Spectrophotometric Determination of Aliphatic Sulfides in Crude Petroleum Oils and Their Chromatographic Fractions HARRY V. DRUSHEL and JAMES F. MILLER M e l l o n Institute o f
lndustrial Research, Pittsburgh, Pa.
Research programs for the study of organic sulfur compounds in crude petroleum by chromatographic techniques have demanded the development of methods for the determination of the sulfur types. A method, based upon a modification of the procedure of Hastings, is described for the determination of aliphatic sulfides in small samples of high molecular weight material such as a crude oil chromatographic fraction. Increased sensitivity has permitted the use of small samples, which substantially reduces sample absorption in the ultraviolet region, thus making possible the analysis of samples containing high concentrations of aromatic hydrocarbons. Interference due to formation of hydrocarbon-iodinecomplexes has been shown to be negligible. This method should be useful for following the course of the separation of aliphatic sulfides from crude petroleum by chromatography or thermal diffusion. A s the method is directly applicable to crude oil, it should be valuable in the characterization of crude petroleum prior to refinery operations.
concentration slightly loxer than that employed in the following procedure. Considering the differences in iodine concentration, solvent, and technique, the results by the two modified pr'ocedures agree remarkably well. PROCEDURE
Apparatus. Beckman DU spectrophotometer equipped with the photomultiplier attachment and one pair of 1-em., matched, fused-quartz cells. Volumetric flasks and pipets. Reagents and Chemicals. Carbon tetrachloride, technical. Iodine, analytical reagent grade. Iodine reagent, prepared by dissolving exactly 10.00 grams of iodine in carbon tetrachloride and diluting to 1 liter. Procedure. A weighed sample is dissolved in carbon tetrachloride and diluted t o the proper volume. Dilutions of this solution may be prepared in order to produce absorption of the sample-iodine blend falling in the range of optimum accuracy (an absorbance of between 0.4 and 1.0). For very small samples, such as chromatographic fractions, a micro- or semimicrobalance may be used to weigh the required quantity of sample for dilution to 25 ml. in order to conserve material. A list of appropriate sample weights is given below.
I
N T H E course of studying sulfur compounds in crude oil, it has become necessary to develop methods for the determination of various sulfur types in crude oil and crude oil chromatographic fractions ( 7 ) . Such methods must be applicable to very small samples of high average molecular weight which are complex in nature and, in manv instances, dark in color. The classical method developed by Ball ( 2 ) for the schematic determination of the sulfur types in petroleum distillates requires large samples (250 ml.). Crude petroleum contains sulfur compounds which exhibit such low reactivities toward the reagents used in the Ball procedure that only a low percentage of the total sulfur in most crudes can be accounted for. The method is also time-consuming. Recently a unique method for the determination of aliphatic sulfides in petroleum distillates boiling below about 450" F. was developed by Hastings (4). This method, which is based upon the formation of intermolecular complexes of aliphatic sulfides and iodine having intense absorption a t 308 mp, cannot be applied to crude petroleum because of the intensity of the absorption due to the sample itself. This paper describes a procedure, based upon a modification of Hastings' method, for the determination of sulfide sulfur in crude petroleum or crude petroleum chromatographic fractions. Since complex formation with iodine follow the equilibrium Aliphatic sulfide
+ I, G aliphatic sulfide.12
Aliphatic Sulfide Sulfur in Sample, %
Approximate n'eight of Sample Diluted t o 25 M1. t o Obtain Absorbance between 0.4 and 1.0, Mg.
