Estimation of Thiophene in Gasoline HERBERT C. McKEE, L. KERMIT HERSDOS, ~ N J.I\IES D K. B-I'THROW Ohio State Cnicersity, Columbus, Ohio A colorimetric method for the direct estimation of thiophene in gasoline, without interference from other sulfur compounds present, has been worked out. The test is based on the blue color produced by the indophenine reaction between thiophene and isatin, in the presence of sulfuric acid. While extreme accuracy is not possible, the method is much quicker and easier than the conventional type of thiophene determination. Two variations are possible: an approximate method in which colors are matched by visual observation, and a more precise method utilizing a spectrophotometer or other photoelectric colorimeter.
THE
signed to remove the thiophene. Samples were analyzed according to the procedure presented here. The proposed method can also be applied to cracked gasolines and gasolines containing some aromatics, by observing certain precautions as explained below.
method of thiophene estimation presented here was developed in conjunction with a study of thiophene removal from petrolwm fractions, particularly gasoline. The conventional methods of sulfur analysis found in the literature involve the removal of the various sulfur constituents by means of selective solvents and precipitating agents, and the determination of total remaining sulfur by the lamp method after each removal (Ball, 1, and Cross, 2 ) . While this type of analysis is accurate, it is rather long and involved, particularly when thiophene is to be determined. Since thiophene is usually the last sulfur constituent t o be determined, it is necessary t o remove all other sulfur constituents (generally stepwise) before making the thiophene determination. However, the authors found that the indophenine reaction can be used as a basis for thiophene determination, making it possible t o determine thiophene directly without interference from other sulfur compounds which may be present in the gasoline, since they have no effect on the reaction producing the blue color. The indophenine reaction was used by Schwalbe (5) to determine thiophene in benzene in 1905, but no evidence of recent work of this type was found in the literature. The proposed method seemed t o ivork satisfactorily with various kinds of gasoline fractions. The presence of aromatic compounds from crude oil occasionally caused some difficulty 15 hich necessitated the use of special precautions, but did not prevent accurate analyses. Concentrations as low as 0.0057, thiophene sulfur (0.013% thiophene) by weight have been readily determined, and it is possible that lower concentrations could be detected \vith little or no decrease in accuracy. Many factors, some of them difficult to control, affected the intensity of color produced, and caused slight variations of the color intensity as measured by a photoelectric colorimeter. Consequentlv, the results obtained JT ere not always accurate to \r-ithin less than 15 to 20% of the amount of thiophene sulfur present, although better accuracy x-as obtained in many cases. Khen the colois nere matched by visual observation, the results weie usuallv accurate t o vithin 25 or 30% of the amount of thiophene sulfur present, although this naturally depended to a large e\teilT upon the individual operator. Experience greatly increased accwracy in this determination. While it may seem that this degree of accuracy is not very pre.cise, thiophene concentrations n-ere very 1011- in many of the .ample< anal) zcd, and this permitted more precicion in measuring The absolute quantity of thiophene present. For example, an ,muracy of nithiii 2 0 5 of the amount of thiophene sulfur prescnt, in analyzing a saniple containing 0.057, thiophene -ulfur, ~r-ouldgive a maximum error of only 0.0170 thiophene d f u r . The larger errors were obtained in analyzing samples of higher thiophene concentration (0.50 to 1.00% thiophene sulfur). The proposed method has been used primarily t o aid in the ytudy of various methods of removing added thiophene from -traight-run gasoline. I n this work, 1% thiophene sulfur (by eight) was added to a substantially thiophene-free gasoline, and the resulting mixture subjected to various reactions de-
PRECAUTIOSS
