ACKNOWLEDGMENT
The authors are indebeted to E. G. E. Shafer for his helpful suggestions, to Jacob Scheiner for carrying out the microbiological assays, to Alice Billmeyer for some of the chemical determinations, and to Esther Critelli for drawing the graph. LITERATURE CITED
(1) Cocking, E. C , Temm, E. W., Biochem. J . 58, 12 (1954). (2) Crokaert, R., Arch. intern. physiol. 56, 189 (1948). (3) Crokaert, R., Bull. SOC. chim. biol. 31,903 (1949).
(4) Crokaert, R., “Contribution a 1’6tude de la 6-alanine et de ses compos& dans les milieux biologiques,” Acta Medica Belgica, Brussels, 1953. (5) Crokaert, R., hloore, S., Bigwood, E. J., Bull. SGC. chim. biol. 33, 1209 (1951). (6) DeRitter, E., Rubin, s. H., ANAL. CHEM.21, 823 (1949). (7) Folin, O., J . Biol. Chem. 51, 377 (1922). (8) Frame, E. G., Russel, J. A , Wilhelmi, A. E., Ibid., 149, 255 (1943). (9) Kalant, H., ASAL. CHEM.28, 265 (1956). (IO) hloore, Stein, W. H., J . Bid. Chem. 176, 367 (1948); 192, 663 (1951); 211, 907 (1954). (11) Skeggs, H. R., Wright, L. D., Ibid., 156,21(1944).
s.,
(12) Szalkowski, C. R., Davidson, J. H., Jr., -4x.4~.CHEM.25, 1192 (1953). (13) Szalkowski, C. R., blader, W. J., Frediani, H. .4., Cereal Chem. 20, 218 (1951). (14) Troll, W.,Cannan, R. K., J . B i d . Chem. 200, 803 (1953).
(1.5) Weiss, M. s., Sonnenfeld, I., DeRitter, E., Rubin, s. H., ANAL.CHELI. 23,1687 (1951). (16) Wollish, E. G., Schmall, M., Ibtd., 22,1033 (1950). (17) Yemm, E. W.,Cocking, E. C., Snalyst 80, 209 (1955)
RECEIVEDfor review April 2, 1957. Accepted May 25, 1957. Division of Bnalytical Chemistry, 131st Meeting, ilCS, Miami, Fla., A4pril1957.
Determination of Trace Amounts of Combined and Elemental Sulfur in PetroIe um Frac t io ns N. W. HOUGHTON American Oil Co. (Texas), Texas City, Tex.
b Trace amounts of sulfur in petroleum liquids have been determined by the technique of burning the sample in an oxyhydrogen flame. After absorption of the combustion products in aqueous hydrogen peroxide, the sulfate ion formed was measured turbidimetrically. Recovery of both combined and elemental sulfur was good. The method, when applied to combined sulfur as 4-thiaheptane, shows standard deviations of 0.6 to 1.0 p.p.m. in the 2 to 1 1 p.p.m. sulfur range. The standard deviation increases gradually to 8 p.p.m. at the 300 p.p.m. sulfur level. Recovery of elemental sulfur at the 50 p.p.m level was 99.270, with a standard deviation of 2.4 p.p.m.
T
accurate determination of sulfur in petroleum distillates is necessary to the petroleum industry to monitor the sulfur content of gasolines, solvents, and other petroleum oils and to aid in control of corrosion in refinery units. Granatelli’s (4) use of the oxyhydrogen flame to burn a variety of petroleum liquids for subsequent sulfur determination a t relatively high sulfur levels is extended to the micro level in this paper. The work presented shows the feasibility of using the oxyhydrogen burner for the quantitative oxidation of microgram quantities of dissolved sulfur and sulfur compounds in petroleum liquids. Sulfate ion formed by the oxidation is measured turbidimetrically as barium sulfate. HE
By the use of oxyhydrogen combustion-large samples can be burned in a much shorter time than by ASTM lamp ( I , 2) methods. This produces larger quantities of sulfate ion and permits easier and more accurate measurements. I n addition, the oxyhydrogen flame can burn olefinic and aromatic materials with ease. It is confirmed in these tests, as reported by Granatelli (4), that elemental sulfur is quantitatively recovered by the oxyhydrogen combustion method. APPARATUS
The oxyhydrogen burner system is described in Figure 1. The air purification system has been changed to consist of three 1-liter mixing cylinders fitted with fritted cylinder glass dispersion tubes. Air to the burner passes, in order, through 600 ml. of 10% sodium hydroxide, 600 ml. of 3% hydrogen peroxide, and 600 ml. of demineralized water. Each cylinder contains 20 ml. of 2-ethylhexanol to minimize frothing. Spectrophotometer, Beckman Model
B.
