Determination of Trace Sulfur in Light Hydrocarbons by a Metal Oxy-Hydrogen Burner-Quartz Combustion Tube Technique DEAN HOGGAN and WlLLlS R. BATTLES Richfield Oil Corp., Wilrnington, Calif.
b For the determination of trace quantities of sulfur in petroleum distillates an oxy-hydrogen burner of improved design followed b y a heated quartz combustion tube gives rapid combustion, increased sulfur recovery, and the elimination of a potential explosion hazard. A unique twostage absorber-evaporator system, in addition to absorbing sulfur oxides, eliminates excess water of combustion and permits uninterrupted burning of large samples. The recovered sulfur i s determined as the sulfate in the evaporated hydrogen peroxide absorbent b y micro-titration with barium perchlorate in 80% alcohol using a mixed thorin-methylene blue indicator, An average standard deviation of 35 p.p.b. sulfur has been obtained when burning 50-ml. samples of synthetic benzene blends in the 50 to 400 p.p.b. range. Sulfur recoveries averaged 98% on a variety of known blends over a wide concentration range. The method i s applicable to determination of sulfur up to 400 p.p.m. or more, burning smaller samples. Fifteen samples can be analyzed per man-day.
T
RECEST emphasis on high purity petrocheniicals makes the accurate determination of trace amounts of sulfur increasingly important to the petroleum industry. The -4SThl modification for trace sulfur (3) was unsuitable because it provided poor accuracy with samples containing a few parts per million of sulfur. In addition, the sample burning time was prohibitive. Other methods n ere therefore investigated. Wickbold (8) has used a quartz aspirating oxy-hydrogen burner and apparatus for the determination of chlorine and sulfur in distillates. Houghton ( 6 ) has adapted the Beckman aspirating burner method of Granatelli (5) and used a turbidimetric determination of the sulfate formed in the hydrogen peroxide absorbent. He reported standard deviations of 0.6 t o 8 p.p.m. for sulfur contents of 2 t o 200 p.p.m. Another method applicable to the 1 to 100 13.p.m. sulfur level is that HE
of Hudy and Mair ( 7 ) . They used a vertical catalyst-packed, heated quartz combustion tube for combustion of the sample, which was added dropwise to the top layers of the quartz. The sulfate formed in the hydrogen peroxide absorbent was determined conductometrically. A precision and accuracy of 1 to 2 p.p.m. was reported. Our specific need was for a rapid and reliable method covering the range of 0.1 to 200 p.p.m. sulfur content. A repeatability of a t least hO.1 p.p.m. in the 0 to 1.0 p.p.m. range vas desired. The elimination of explosion hazards and simplicity of operation were also important goals. DEVELOPMENT OF METHOD AND DESCRIPTION OF APPARATUS
Initial attempts to modify the M'ickbold apparatus were abandoned because explosions within the quartz burner destroyed several burners and our confidence in the method. A metal burner was then designed and constructed by modifying a Beckman oxy-hydrogen burner as follows : h cylindrical stainless steel sleeve TT as machined to the specifications shown in Figure 1. The Beckman burner was inserted into the rear of the sleeve and silver soldered in place (Figure 1, detail .XI while holding it in centered position with four narrow pieces of 0.004-inch shini stock. The annular clearance between the Beckman burner and the outer sleeve was carefully maintained t o assure proper distribution of auxiliary oxygen to the burner. Silver solder was also used to fasten the auxiliary oxygen tube in place and to fasten the adapter sleeve to the Beckman burner capillary. Plugging of the capillary was prevented by gently blowing air back through the capillary while applying the silver solder. A sample control needle valve (Kuclear Products Corp., Type SS-411.4) and sample delivery tube were then attached. The burner O-ring makes a tight seal in the quartz tube which has an entrance with ail inside diameter of 0.730 inch. The maximum i.d. of the tube is 25 mm., and the over-all length is 54 em. The front combustion chamber is 23 cm. long and is followed by a furnaceheated, 'quartz bead-packed section 18 cm. long. The heated quartz beads
