Determination of Elementary Sulfur in Gasoline
and Naphtha A Quantitative Method CHARLES WIRTH I11
AND
JAMES R. STRONG, Universal Oil Products Company, Riverside, Ill.
T
HE presence of excessive free or elementary sulfur in gasoline or naphtha has been avoided for many years. The corrosion of the metal parts of automobiles (2, IO), particularly copper and its alloys, has been attributed to the effect of this component. A recent study (3)of color stability of gasoline has shown the influence of elementary sulfur upon this property. Sulfur may be present, normally, through oxidation of hydrogen sulfide to sulfur, or addition of sulfur in the doctor, or sodium-plumbite, sweetening process. Since hydrogen sulfide is usually removed from gasolines before much oxidation has occurred, the latter source accounts for the greater proportion. I n the sweetening process, the elementary sulfur is added during the treatment of the gasoline with doctor solution to convert the malodorous mercaptans to disulfides. In order to assure rapid settling of the lead sulfide formed in the reaction, an excess of sulfur must be used. A careful control of the sulfur addition must be exercised to avoid a corrosive gasoline. The criterion in specifications of a corrosive gasoline is based upon the results of a copperstrip test, A. s.T. M. Designation D130-27T. This test usually requires 3 hours for completion and the result is only qualitative. In most gasolines the amount of excess elementary sulfur that is required to give a corrosive copper-strip test is between 0.003 and 0.004 per cent by weight. There have been many methods (4-9,II-IS) recommended for the quantitative determination of elementary sulfur in gasoline. Most of these are long or too involved to be satisfactorily used in a refinery doctor treating plant. Time is the essence of most refinery operations and a rapid determination is extremely advantageous. Mercury is often used as a quick method for the detection or estimation of elementary sulfur. It is sensitive to extremely small quantities which are much lower than the amounts required for a corrosive copper strip. A quantitative determination of the precipitated mercury sulfide involves a long and tedious analytical procedure. I n addition, mercury reacts with organic peroxides in cracked gasolines with the formation of a black precipitate similar in appearance to mercury sulfide. The method proposed in this paper allows a rapid quantitative determination of elementary sulfur which involves a simple laboratory technic readily usable by the small refiner. It is particularly applicable to a ready control of the doctor sweetening process and it may in time serve as a substitute for the copper-strip test in gasoline specifications. In the determination a known quantity of a mercaptan is added to the gasoline. The mixture is agitated with doctor solution until complete reaction of the sulfur and the added mercaptan has occurred. Since the excess mercaptan is now present as lead mercaptide, it is restored to the original mercaptan by dilute sulfuric acid. Following a wash with acidified cadmium chloride solution to remove the liberated hydrogen sulfide, the gasoline is titrated with silver nitrate solution ( I ) to determine the excess mercaptan. The used mercaptan, obtained by difference, permits a calculation of the free sulfur content of the original gasoline. The method is not applicable to oils heavier than kerosene because of the difficulties involved in the determination of their mercaptan sulfur contents. 344
Reagents The only special reagent required for the determination is a standard butyl mercaptan solution in a cleaners' naphtha. The preferred butyl mercaptan is of the chemically pure Merck grade, boiling point 98' C. and free of elementary sulfur. The cleaners' naphtha must not contain any mercaptans or elementary sulfur. The former may be removed by precipitation by agitation with a silver nitrate solution. By agitation of the naphtha with mercury, the elementary sulfur may be precipitated as mercury sulfide which may be readily removed by filtration. The standard (0.5 M ) solution of the mercaptan is prepared by the addition of 45.07 grams of butyl mercaptan in sufficient of the cleaners' naphtha to obtain 1 liter of the solution. The solution should be retained in a tightly stoppered dark bottle to avoid deterioration. The remaining reagents are common to the usual petroleum laboratory and consist of a 30" B6. sodium plumbite solution, sulfuric acid (20 per cent concentration), and cadmium chloride solution (10 per cent, containing 1 cc. per 100 cc. solution of concentrated hydrochloric acid). The reagents used in the mercaptan titration ( I ) are 0.05 N silver nitrate, 0.05 N ammonium thiocyanate, and ferric alum indicator.