Absorption cells, 1-cm., Fisher No. 14-381-12, 5-cm., Cenco No. 29360. REAGENTS
Barium chloride, dihydrate, c.P., 20 to 30 mesh crystals. Desiccant, indicating Drierite. 2-Ethylhexanol. Hydrochloric acid, c.P., 1 to 1 aqueous dilution of 1.19 specific gravity. Hydrogen. Hydrogen peroxide, c.P., 30%. ~~
Hydrogen peroxide, c.P., 3y0, Iso-octane (2,2,4-trimethylpentane). Iso-octane n as desulfurized over cobalt molybdate catalyst. Space velocity was 2 pounds of iso-octane per hour per pound of catalyst with a hydrogeniso-octane molar ratio of 4. The reaction temperature was 357” C. with the pressure a t 47.8 atmospheres. Hydrogen was bubbled through the product to remove hydrogen sulfide. The isooctane was then passed through a 2foot column of silica gel. Oxygen. Sodium chloride, C.P. (Mallinckrodt), 10% aqueous solution. Sodium hydroxide, c.P., 10% aqueous solution. Water, distilled water was demineralized bv ion exchange to contain less than 0.3 p.p.m. of -salt (as sodium chloride), PROCEDURE
Burner Operation. The apparatus is assembled as shown in Figure 1. The absorber is placed in a 3-liter beaker and the beaker, knock-back, and condenser are charged with a water-ice mixture as coolant. The coolant must be replenished periodically during operation. The absorber is charged with 10 ml. of 3070 hydrogen peroxide and 50 ml. of demineralized water; 2 ml. of 2-ethylhexanol are added to decrease foaming. One inch of a 6-inch B and S gage Nichrome 77-ire is inserted into the lower end of the burner capillary. The external 5 inches is bent up alongside the capillary, so that most of it is outside the test tube. Manipulation of the wire proves useful in unstopping VOL. 29, NO. 10, OCTOBER 1957
1513
the capillary, if necessary, without shutting down operation. Too rapid aspiration of hydrocarbons which occurs when the wire is not in the capillary extinguishes the flame. Suggested pressures a t the pressure regulators are 10, 4, and 10 pounds per square inch for oxygen, hydrogen, and air, respectively. Final control of each rate of flow is accomplished by needle valves between the pressure regulators and burner. The oxygen supply is turned full on by means of the needle valve and the burner is ignited as the hydrogen flow is started. The flow of hydrogen is adjusted a t the needle valve t o produce a flame about s/g inch high. The vacuum pump is started and allowed to operate for 15 seconds before the burner base is positioned, in order to clear the chimney of possible explosive hydrogen-air mixtures. The burner base is clamped securely in position to ensure an air-tight system. Large leaks in the system may be detected by failure of the manometer t o show immediately an increasing vacuum in the system. Small leaks must be detected and corrected before the burner is ignited. When the system is found to be airtight, the air supply is turned on a t the needle valve and adjusted to produce a pressure of approximately 5 mm. of mercury below atmospheric. The weighing buret containing the weighed sample is attached to the test tube and the sample delivery system is raised into the position shown in Figure 1. A few milliliters of sample are admitted into the test tube and aspiration of the sample into the flame begins. At this point the rate of a sample combustion may be adjusted to a maximum (ca. 0.5 ml. per minute). This maximum has been passed when the flame shows signs of excess flickering or jumping, or is extinguished. The rate of burning is controlled by adjusting the pressure in the system. This is done by changing the air flow, or in the case of heavy or light samples, by varying the length of the wire inside the capillary tube. Occasionally, it is necessary to operate with the pressure in the system at, or slightly above, atmospheric. It is helpful during operation to have an air stream blowing on the burner base and another on the chimney. The air cools the chimney and, by cooling the sample, reduces its volatility, thereby giving better flame characteristics. As the sample in the test tube is consumed, more is admitted until the supply in the buret is exhausted. The buret is then washed with 1 ml. of sulfur-free iso-octane and the washings are added to the test tube just before the last portion of sample is consumed. After the iso-octane has been burned, the sample delivery system is lowered, the hydrogen, then the oxygen are turned off, and the vacuum pump is stopped. This allows a slight pressure build-up in the system and the burner base is lowered. If the flame goes out during operation, the sample delivery system is lowered and the hydrogen supply is turned off immediately. The absorber solution is quantitatively transferred to a 600-ml. beaker and the 1 5 14
ANALYTICAL CHEMISTRY