One factor which caused some difficulty was a secondary reaction that took place between the gasoline and the isatin-acid reagent. This reaction proceeded over a period of hours or days, and produced a red color. This secondary reaction was not studied completely, but seemed to be more evident in the presence of compounds of aromatic or cyclo- structure than in the presence of aliphatic compounds. When a blank solution of thiophene-free gasoline was used for comparison, the same color change took place in the blank during the same period of time. Therefore, no error in analysis was caused by this secondary reaction, since the analysis is based on a comparison of the intensity of monochromatic light passed through the blank and unknown solutions. It was important to use a blank solution containing similar gasoline and the same amount of gasoline as the solution being tested M henever possible. Hoyever, it was not always possible to use a blank solution containing gasoline identical to the sample, since the reactions used in attempting to remove thiophene sometimes changed the gasoline by removing some aromatics. I n the case of hightemperature reactions, there was occasionally some evidence of cracking or other change in the molecular constitution of the gafoline. These changes (particularly the cracking) sometimes caused the secondary reaction t o be more noticeable with the sample being tested than with the blank solution prepared from the original gasoline. I n no case was this secondary reaction noticeable Tyhen the te>t solutions were first prepared, but appealed some time latei, usually in from 2 to 24 hours. Therefore, the best criterion for judging the effect of thi-; secondary reaction seemed to be the factor of time. The original reaction producing the indophenine blue color appealed to bc complete nithin 20 to 30 seconds. -4s long as no additionaI color changes took place after this period, no errors in analysis xcre noted in any cafe, although test solutions neie occa-ionally analyzed after standing for several hours. Errors due to the reaction between gasoline and isatin-acid ieagent were avoided by obseiving the following precautions : (1) h blank solution containing gasoline similar to that being tested was used whenever possible, a d both solutions nere prepared in the same n a y a t the same time. (2) If this waq impossible, all matching and checking of colors T+ ere completed as soon as possible after the solutions were made up. X o samples of gasoline mere encountered in which this secondary reaction proceeded rapidly enough to make it impossible t o complete a thiophene analysis before the reaction became evident. However, such gasolines might be encountered in
301
ANALYTICAL CHEMISTRY
302 Table 1.
Data for Spectral Transmittance Curve
[Wave length us. per cent transmittance for A 0.80%, E 0.50%, C 0.30% and D 0.15'% thiophene sulfur] Wave Length, Per Cent Transmittance mp 0.80% 0.50% 0.30% 0.15% 400 44 56 64 76 425 26 33 46 60 450 11 21 30 49 475
500 525 530 540 550 560 570 585 600 625 650 675 700 725 750 775 800 825
5
3.1 1.8 1.75 1.5 1,35 1.40 1.5 1.5 1.8 2.7 3 .08 3.95 1.32 4.8 29 56 66 71
15 . .
27
48
12 7.5
24 19 18 16 16 15.5 15.2 17 21 30 35 38.5 36.7 56.3 75.3 83.5 87.8 91
47.8 44 42.5 41.8 41.2 42.1 43.4 47 51.7 61.7 68 72 75.3 84 91 94 96 98
5; 5.0 5.0 5.1 5.5 7.8 15 19.5 18.7 18.7 38 63 75 80 84
working with cracked gasolines or those high in aromatic content. If such a gasoline were encountered, the' proposed method of thiophene analysis could not be used. According to the suggestion of Wray (67, one drop of concentrated nitric acid was added to the isatin-acid reagent to act as an oxidizing agent and t o aid in the formation of the characteristic blue color. However, if more than one drop was added, the reaction was inhibited and results were inaccurate. When the nitric acid was added to isatin-acid reagent, the dark brown color of the latter was changed to a lighter shade upon agitation.
For analytical work, the transmittance at 540, 550, and 560 mp was determined, and curves of concentration us. transmittance were prepared. The curve for 550 mp is shown in Figure 2. For good results, only readings of transmittance between 5 and %yo were recorded, With solutions containing 1 ml. of thiophene-gasoline, this included concentrations between approximately 0.50 and 0.025% thiophene sulfur. Best results were obtained by considering only that portion of the curve which showed the greatest change in transmittance in proportion t o concentration change-namely, from 50 t o The irregularities which appeared from time to time also seemed to be in the darker solutions where transmission was less than 50%. Tables I and I1 give the data used in preparing the curves of Figures 1 and 2. I t is suggested that calibration curves be prepared for the particular instrument to be used, before attempting analytical work. It nil1 then be possible to analyze unknown solutions by measuring the per cent transmittance against a blank solution containing thiophene-free gasoline, and referring to the calibration curves to find the concentration of thiophene sulfur in the solution being analyzed. The usual method was to take readings a t 540, 550, and 560 mp, and average the results obtained from calibration curves at these wave lengths. If the concentration of thiophene sulfur was less than 0.025%, 2 ml. of the gasoline being tested were used instead of 1 ml. The result as read from the calibration curve mas then divided by 2 t o obtain the actual concentration present in the sample. By such a process, amounts of-thiophene-gasoline solution up to 5 ml. have been used with no decrease in accuracy. At 96% Table 11.
