1852
A N A L Y T I C A L CHEMISTRY
Table 11. Recoveries of Radioactive Iron Added to Biological Tissues Wet Weight of Tissue, Grams 10 (rat) 50 (rat) 50 (rat) 75 (rat) 75 (rat) 100 (rat) 10 (feces) 75 feces) 6-Lour urine 24-hour urine 15 ml. of serum
Recovery of Fe69 Counts/Minute‘ 840 1450 766 860 3890 1280 1820 1650 1450 1800 1500
ash, recoveries average between 85 to 95%. samples the error of the method is 1 5 % .
Feao Recovered, % ’ 97.0 98.5 95.5 98.0 99.5 91.0 100.3 93.5 96.5 87.0 100.0
for counting. The radioactivity is measured with a thin mica end-window Geiger counter. RELIABILITY OF THE SEPARATION
Table I1 lists the recoveries obtained from various amounts of biological tissue with added radioactive iron 59. With serum, red blood cells, small rats, mice, and 10- to 20-gram feces samples, 95 to 100% recoveries are consistently obtained. However, with 12- to 24-hour urines that contain 15 to 40 grams of
With duplicate
LITERATURE CITED
Baudisch, O., and King, 1‘.L., J . I n d . Eng. Chem., 3 , 6 2 9 (1911). Dunn, R . W., J . Lab. Clin. Med., 37, 644-52 (1951). Furman, N . H., Mason, IT. B., and Pekola, J. S., ANAL.CHEX., 21,1325-30 (1949).
Greenberg, G. R.. Humphreys, S.R., Ashenbrucker, H., Lauritsen, bl.,and Wintrobe, AT. AT., Blood, 2, 94-100 (1947). Hahn, P. F., IND.ENG.CHEV., - ~ A L ED., . 17,45-6 (1945). Lundell, G. E. F., and Hoffman, J. I., “Outlines of Methods of Chemical Analysis,” Kew S o r k , John W l e y & Sons, 1938. Moore, C. V., Dubach, R., LIinnich, V., and Roberts, H. K., J. Clin. Invest., 23, 755-67 (1944). Peacock, W. C., Evans, R. D., Irvine. I. \I-., Good, IT. XI.. Kip, -4. F., Weiss, S., and Gibson, J. G., Ibid., 25, 605-15 (1946). Rodden, C. J.. “Analytical Chemistry of the Manhattan Project,” New York, McGraw-Hill Book Co. 1950. Ross, J. F., and Chapin, M. d.,Rev. Sci. Instruments, 13, 7 - 9 (1942)
Sandell, E. B., and Cumniings, P. F., AN.AL CHEM.,21, 1356-8 (1949).
Vosburgh, G. J., Flexner. L. B.. and Cowie, D. B., J . B i d . C‘iienz., 175,391-404 (1948). RECEIVED for review May 7, 1952.
.iccevted August 18, 1952.
Determination of Sulfur in Petroleum Distillates by X=Ray Absorption A. Y. MOTTLAU AND C. E. DRIESENS, J R . Standard Oil Development Co., Esso Laboratories, Linden, .V. J .
K T H E past fex years, considerable work has been reported
I (2-5) on the adaptation of the x-ray Geiger counter spectrom-
eter to the determination of sulfur in petroleum distillates by x-ray absorption. While these procedures fail to match the accuracy of chemical methods, their timesaving features make them desirable for many applications. The Geiger counter spectrometer is not, however, ideally suited to x-ray absorption measurements. Because it is a single-beam instrument, unknown and standard must be compared in succession rather than simultaneously, thus introducing the possibility of error due to fluctuations in the beam intensity during the time required to make these absorption measurements. Furthermore, the geometry of the instrument is such that the beam must pass through the cell in a horizontal direction, necessitating a constant thickness of sample. Under these conditions, it is necessary to correct absorbance measurements for changes due to variations in sample density. This paper describes a procedure for the determination of sulfur in certain petroleum distillates using the General Electric x-ray photometer. Any improvement in speed, simplicity, and accuracy shown by this method over those cited is due entirely to instrumentation. The x-ray photometer was designed specifically for absorption work and, therefore, does not suffer from some of the limitations imposed by the Geiger counter spectrometer which was designed primarily for x-ray diffraction. The present method of analysis was named the “comparative method” by Zemany et al. (8), and their exploratory Rork demonstrated, as did that of S’ollmar et a2. ( 7 ) , the feasibility of applying it to the determination of sulfur in petroleum, as well as tetraethyllead in gasoline. Calingaert et al. (1) have applied the comparative method to the determination of tetraethyllead in gasoline, and the authors have profitably used their procedure, with modifications, for over a year. The method to be described has also been used for almost a year for the routine determination of sulfur in gas oils, heating oils, Diesel fuels, and lubricating oils containing no additives-any petroleum distillate whose normal sulfur content does not run much less than 0.1% and which does not contain any interfering substance.