Procedure Add by means of a buret exactly 3 cc. of the standard butyl mercaptan solution to 147 cc. of the gasoline to be tested. Agitate thoroughly in a small separatory funnel for 5 minutes with 20 cc. of the sodium plumbite solution. Allow to settle and drain the plumbite. Wash the oil by agitation with 50 per cent by volume of 20 per cent sulfuric acid until all the black lead sulfide is dissolved and the oil is restored approximately to its original color. Settle and withdraw the acid. Wash the oil by agitation with 50 cc. of a 10 per cent solution of acidified cadmium chloride t o remove the hydrogen sulfide. Remove the cadmium chloride solution by drainage. Determine the mercaptan sulfur content on 100 cc. of the residual oil (1) by agitating the gasoline with an excess of silver nitrate solution and back-titrating with ammonium thiocyanate using ferric alum as an indicator. Express TABLEI. MIDCONTINENT STRAIGHT-RUN GASOLINE Weight of Elementary Sulfur Present Found Gram Gram 0.015 0.0164 0.01125 0.01042 0.00733 0.0075 0.00559 0.00662 0.00458 0.0045 0,00374 0.00376 0.0028 0.0030 0.001875 0.00198 0.001125 0.00129
Elementary Sulfur Present Found
Error
%
%
%
0.02 0.016 0.01 0.0075 0.006 0.006 0.004 0.0026 0.0015
0.0218 0.0139 0,0098 0.00745 0.0061 0.00499 0.00373 0.00264 0.00172
+0.0018 -0.0002 -0.0011 -0.00006 +O.OOOl -0.00001 -0.00027 +o. 00012 +o. 00022
TABLE 11. MIDCONTINENT CRACKED GASOLINE Weight of Elementary Sulfur Found Present Gram Gram 0.0168 0.015 0.01158 0.01125 0.0075 0.0073 0,00561 0.00562 0.0044 0.0045 0.00375 0.00399 0.00299 0.0030 0.001875 0.00198 0.00132 0.001126
Elementary Sulfur Found Present
Error
%
%
%
0.02 0.015 0.01 0.0075 0.006 0.005 0.004 0.0026 0.0015
0.0224 0.0154 0.0097 0.00748 0.00686 0.00532 0.00399 0.00264 0.00176
+0.0024 +0.0004 -0.0003 -0.00002 -0.00014 +0.00032 -0.00001 +0.00014 + O . 00026
SEPTEMBER 1.5, 1936
ANALYTICAL EDITION
TABLE111. MIDCONTINENT STRAIGHT-RUN GASOLINE Per Cent by Weight of Elementary Sulfur 0.025 0.0125 0.00625 0.004 0.003125 0 0015625 0 00078126
Corrosion Test Positive Posjtjve Positive Questipnable Negative Negative Negative
TABLEIV. MIDCONTINENT CRACKED GASOLINE Per Cent by Weight of Elementary Sulfur 0,026 0.0125 0.00625 0.004 0.003125 0.0015625 0.00078125
Corrosion Test Positive Positive Positive Questionable Negative Negative Negative
the mercaptan sulfur content of the residual oil in grams of sulfur per 100 cc. of gasoline, which is equal to cubic centimeters of silver nitrate solution multiplied by its normality and the conversion factor of 0.032. By using 100 cc. for the titration and calculating the mercaptan content in terms of grams of sulfur, the following calculations are simplified. The elementary sulfur content of 100 cc. of the gasoline is calculated by means of the following formula: sulfur X grams of butyl mercaptan used x = 1/2 mol. wt. ofmol. wt. of butyl mercautan where X = grams of sulfur in 100 cc. of the sample, and grams of butyl mercaptan used = grams of butyl mercaptan added - grams of mercaptan sulfur in 100 mol. wt. of butyl mercaptan cc. of residual oil X mol, u,t. of sulfur ~
Experimental In the experimental study of this procedure several synthetic mixtures of known elementary sulfur contents were prepared. Concentrated stock solutions of elementary sulfur in midcontinent straight-run and cracked gasolines were made. Elementary sulfur and mercaptans were removed from each of these solvents by mercury and silver nitrate, respectively, before use. The stock solutions were used in the preparation of a sample of any desired percentage of elementary sulfur by suitable dilution with the proper solvent. Determinations were made on each of these prepared samples, and the results are shown in Tables I and 11. The results indicate that the proposed method gives an accurate quantitative determination of elementary sulfur in cracked and straight-run gasolines. The method is suitable as a control of the addition of elementary sulfur during the sweetening process, since the accuracy in the concentration range of a corrosive copper strip is satisfactory.
Investigation of the Determination ELEMENTARY SULFURRBQUIREMENT FOR A CORROSIVE COPPER-STRIPTEST. The stock solutions of elementary sulfur in cracked and straight-run gasolines were properly diluted to extend over the desired range of sulfur contents. Copper-strip corrosion tests, A. S. T. M. Designation D130-27T, were made on each prepared sample. The results of these tests are shown in Tables I11 and IV. In each of these two gasolines, an elementary sulfur content of approximately 0.004 per cent by weight was required t o obtain a corrosive copper-strip test. From the authors’ experience, the amount may show a slight variation with different gasolines, although this is probably partially a function of the accuracy of the test method.
