November, 1927
INDUSTRIAL AND ENGINEERING CHEMISTRY
1281
Quantitative Determination of Sulfur and Sulfur Derivatives of Hydrocarbons in Naphtha Solutions and in Petroleum Distillates' By W. F. Faragher, J. C. Morrell, and G. S. Monroe UNIVRRSAl 0% PRODUdS CONPAXY, CIIICAGO, I L L
SYSTEMATIC procedure for the quanbitative determination of sulfur and sulfur deriratives of hydrocarbons in petroleum products is of great importance in petroleum research. I t is of particular importance in the investigation of the corrosion of petroleum equipment during atmospheric and pressure distillation, and in the study of the reactions that occur during cracking and in the refining of petroleum distillates. I n addition to these uses of practical interest, the quantitative determination of the various sulfur compounds which occur in petroleum has many scientific applications. This paper presents a procedure for the quantitative determination of sulfur present in naphtha as hydrogen sulfide, elementary sulfur, mercaptans, sulfides, disulfides, and, by difference, residual sulfur. Materials
A
The sulfur compounds used were the purest obtainable. Hydrogen sulfide was prepared by the usual laboratory method and the elementary sulfur was pure flowers of sulfur. The naphtha used in making up the solutions of the sulfur compounds had the following physical properties : Color
Specific gravity at 60°F. (15.6" C.) Initial boiling-point End point Sulfur by lamp method
Water-white 0 740 190' F. (88' C.) 330' F. (166' C . ) None
Preparation of Naphtha Solutions
HYDROGEN SULFIDE-The naphtha solutions of the sulfur compounds were saturated with hydrogen sulfide by passing the gas into the solutions for approimately 5 minutes. ELEMEKTARY SuLFuR-sulfur was dissolved in the naphtha solutions of the sulfur compounds. MERCAPTAXS-Ethyl, n-propyl, n-butyl, isoamyl, and benzyl mercaptans were used. DIsuLFIDEs-E'thyl, n-propyl, n-butyl, isoamyl, and diphenyl disulfides were used. SULFIDES-&kthyl, ethyl, n-propyl, isoamyl, and benzyl sulfides were used. THIoPHEm-ResiduaI sulfur wis represented by thiophene. The percentage of sulfur in the solutions as determined by the lamp method2 is as follows: mercaptans 1.73, sulfides 1.10, disulfides 1.52, and thiophene 0.52 per cent. Method of Analysis
HYDROGEN SULFIDE-Hydrogen sulfide must be eliminated from the sample before proceeding with the determination of the other groups, as it interferes with the determination of the elementaiy sulfur and the mercaptans. In the determination of hydrogen sulfide, the sample is shaken for 2 to 3 minutes with the quantity of cadmium chloride solution necessary for complete precipitation. The 1 Presented before the Division of Petroleum Chemistry at the 73rd Meeting of the American Chemical Society, Rjchmond, Va., April 11 to 16, 1927. 2 Bur. of Miner, Tcth. Paper 323-A; Method 520 1, A. S. T. M. D90ZIT.
cadmium chloride solution is made by adding 10 grams of cadmium chloride to 100 cc. of water and subsequently adding 1 cc. of concentrated hydrochloric acid. This quantity of acid gives the solution an acidity of approximately 0.32 per cent, which has been found sufficient to prevent the formation of cadmium mercaptide without preventing the quantitative precipitation of cadmium sulfide. The percentage of hydrogen sulfide is calculated from the weight of the cadmium sulfide after washing and drying. An attempt was made to remove the hydrogen sulfide by refluxing the sample, but the removal could not be completed with 3 or 4 hours of refluxing. The use of a solution of sodium hydroxide to remove the hydrogen sulfide is open to the objection that sodium hydroxide reacts with the mercaptans and with elementary sulfur. Elementary sulfur and sodium mercaptides would produce the disulfides, thereby complicating the analysis. The sodium salts of the mercaptans of lower molecular weight, including the thiophenols, are removed in part by sodium hydroxide. The suggestion of Wendt and Diggs? to use acidified lead acetate to remove hydrogen sulfide cannot be applied here, as the lead acetate solution acidified with acetic acid up to 30 per cent will react with the mercaptans in the presence of sulfur to form the disulfide and lead sulfide. (While a solution of lead nitrate acidified with a small amount of nitric acid should be satisfactory, the acidified cadmium chloride solution is preferable.) ELEMENTARY SULFUR-It is necessary to remove the elementary sulfur after the hydrogen sulfide so that the determination of the mercaptans will not be complicated. The use of mercury for the removal of elementary sulfur is a modification of the method suggested by Ormandy and Craven.4 A naphtha solution containing 0.2 per cent of elementary sulfur shopred nqsulfur by the lamp method after shaking with mercury until no further reaction was observed by filtering off a small sample and shaking with a fresh supply of mercury. A qualitative test upon the same sample, made by adding ethyl mercaptan and sodium plumbite solution, gave no precipitate of lead sulfide. The only observable reaction was the formation of the bright yellow mercaptide, indicating the absence of elementary sulfur. The method of Ormandy and Craven is tedious. There are several operations in their determination, including the oxidation of the sulfide of mercury to the sulfate and the determination of the sulfate gravimetrically as barium sulfate Wendt and Diggs3 propose the estimation of elementary sulfur by adding ethyl mercaptan to the sample after the removal of hydrogen sulfide. The sample is subsequently shaken with plumbite solution and the lead sulfide formed furnishes the basis for calculating the amount of elementary sulfur present. This method is open to several objections, principally because a known quantity of ethyl mercaptan must be added, or an aliquot of the solution must be used. The reactions involved are' *Trim 4
JOURNAL, 16, 1114 (1924). J . I n s f . Pctrokum Tech., 9, 135 (1923).