REAGENTS
Data for Calibration Curve for 550 m p
(Concentration, % thiophene sulfur vs. per cent transmittance for 1-rnl. sample) Concentration 70Transmittance 0.02 98.5 0.04 88.8 75.0 0.07 64.5 0.10 41.2 0.15 35.7 0.20 15.5 0.30 0.40 5.5 5.0 0.50
Isatin Acid. Isatin (0.400 gram) was dissolved in enough concentrated sulfuric acid (sp. gr.>1.84) to make 1000 ml. of solution. If the concentration of isatin was too small, the resulting solutions showed irregular color characteristics and accurate analysis was impossible. If the concentration of isatin was too great, the total range of color change took place within a very narrow range of thiophene concentration. The amount specified above seemed to be the best average between these two factors. Nitric Acid, concentrated, C.P. Thiophene-Gasoline. Solutions of C.P. thiophene in gasoline were used as a standard. These were made up volumetrically and the thiophene concentration was converted i o a weigHt basis, using the density of the gasoline as determined by a pycnometer, and the data of Fawcett and Rasmussen (3) concerning the density of thiophene. All concentrations were expressed as per cent by weight of thiophene sulfur.
100
80
PROCEDURE
In making up test solutions, 50 ml. of isatin-acid reagent were measured by pipet and placed in a 200-ml. flask or bottle. One drop of concentrated nitric acid was added, and the mixture was shaken. The required amount of thiophene-gasoline solution (1 ml. in most cases) was added with a graduated pipet. Upon shaking, the characteristic blue color appeared almost immediately, and the reaction appeared t o be complete within 20 to 30 seconds. PHOTOELECTRIC METHOD
Because of interference between the brown isatin color and the blue indophenine color, visual matching of colors was sometimes difficult for an inexperienced observer. Better results were obtained by measuring the color with a photoelectric instrument, using monochromatic light. The tests described here were made with a Beckman quartz spectrophotometer, after the method of Mellon (4). To determine the spectral characteristics of the solutions, spectral transmittance curves were prepared for four concentrations, using 1 ml. of thiophene-gasoline solution (Figure 1). As can be seen, the minimum transmittance occurred a t 550 mp wave length. Partial minima occurred at 475 and 710 mp, but these were not considered in detail.
a
*
60
'E
2
E
g
40
20
400
500
Figure 1. A.
0.80%
600 700 Wave Length, mp
800
Spectral Transmittance Curve B.
Thiophene sulfur 0.50% C. 0.30%
D.
0.15%
303
V O L U M E 20, NO. 4, A P R I L 1 9 4 8
Khen the two solutions appeared identical in color, the amount of thiophene sulfur present in the unknown was computed according to the formula:
CIMl = CnM2
where C1 = concentration of thiophene sulfur in unknown sample bfl = ml. of unknown sample used Cz = concentration of thiophene sulfur in standard solution M? = ml. of standard solution used For example, if a standard solution containing 1 ml. of a solution of 0 . 1 0 ~ ' thiophene sulfur was used, and 1.2 nil. of unknown solution were required to produce the same shade of color, the thiophene concentration was found as follows:
C,) (1.2) = (1.0) (0.10) c1=
0
Figure 2.