serve a dual purpose. They assure more complete combustion of refractory samples and provide a n automatic igniter in case the flame should be extinguished accidentally. The apparatus assembly is shown in Figure 2. I t includes a sample reservoir, the combustion section, a condenser, a n absorber, and an evaporator. The liquid sample reservoir-flow measurement section attaching to the burner was made from a 100-ml. borosilicate glass centrifuge tube, a 2-ml. pipet, and a stopcock, designed so that the exact sample combustion rate could be determined a t any time by merely closing the stopcock (KO. 4, Figure 2) and timing the fall of the liquid level for 0.10 ml. in the 2-ml. pipet tube. The large chamber of the absorber is 40 mm. id. and 160 mm. high above the coarse sintered plate. The evaporator is of the same dimensions with the addition of a dip tube 7 mm. i d . by 60 mm. long. The eraporator, used only when burning samples of more than 20 ml., provides for evaporation of the water from combustion as it is formed. After each 20 ml. of sample is burned, additional hydrogen peroxide absorbent is added through stopcocks 1 and 3 (Figure 2) during operation. When burning large samples, knockback of the hydrogen peroxide-rich absorbent rising into the evaporator is performed every 12 to 15 minutes vithout shutdown of the flame by momentarily by-passing the combustion gases from the absorber-evaporator system through stopcocks 1 and 2 and the by-pass tube. This permits the liquid in the evaporator to drain back into the absorber by gravity. The original gas flow is then restorcd. The quartz combustion tube is fitted nith a '/*-inch i.d. blow-out port and a manometer fitting. During operation the ground glass top surface of the blowout port is closed by a flat, 1-inch square piece of 14-gage stainless steel (nitric acid cleaned) held in place hy a thin coating of sulfur-free, heavy, high temperature silicone grease. (Hi Tern Vac made by Glass Engineering Laboratories, Belmont, Calif., is satisfactory). On several occasions this cover has been blown off by backfire with no damage to the equipment or operator. A pressure-actuated switch in the vacuum system operates a buzzer in case the house vacuum falls below 15 inches of mercury. This frees the operator from VOL. 34, NO. 8, JULY 1962
1019
Auxiliary Oxygen Tube Edrner eckrnan Burner
'-Flame Oxygen Tube Of Beckrnan Burner
3/4"O.D. x 9/16" I D .
3/32''Cross Section
Sample Supply Tube
- Defail "Z"-
----- D e ta
,060'' I D II
"Y"-
Burner Body, For Hydrogen Supply T u b e -Detail
'f-
S = Silver
Figure 1 .
continuously watching the apparatus, as most operational difficulties arise from a loss of vacuum. An air jet was necessary to cool the burner and entrance to the quartz combustion tube. This prevented overheating and decomposition of the neoprene O-ring and also provided smoother burning of the sample. The sulfur dioxide from combustion is absorbed in the conventional manner in hydrogen peroxide. A micro adaptation of the barium perchlorate titration method for sulfate of Fritz and Yamamura (4) was developed, was more reliable, and covered a wider range with greater sensitivity than nephelometric or turbidimetric sulfate methods.