chimney, connecting arm, spray trap, absorber, and absorber knock-back are rinsed into the beaker with hot demineralized water. Twenty milliliters of 10% sodium chloride are added to the beaker, the beaker is covered with a watch glass, and the contents are evaporated to about 20 ml. Evaporation also removes the 2-ethylhexanol. Evaporation of the absorber solution to dryness must be avoided. In every case of evaporation to dryness, high results were obtained. The contents of the beaker are uantitatively transferred to a 100-2. volumetric flask; exactly 1 ml. of 1 to 1 hydrochloric acid is added; and the contents of the flask are diluted to volume with demineralized water. Because operation of the burner is somewhat complicated, the operator must be present during the entire combustion. It is easily possible, however, for one operator to handle two burners simultaneously, except with the most difficult samples. Turbidimetric Measurement. After the solution is thoroughly mixed in the volumetric flask, 50 ml. are measured into a 250-ml. glass-stoppered Erlenmeyer flask. T o this, excess barium chloride crystals are added (0.6 gram was used) and the flask is swirled for exactly 1 minute to dissolve the barium chloride and produce a suspension of barium sulfate. The absorption cell is rinsed with a small portion of the mixture and the remainder is poured into the cell. The spectrophotometer is adjusted to zero absorption a t 410 mp on a reagent blank. The absorption of the sample
is measured a t 410 mp exactly 3 minutes after the addition of barium chloride. The quantity of sulfur is obtained from a calibration curve. Calibration Curve. Both sulfuric acid and sodium sulfate have been found satisfactory as standards for construction of a calibration curve. The calibration standards were prepared using all reagents added in the actual determination, except 2-ethylhexanol. A reagent blank was prepared in the same manner. A range of 0 to 700 y of sulfur per 100 ml. of absorber solution was measured in a 5-cm. cell, a range of 500 to 2150 y of sulfur per 100 ml. of absorber solution was measured in a 1-cm. cell. EXPERIMENTAL RESULTS
To test precision and accuracy of the method, six solutions containing known amounts of combined sulfur in the range of 2 to 300 p.p.m. in iso-octane were prepared and analyzed. The sulfur compound added was 4-thiaheptane supplied by the American Petroleum Institute and reported t o be 99.96 =t 0.1 mole yo pure. To test the recovery of elemental sulfur, a solution containing 51.9 p.p.m. of recrystallized sulfur in iso-octane was prepared and analyzed. Table I shows the results of ten determinations of various levels of combined sulfur. Standard deviations for sulfur levels below 55 p.p.m. range from 0.60 to 1.0 p.p.m. of sulfur. Standard deviations for the two higher levels, 100
AIR
I
2
Figure 1.
3
4
5
Assembly for burning liquid petroleum samples
1. Air scrubber, 10% NaOH 2 . Air scrubber, 370 H202 3. Air scrubber, H20
4. Spray trap with glass wool 5. Surge bottle 6. Manometer, mercury 7. Needle valve 8. Glass connecting tube 9. Metal tube, a//le inch diameter 10. f71/60 glass joint
11. Oxyhydrogen burner 12. Chimney 13. Blow-out port 14. Connecting arm
15. 16. 17. 18. 19. 20. 21.
Absorber Spray trap
Knock-back Condenser Weighing buret Delivery tube Test tube
and 300 p.p.m. of sulfur, are 3 and 8 p .p.m., respectively. Table I1 contains the results of ten deterniinations on the sample containing elemental sulfur. The standard deviation was found to be 2.4 p.p.m. sulfur a t the 51.9 p.p.m. level. Recovery averaged 99.2% of the sulfur added. The precision and accuracy of the method and tlie recovery of elemental sulfur were determined by four technicians and the author, operating four separate burner systems. Various parts of each apparatus rvere replaced during the 2 months this test was in progress. Analysis of all test samples was interspersed with the analysis of approximately 200 refinery samples submitted for trace sulfur determination by this method.
Table I.
Sulfur, P.P.M. Samnle Dev. Si&, from mean S O . Grams Addedb Found la Ih IC Id le If 1g Ih li 1j
2a 2b
2c 2d 2e 2f
DISCUSSION
Granatelli (4) has shown that elemental sulfur is quantitatively recovered a t the 0.15% level by oxyhydrogen combustion followed by a gravimetric finish. The present investigation substantiates his work a t the 50 p.p.m. level, using a turbidimetric finish. There can be no doubt that the oxyhydrogen burner is superior to current lamp procedures for the complete recovery of elemental sulfur (6). I n the development of this method it was found that the air purification system used in the ASTlI lamp sulfur determination (2) and in the original paper on the oxyhydrogen burner (4) was unsatisfactory for use in trace work. Spray from the hydrogen peroxide scrubber contained sulfuric acid, which eventually was found in the absorber solution and/or chimney and connection arm. This amounted to 30 to 35 y of sulfur per hour of burner operation. Use of the air purification described above reduces sulfur contamination from the air to 5 y per hour or lessapproximately 0.2 p.p.m. on the longest operations. The oxygen and hydrogen used in these tests contained no appreciable amounts of sulfur as contaminants. Therefore, no purification system was used on these gases after the first few trials. Several conditions and media for the development of a barium sulfate suspension were examined. The use of ethyl alcohol (S), glycerol (S), dipropylene glycol (e), and mixtures of these to increase the reproducibility of the barium sulfate suspension was not found necessary. I n the procedure described above, hydrochloric acid was used to obtain a reproducible pH, sodium chloride was added to ensure a constant ionic strength. Calibration curves developed by this technique were reproducible.