EXPERIMENTAL
Apparatus. Absorption measurements were made with a General Electric x-ray photometer, Catalog No. 53283500-1, This instrument is described in detail by Rich and hlichel (6). Current was supplied by a 10-kv.-amp. Sorensen voltage regulator. A dummy load of 1200 watts in the form of tungsten lamps was connected to the regulator in parallel with the photometer. The sample, 85 grams, was held in the right-hand side of the double aluminum cell supplied with the instrument. (Each side of cell is 4.25 cm. square by 15.3 cm. deep.) A standard absorber consisting of a block of polystyrene, 3.5 cm. thick, was placed over the left-hand (reference beam) port of the photometer. A 50-mil aluminum block was placed over the right-hand (sample beam) port. A torsion balance (KO. 3015, The Torsion Balance Co., Clifton, N. J.), having 2-kg. capacity, and 0.1-gram sensitivity, was used to weigh the samples directly into the sample cell. Instrument Settings. Primary voltage 80. Emission current 10 ma. Amplification level 90. Calibration. Some of the samples used for calibration were prepared synthetically, while others were regular samples of known sulfur content. The sulfur contents of all the calibration samples were determined by a gravimetric procedure involving theuse of either the A.S.T.M. lamp (D 90-47T) or the Parr oxygen bomb (D 129-44). I t will be noted from the description given in Table I that these samples covered a wide range of carbonhydrogen ratios. The total x-ray absorption of each calibration sample was measured by the following procedure: The sample(85.0 i 0.1 grams) was weighed into the right-hand side of the sample cell. With the cell in position in the photometer, the null meter was brought to balance Kith the aluminum attenuator. The total absorbance of the sample was read in terms of the “drum reading,” which in turn signifies the mils of aluminum required from the attenuator to achieve balance. The drum reading is not the mils of aluminum equivalent to the sample in absorbance. The absorbance of the sample is actually equivalent t o the absorbance of the attenuator, plus the absorbance of the 3.5-cm. polystyrene block, minus the absorbance of the 50-mil aluminum block placed in the sample beam. Because, for this procedure the polystyrene block and the 50-mil aluminum block remain unchan ed, it is simpler and less confusing to think of them as part of t%e instrument, and to refer to absorbance simply in terms of drum reading. The difference between two drum readings can be correctly referred to as mils of aluminum.
1853
V O L U M E 2 4 , NO. 11, N O V E M B E R 1 9 5 2
2
92
-
88
-
84-
w I
g
80-
76
72
-
I/
68P
I
04
,
08
1
I
I
I 2
16
20
I
%SULFUR
Figure 1. Sulfur Calibration at 80 Primary Voltage
The drum readings of the calibration samples, recorded in Table
I, were plotted against the per cent sulfur obtained by the gravi-
metric method. The calibration curve thus obtained is given in Figure 1. Analytical Procedure. An unknown sample is analyzed by obtaining its drum reading in esactly t h e same manner as t h a t used t o obtain the drum readings of the calibration samples and by determining the per cent sulfur from the calibration curve, corresponding t o that drum reading.
such that the use of a constant sample uTeight would automatically compensate for changes in sample absorbance due to changes in hydrocarbon density. The x-ray beam passes up through the bottom of the sample cell and out through the surface of the absorbing liquid. A given weight of sample of high density offers a shorter path length than one of low density and the same weight; but the number of absorbing atoms in the path remains essentially the Bame in either case. Assuming the effective wave length of the x-rays used is that for which carbon and hydrogen have the same mass absorption coefficient, the constant weight system of measurement described assures that all hydrocarbons, regardless of type or density, will show the same total absorbance. Any increase, then, that a sample might show over this basic total absorbance, is due to the one additional element, sulfur. The specific weight of sample (85 grams) was chosen because it approximates, in absorbance, the amount of hydrocarbon equivalent to 3.5 em. of polystyrene, the standard absorber already in use for the previously developed tetraethyllead analysis. Standard Absorbers. Calingaert et al. (1). have pointed out the usefulness of a standard hydrocarbon absorber in the reference beam. The absorbance of all substances may be described as equivalent to the absorbance of a certain thickness of aluminum. The absorbances of aluminum and hydrocarbon, however, do not change the same with change in x-ray frequency, and aluminum equivalence, as a measure of absorbance, holds for only a very specific wave length. The constancy of effective
DISCUSSION
75
Voltage Regulator. Calingaert et al. ( I ) demonstrated the need for a voltage regulator in conjunction with the General Electric photometer, and showed that a Sorensen Zkv.-amp. regulator did not have sufficient capacity to handle the peak voltage load. The lO-kv.-amp. Sorensen regulator used in this work had, however, such excess capacity that a dummy load of 1200 watts was added to bring the regulator into a more satisfactory working range. It would seem that a regulator of intermediate capacity (3 to 5 kv.-amp.) should give undistorted voltage without the need for dummy loading. Sample Size. The amount of sample to go into the absorption cell was measured on a weight basis for two reasons. Sulfur is reported on a weight basis. The geometry of the instrument is
-
0
z w 4
;
70-
Y 0
4
a
0
m
65
X BENZENE
Table I.