0.05 0.10 0.20 0.40 0.00 0.80 1.0 1.5 2.0
60-140 30-70 15-33 8-18 5-12 4-9 3-7 2-5 1-3
Dilute 1 ml. of the iodine reagent to 10 ml. with the above sample solution a t the proper concentration. Dilute 1 ml. of carbon tetrachloride to 10 ml. with the same sample solution and use this solution as the reference solution for the spectrophotometric measurement. Immediately after preparing the sample-iodine blend and the reference solution, obtain the absorbance of the iodine blend a t 310 mp. Make the measurements as soon as the solutions have been placed in the cell compartment. S s the absorbance has been noticed to increase slowly in the case of crude oils if the iodine blend is allowed to stand, it is recommended that measurements consistently be made immediately after the preparation of the blend. A blank value for the iodine reagent is obtained as follows: One milliliter of the iodine reagent is diluted to 10 ml. with carbon tetrachloride and the absorbance of this solution measured a t 310 mp using carbon tetrachloride as the reference. Calculations. The weight percentage of aliphatic sulfide sulfur is calculated as follows:
(1) Wt. % aliphatic sulfide sulfur =
consideration of the mass action law has led to the supposition that the sensitivity of the Hastings' procedure could be increased considerably by using much higher iodine concentrations. This increased sensitivity, together with the use of a Beckman DU spectrophotometer with photomultiplier attachment, made possible the analysis of crude oils through the use of lower sample concentrations which, in turn, decreased the sample background absorption. The rapid decrease in the absorbance of the iodine blend after being placed in the dark (cell compartment) has not been observed. Hastings and Johnson ( 5 )have recognized the mass action law effect and have proposed a similar modification using an iodine
-
loo
400 X 0.9 X c
where A1 is the absorbance of the sample-iodine blend a t 310 mp and A, is the absorbance due to iodine (blank). Subtraction of this blank not only accounts for absorption due to iodine in the blend but also corrects for differences in cell absorbances, as pointed out by Hastings ( 4 ) . The average absorptivity is taken to be 400 liters per gram-em. The absorptivity for the sulfideiodine complex is calculated on the basis of the total aliphatic sulfide concentration rather than on the true concentration of the complex. The factor 0.9 is the dilution factor necessary to correct for diluting 1 ml. of iodine to 10 ml. with the sample solution, the concentration of which in grams per liter is represented by c in the equation above
495
ANALYTICAL CHEMISTRY
496
Absorptivity of the Aliphatic SulfideTable I.
Absorptivities of Iodine Complexes of Sulfur Compounds in Carbon Tetrachloride at an Iodine Concentration of 1 Gram per Liter Bbsorptivity, Liters/Gram-Cm.'" 433 4 403 13 185 11 265 i 2 262 i 1 308 1 2 364 f 1 301 f 1 198 11 324 11 379 1 280 f 2 385 3= 3 391 12 403 14 402 f 3 72 f 0 l.5fO.O 1710 32 f 1 4.810.1 0.3 2.5 f0.0 4.0 f 0 . 2 6.9 13.1 f0 . 2 11.6 f0 . 1 9 . 9 10 . 2 9 . 0 10 . 3 10.6 f0 . 3 1 2 . 5 i0 . 0 4 . 2 f0.5 1 2 . 2 zt 0 . 1 5.31 0 . 2
Iodine Complex. The absorptivities of the iodine compleses of a number of
sulfur compounds were determined a t an iodine concentration of 1 gram per liter. These values are given in Table I. The absorptivities of the sulfideiodine complexes are also plotted in Figure 3. Values for the symmetrical di-n-alkyl sulfides (except 2-thiapropane) Eastman Kodak Co. fall on a straight line as shown. As is Eastman Kodak Co. Eastman Kodak Co. observed so often in physical data of a Eastman Kodak Co. homologous series of compounds, the Eastman Kodak Co. Eastman Kodak Co. compound containing the methyl group Eastman Kodak Co. Eastman Kodak Co. (or groups) is out of line. I n general, Eastman Kodak Co. Eastman Kodak Co. it is also observed that absorptivities for Eastman Kodak Co. the sulfides with sec- or tert-alkyl groups Eastman Kodak Co. Eastman Kodak Co. are somewhat low. The five-membered cyclic sulfides have slightly higher ab.amie w y o . sorptivities than the acyclic sulfides. -4s it is unlikely that sulfides of low molecular weight, such as 2-thiapropane hlatheson Coleman & Bell Eastman Kodak Co. and 2-thiabutane, are to be found to Matheson Coleman & Bell any great extent in crude oil, and Eastman Kodak Co. Eastman Kodak Co. 0.6 cyclic sulfides such as thiacyclopentane 0.5 Eastman