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381
apply the method to an actual smokeless powder sample of known volatiles content. Unfortunately, it 1% as impossible to prepare such a sample. Therefore, a plant sample of powder was first dissolved in dibutyl phthalate, evacuated for 1 hour at 5 mm. to remove the volatiles already present, and weighed, and the total volatiles were calculated as show-n in Table 11. The triplicate results thus obtained TTere concordant. Further evacuation for an additional hour under the same conditions had a negligible effect upon these results. Known amounts of ether, alcohol, and water were added and the tubes were again evacuated and weighed. The agreement between the volatiles added and found was as good as that previously obtained in the analysis of synthetic mixtures. Therefore, it was concluded that the use of a temperature of 85" C., a pressure of 5 mm., and a time of 2 hours was satisfactory. Table I11 shows the average results found when three check samples were analyzed in six ordnance laboratories. Although the variation among these results is greater than that observed in any single laboratory, the concordance among them is considered satisfactory. A comparison of results found in one laboratory by the solution-evacuation and precipitation methods is CORtained in Table Is'. The maximum difference between average results found by the two methods when applied t o 91 samples is
0.12%. However, the weighted average difference is only 0.027c, which is negligible. A consideration of the results of replicate determinations on hundreds of samples by the solution-evacuation method indicates that these results, expressed as per cent total volatiles, have an accuracy of a t least 0.20% and a precision of 0.10%. The average value for standard deviation, calculated in accordance with the equation of Moraii ( 2 ) from the results found when t v o samples were analyzed 14 and 15 times, respectively, is 0.04'%. As has been pointed out ( I ) , this general method can he used for the determination of any readily volatile substance in admixture with ani' nonvolatile heat-stable material. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of Harold M. Spurlin in devising and constructing the first apparatus which embodied the principle of the solution-evacuation method. LITERATURE CITED
(1) McKinney, C. D., Jr., Turk, E., and Shaefor, W. E., ISD. Erio. C H E M . , -4XAL. ED.,18, 14 (1946). (2) Moran, R. F., Ibid., 1 5 , 361 (1943). (3) K e w m a n , M . S., Ibid., 12, 274 (1940).
Determination of Sulfur by Combustion in a Vertical Tube DUDLEY B. H.IGERRI.AN, Socony-Vacuum Laboratories, Socony-Vacuum Oil Co., Znc., Research and Development Department, Paulsboro, N. J . A method is described w-herein sulfur is determined in a wide range of organic liquids by allowing the sample to drop into a verticall?.held tube containing a bed of hot porous material. The resulting sulfur gases are draw-nfrom the lower end of the tube and are absorbed in a solution of hydrogen'peroxidein which the sulfur is determined alkalimetrically. Rapidity, accuracy, and simplicity are among the advantages of the new method.
S
EVERBL accurate and reliable methods for the determination of sulfur in organic compounds are in common use. The Carius ( 5 ) ,Parr oxygtn bomb ( I ) , and peroxide bomb (6) methods all give dependable results, but the number of steps involved presents numerous opportunities for error and consumes much valuable time. Considerable success has also been claimed for the Braun-Shell method (S),wherein the sample is placed in a heated quartz tube and slowly ignited, the combustion products being absorbed in a suitable solution. Still another method suggests burning the sample in a lamp after blending with a sulfur-free, low-boiling solvent (4). Sumerous other methods have been devised, none of which provides the simplicity, rapidity, and accuracy of the new method described here. This method yields results which agree well with theoretical values on pure chemical compounds. With oils and synthetic additives, the values agree with those obtained by the Parr oxygen bomb method. A single sulfur determination can be completed by the new method in 20 to 30 minutes. When a multiple unit apparatus was used (Figures 4 and 5 ) with two operators sharing the work, forty-eight sulfur determinations were completed in an 8-hour working day. APPARATUS
The apparatus required consists of a tube of stainless steel, 3 mm. thick and 370 mm. long, with a 25-mm. inside diameter, to one end of which is welded a stainless steel tube, 1.5 mm. thick and 230 mm. long, with a 6.5-mm. inside diameter (Figure 1). At the other end of the 25-mm. tube are attached outwardly two small metallic wings or shoulders placed at right angles to the tube and opposite each other. These serve as supports by which the tube is suspended in an electric furnace and also as a base for attaching the springs which hold the glass chimney in position.