I N D U S T R I A L AND ENGINEERING CHEMISTRY
1282 NazPbOz
+ 2RSH+
R--S. R--S/ __
+
+ 2NaOH
'Pb
R-S
+
R->Pb S+ I PbS R-S R-S MERCAPTANS-The mercaptan sulfur is determined after the removal of hydrogen sulfide and elementary sulfur. Alcoholic Plumbite Method. A small portion of the sample is shaken with about one-half its volume of alcoholic plumbite solution made by mixing equal volumes of 95 per cent alcohol and an aqueous solution of sodium plumbite. The aqueous solution is made by saturating a 20 to 25 per cent solution of sodium hydroxide with lead monoxide (litharge) and filtering off the excess of lead oxide. The sample of oil is shaken with the alcoholic plumbite solution until it is sweet to the doctor test-i. e., until no discoloration. occurs upon the addition of a small amount of sulfur to a portion of the sample agitated with aqueous plumbite solution. Usually 'two separate treatments are sufficient. The lead mercap tides are removed by the alcoholic plumbite solution. The treated sample is washed with water and filtered. The percentage of mercaptan sulfur is equal to the difference between percentages of sulfur in the oil before and after the treatment as determined by the lamp method. Titration Method. From 200 to 300 cc. of c. P. benzene o r of 90 per cent benzene from which the elementary sulfur has been removed by mercury are put into a 500-cc. separatory funnel and a known weight of the sample (5 to 10 grams) is added. From 40 to 50 cc. of basic lead acetate suspension are added to the benzene solution. This suspension is made by dissolving 0.1 mol of lead acetate (37.93 grams containing 3 molecules of water of crystallization) and 0.1 mol of sodium hydroxide in 1000 cc. of water. The basic lead acetate formed is not completely soluble in water, hence it is called a suspension rather than a solution. The basic lead acetate reacts with the mercaptans, forming the corresponding mercaptides, which dissolve readily in the benzene. The aqueous layer is separated from the benzene layer, which is filtered into a separatory funnel. Pb(OH)-(GHsOZ)
+ 2RSH-R->PbR-S
+ H-(CzH302) + H10
A known quantity of 0.1 N sulfuric acid is added to the funnel and the funnel shaken until the greenish yellow color of the benzene solution of the mercaptides disappears. The mercsptans are regenerated as a result of this reaction. The lead sulfate formed is a direct measure of the lead mercaptides, and hence of sulfur present as mercaptans. R%Pb H2S04+2RSH PbSOc R-S/ The acid is filtered free from lead sulfate and the washings from the precipitate, as well as from the benzene, are combined with the main portion and titrated with 0.1 N sodium hydroxide. The mercaptan sulfur is calculated from the difference between the acid used and the acid remaining as determined by titration. A formula for this calculation is shown. (Cc. 0.1 N HlSO, - cc. 0.1 N NaOH) X 0.003206 X 100 Weight sample per cent mercaptan sulfur The lead sulfate may be determined gravimetrically by the usual method instead of by titration, but the gravimetric method is not so convenient. I n the preparation of the naphtha solution containing ethyl, n-propyl, n-butyl, isoamyl, and benzyl mercaptans, and p-thiocresol, approximately equal percentages of the mercaptans calculated as mercaptan sulfur were added to make the total amount of sulfur 1.73 per cent.