Kodak Co. derivatives are thought t o be present Socony-Vacuum Oil Co., Inc. 0.6 0.3 Socony-Vacuum 011 Co., Inc. in relatively high concentration, it is concluded that an average absorptivity of 400 liters per gram-em. should be used for the determination of sulfides DISCUSSION OF METHOD in crude oil and crude oil chromatographic fractions. This value, which is based on limited data, must suffice until more work Wave Length of Maximum Absorption. The average wave is done on the separation and identification of sulfides in crude length of masimum absorption for the aliphatic sulfide-iodine comoil by techniques such as chromatography or thermal diffusion plexes at an iodine concentration of 1 gram per liter (in carbon where no extensive thermal degradation takes place. tetrachloride) was found t o be 310 mp (Figure I), in good agreeTable I shows that the sulfur types other than the aliphatic ment with the value given by Hastings for an iodine concensulfides form iodine complexes with absorptivities which are only tration of only 0.1 gram per liter in iso-octane (2,2,4-trimethylpentane). Effect of Iodine Concentration. A standard solution of iodine in carbon tetrachloride was added from a buret to 95 ml. of a 0.000159M solution of thiacyclopentane (tetrahydrothiophene) in carbon tetrachloride. The absorbance a t 310 mp was measured after each increment of iodine solution was added. The small quantity of liquid required for the measurement was returned to the bulk of the solution. No correction was made for small changes in the concentration of the solution owing to evaporation of the solvent. The results, shown in Figure 2, indicate that a large increase in sensitivity is obtained by increasing the concentration of iodine in the sulfide-iodine blend. An iodine concentration of 1 gram per liter in the sample-iodine blend was arbitrarily chosen as desirable for the method. Because of the greater solubility of iodine in carbon tetrachloride than in iso-octane carbon tetrachloride was chosen as the solvent for these investigations. Hildebrand and Scott (6) list the solubility of iodine in iso-octane and carbon tetrachloride as 0.592 and 1.147 mole 70,respectively, which correspond to about 9 and 30 grams per liter. An iodine concentration higher than 1 gram per liter would involve the use of iodine blanks which are objectionably high. The possibility of lowering the blank value has not been investigated, but it might be accomplished through purification of the carbon tetrachloride by passing it through a silica or alumina column. W A V E L E N G T H rnp) A higher iodine concentration than 1 gram per liter, even if the Figure 1. Absorption spectra of typical aliphatic blank could be reduced, would not increase the absorptivity sulfide-iodine blends in carbon tetrachloride significantly . 1. Iodine blank (1 gram per liter) Theoretical treatment of these data shows that the Eystem 2. Curve 3 minus curve 1 3. 2-Methylthiacyclopeutane-iodine hlend follows the equilibrium formation of a 1 to 1 complex between the 4. Curve 5 minus curve 1 5. 7-Thiatridecane-iodine hlend sulfide and iodine. Compound Thiacyclopentane 2-Lfethylthiacyclopentane 2-Thiapropane 2-Thiabutane 2-Thiapentane 3-Methyl-2-thiabutane 3-Thiauentane 2,2,4,4:Tetramethyl-3-thiapentane Thiacyclobutane Thiacyclohexane 5-Thianonane 2,6-Dimethyl-4-thiaheptane 6-Thiaundecane 2,8-Dimethyl-5-thianonane 7-Thiatridecane 8-Thiapentadecane 1,3-Diphenyl-Z-thiapropane Diphenylthiamethane 1-(p-hlethylpheny1)-1-thiaethane 1-(p-Methylpheny1)-1-thiapropane 5,6-Dithiadecane 1,2-Diphenyldithiaethane 1,4-Dipheny1-2,3-dithiabutane tert-Butyl trisulfide tert-Butyl polysulfide 1-Dodecanethiol 1-Deoanethiol 1-Tetradecanethiol 1-Hexadecanethiol 1-Octadecanethiol 1,I-Dimethyl-1-hexanethiol p-Methylbenaenethiol 2-Phenyl-1-ethanethiol Phenylmethanethiol 9.10-Dithia-9,lO-dihydroanthracene Dibenaothiophene 2.5-Di-tert-butylthiophene 2.5-Di-tert-octylthiophene Determined a t 26' i. l o C.
Source API, RP48A, Bur. Mines, Laramie, Wyo. API, RP48A, Bur. Mines, Laramie, Wyo. API-USBM standard sample 13-5s API-USBM standard s a m d e 902-5s SPI-USBM standard samble 18-88
*
*
'
V O L U M E 27, NO. 4, A P R I L 1 9 5 5
497 Table 11. Equilibrium Constants for Some Aromatic Hydrocarbon-Iodine Complexes in Carbon Tetrachloride at 25" C."
W
e
Concentrotion o f in Hostings'
9 5 m l . o f 000016M
2rocedure
Thtocyclopentone
in CC14
Em 0.8a
0.6l.lMalar
a
Blonk ( 9 5 m l of CCI.)
c
0
-
.