This combustion tube is packed with quartz beads, or a suitable inert substitute, and coarse, clean sand supported a t the bottom of the tube by a perforated disk. The iroportion of beads to sand is approximately 5 to 1 in six or eight alternate layers, starting with 80 mm. of beads at the bottom and leaving 150 mm. of unfilled tube a t the top. The beads should not be over 5 mm. in diameter and the sand should be 20- to 40-mesh. The furnace consists of a core of stainless steel, 3 mm. thick, and 430 mm. long, vith a 38-mm. inside diameter. One end of this core is welded to the center of a circular disk of sheet iron, 3 mm. thick and 250 mm. in diameter, with a 12.5-mm. hole in its center. To the opposite side of the disk are welded three legs 230 mm. long which support the furnace and combustion tube in a vertical position. The core is wound with 12.8 meters of KO.19 Nichrome V resistance wire which is first coiled to form a helix havine a 4.&mm. diameter and then covered with fishspine interlockiig ceramic insulators (Figure 2). A heating element so designed will carry a current of 600 to 650 watts. The core is insulated with 50 mm. of Super X, then 50 mm. of 85% magnesia, and finally a thin layer of flaked asbestos and cement. Thirty millimeters of insulation are affixed to the bottom of the furnace. A 6.5-ohm slidebar rheostat is placed in the circuit as a means of adjusting and controlling the furnace temperature. Optimum operating temperature is 1526' F. 4 furnace so constructed (Figure 3) can be operated continuously a t the required temperature for a t least 6 months. The loT-ier or exit. end of the combustion tube is connected by means of a short piece of rubber tubing to the type of absorber described by A.S.T.M.(2). At the top of the tube is mounted a Pyrex chimney, 25 mm. in inside diameter and 150 mm. long, slightly flared at the bottom and having two hooks diametrically opposite each other, to which are attached two small springs connecting with the combustion tube to hold the chimney firm. A mirror held above the furnace at the proper angle permits the operator to observe the interior of the tube. The dropper from which the sample is automatically fed into the combustion tube is made by drawing out the end of a piece
382
V O L U M E 19, N O . 6 of 7-mm. glass tubing into a fine capillary. The other~endis bent itt a slight~anglc, so that when placed in a stopper on a balance Dan in a horizontal Dosition for weighing.' the slight upward d i n t orevents tbe-sample from-ruGing out of the large end of the dropper. The over-all length of the dronncr should he ahout 100 mm. and a suppiy having capillary openings of various size should be kept on hand t o accommodate Sample8 of ;%rims viscosities. A fine wire extending into the tip of the dromer with ahout 2.5 cm. i l inch) protrudcig from the umer or open end is "sed to dose the outlet pmtly Lnd thereby control the drop rate of the sample. If the air temoerature directlv above the furnace is hiah'enourh to c a k Dossihle sample loss by evaporation, cooling is effected by means of a low-speed electric fan or b y ~ astream of air from a compressed air outlet DESCRIPTION O F THE METHOD