+
+
Vol. 19, No. 11
Five determinations by the titration method gave an average of 1.74 per cent, the minimum being 1.70 per cent and the maximum 1.76 per cent. Similarly, five determinations by alcoholic plumbite solution gave an average of 1.70 per cent, the minimum being 1.68 per cent and the maximum 1.73 per cent. The use of aqueous sodium plumbite solution instead of basic lead acetate for the formation of the mercaptides gave high results when the relative proportion of sodium plumbite solution to the sample was increased beyond a definite value. For example, with the naphtha solution of isoamyl mercaptan, containing 0.60 per cent of sulfur, when the solution was shaken with 20 per cent of its volume of a 30" BB. plumbite solution (23 per cent of sodium hydroxide) it showed 0.60 per cent mercaptan sulfur by analysis. Using 40 per cent by volume of the plumbite solution showed 0.70 per cent of mercaptan sulfur, while using 80 per cent by volume showed 0.80 per cent of sulfur. Benzene is used as a solvent for the naphtha or oil solution of the mercaptans because lead ethyl mercaptide, if present in appreciable amounts, will precipitate from the naphtha solution. This mercaptide is soluble in benzene. The results obtained with lead acetate dissolved in water were low. Wendt and Diggs3 have indicated that alcoholic plumbite will react with mercaptans of high molecular weight because of the closeness of contact between the alcoholic solution and the oil, whereas an aqueous solution of sodium plumbite will not react satisfactorily. They did not state that the mercaptans pass into the alcohol layer. The preferential solubility of the mercaptans in the alcoholic reagent used in this work has been utilized in a concurrent research by Faragher, Morrell, and Comay. This method has been adopted as one step in the present scheme of analysis, and its use is recommended in the determination of mercaptans, especially in the indirect method for the determination of disulfides, which will be described later. Wendt and Diggs3 have proposed the addition of elementary sulfur to the sample and agitation with aqueous plumbite solution for the determination of mercaptans. The reaction is the same as for their determination of elementary sulfur by the addition of ethyl mercaptan. This procedure, however, is open to the objection raised in connection with the addition of mercaptans, in addition to others. The use of iodine for the determination of mercaptans according to the following reaction is old:' R-S 2RSNa 21-2NaI 1
+
+ R-S
The reaction is adapted to lead mercaptides in the same manner, It is obvious that it cannot be applied to cracked petroleum distillates because of the olefins present. DIsuLFIDES-The disulfides are determined upon the major portion of the sample from which the hydrogen sulfide and elementary sulfur have been removed. I n reducing the disulfides to mercaptans with zinc and glacial acetic acid, 50 cc. of the sample are mixed with 50 cc. of glacial acetic acid, and the mixture is refluxed for 3 hours with 10 grams of zinc dust. R-S
R-S
I
+ &+2RSH
The excess of zinc is removed by filtration. The sample containing the acetic acid is washed with water in a separatory funnel; the acetic acid passes into the water layer and the oil remains in the upper layer, The oil is washed several 6
Kekuld, Sinneman Ann., 193, 277 (1862).
INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1927
1283
Schematic Diagram of Analytical Procedure Sample contains HzS, S, RSH, RzSt, R&, residual sulfur
I
Treat with acidified CdClz solution
1
oil layer contains S, RSH, ~ 9 2 R , ~S, residual sulfur; make lamp detn. ( 1 )
Treat with Hg
i
Ppt. HgS
1.
I
1
Oil layer contains RSH, RzS2, R t S , residual sulfur; make lamp detn. ( 2 )
I
I
Divide i n 2 parts Part I L R S H detn.