- -
,
8
12
+
4 M L 12
IN
y
c
r "
CCl~(00198Ml
Figure 2. Effect of iodine concentration upon absorption at 310 mp due to complex formation between thiacyclopentane and iodine
. a
02,2,4,$-Teframethyl- 3-Thiopentane O2,6 3 , T e t h y i - 4 - T t i c h e p t o n e
'
LEGEh?
'
10-3 16.4 16.7 12.5 12.5 10.1 8.85 9.00 9.26 8.20 16.7 10.4 13.0 7.64 7.35 7.14 11.2
KC 0.15 0.16 0.27 0.31 0.31 0.82 0.63 0.88 1.35 0.13 0.13 0.15 0.25 0.31 0.31 0.46
Ethylbenzene p-Di-tert-butylbenzene Naphthalene Acenaphthene Determined b y authors.
303 308 360 399
I
12.1 9.9 6.2 5.7
,
,
3
4
0.22 0.30 0.28 0.43
,
0 - n-ulftdes A - sec - A l k y l S u l f i d e s
VThiacyclobufone
V 1001 2
tma.
Table 111. . Equilibrium Constants for Some Aromatic Hydrocarbon-Iodine Complexes in Carbon Tetrachloride at 26" C."
20
16
x
Bromatic Amax, mlr Benzene 292 Toluene 302 +Xylene 316 m-Xylene 318 p-Xylene 304 332 Mesitylene Durene 332 Pentamethylbenzene 357 Hexamethylbenzene 375 Hexaethylbenzene 378 Bromobenzene 290 Phenanthrene 396 Naphthalene 360 Styrene 330 Stilbene 373 Bibenzyl 304 Determined by Andrews and Keefer ( 1 ) .
6
- f e r t . A l k y l Suif d e s - C j c ' c Sul!ices
8 NO O F C - A T O M S
0
12
14
Figure 3. Absorptivities of some aliphatic sulfide-iodine complexes in carbon tetrachloride at an iodine concentration of 1 gram per liter Dotted line d r a w n through values for di-n-alkyl sulfides
very small percentages of the average absorptivities for aliphatic sulfides. Sitrogen- and oxygen-containing compounds which would also form complexes with iodine are expected to be present in only relatively small concentration and would not cause serious interferences. Interference Owing to Formation of Aromatic HydrocarbonIodine Complexes. Even though small absorptivities were reported by Hastings for the aromatic hydrocarbons, it was feared that crude oils and especially the latter fractions from a chromatographic fractionation would contain such high concentrations of aromatics in proportion to the aliphatic sulfides that interference because of the formation of aromatic-iodine complexes would be serious. Andrews and Keefer ( 1 ) have determined the equilibrium constants and molar absorptivities of a number of aromatic-iodine complexes. These values, along with the wave lengths of maximum absorption, are given in Table 11. The equilibrium constants and molar absorptivities for iodine complexes of several additional hydrocarbons readily available in pure form have been determined. The method used by Bndrews and Keefer and by Blake, Winston, and Patterson (3)was followed. This method involves the following easily derived equation:
,
Oi O
I
2
,
5
I/[ETHYLBENZENE] (L,TERS/MOLE1
Figure 4. Plot of equilibrium data for ethylbenzene-iodine complex
=
1
1
+1
~ , eX , [4r1 E, for [Ar] >> [I,]
(2)
where [I4 and [Ar] are the molar concentrations of iodine and aromatic hydrocarbon, respectively, 1 is the cell path length, A is the absorbance a t the wave length of maximum absorption, K , is the equilibrium constant, and ce is the molar absorptivity of the complex. Five or more points were obtained a t different aromatic and iodine concentrations, and the values for e, and K , for Equation 2 were obtained by the method of least squares. A typical plot of values for the ethylbenzene-iodine complex is
ANALYTICAL CHEMISTRY
498 presented in Figure 4. The results of these measurements are given in Table 111. On the basis of values in the above tables, it is estimated that e, (for A), is about 15,000 liters per mole cm. and K , is about 1.0 liter per mole for the aromatic hydrocarbon-iodine complexes. All assumptions or approximations here were chosen so as to maximize the interference4.e., the average values for eo and K , were chosen to be higher than they probably are for most samples which will be encountered. Using these values, and assuming that the entire sample con&ts of aromatic hydrocarbons, the interference, in terms of apparent weight per cent aliphatic sulfide sulfur can be calculated as follows: For the equilibrium Aromatic
+ Ip + aromatic.12,
(3)
the equilibrium constant is written
K, =
[ arornatic.I~] [aromatic] [Is]
(4)
Thus, [Aromatic.In] = K , [aromatic] [Iz]
Experiment Station, Bartlesville, Okla. ) which was reported to contain 0.12y0 by weight total sulfur for the whole crude and 30 to 32% by volume aromatics in the 300" to 600" F. boiling range fractions. This was considered a representative sample relative to aromatic content, and was assumed for the purpose of this investigation, to contain no aliphatic sulfides. The absorbance of the iodine complex of this sample was determined a t several sample concentrations and the apparent sulfide content calculated to be 0.016 weight %. This should represent a reasonable "interference value" for aromatics for most samples. If anything, it may be slightly high, as there is no reason to believe that no aliphatic sulfide sulfur was present. On the basis of these studies, it is suggested that the interference due to aromatics may reasonably be assumed to be 0.01 to 0.02 weight % aliphatic sulfide sulfur. Referring to Table IV and Figure 9, it is apparent that this would constitute negligible interference for most crude oils and chromatographic fractions. However, until the aromatic content of a sample and its interference can be more precisely determined, it is not recommended that this method be applied to samples containing less than 0.05 weight % aliphatic sulfur.