With the furnaoe adjusted t o the re scribed temperature eonneot the ouilet end of the cornhuskion tube to the absorber using an S-shaped glass tube. Place 40 ml. of a 10% solution of 30% hydrogen peroxide in the absorber, place the spray trap in position on the absorber, and apply vacuum through the spray-trap outlet, until air psases briskly but not violently through the peroxide solution. Continue this for about a minute t o heat the lower part of the combustion tube before admitFigure 1. ting the sample. CornbusDraw a suitable amount of sample into tion T u b e a dropper which has a capillary opening of such size as to nermit a dron to e s c a ~ enot oftener than once every 3 seconds. Wipe with a clean cloth and fix the dropper into a No. 6 ruhberstopper having a V-shaped vent cut into its side. Weigh accurately to 0.1 mg. and place in position in the chimney. The amount of sample t o be burned will vary with the anticipated sulfur content. For lessthan I%sulfur, use 0.8 t o 1.0 gram, for samples up to 20Yo sulfur, use 0.2 to 0.5 gram and less amounts for higher percentages. When enough of the sample has been burned, remove the stopper and set aside for reweighing. Add slowly t o the combustion tube 8 t o 10 drops of water to purge the tube of any entrained sulfur vapors. Allow air to he drawn through the system for 1or 2 minutes to expel carbon dioxide from the solution. The dropper may he reweighed during this period to ascertain the weight of sample burnod. Discontinue the vacuum and .rinse the connecting tube and
manner without using any sample and subtract the amount from the total titration. One millilitcr of 0.1 N alkali is equal to 0.001603 gram of sulfur. EXPERIMENTAL
I n Table I are listed a variety of materials, showing their sulfur content as obtained by both the Parr oxygen bomb and the wrtical tube method. Where duplicate determinations were made, the results are included. In a series of tests designed to show the reproducibility of the method, a group of sixteen oils was run in triplicate and all results obtained were reported. See Table 11. In Table I11 are shown the results of several pure organic cornpounds, some of which were blended with an oil of known sulfur content and analyzed by the proposed method. Samples 5 and 6, being solids, were first formed into pellets by means of a pellet press. These were crushed into smaller pieces and dropped one a t a time directly into the combustion tube from the end of B small spatula. This was found more desirable than adding the material in a powdered form or in solution in a solvent. It was thought that some advantagemight be gained byusing a quartz combustion tubeor one made from heat-resistant glaes. Both materials were found to soften somewhat under prolonged exposure to the temperature required. Their heat conductivity being low and their breakage high, i t was decided t o discard them in favor of stainless steel Type 309, designation 25Figure 2. Heating Ele12. ment on Steel Core beAlthough a solution of 3% hyfore Insulation Is Applied drogen peroxide can be obtained WIRE FOR DROP-RATE CONTROL DROPPER CONTAINING SAMPLE PYREX GLASS CHIMNEY
absorber c o h n n usina 0.1 N sodium hydroxide. Mix the
SPRINGS TO HOLD CHIMNEY
tin-
COMBUSTION TUBE INSULATION
absorber. Make a blank deterrmnstian in exactly-the same
WINDINGS OF HEATING ELEMENl
Table I. Description of Sample Experimental additive
Turbine oil Mineral oil Medicinal oil Lubricating oil
'Tar separator overhead Cyole stock SO,-treated raffinate Sulfurized fat No. 6 fuel oil
.ALTERNATE LAYERS OF BEADS AND SAND
Sulfur Content Per Cent Sulfur bomb Vertical tube 8.95 8.98 7.73 7.63 1.65 1.62
Oxygen
2.93 0.86 1.29
0.08 0.10 0.29
0.01 0.21 0.19 0.20 0.01 0.20 0.07
!RON DISK WITH ? I N . HOLE IN CENTER
2.86 0.86.0.81 1.20
0.07 0.12,O.ll 0.31 0.01 0.16.o.16 0.21.0.22 0.20.0.24 o. 02.0.03 0.24.0.19
0.96
0.07.0.08 1.04.0.9~
9.96 2.06
9.96,9.93 2.00
Figure 3.