Part 2-Rkduced
Method 1 or 2
I
I Method 2 (Titration)
Method 1 Treat with alcoholic plumbite
I
contains RSH, RzS, residual sulfur (RzSz has been reduced to RSH)
I
I
Treat with alyoholic plumbite
I
I
I
Oil layer contains RzSz, RzS, residual sulfur; make lamp detn. (3)
Alcoholic plumbite removes RSH I
I
I
Hydrogen sulfide Elementary sulfur Mercaptan: Method 1 XIethod 2 Disulfide sulfur Disulfide sulfur Sulfide sulfur Residual sulfur
I
Treat
= calcd. from weight CdS = lamp (1) lamp ( 2 )
-
= =
= = = =
lamp (2) by titration lamp (2) lamp (3) lamp (4) lamp (5)
lamp (3)
R-S
+
Textbooks state that disulfides may be reduced with alcoholic potassium sulfide to the mercaptans.' R-S I 2KzS+2RSK KzS2 R-S
I
- lamp (4) - per cent RSH - lamp ( 4 ) - lamp ( 5 )
times with water and then treated with the alcoholic plumbite solution to remove the mercaptans. The percentage of disulfide sulfur is the difference between the total percentages of sulfur by lamp determination before and after the reduction [(3) minus (4) on chart]. This method of determining disulfide sulfur was applied to the naphtha solution containing 1.52 per cent of total disulfide sulfur that was made by adding approximately equal parts of ethyl, n-propyl, n-butyl, isoamyl, and diphenyl disulfide, The average of three determinations was 1.50 per cent of disulfide sulfur. As a further experiment the naphtha solution of disulfides was refluxed with metallic sodium, but no mercaptans were formed. Moses and Reid6 have found that in ether solution this reduction takes place very slowly, but that in alcohol solution the disulfides are reduced more rapidly. R-S 1 ,2Na+2RSNa
+
+
Barely a trace of mercaptans was found after persistent refluxing of the naphtha solution with alcoholic potassium sulfide and acidifying. Dilute hydrochloric, sulfuric, or glacial acetic acid gave a quantitative reduction of the disulfides in naphtha solution to mercaptans in the presence of zinc. By the use of the glacial acetic acid, the spent zinc was more readily separated than when the other acids- were used. SULFIDES-AftW the removal with alcoholic plumbite solution of the mercaptans formed by the reduction of the disulfides, the sample is washed and filtered. It is then treated
I
wid
HgNOr
'
I
Oil layer contains residual sulfur; make lamp detn. (5) I
I
with an equal part by weight of powdered normal mercurous nitrate. (The normal mercurous nitrate contains 1 mol of water of hydration, whereas the common mercurous nitrate contains 2 mols; there is a marked difference in the results obtained by using the two forms.) The mixture is shaken for 10 to 15 minutes. The mercurous nitrate forms an insoluble complex with the sulfide sulfur, probably according to the formula R2S.HgP\IO:. This product is analogous to the complex foi-med between the alkyl sulfides and mercuric chloride. Mercuric chloride did not give satisfactory results. The percentage of sulfide sulfur is the difference between the percentages of sulfur as determined by lamp before and after the treatment with mercurous nitrate. A naphtha solution containing a total of 1.10 per cent of sulfide sulfur that was made from approximately equivalent weights of methyl, ethyl, n-propyl, isoamyl, and benzyl sulfides showed 1.07 per cent of disulfide sulfur as an average of three determinations. i\laberys used alcoholic mercuric chloride for the removal of sulfides from Ohio petroleum. Thierrys used mercuric chloride to form the corresponding derivatives of methyl, ethyl, and other sulfides and for the identification of these substances in Persian petroleum. Birch and Norrislo used' mercuric chloride in the identification of the mercaptans in the distillate from Maidan-i-Naftun crude oil. I n the present work mercuric chloride could not be used for the quantitative determination of sulfides because of the solubility of the complex in naphtha. I n some cases 40 per cent of sulfide sulfur remained in the naphtha. Mercurous nitrate, however, which has n o t been used heretofore, removes sulfides practically quantitatively. Mercurous nitrate is very effective for the removal of mercsgA m . Chom. J., 13, 233 (1891). J . Chrm. SOC.(London), 127, 2757 (1926). 10 Ibid., 197, 904 (1925). 8
A m . Chcm. Soc., 48, 776 (1926). Otto and Rossing, Bcr.. 18, 3129 (1886).
8J.