(5)
However,
A
(6)
= e k
where A is the absorbance, E is the molar absorptivity of the aromatic-iodine complex, in liters per mole centimeter, 1 is the cell path length in centimeters, and c is the concentration of the complex in moles per liter. Substituting Equation 5 in Equation 6,
A = elK, [aromatic]
[I21
(7)
But Apparent wt. % aliphatic sulfide sulfur =
A X 100 I X grams of sample/liter X a (8) where a is the average absorptivity of aliphatic sulfide-iodine complexes in liters per gram (of aliphatic sulfide sulfur) cm. Substituting Equation 7 in Equation 8, Apparent wt. % aliphatic sulfide sulfur = d K , [aromatic] [I21 X 100 1 x grams of sample/liter X a
(9)
Since it was assumed, for simplicity, that [aromatic] =
grams of sample/liter mol. wt.
Table I\'.
Sulfide Sulfur Content of Several Crude Oils Total
Crude Oil Sulfur, % Light M a r a , Venezuela 1.91 4.67 Heavy hlara, Venezuela Kuwait 2.46 0.87 Light Eastern, Venezuela Heavy Eastern, Venezuela 2.40 2.37 Permian McElroy. West Texas 1.94 Coniferous sulfur, Kentucky Santa, Maria Valley, California 2.34 Wilniington, Calif. 1.39 1 40 North Ward Estes, Tex.
Aliphatic
Relative Percentage of Sulfide
Sulfide Sulfur, % 0.45 0.79 0.U 0.27 0.78 0.41 0.26 0.93 0.82 0.46
Sulfur 24 17 23 31 31 17 13 40 59 33
Interference Due to Sample Absorption. If a sample solution containing high concentrations of certain aromatic hydrocarbons is to be analyzed, interference attributed to background absorption may be encountered. Fortunately, this interference is greatest below 300 mp. Above the analytical wave length, 310 mp, interference owing to background absorption is not serious for most samples. As can be seen from Figure 5, stray light effects produce anomolous absorption below 300 mp when the sample concentration is too high. By decreasing the sample concentration stray light effects are reduced and a more typical
Equation 9 becomes Apparent wt. % aliphatic sulfide sulfur =
e K c [I21 X 100 mol. wt. X a (11)
Substituting the values: = = = [I2] = a mol. art. = E
KC
15,000 liters per mole cm. 1.0 liters per mole 0.00394 mole per liter 400 liters per gram-cm. 300 grams per mole
a 06w 0
the apparent weight % aliphatic sulfide sulfur = 0.049 for a sample of average molecular weight 300. For samples of average molecular weight 200, 400, and 500 it becomes 0.074, 0.037, and 0.030, respectively. These data represent greater than maximum interference which would be encountered in any actual sample of crude oil. It was presumed that actual interference would be nearer 0.01 to 0.02 weight % aliphatic sulfide sulfur (absolute). A sample of crude oil low in sulfur and high in aromatics was obtained from H. M. Smith (U. S. Bureau of Mines, Petroleum
m a
n
0.4 -
02-
0
260
280
300 320 340 360 WAVELENGTH (m,u)
380
400
Figure 5 . Ultraviolet spectra of chromatographic fraction 9 and its iodine blend
499
V O L U M E 2 7 , NO. 4, A P R I L 1 9 5 5
I.o
n
0.8 -
0 0 2 6 5 G/L.