Solder
Aspirator burner construction details
acid with a sulfate-free mixture of 80% 10s EXCHANGE COLUMN.Prepare a isopropanol, 20% water. 1.00 ml. = bed 7 inches by 15 mm. of Dowex 100 pg. of sulfur, or 300 pg. of sulfate. 50- x8 resin, 100 to 200 mesh. Full THORIN-METHYLEKE BLUE IKDICA- details of use are given by Fritz and TOR. Mix equal volumes of aqueous Yamamura (4). A SARGENT MALMSTADT MODELSE 0.2070 thorin (Hach Chemical Co., SPECTROPHOTOMETRIC TITRATOR faciliAmes, Iowa) and 0.02% methylene blue. This indicator gives sharper end tates titrations. Before use, all glass portions of the points than thorin alone. STANDARDBARIUM PERCHLORATEapparatus, but not the quartz tube, SOLUTIONSIN 80% ISOPROPANOL, should be thoroughly cleaned with hot 0.00551 and 0.002M. Prepare according nitric acid, doubly rinsed with det o the instructions of Fritz and Yamaionized m t e r , and given a final rinse mura (4) and standardize against known with ion-free (quartz-distilled) water. amounts of standard sulfate solution by Procedure. Determine the specific the sulfate titration method outlined gravity of liquid samples by any convenient method a t the temperabelow. For the 0.005M solution titrate ture of test. five sets of duplicates over the range of 20 to 500 pg. of sulfur in 15 ml. of COMBUSTIOS TECHNIQUE FOR LIQUID SAMPLES.Assemble the apparatus as total solution. For the 0.002M soluANALYTICAL METHOD shown in Figure 2. Adjust the powertion, use samples containing 20, 40, 80, stat to give a temperature of 975' to and 120 pg. of sulfur in a total volume Reagents. All reagents should be of 15 ml. Plot standard curves and 1000" C. in the quartz tube. Add 25 of the purest grades obtainable, ml. of 3% hydrogen peroxide absorbent check several points every 10 days. especially with respect to sulfur or to the absorber and replace the spray SODIUMHYDROXIDE, 0.03iV. Consulfate content. trap. If more than 20 ml. of sample is centrate 400 ml. by boiling to 30 ml., ION-FREE WATER. Distil deionized to be burned, insert the evaporator and determine any sulfate present by water in a quartz still and store in section between the absorber and the the ASTM (5) turbidimetric procedure tightly capped polyethylene bottles. spray trap, and use 6% hydrogen for sulfate. If any sulfate is found, SCAVENGER-RISSE SOLUTION. Preperoxide. Set the oxygen regulator a t corrections must be made for any sulfur pare a mixture of equal volumes of the cylinder a t 15 pounds pressure, and introduced by this reagent in the alkali acetone and isopropanol. the hydrogen regulator at the cylinder HYDROCHLORIC ACID,0.LV. Prepare titration following combustion. a t 4 pounds pressure. With the burner by dilution in a mixture of 80% isoApparatus. All combustion apin the open, adjust the panel micrometer propanol and 20% water. paratus glassware is available as valves to a hydrogen flow of 2 liters per STANDARDSULFATE SOLUTIOK, stock items from Greiner Glassblowminute and an auxiliary oxygen flow of ing Laboratory, 3604 E. Medford 0.00624N sulfuric acid. Prepare by 3 liters per minute. Ignite the burner St., Los Angeles 63, Calif. dilution of standard 0.100N sulfuric
1020
0
ANALYTICAL CHEMISTRY
Cambustior: ;as B y p a s s Tube
~
IIOV,-A.C.
Absorber
-Mercury Manometer
Vacuum Alarm System
V-l To V-5: Micrometer Needle Valves W i F l a m e Arrestors
Figure 2.
and plug a cork equipped with a 5-inch by 1-mm. capillary tube into the burner entrance of the quartz tube. Adjust the vacuum with the panel micrometer valve to 40 mm. on the mercury manometer. Remove the cork and capillary orifice from the quartz tube. As soon as the burner flame is steady, plug the burner into the quartz tube inlet. The vacuum should now increase to 100 to 140 mm. on the manometer. Open the flame oxygen to 2.5 liters per minute flow rate. Open the cooling air jet which is directed a t the burner and inlet to the quartz tube. Readjust all flow rates so that the vacuum manometer will read 60 mm. and the hydrogen 2, auxiliary oxygen 3, and flame oxygen 2.5 t o 3 liters per minute. Add the sample to the large sample reservoir (stopcock closed), using a volumetric pipet. The following are recommended sample sizes: Sulfur Content, p.p.m.
Sample Size, ml.
