were distributed by Committee D-2, Subcommittee A, Section VII, for cooperative testing of certain variables in the gravimetric procedure D 526. Table I1 shows that the aviation gasolines gave results which were in close agreement with those obtained by the A.S.T.M. method. However, motor fuels analyzed by the silver nitrate method consistently gave high rc-
Figure 1. Typical Calibration Curve
Table I.
Analysis of Iso-octane Samples of Known Tetraethyllead Content T E L Concn., Ml./Gal. Found Known 0.20 0.20 0.40 0.38 1.04 1.03 1.20 1.24 1.18 1.27 1.73 1.80 1.80 1.80 2.00 2.00 2.00 1.90 2.43 2.40 3.04 3.05
CONCLUSION Deviation, % ’
The turbidimetric technique provides a simple and rapid procedure for the determination of tetraethyllead in aviation gasolines or motor fuels which do not contain free sulfur or polysulfides. With established calibration curves, an average analysis can generally be made in less than 10 minutes. Results on aviation fuels check closely with A.S.T.M. method D 526.
0.00
-0.02 -0.01 4-0.04 -0.02 +0.07 -0.07
0.00 0.00
n on 0.00 -0.10 +0.03 10.01 Av. 1 0 . 0 2 6
LITERATURE CITED
Table 11. Results of Tetraethyllead Analysis by Silver Nitrate Method and A.S.T.M. Method Sample Aviation fuel No. 1 2 3 4 5 R
Motor fuel, Type A, No. 1 A No. 2 B No. 1 B No. 2 A.S.T.M. sample PPC-1 PPC-2 PPC-3 PPC-4
sults. This was due to the presence of ail interfering substanre which reacted slowly with silver nitrate and gave an unstable and high colorimeter reading. In every case these motoi fuels were known to contain coinponents which had been subjected to the “doctor” treating process. This process normally involves treating the stock with sulfur and alkaline sodium plumbite solution which converts mercaptans to alkyl disulfides; in the presence of excess sulfur, higher polysulfides may be formed. In stocks which are sweetened by the ropper chloride process, the mercaptans are converted to disulfides but no polysulfides are formed. Copper chloridesweetened stocks containing known quantities of tetraethyllead gave the expected value by the silver nitrate procedure. This pointed to polysulfides or free sulfur in doctor-sweetened stocks as the interfering substances responsible for the high values. In confirmation, it was found experimentally that small quantities of free sulfur or polysulfides, dissolved in iso-octane containing tetraethyllead, caused the same characteristic colorimeter drift and high tetraethyllead values as had previously been obtained with motor fuels containing doctor-sweetened components. Polysulfides are known to be rather unstable compounds in which sulfur atoms are loosely held in combination. It is assumed, therefore, that silver liberated in the colloidal state in the reaction between silver nitrate and tetraethyllead would react Mith the loosely held sulfur atoms to form silver sulfide. Such a reaction would continue until the reactive sulfur was removed and a stable polysulfide (probably a disulfide) was formed. The course of this reaction would account for the gradual drift of the colorimeter reading and the high tetraethyllead result. Many attempts have been made to remove the interforing s u b stances from fuels containing doctor-sweetened stock without affecting the tetraethyllead, but all have been unsuccessful. Oxidizing and reducing agents under various conditions eithrr partially destroyed the tetraethyllead or had no effect on the interfering substances. Treatment with various reagents including triethylenetetramine, chloramine-T, metallic mercury, lime and hydrogen sulfide ( 5 ) , sodium hydroxide and hydrogen peroxide (IO), alkali-metal stannite solution (Z), and piperidine (9) all failed to accomplish the desired end. Adsorption on activated charcoal, Attapulgus clay, or aluminum oxide was likewise unsuccessful.
T E L Concn., Ml./Gal. A.S.T.M. AgNOi method method 2.99 3.02 2.92 2.92 3.75 3.79 3.68 3.66 3.75 3.74 3.61 3.60 1.30 2.17 2.09 2.35 2.06 1.89 2.91 3.25 4.34 4.36 2.99 3.45 0.99 0.95 1.97 2.10
Deviation, % +0.03 0.00
+O. 04
-0.02 +0.01 -0.01 +0.87 +0.26 + O . 17 +0.34 -0.02
+0.46
-0.04 4-0.13
(1) Am. SOC.Testing Materials, “A.S.T.M. Standards on Petroleum Products and Lubricants,” Committee D-2, 1948. (2) Ayers, G. W., and Lyon, P., U. S. Patent 2,435,732 (Feb. 10, 1948). (3) Borup, R., and Levin, H., Proc. Am. SOC.Testing Materials, 47, 1010 (1947). (4)Bykhovskaya, M. S., GiQiena i. Sanitariya, 10, 17 (1945). (5) Espach, R. R., Blade, 0. C., and Rue, H. P., Refiner Natural Gasoline Mfr., 13,65 (1934). (6) Frediani, H. A., and Bass, L. A., Oil Gas J . , 39, No. 20, 51 (1940). (7) Krause, E., and von Grosse, A., “Die Chemie der metallorganischen Verbindungen,” p. 792, Berlin, Gebruder Borntraeger, 1937. (8) Lykken, L., Treseder, R. S., Tuemmler, F. D., and Zahn, V . , IND.ENQ.CHEM.,ANAL. ED.,17, 353 (1945). (9) Riding, R. W., and Thomas, J. S., J . Chem. SOC.,1923,3271. (10) Szeberenyi, P., 2.anal. Chem., 78, 36 (1949). (11) Vollmar, R. C., Petterson, E. E., and Petruzzelli, P. A,, ANAL. CAEY.,21, 1491 (1949). RECEIVED May 8 , 1950.