Determination of Combined Sulfur" in Iso-octane
3 2i 2j
34 34 34 34 34 34 29 31 28 45
1 2 7 9 3 4 6 1 6 7
32 33 31 34 31 31 31 35 29 34
7 7 0 3 3 1 8 0 5 7
2.46
7 04
1.76 2.05 1.73 1.72 1.89 3.34 2.36 3.22 3.15 2.19
-0.58 -0.87 -0.61 -0.62 -0.45 +1.00 $0.02 +0.88 +0.81 -0.15
Mean
Std. dev.
2.34 8.42 8.46 7.27 7 13 7.19 8.52 7.39 7.15 7.12 7.36
0.65 +O. 82 +O. 86 -0.33 -0.47 -0.41 10.92 -0.21 -0.45 -0.48 -0.24
hlean 3a 3b 3c 3d 3e 3f 3g 3h 3i 3j
19 20 20 20 20 19 19 20 21 21
9 5 1 4 7 8 7 7 5 4
11.0
7.60 12.1 11.9 12.9 10.8 13 3 10 1 10.9 12.3 11.6 11.0
Mean 11.7
Sulfiir, P.P.31.p Samole
Dev.
Size,
KO. Grams Addedb Found 4a
4b 4c 4d 4e 4f 4g 4h 4i 4j
16 12 11 11 12 11 11 11 11 11
2 9 4 8 5 9 3 3 6 9
54.3
Mean 5a 5b 5c 5d 5e 5f 5g 5h 51 55
6 4 9 8 10 9 18 2 11 2 13 6 9 1 17 0 18 4 18.4
110.3
Std. dev. 0.60
+0.4
6a
+0.2 +1.2 -0.9 11.6 -1.6 -0.8 +0.6 -0.1 -0.7
6c 6d 6e 61 6g 6h 6i 6j
Std. dev. 1.0
56.7 54 1 53 8 53 5 56 0 543 54 7 53 7 53 8 544
6b
7 . 2 . 287.0 5.1 4.2 3.6 4.5 5.0 4.8 4.6 4.6 4.2
54.5 106 10s 114 108 109 110 109 108 115 111
from mean t2.2 -0.4 -0 7 -1 0 1-1 5 -0 2 -0 2 -0s -0 5 -0 1 Std. dev. 1.0 -4
-2 J-4 -2 -1 0 -1 -2 +5 1-1
1Zean
Std. dev.
110 265 278
3 - 13 0 - 12
266 _
.
283 280 285 286 274 289 273
Mean 278
+ 5
:; + 8 - 4
$11 - 5
Std. dev. 8
Added as Pthiaheptane. 0.58 P.p.m. of sulfur found in reference fuel iso-octane used as solvent for standard solutib6s. a
* Included
ACKNOWLEDGMENT
Table II.
Determination of Elemental Sulfur in Iso-octane
Sulfur, P.P.M. Sample Dev. Size, from S o . Grams Added" Found mean 7a 7b 7c 7d
7e 7f
;ETi
7j
10 6 10 1 10.9 10.8 12.8 11.3 12.2 9.7 12.3 10.2
51.9
52 0 51 7 55.1 47.8 48.4 48.7 52.1 53.9 52 7 52.3
$0 5 +O 2 $3.6 -3.7 -3.1 -2.8 +0.6 +2.4 f1.2 +0.8
Mean
Std. dev.
51.5
2.4
a Included 0.58 p.p.m. of sulfur found in reference fuel isc-octane used as:solvent for standard solutions.
The author wishes to thank Lawrence Granatelli and R. L. diken for many helpful suggestions during this investieation. W. 1 4 . Lowev, 11. 0 . Curle;,, A. R. Clegg, and R. E. Hewitt n e r k especially helpful in performing most of the analyses. LITERATURE CITED
(1) Am. SOC. Testing Materials, Phil-
adelphia, Pa., "ASTM Standards on Petroleum Products and Lubricants,' ' D 90-50T, 1954. (2) Zbid., D 1266-53T, 1954. (3) Becchold, H. von, Hobler, F., Kolloid2.31, 70 (1922). (4) Granatelli, L., ANAL.CHEM.27, 266 (1955). (5) Lane, W. H., Ibid., 20,1045 (1948). (6) Toennies, G., Bakay, B., Zbid., 25, 160 (1953).
RECEIVED for review November 13, 1956. Accepted February 21, 1957. VOL. 29, NO. 10, OCTOBER 1957
1515