Calibration Samples
% Sulfur by Drum Gravimetric Sample Description Reading Method Cetane 68.8 0.028 Cetane di-tert-butylthiophene 7 1 . 9 0.26 Cetane di-tert-butylthio hene 8 1 . 4 0.99 92.6 Cetane i 2-tert-octylthiopRene 1.89 Toluene 68.2 0.0041 Toluene di-tert-butulthio70.8 0.21 phene Toluene di-tert-butylthio79.3 0.85 phene Toluene 4-2-lert-octylthiophene 90.0 1.68 Methylcycloliexane diphenyl 7 5 . 1 0.48 sulfide Benzene thiophene 80.6 0.98 Benzene t diphenyl sulfide 69.2 0.088 n-Heptane thiophene 80.2 0.92 Piaphtha 68.8 0.026 Lube oil base 72.4 0.31 Forum 40 71.7 0.27 Forum 50 (A) 71.0 0.23 Fqrum 50 (B) 71.1 0.22 Diesel fuel (A) 80.2 0.95 Diesel fuel (B) 80.4 0.94 Diesel fuel (C) 80.5 0.96 Diesel fuel (D) 80.5 0.96 Diesel fuel (E) 78.4 0.75 Diesel fuel (F) 78.6 0.75
++ + +
+
+
+
0 n-HEPTANE
% Sulfur Deviby X-Ray ation 0.02 -0.01 0.26 0.00 1.01 $0.02 1.89 0.00 0.00 0.00 0.18 -0.03 0.85
0.00
1.69
+0.01 +0.04
0.52
0.95 0.05 0.92 0.02 0.30 0.25 0.20 0.20 0.92 0.94 0.95 0.95 0.78 0.79
-0.03 -0.04 0.00 -0.01 -0.01 -0.02 -0.03 -0.02 -0.03 0.00 -0.01 -0.01 + O . 03 4-0.04 Av. 0 . 0 1 7
A
METHYLCYCLOHEXANE
PRIMARY VOLTAGE
Figure 2.
Effect of Voltage on Absorbance
wave length of the x-rays produced by the photometer, when fed by a voltage stabilizer, is good, but not perfect, and the amount of this fluctuation is sufficient to cause a material error in absorbance readings. When, however, most of the hydrocarbon of the sample is balanced by a hydrocarbon absorber in the reference beam, this chromatic error is markedly reduced. When the polystyrene block was balanced against 85 grams of pure hydrocarbon, it was found that the amount of aluminum required from the attenuator to achieve this balance was off scale a t the lower end. A 50-mil aluminum block was therefore placed in the sample beam to move the attenuator requirements on scale. Choice of Accelerating Voltage. The General Electric x-ray photometer uses a tungsten target tube which gives, a t the ac-
ANALYTICAL CHEMISTRY
1854 celerating voltages available, only white radiation. Furthermore, the effective wave length of this white radiation is dependent on the accelerating voltage used. It m-as desired that the effective wave length used in this procedure be adjusted as closely as possible to the point a t which carbon and hydrogen have the same absorption coefficients. This was done empirically by determining the absorbances, at a number of different primary voltages, of three pure hydrocarbons covering the extremes of carbon-hydrogen ratios; benzene and n-heptane represented the extremes, and methylcyclohexane represented an intermediate ratio. The absorbances, as drum readings, were plotted against primary voltages (Figure 2). The curves representing this relationship cross at a primary voltage of 76. 9primary voltage of 80 instead of 76 n a s chosen for this procedure for two reasons. Because of the design of the meter, it was easier to set the primary voltage reproducibly a t 80 than a t 7 6 . Secondly, the maximum error a t 80 volts that could be caused by variations in carbonhydrogen ratio was about the same as the precision of the method-namely, -I 0.02% sulfur. Accuracy and Precision. Accuracy in this paper refers to the average deviation between the per cent sulfur obtained by the gravimetric procedure and that obtained by the x-ray method. The gravimetric procedures used by this laboratory are well established and regularly checked. Their accuracy is knov-n to be better than the precision of the x-ray method. Furthermore, all the gravimetric sulfur values given in Table I were run a t least in duplicate. To obtain the accuracy of the present method, the deviation of the drum reading from the calibration curve, in terms of per cent sulfur, was recorded for each sample in Table I. The maximum deviation was &0.04% sulfur and the average was 10.02%.