Lese than 1 . 0 1 to 10 Over 10
50 10 t o 20
10
As soon as the front plug section of the burner has been cooled by the air jet to about 120' F., estimated by touching it, connect the sample reservoir-flow rate tube assembly to the ground-glass joint on the burner sample tube. Open the stopcock on the sample reservoir assembly and allow the flow rate tube to fill. Close the stopcock t o the sample reservoir. Slightly open the sample micrometer valve on the burner. The sample will start burning at a very low rate, as observed by fall
Trace sulfur appartus flow diagram
of the liquid level in the calibrated flow rate tube. Gradually increase the sample feed rate to 1 to 2 ml. per minute, maintaining the above specified vacuum and gas flow rates throughout. Open the stopcock to the sample reservoir. Readjust the auxiliary oxygen and vacuum if necessary, to maintain an excess of oxygen a t all times, as indicated by the following: The flame \vi11 be clean, steady, and blue with no yellow portions, except when burning aromatic materials. Any decrease in either flame or auxiliary oxygen will cause an increase in vacuum. If the evaporator tube is to be used, avoid thermal shock t o the system by allowing 5 to 10 ml. of liquid to accumulate in the tube before applying heat. When only a few milliliters of sample remain to be burned, increase the hydrogen flow rate to 3 liters per minute. Rinse the sample reservoir and the flow rate tube with 2- and 1-ml, portions of scavenger-rinse solution, respectively, as soon as the sample level falls below the reservoir stopcock. If all of the sample is not burned before scavenger rinse is added, the transition from one fuel to the other will be smooth with less chance of a flame out and backfire, which sometimes results from a too rapid surge upon startup with volatile materials. When all of the rinse has burned out, shut down the apparatus by closing the sample micrometer valve, removing the sample reservoir, and gradually decreasing the vacuum. When the vacuum approaches zero on the manometer, unplug the burner from the quartz tube and close both panel oxygen valves. Finally close the hydrogen valve.
CONBUSTION OF LIQUIFIEDPETROGASES. Follow ASTM procedure D-1266, Section 10 ( d ) , relative t o sample container and vaporizing the sample. Open the burner micrometer valve two full turns. Cse the same hydrogen, oxygen, and vacuum settings as used for liquid samples, and burn the sample in a flame with the inner blue cone about inch long. This provides complete combustion with a burning rate of about 1 gram per minute. Omit scavenger-rinse burning. ABSORBEXTTREATNEXT. Rinse the outside of the drain tube of the absorber with ion-free water and transfer the absorber contents to a 250-ml. iodine flask. Rinse the absorber and gas cooler section with ion-free water into the flask. Add two drops of Fleisher's methyl purple indicator to the solution and titrate to a faint green with 0.03X sodium hydroxide. Record the volumes used. If more than 2 nil. are required per 10 ml. sample burned, there is probably an air leak into the apparatus, forming nitrogen acids, or else the sulfur or halogen content is high. Add 1 ml. of excess 0.03N sodium hydroxide to the solution and reduce the volume to 2 t o 3 ml. by gently boiling on a hot plate in a sulfate-free environment. Caution: Do Not Boil Dry. Cool the solution t o room temperature. lIeasure the final solution volume in a IO-nil. graduate and transfer the solution to a 125-ml. Erlenmeyer flask. Rinse the boiling flask and graduate thoroughly with two 5-ml. portions of isopropanol and transfer the rinses to the titration flask. If the range of sulfur content of the sample is completely unknown, rinse the flask into a 10-ml. volumetric flask LEUM
VOL. 34 NO. 8, JULY 1962
1021
with ion-free water and adjust the volume to 10 ml. Titrate a 5-ml. aliquot M ith 0.005X barium perchlorate as described below A second titration may then be performed, either with 0.002M barium perchlorate on a 5-ml. aliquot when less than 50 ,ug. of sulfur are present, or on a 1.0- or 2.0-ml. aliquot with the 0.00531 reagent if the first titration reauired more than 4.0 ml. of titrant. Add 20 UE. of sulfur 10.20 ml. of standard s u h t e solution) and additional isopropanol to the flask to increase the concentration of the alcohol to 80% by volume before titration. Add three drops of thorin-methylene blue indicator for each 15 ml. of final solution. Adjust the orange-pink color by adding about 0.20 to 0.50 ml. of 0.1.V hydrochloric acid dropwise to the solution until the color changes to greenish-yellow. This is the typical pre-end point color at the correct pH, as thorin acts as a n acid-base indicator under these conditions. Titrate with either 0.005M or 0.002M barium perchlorate, using a fine tipped microburet dispensing 0.01-ml. drops. The end point is indicated when the greenishyellow color changes to a golden or pinkish yellow. Titrate all solutions to the point where blank and sample solutions have the same golden shade. Add 0.20 ml. of standard sulfate solution and again titrate to the end point. Record all volumes of reagents used and calculate the total net micrograms of sulfur present in the absorbent for each end point. Use the average value in calculations. Perform two blank determinations on the hydrogen and oxygen gases in a n identical manner, burning the oxyhydrogen burner for 20 and 40 minutes. Burn the 3-ml. scavenger-rinse solution in each case. Calculate the volume of barium perchlorate needed for a blank determination of the same burning time as the samples. This method is not intended for use on leaded gasolines, but if traces of metals such as lead or barium are present, the absorbent should be passed through the ion exchange column before titration, following evaporation to 2 to 3 ml. Rinse the flask and ion exchange column with three 5-ml. portions of
Table I.