W
u z
-
4
E06L o 0
-
piex i s a m p i e
'
rn
Concentration
a
o 1\92 G/L)
04-
of sulfide sulfur for the Wilmington crude oil analyzed a t 29.8" C. (Table V) and a t 26' C. (Table IV) when the same absorptivity is used, A convenient reliable procedure for temperature correction when the spectrophotometer is not equipped with thermospacers consists of using the absorptivity of the 7-thiatridecane-iodine complex R hich is near 400 liters per gram-cm. a t 26' C., a t the same temperature a t which the sample is analyzed. Figure 8 shons the variation of the absorptivity of the 7-thiatridecane-iodine complex with temperature. Reproducibility of Method. A crude oil and a combination of several chromatographic fractions 1% ere analyzed for aliphatic sulfides at several different concentratlons. The results are set forth in Table V. Reference to these results shows that the reproducibility is about 1 or 27,. The Wilmington crude was analyzed a t 29.8" C. and the combined fractions a t 25.5" C. Recovery of Added Sulfide. Different amounts of 7-thiatridecane m-ere added to a crude oil (Wilmington), and the procedure for the determination of aliphatic sulfides R as applied to the mixtures. After correcting for the aliphatic sulfides present
1
02-
0 260
8
1
'
280
300
1
,
320 340 360 WAVELENGTH in/)
360
400
Figure 6. Ultraviolet spectra of chromatographic fraction 8 and its iodine blend
3:
d
/ / 7-Thlatridecane . . ,!; presence of crude 011 [30.4'$1
02
4
i
1
0 - 7-Thiatridecane I24 8°C) 0 - 7-Thiatridecane 129.9'C.I
A- 7-
Thiatridecane in presence of Oil but corrected for absorption due io sulfides in the crude oil(30.4°C.)
--
3c
0'
z
.
c 0
0,
E
f Figure 7. Absorptivity of 7-thiatridecane at an iodine concentration of 1 gram per liter under various conditions
2:
I
spectrum is obtained, as illustrated in Figure 6. Even though background absorption for most samples is more intense than is usually encountered in routine spectrophotometry, reliable
420
440
I 400
I
I
I
380
360
340
AbsorDtivitv
( L i t e r s I G r a r n Cm.)
Figure 8. Effect of temperature upon complex formation between 7-thiatridecane and iodine analytical results may be obtained with the use of the photomultiplier attachment. Effect of Temperature. The Beckman DU - 700 spectrophotometer used for this investigation as not equipped with thermospacers for precise tem- 600 perature control, but all measurements w ere made immediately after the solutions had been placed in the cell compartment, before any appreci-500 I able temperature change occurred. Tap m-ater 0 r m was circulated through the lamp housing to keep -400 2 the cell compartment as near room temperature ia as possible. Therefore, any relationships concern-300 ing temperature reported here are based upon 3 0 room temperatures a t the time of measurement. - 200 From the data in Figures 7 and 8 and other data from the analysis of certain crude oils, it is estiA - Molecular Weight U-L - 100 mated that the absorptivities decrease about 2 0 - Aliphatic Sulfide Sulfur or 3% per degree Centigrade increase in tem0 - Relative Percentage of Aliphatic Sulfide Sulfur ( I n Total Sulfur) perature. This large coefficient is probably at2 f 4 b 6 B F R A CIbT I O 1'N1 N U1'2M B1E'3R Id ; 1 16 1' 7 IS i9 20 21 - 0 tributable to the temperature dependence of the equilibrium constant and very slightly to the Figure 9. Aliphatic sulfide sulfur content and molecular weight of a thermal expansion of carbon tetrachloride. There series of chromatographic fractions from North Ward Estes (West is a significant difference between the percentage Texas) crude oil
t
4
ANALYTICAL CHEMISTRY
500 Table V.
Reproducibility of Method Over Range of Sample Concentrations
Concentration of Sample in Iodine Blend, Grams/Liter
Sulfide Sulfur Found, % '
Mixture of Several Chromatographic Fractions" 0.0371 0.674 0.1112 0.683 0.1483 0.705 0.1854 0.698 Average O.fi9 f 0 . 0 1 Crude Oil, Wilmington Field, California 0.0678 0.775 0.1386 0,753 0.2038 0.748 0.2713 0,740 0.750 0.3391 Average 0 . 7 5 i 0 01
0.8. w
u
,
z a
m 0.6. a Sm I ,
a 0.4
a Aliphatic sulfide sulfur content, calculated from experimentally determined aliphatic sulfide sulfur content of each fraction used in preparing the mixture, was also 0.69%.