345
USE OF SULFURIC ACIDTO CONVHRT LEADMERCAPTIDE. The concentrated butyl mercaptan solution was diluted with sufficient sulfur-free gasoline to prepare solutions containing several percentages of mercaptan. Each of these samples was shaken with plumbite until all the mercaptan was converted to lead mercaptide. The plumbite solution was separated by drainage and the gasoline was washed with 20 per cent sulfuric acid until all the yellow color had disappeared, indicating that no lead mercaptide remained. A mercaptan titration was made on the residual oil to determine the loss of butyl mercaptan, if any. A comparison of the mercaptan content found after this treatment with that present in the original solution is shown in Table V. Based upon this comparison, no butyl mercaptan is lost by this treatment with plumbite and sulfuric acid solutions. The mercaptan sulfur contents of the gasolines after treatment check the actual values of the original solutions well within the accuracy of the titration. EFFECT OF HYDROGEN SULFIDE. In the presence of elementary sulfur, lead mercaptides are converted into disulfides with the formation of lead sulfide. The addition of dilute sulfuric acid results in the formation of hydrogen sulfide. The removal of this gas is necessary to permit a satisfactory mercaptan titration for the determination of the unused butyl mercaptan. Several gasolines of known mercaptan contents determined by titration were agitated with plumbite solution. The plumbite was properly drained and a small amount of powdered lead sulfide added to each gasoline. These mixtures were shaken with a 20 per cent sulfuric acid solution until the lead sulfide was converted to lead sulfate. The acid was drained and each sample of oil was washed with a cadmium chloride solution to remove the hydrogen sulfide. A mercaptan titration was conducted on each of the residual oils and compared to a similar determination on each original gasoline as shown in Table VI. The results show that the hydrogen sulfide may be removed with the acidified cadmium chloride solution without any effect on the mercaptan content of the gasoline. TABLEV. MERCAPTAN SULFUR Mercaptan Sulfur Added
Mercaptan Sulfur Found
%
%
0.246 0.129 0.060 0.031 0,0163 0,0082
0.240 0.127 0.059 0.030 0.0161 0,0081
TABLEVI. MERCAPTAN SULFUR Mercaptan Sulfur in Original Gasoline
Mercaptan Sulfur in Reeidual Oil
%
%
0.072 0.0528
0.070 0,0523 0.0178 0.0147 0,0047
0.018
0.015 0.005
TABLEVII. MERCAPTAX SULFUR Mercaptan Sulfur in Original Gasoline
Mercaptan Sulfur in Residual Oil
%
%
O.OS3 0.060
0.081 0.058 0.020 0,009 0.004
0.021
0.01 0.005
INFLUENCE OF THE PRESENCE OF DISULFIDES.Since butyl disulfide is formed in the elementary sulfur determination, the effect of its presence on the mercaptan titration was determined. Normal butyl disulfide (0.10 per cent by weight) was added to several unsweetened gasolines of known mercaptan contents. These gasolines were agitated with plum-
346
INDUSTRIAL AND ENGINEERING CHEMISTRY
bite and settled, and the plumbite was drained. They were then agitated with dilute sulfuric acid, and the oil was separated and titrated for mercaptan content with silver nitrate. Table VI1 shows a comparison of the mercaptan contents of the original gasolines, free of disulfides and elementary sulfur, with those obtained on the treated oils. These mercaptan contents check each other satisfactorily. The presence of disulfides has no effect on the determination. OTHER APPLICATIONS. This method may be used to determine quantitatively the theoretical amount of sulfur required to sweeten a gasoline. The same procedure is followed with the exception of using the sour gasoline and without the addition of a known quantity of butyl mercaptan. The unsweetened gasoline is agitated with plumbite, settled, and the plumbite drained. The gasoline is treated with dilute sulfuric acid, separated, and washed with cadmium chloride solution. The residual oil is titrated with silver nitrate for mercaptan sulfur content. The remaining mercaptans are a measure of the additional sulfur required which is calculated by the following equations: cc. of silver nitrate X normality X 0.032 %of mercaptan sulfur = wt. of sample % of mercaptan sulfur in residual oil % of sulfur required = 2 The actual amount of sulfur required will be greater than the determined theoretical value by an amount necessary to give a sharp break of the lead sulfide from the gasoline. This amount should not be allowed to exceed the quantity which will cause a corrosive copper-strip test. A knowledge of the
VOL. 8, NO. 5
theoretical requirement will aid in preventing the refiner from using excessive sulfur.