Sulfur content, p.p.m.
=
AC - BC ~
w
where A = milliliters of barium perchlorate
B
=
C =
W
=
solution needed to titrate sample milliliters of barium perchlorate solution needed t o titrate blank micrograms of sulfur equivalent to 1 ml. of barium perchlorate solution grams of sample burned RESULTS
The accuracy of this method in a representative Ion- sulfur range (6 to 60 p.p.m.) was tested by analyzing synthetic blends containing thiophene or elemental sulfur. THIOPHENE BLEXDS. A stock solution of about O.lyosulfur content was prepared by dissolving thiophene (Eastman Kodak Co., b. p. 83" to 85" C.) in a desulfurized naphtha (0.5 p.p.m. sulfur). This blend was analyzed in quadruplicate for total sulfur by the lamp method ( 2 ) . The true sulfur content was then calculated assuming these results to be 4y0low, as found by an ASTM cooperative survey of the lamp sulfur method ( 1 ) . Blends of varying known sulfur contents were then made up by dilution of this blend with the desulfurized naphtha, or with benzene of known low sulfur content. ELEVENTAL SULFUR BLENDS. Elemental sulfur was purified by recrystallizing from toluene, air drying, then vacuum drying a t 120" F. -4 stock solution containing 593 p.p.m. sulfur was prepared by dissolving a weighed amount of this purified sulfur in toluene (reagent grade: 0.4 p.p.m. sulfur content). Other blends of elemental sulfur were then made by dilution of this stock solution with desulfurized naphtha
of 0.5 p.p.m. sulfur content. ilnalytical results obtained on these known blends are summarized in Table I. Pa4RTS PER BILLIONSULFUR BLENDS. A number of analyses were made on high purity benzene, burning 50-ml. samples. An average sulfur content of 63 (115) p.p.b. n a s found. 1 dilute thiophene solution n a s added to give a benzene blend containing 383 ( =t15) p,p.b, total sulfur. Similar blends were made nith high purity toluene. - h a lytical results obtained on t h v e blcnds are shown in Table 11. ADSORPTIONEXPERIMENTS. There v a s some question as to whether lowsulfur blends of polar sulfur compounds might lose sulfur by adsorption on the nalls of glass containers upon standing. To test this possibility, blends of butyl mercaptan in isooctane containing 7 and 170 p.p.m. S vr-ere made, analyzed for total sulfur, and poured into glass bottles which had been freshly cleaned with hot nitric acid, washed, and dried. These blends were then stored in a cold room. Analysis of these blends and of desorption hot isopropanol rinses of the emptied bottles shon-ed that no sulfur was adsorbed on the ~ ~ 1 of1 sthe glass bottles even after storage of 65 days a t 40" F. DISCUSSION
The speed of this method makes it ideal for routine sulfur analysis in the parts per million range. Total elapsed time for a single analysis on a 10-ml. sample is about 1 hour, with following analyses finishing up on a 20- to 25minute time cycle, permitting the analysis of 15 to 20 samples per man day, with one apparatus. The ultimate accuracy of this method is dependent upon the performance of the combustion-absorption step, the accuracy of the sulfate titration, and the accuracy of the sulfate standards. The accuracy of the sulfate titration is increased by titrating in a small volume, made possible by the evaporation step
Sulfur Content of Synthetic Blends in Parts per Million Range by Richfield Trace Sulfur Method
Material A.