0.2
'
1
I
I t Complex (Sample Concentration - 02195 G / L l
260
200
1
I
300 320 WAVELENGTH ( r n p )
1
340
360
Figure 11. Typical spectra of crude oil-iodine blends 1. 2.
Iodine blank (1 grnm per liter) Light Mara, Venezuela Light Eastern. Venezuela 4. Corniferous sulfur, Kentucky 5. Heavy Enntern, Veuezuela 6. West Texas, Permian McElroy 7. Kuwait 8. Heavy Mara, Venrzuela 9. North Ward Estes, West Tevas 10. Light Mara, Veneruela 11. Wilmington Field, California 3.
WAVELENGTH ( m r )
Figure 10. Spectra of Santa Maria Valley, Calif., crude oil and its iodine blend
in the crude oil, complete recovery of the added sulfides was obtained (Figure 7 ) . The average absorptivity for 7-thiatridecane alone (at 29.9" C.) was 362 =k 4 liters per gram-cm. while the absorptivity for 7-thiatridecane in the presence of crude oil (at 30.4" C . )was 363 =k 9 liters per gram-cm. Effect of Time in the Dark. The rapid decrease in the absorbance of the iodine blend when placed in the dark (cell compartment), as reported by Hastings, was not observed under the conditions used in this investigation. Table VI gives some absorbance us. time in the dark data for several aliphatic sulfide-iodine blends. Variations in absorbance are of the order of magnitude of the accuracy of instrumental measurement. Estimated Over-all Accuracy of Method. Considering the estimated interferences from formation of aromatic hydrocarbon-iodine complexes and from sample absorption and the uncertainty in establishing an average absorptivity for the iodine complexes of aliphatic sulfides likely to be present in crude oil, the accuracy of the method is believed to be of the order of 10%. This degree of uncertainty must be considered tolerable in the analysis of extremely complex samples such as crude oil. RESULTS
Results of the analysis of a series of chromatographic fractions from Sorth Ward Estes (West Texas) crude oil for aliphatic sul-
fides are shown graphically in Figure 9. These fractions were obtained by Clarence Karr, Jr. (Multiple Fellowship on Petroleum of the Gulf Research & Development Co.), by means of a chromatographic technique which has not, as yet, been published. Typical ultraviolet absorption spectra of a sample and an iodine blend are presented in Figure 6. Typical spectra for a crude oil and a crude oil-iodine blend are illustrated in Figure 10. The spectra of the iodine blends of some additional crude oils are shown in Figure 11. The calculated aliphatic sulfide sulfur contents of these crudes are also given in Table IV. The data in Table IV, although not extensive, indicate that the relative proportion of aliphatic sulfide sulfur is probably very similar for different crude oils from the same general field, regardless of the total sulfur content, as shown by a comparison of the
Table VI. Time in Dark, Min.
iibsorbance t's. Time in Dark Data for Several Aliphatic Sulfide-Iodine Complexes 1"
2b
Absorbance a t 310 mp 3c 4d
56
6f
0.613 0.612 0.612 0.612 0.615 0.617 0.617 0.617
0.672 0.670 0.670 0.667 0.668 0.668 0.668 0.667
V O L U M E 27, NO. 4, A P R I L 1 9 5 5
so1
relative percentages of sulfide sulfur in the Light Mara and Heavy Mara of Venezuela, the Light Eastern and Heavy Eastern of Venezuela, and the Santa Maria Valley and Wilmington crudes of California. This method should prove valuable ih following the course of the isolation or concentration of aliphatic sulfides from crude petroleum by techniques such as chromatography and thermal diffusion, and should be helpful in characterizing crude petroleum prior to refinery operations.
Gulf Research & Development Co., which has supported the work described herein, for permission to publish this material.
ACKNOWLEDGMENT
lytes,” 3rd ed., p. 274, Reinhold, New York, 1950. Karr, Clarence, Jr., Weatherford, W.D., Jr., and Capell, R. G . , AN.4L. CHEM.,2 6 , 2 5 2 (1954). RECEIVED for review August 12, 1954. Accepted December 15, 1954.