Conclusions The proposed method is a rapid, accurate quantitative determination of the elementary sulfur content of gasoline. The method facilitates the testing and control of gasolines or copper-strip corrosion during refinery sweetening operations by minimizing the time required for the determination. The test can be readily completed in 0.5 to 0.75 hour. The proposed method may be used to determine the theoretical quantity of elementary sulfur required in the sweetening of a gasoline. By this means the addition of excessive sulfur over the amount necessary for a satisfactory break may be avoided.
Literature Cited (1) (2) (3) (4) (5) (6) (7)
(8) (9) (10) (11) (12) (13)
Borgstrom and Reid, IND.ENQ.CHEM.,Anal. Ed., 1, 186 (1929). Diggs, IND.ENQ.CHEM.,20, 15-17 (1928). Egloff, Morrell, Benedict, and Wirth, Zbid., 27, 323 (1935). Faragher, Morrell, and Monroe, Ibid., 19, 1281 (1927). Garner, J . Inst. Petroleum Tech., 17, 451-63 (1931). Hoffert, J., Rept. Nut. Benzols Assoc., 30 (March, 1925). Inat. Petroleum Tech., “Standard Methods for Testing Petroleum and Its Products,” 2nd ed., 1929, p. 18. Standard Method G. 4b. Kattwinkel, Brennstof-Chem., 8, 259-60 (1927). Lomax, J . Inst. Petroleum Tech., 4 , 19 (1918). Mangey, IND.ENQ.CHEM.,20, 18-21 (1928). Morrell, Zbid., 18, 733 (1926). Ormandy and Craven, J . I n s t . Petroleum Tech., 9, 135 (1923). Wendt and Diggs, IND. ENQ.CHEM.,16, 1114 (1924).
R E C ~ I V EJune D 26, 1936.
A System for the Qualitative Analysis of the Alkaline Earth and Alkali Groups CHARLES H. GREENE, Harvard University and Radcliffe College, Cambridge, Mass.
A
LTHOUGH the procedures recommended by Noyes (3) and Bray (9) for detecting the alkaline earth and alkali metals for the most part give accurate results with careful handling, it is found in practice that some of the filtrations are time-consuming. It is the purpose of this paper to describe a system which is more convenient and rapid and which also gives accurate results. All the separations in the two groups have been tested carefully and where it has not been found possible to make improvements, as in the separation and detection of barium and strontium, the methods employed by Woyes and Bray have been adopted. The new system has been used with success for two years by a class in qualitative analysis in Harvard College. The number of errors made by students in identifying the ions of these groups has been decidedly smaller than i t was previously with other systems. With the new procedures the precipitates which must be filtered in order to separate ions or groups of ions are of such a nature as to promote rapid filtration and to give clear filtrates. Prompt and decisive results are obtained in the final tests even when only traces of the ions are present. I n this system advantage is taken of the fact that neither the very delicate p-hydroxybenzene-azoresorcinol test for magnesium described by Suitsu and Okuma (7) nor the magnesium uranyl acetate test for sodium described by Blanchetiere (1) is interfered with by traces of the alkaline earth metals. The convenient process recommended by Rawson (5) for the quantitative separation of barium and strontium from calcium is employed.
The detection of small quantities of calcium in the presence of much magnesium proved to be a difficult problem, particularly when strontium was also present. It was finally solved by evaporating the solution of calcium and magnesium nitrates in nitric acid to dryness and igniting the residue. The magnesium was thus rendered sufficiently insoluble so that when the calcium was extracted with water a delicate test for it could be made with potassium oxalate. With much magnesium in the solution the oxalate test for calcium is both insensitive and unreliable.
Materials and Reagents In general, solutions and reagents were prepared from reagent-grade chemicals. It was necessary, however, to purify calcium nitrate and magnesium nitrate by repeated recrystallization. Magnesium uranyl acetate reagent was made up according to the directions of Noyes and Bray (4). The magnesium reagent was prepared according to the directions of Ruigh (6). Other reagents were prepared according to Noyes’ directions (3).
Procedure Prepare a solution free from members of the other groups and containing approximately 45 milliequivalents of ammonium salts and not more than 500 mg. of the elements of the two groups in 25 ml. Heat to boiling, add 10 ml. of ammonium carbonate reagent, and keep at the boiling point for 3 minutes. A white precipitate indicates the presence of calcium, strontium, barium, or more than 50 mg. of magnesium. Filter off the precipitate, wash with hot water, and test for these elements.