After long experience with repeat samples, the precision of the method has been determined t o be the same as the accuracynamely, &0.02% sulfur. COYCLUSIOh s
It is estimated that the man-hours saved by use of the sulfur analysis just described, and the tetraethyllead analysis using the same instrument, has been sufficient to pay for the instrument in less than 8 months. One operator can run between 30 and 40 sulfur determinations in a day, alloffing time for instrument warm-up and check, notebook work, and reporting. One determination, including cleanup, requires about 10 minutes. The accuracj- of the method is well within the limits required for the great bulk of the samples submitted to the laboratory, not including, of course, gasolines-the sulfur contents of n-hich are too lox to be handled by the present x-ray method. LITER4TURE CITED (1) Calingaert, G., Lamb, F. W,, hiiller, H. L., and Xoakes, G. E., A N A L . CHEY., 22, 1238 (1950). (2) Hughes, H. K., and Hochgesang, F. P., Ibid.,22, 1248 (1950).
(3) Hughes, H. K., and Wilczemki, J. W., Proc. Mid-Year Meeting Am. Petroleum Inst., 3051 IIII), 11 (1950). (4) Kehl, TT. L., and Hart, J. C., Proc. 28th Ann. Meeting A m . Petroleum Inst., I11 (1948). ( 5 ) Levine, S.W., and Okamoto, A. H., ANAL.CHEM.,23, 699 (1951). (6) Rich, T. .4.,and llichel, T. C.. Gen. Eke. Rec., 50, So. 2, 45 f 1947). (7) Vollmar, R. C., Petterson, E. H., and Petruaaelli, P. A , .%SAL. CHEM., 21,1491 (1949). (8) Zemany, P. D., Winslow, E. H., Poelimita, G. S., and Liebhafsky, ~I
H. h.,Ibid., 2 1 , 4 9 3 (1949). RECEIVED for review June 17, 1952. Bccepted August 23. 1952.
Estimation of Elemental Sulfur by Ultraviolet Absorption X. G. HEATLEY AND EILEEN J. PAGE Sir William Dunn School of Pathology, University of Oxford, Oxford, England
N C O N S E C T I O S with other work it was necessary to estimate
1 small amounts of elemental sulfur in aqueous suspensions, in deposits, and in solution in organic solvents, Gravimetric methOb
0%
04
D 03
0 2(
04
IGTH,
LITERATURE CITED ~
I
I
Figure 1. Optical Density of Ethanolic Sulfur Solution Containing 25 Micrograms per hll. E: y&.
= 194.5 a t 250 mp ( A ) , 239 at 264 mp (B),230 at 274 m p (C), 117.5 at
300
mp
ods were sometimes not applicable and in any case were tedious and seldom as accurate as desired. However, ethanolic solutions of sulfur were found to have a strong ultraviolet absorption. Figure 1 shows the absorption of an ethanolic solution of sulfur recrystallized from pyridine and ethanol, as measured in the Beckman absorption spectrophotometer Model DU. The solution contained 25 micrograms per milliliter. The absorption was not appreciably changed by the presence of up to 40% water in the ethanol. Beer’s law was obeyed a t concentrations of 0 t o 40 micrograms per milliliter; at 70 micrograms per milliliter the observed values were slightly lower than expected, though the instrumental errors of density measurement at this level I > 1.5) n ere high. Some other sulfur compounds ( h j drogen sulfide, sulfate, thiosulfate, etc.) have broad absorption bands in the ultraviolet and interfere. For the estimation of sulfur in biological material it is unfortunate that its peak absorption covers the wave-length range in ivhich nucleoproteins, proteins, and their components absorb most strongly However, interfering substances can probably be eliminated or allowed for by appropriate procedures. iilthough absorption curves for sulfur have been given, incidental to other studies, by Baer and Carmack ( 1 ) (in ethanol and hexane) and by Koch ( R ) (in chloroform:, no reference has been found t o Lhe use of ultraviolet absorption measurements for ita estimation.
(D)
(1) Baer, J. E., and Carmack, M.,J . Bm. Chem. Soc.. 71, 1215 (1949). (2) Koch, H. P., J . Chem. Soc., 1949,394. RECEIVEDfor review March 21, 1952. Accepted August 16, 1952.