The authors wish to express their appreciation to the Multiple Fellowship on Petroleum sustained a t Mellon Institute by the
LITERATURE CITED (1) Andrews, L. J., (1962).
and Keefer, R. M., J . A m . Chem. Soc.,
7 4 , 4500
(2) Ball, J. S.,U. S. Bur. Mines, Rept. Inrest. 3591 (1941). (3) Blake, K. W., Winston, H., and Patterson, J. A., J . Am. Chem. Soc., 7 3 , 4 3 3 7 (1951). (4) Hastings. S. H., ANAL.CHEM.,2 5 , 420 (1953). (5) Hastings, S. H., and Johnson, B. H., I b i d . , in press. ( 6 ) Hildebrand, J. H., and Scott, R. L., “Solubility of Non-Electro-
(7)
Manual and Continuous Recording Attachments for the Beckman Model DU Spectrophotometer G. L. ROYER, H. C. LAWRENCE, and S. P. KODAMA American Cyanamid Co., Bound Brook,
N. 1.
and C. W. WARREN W a r r e n Electronics, Inc., Bound Brook,
N. 1.
The Beckman iModel DU spectrophotometer has found wide application in the field of visual and ultraviolet spectrophotometry. To reduce the time required to plot a spectral curve with this instrument, the authors built a manual plotting attachment which would record the data from the spectrophotometer directly on a plotting paper. This saves time but suffers from the inherent disadvantages of point-by-point plotting in that one can only obtain spaced points on the curve. The continuous recording attachment described in this paper has the advantage of giving a complete record of the per cent transmittancy at all wave lengths through which the instrument is operated.
T
H E Beckman Model DU spectrophotometer was first described by Cary and Beckman in 1941 ( 5 ) . Since that time about 10,000 instruments have been built for use in the field of visual and ultraviolet spectrophotometry. The authors believe this large use has resulted because, as stated by Gibson and Balcom (9) of the U. S. Bureau of Standards in 1947, “The instrument is capable of precise and accurate measurement of spectral transmittance or transmittancy.” The Beckman Model DU instrument was one of the instruments used by Haupt ( 1 0 ) of the Bureau of Standards for establishing an alkaline solution of potassium chromate as a transmittancy standard in the ultraviolet. This solution along with a Tropex neutral glass filter was used by Brode, Gould, Khitney, and Wyman (d), also of the Bureau of Standards, to carry out a comparative survey of Beckman Model DU instruments located in a number of cooperating laboratories. They report that, in general, the results show good agreement among the spectrophotometers. A paper by Caster (6) presents and discusses data on the variability in the Beckman Model DU spectrophotometer and refers to a number of publications which report the precision of both the instrument and the over-all analytical methods in iyhich the instrument is used. The performance of the Beckman Model D U instrument, as indicated by these reports and the
experience in many laboratories, has made it a required analytical tool for chemical research and process control. Its reasonable cost has also made it available within the budget of the average laboratory. Because of the wide availability of the Beckman Model DU, a number of systems for the automatic recording of spectral data which incorporate this instrument have been devised and described in the literature. Coor and Smith ( 7 ) have described an automatic recording spectrophotometer which gave high-speed recording of per cent transmittancy. This instrument was reported to have Lyorked satisfactorily for a number of years, but it has the disadvantage that the “correction device” is not readily adjustable. The Beckman Model D K recording spectrophotometer is based on the design originally made by Kaye, Canon, and Devaney (11) and Kaye and Devaney (12) of the Tennessee Eastman Co. With change in light sources and detectors and by a change in the Beckman Model D U Spectrophotometer itself, it is possible to cover the range of 200 to 2700 mp. Etzel (8) has also decribed a single detector split-beam automatic recording instrument nhich incorporated the Beckman Model DU spectrophotometer. Per cent transmittancy versus wave-length data have been recorded automatically in the region 205 to 400 mp with the hydrogen lamp as a source and 320 to 700 mp with a tungsten lamp as a source. Beckman ( 1 ) has described a means of recording for process control the per cent transmittancy a t a set wave length using the Beckman hlodel DU spertrophotometer as a source of monochromatic light. Cahn and Gale have described ( 4 )the Beckman automatic operator, which can be attached to the Beckman Model DU spectrophotometer to produce point-by-point plots. The time for obtaining a normal curve of 22 points is about 3 minutes, and for a curve of 400 points about 60 minutes. The Model D U spectrophotometer can also be used for manual operation by the usual procedure. Recently, Muller ( 1 4 ) described the Process and Instruments recording spectrophotometer which a,lso incorporates the Beckman Model DU monochromator. A tenfold increase in resolution and low scattered light over the