I
Oil layer contains RzS, residual sulfur; make lamp detn. (4)
Alcoholic plumbite removes RSH
9
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1284
tans, but this does not interfere with the analysis a t this stage. Likewise, its reaction with the disulfides presents no difficulty in the analysis. RESIDUALSULFUR-The residual sulfur represented by thiophene is all the sulfur that remains after all the foregoing operations. 'None of the reagents used throughout the analysis react with thiophene, with the exception of alcoholic plumbite, which has a slight tendency to remove thiophene; e. g., a naphtha solution of thiophene containing 0.52 per cent sulfur upon treatment with mercurous nitrate showed 0.52 per cent by several checked tests. Treatment of a naphtha solution containing the same a m o ~ n tof thiophene sulfur with alcoholic plumbite solution reduced the sulfur content to 0.48 per cent. This reduction is practically negligible. Analysis of Mixture
HYDROGEN SULFIDE-Hydrogen sulfide is determined as cadmium sulfide b y precipitation with an acidified solution of cadmium chloride. ELEMENTARY SULmR-Elementary sulfur is determined by shaking the sample with metallic mercury and filtering off the mercury suEde. A lamp determination before the treatment with mercury and another after the treatment give by difference the percentage of elementary sulfur. MERcAPTANS-The mercaptans may be determined by one of two methods: Method 1 (Faragher, Morrell, and Comay), The sample is dissolved in benzene after removing sulfur and hydrogen sulfide and is shaken with alcoholic sodium plumbite. The lead mercaptides formed dissolve in the alcohol, and a lamp determination upon the oil gives by difference the sulfur in mercaptans. Method 8. The oil freed from hydrogen sulfide and sulfur is dissolved in benzene and is treated with a suspension of basic lead acetate; the lead sulfate is filtered. The acid remaining in the solution and that in the washings are titrated, and the mercaptans equivalent to the acid which combined with the lead mercaptides are calculated. Both of these determinations are made upon aliquot portions. DISULFIDES-The oil, freed from hydrogen sulfide and sulfur, is refluxed with a dilute acid solution, using either hydrochloric or sulfuric acid (approximately 10 per cent) together with zinc. Glacial acetic acid, which is soluble in the oil, may be substituted for the hydrochloric and sulfuric acids. The disulfides are reduced to mercaptans. The oil is then treated with alcoholic sodium plumbite to remove the mercaptans that have been formed. A lamp determination gives the sulfide and residual sulfur in the reduced oil. The sulfur in disulfides is found by subtractiiig the mercaptan sulfur previously determined from the combined disulfide and mercaptan sulfur. The use of the titration method a t this stage gives unexplained results, in that the percentage of mercaptans found must be multiplied by the factor 3//z in order to obtain the correct result. Hence, it is advised that the extraction method be used in preference to the titration method. SuLFIDES-The 02 a t this stage is treated with mercurous nitrate, which removes the sulfides. A lamp determination gives the sulfur in residual sulfur. m e sulfide sulfur is determined by difference. Results
The accompanying tables show the results obtained by the analysis of a known mixture of elementary sulfur. mercaptans, sulfides, disulfides, and thiophene. The solution was
SULFUR
Vol. 19, No. 11
FOUND^
PRESENT (1)
Per cent 0.05
Elementary Mercaptan Sulfide Disulfide) Thioohene (residual)
Per cent 0.05
0.38 0.13
(3)
(2)
Peu cent
Per cenl
0.05 0.46
0.41
0.05
0.20 0.38
0.45 0.10
0.24
0.38
0.16
0.16
I n column (1) the mercaptans, both those present a s such and those formed by the reduction of disulfides, were 'determined b y titration. I n column (2) they were determined by extraction with alcoholic plumbite. In column (3) the mercaptans present as such were determined by titration while those made by reduction from the disulfides were determined b y alcoholic plumbite. b The factor '/Z was used in the calculations of column (3) for the disulfides.
I n order to determine the accuracy of the method for dilute solutions, a new mixture was made up, and the results in Table I1 xere obtained. Table 11-Analysis
of Dilute S o l u t i o n s FOUNDO
SULFUR
PRESENT'
Elementary Mercaptan Sulfide Disulfide) Thioohene (residual)
(2)
(3)
Per cent
Per cent 0.02
0.19
0.22 0 05
Per cent 0.02 0.23 0.12 0.22
0.06
0.05
STRAIGHT-RUN GASOLINE
CRACKED GASOLINE
SULFUR
Calif. (Midway) Smackovercrude crude
Per cent Elementary Mercaptan (titration) Disulfide Sulfide Residual Hvclroeen sulfide
1
Ptr cent
pd'ot'2" 0.03
0.02 0.95 Present
0.00 0.15 Present
Panhandlecrude
I
I
Per cent 0.05 0.04
.
None None None Present
Examination of the results (Table 111) reveals some very interesting facts in connection with the composition of petroleum distillates. The cracked gasoline from the heavy California crude cont,ains 1.1 per cent of sulfur, 86 per cent of which is residual sulfur as defined here. The cracked gasoline from Smackover crude contains 0.2 per cent of sulfur, 80 per cent of which is residual sulfur; while the straightrun gasoline from Panhandle crude contains 0.09 per cent of sulfur, all of which is elementary and mercaptan sulfur. A knowledge of the distribution of the sulfur groups is useful in desulfurizing and refining gasolines. Furthermore, with a more complete knowledge of the behavior of the individual groups, such analyses will give much information about the corrosion of equipment and other problems of technologic interest. In addition to the pure scientific aspects of the distribution of various sulfur groups in petroleum distillates, information of value may be obtained by using the method in the separation and identification of the sulfur groups in mixtures other than petroleum distillates.