0.1 0.2 Concentration,
0.3 0.4 Thiophene Sulfur
0.5
Calibration Curve of Thiophene Sulfur us. % Transmittance
transmittance, this corresponded to a concentration of X 0.025, or 0.005% thiophene sulfur. I n the same manner, concentrations of thiophene sulfur greater than 0.50y0 were measured by using 0.5 ml. of thiophene-gasoline solution and multiplying the result by 2, etc. In each case, a blank solution was used containing the same amount of gasoline (thiophene-free) as the test solution, and both solutions were made up a t approximptely the same time. This eliminated inaccuracy due to the reaction between the gasoline and the isatin-acid reagent. VISUAL OBSERV4TION METHOD
If no photoelectric colorimeter was available, unknown solutions of thiophene in gasoline were analyzed by visual observation. This could have been accomplished by using a simple colorimeter in which the intensity of color is matched by looking through two samples, and diluting one t o a measured extent, obtaining the same depth of color in each sample, However, in an attempt to simplify €he analytical procedure as much as possible and t o eliminate the necessity for any type of special equipment, most of the work was done by purely visual observation. With such a method, sufficient accuracy was obtained for the work being done. Two solutions were used, each containing 50 ml. of isatin-acid reagent and one drop of concentrated nitric acid. To one solution was added 1 ml. of a standard thiophene-gasoline solution containing approximately the amount of thiophene expected in the unknown. This standard solution replaces the blank solution of thiophene-free gasoline that was used in the photoelectric method. To the other solution the unknown gasoline sample was added until the indophenine color was observed. Both solutions were agitated and their intensity of color was compared. If the solution containing the unknown was not so dark as the one containing the standard thiophene-gasoline s o h tion, more of the unknown sample was added. After agitation, the color was again checked against the standard solution. Final comparison vias made under a strong light o n a white background, after both solutions had stood for 4 or 5 minutes. This addition of the unknolvn sample can be made readily with a 1-ml. graduated pipet, and the quantity which is added can be measured to within 0.1 ml. If too much of the unknown sample was added by accident, the standard solution was made darker by adding a measured amount of the thiophene-gasoline standard solution, to compare with the darker unknown.
("O)
(o'lo) = 0.083% thiophece sulfur in unkno n 1.2
The secondary reaction between gasoline and isatin-acid reagent is more noticeable as the relative amount of gasoline is increased, giving a more rapid color change in the solution containing the larger amount of gasoline. Therefore, it is advisable to have an approximately equal amount of gasoline in the standard and unknown solutions, and all checking of colors should be completed within 30 t o 45 minutes of the time the solutions are first prepared. CONCLUSIONS
While not extremely precise, the method of analysis preaented here is very simple and easy of execution. After the necessary reagents and standard solutions are a t hand, an unknown sample can be analyzed in a few minutes by either of the t r o methods described. Another advantage of the proposed method is that only 1 or 2 ml. of the gasoline being tested are required to prepare a test solution; this enables one to work with very small amounts of gasoline if necessary and still be able to analyze the results properly. In all, about 100 determinations of thiophene in gasoline have been made to date. The majority of the samples were analyzed by the visual observation method. The unknown concentrations varied from 1.00% thiophene sulfur down to a mere trace of thiophene, of the order of 0.0027, thiophene sulfur. A detailed study of the data obtained indicated that the results were consistent in all cases within reasonable limits, as shown by the fact that smooth curves were obtained with the results of each series of tests. Such curves would obviously have been impossible without accurate analyses, regardless of the nature of the reactions being studied. Therefore it is seen that the proposed method of analysis is accurate enough for experimental work, although the accuracy is not equal t o that obtained by other methods. ACKNOWLEDGMENT
The writers wish to acknowledge the assistance of J. A. Shenk and R. J. Morris of the Chemistry Department, Ohio State University, in making the spectrophotometer tests. The writers also wish t o express appreciation to Ralph G. Pearson, Chemistry Department, Northwestern *University, whose preliminary suggestions aided in organizing the work. LITERATURE CITED
(1) Ball, J. S., U. S. Dept. Inteiior, Bur. Mines, Rept. Investigatzons 3591 (December 1941). (2) Cross, Roy, "A Handbook of Petroleum, Asphalt, and Natural Gas," p. 679, Kansas City, R l o . , Kansas City Testing Laboratory, 1931. (3) Fawcett, F. S., and ,Rasmussen, H. E., J . Am. Chem. Soc., 67, 1705-9 (1943). (4) Mellon, M .G., IAD. EXG.CHEM..ASAL. ED.,9, 51 (1937). (5) Schwalbe, C., J . SOC.Chem. Ind., 24, 988 (1905). (6) TT'ray, Edward, Ibid., 38, 83-4T (1919). RECEIVED July 22,1947.