Cross Section of Sulfur Furnace Ready to Operate
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383
VACUUM LINE ACROSS FRONT
HEATING ELEMENT IS WOUND ON EACH OF.THESE STAINLESS STEEL InMM. CORES
WITH PET-COCK CONNECTIONS FOR EACH TUBE
,
THREE SLIQE-BAR RHEOSTATS MOUNTED A ON EACH END
commercially, its use in this method found undesirable because of its acidity. Although the 30% product recommended also contains some acidity, its solution in water in the proportion of 1 to 9 reduces the acid content per unit volume to such a degree that the 40 ml. recommended per sample usuilly require only 0.10 to 0.20 ml. of 0.1 N sodium hydroxide in the titration of the blank. The retention of some of the sulfur within the combustion tube WBS considered as distinctly possible and experiments were conducted to determine whether such a condition existed. The procedure consisted of completing a n aualysis including the titration, then reattaching the absorber and again turning on the vacuum. After 10 or 12 drops of water were added a t about onesecond intervals, the additional sulfur found by a second titration varied from none to as much as 0.3% on the hasis of the weight of the samd e consumed, the higher values being obtained on samples having 8. relatively high sulfur content. It was considered imperative, therefore, that the tube he purged with water after each combustion, unless i t was found by experiment that such a procedure was unnecessary on the particular type of sample being analyzed.
165 MM,
WBS
11"
Figure 4.
Diagram of Multiple-Unit Furnace
LIMITATIONS
This method does not apply to compounds which contain nitrogen, phosphorus, the halogens, barium, calcium, or any metals which form stable compounds with sulfur a t 1500' F. Neither does i t apply in its present form to low-gravity hydrocarbons of the nature of gasolines. DISCUSSION AND SUMMARY
Figure 5.
Multiple U n i t
T a b l e 11. Reproducibility of Method Oil
Per Cent Sulfur by Vertical Tube Method 1 2 3
During the past two years the vertical tube method has been applied successfully to over a thousand samples of various nar tures and compositions. It is particularly well adapted t o multiple unit operation (Fiskres 4 and 5). Untrained analysts can l e m to operate the apparatus in one day. Initial costs are low and maintenance requirements are negligible. The manner in which the sample is automatically fed into the combustion tube saves considerable time and makes i t possible to obtain rezults more quickly then by any other known method. The following are the most important factors for the successful operation of the furnace: air rate, drop rate, drop siee (the smaller the better), furnace temperature, size of particles in the cornbustion tube, and air temperature above the furnace. The application of this method in a modified form to the determination of sulfur in low-boiling hydrocarbons is eonsideied entirely possible and is being investigated. Similarly, the principle of the method may be adapted to the determination of carbon and hydrogen. Preliminary tests directed a t these constituents gave promising results. ACKNOWLEDGMENT
The author wishes to express his appreciation to C. W. Brown and G. H. S. Snyder for their helpful suggestions and e n c o u r a p meut in the development of this method. LITERATURE CITED
Table 111. Sulfur in Organic Compounds ~
Description of Sample
Sulfur by Vertioal Tube
Method
._
% 1 10% @ootyl thio ether in pi1 2 4% d h n i o t b i o n b e n e i n p i l ,
1.38 0.86 0.20 a 0.4% dibenrothiophenem 011 12.34 4 Diootvl thio ether 8 bia [ ( 2 - H y d r o r y - n a p h t h y l ~ ~ ~ l f i d ~ ~ I10.01 )l 25.99 6 Benzyl disulfide
. Sulfur TheoretiOal % .. 1.40 0.86 0.23
12.40 10.05
26.04
Testing Materials, Standards on Petroleum Produota and Lubricants, pp. 307-9, 1942. (2) Ibid., p. 310, 1943. (3) Braun Corp., Los Angeles. Calif., Analytical Methods, No. S28(1) Am. Soo.
1-41, 1941. (4) Korb, E. L..unpublished paper, E. I. du Pont de Nemours snd Co.. Wihninpton, Del. ( 5 ) Niederl, 3. B., and Niederl, V., "Miemmethods of Quantitative Organic Elementary Analysis," New York. John Wiley & Sons. 1942. (6) Parr Instrument Co., Moline. Ill.. Dkeclion Booklet 116, 1839.