ion-free water. Measure the final solution volume accurately and titrate a 5-ml. aliquot after adding 20 ml. of isopropanol and the indicator. (Omit the 0.1N hydrochloric acid.) Calculations :
Blend
Elemental Sulfur Blends: Elemental Sulfur 519 B Solutions in Desulfurized Naphtha 519 C
M1. Burned
Present
10
61.6"
20
6 .50a
Sulfur, P.P.R.I. Found
Recovery,
Std. Dev., P.P.M.
-4V.
70
59.4, 59.5, 59.7, 6 0 . 1 , 59.8 6 . 5 1 , 6.32
59.7 6.41
97 99
0.32
11.18, 11.32, 10.93 11,27, 11.07, 10.93
11.12
101
0.17
8.01
100
B. Thiophene Blends: 20 Thiophene Solution 517 B in Desulfurized Naphtha Thiophene Solution 615 A 20 in Benzene a Includes 0.49 p.p.m. sulfur in naphtha. * Includes 0.06 p.p.m. sulfur in benzene.
1022
ANALYTICAL CHEMISTRY
11.O"
8.0*
8 . 0 , 8 . 2 6 , 7.78
:ind eliminat,ion of the ion exchange with its necessary rinses. The total volume titrated can thus be kept down to 15 nil., making possible the use of 0.0023f barium perchlorate and replicate titrations with a standard deviation of 0.6 pg. of sulfur. The use of the methylene blue in the mixed thorin indicator helps the eye to distinguish slight changes in color at the end point. This internal color filter was essential !\-hen titrations were performed on the Sargent-Malmstadt automatic spectrometric titrator. ‘I’his unit gave about the same accuracy as the best visual results obtainable, iising 0.005.1l bariuni perchlorate, which was added a t a rate of 3 nil. per minute. standard deviat’ion of 0.6 pg. of sulfur might, be expected to give the accuracy tabulated beloiv-, provided the combustion absorption steps introduced 110 wrors. Standard deviations :tpproaching these values are obtained in the extreme low range. (Also see Table I1 ‘i
Grams Sample Burned 10 c50 100
Expected Standard Deviation 60 p.p.b. 12 p.p.b. 6 p.p.b.
Sulfur recovery on k n o m blends of a ?vide r a n g of sulfur content averaged about 987& which indicates the completeness of the combustion and absorp-
Table II.
Analyses of Synthetic Aromatic Blends Containing Sulfur in Parts per Billion Range by Richfield Trace Sulfur Method
Materiala Benzene Blend 75 -4 (Benzene & Thiophene) Blend 711 A Toluene (Toluene & Thiophene) a
b c
Sulfur, P.P.B. Present Found 69, 80, 58, 46 383b 391, 385, 382, 488, 330 790c
530, 320, 540, 620 830, 820, 780, 770, 810, 760
Recovery, AV.
%
63 395
103
470 780
Std. Dev., P.P.B. 15 57 134
98
30
Burned 50-mi. samples. Includes 63 p.p.b. sulfur in benzene. Includes 470 p.p.b. sulfur in toluene.
tion. This is much better than the 85 to 95% recovery we had obtained with the Wickbold apparatus. No loss of sulfur occurred during the evaporation step as long as the solution was slightly alkaline and was not boiled dry with resultant splattering. ACKNOWLEDGMENT
The authors acknodedge the help of
J. C. Marantette, J. A. Sandefur, and K. A. Heath in developing the appatatus, and the useful criticism of G. R. Meador in preparation of this paper. They also thank the management of Richfield Oil Corp. for permission to publish this paper.
LITERATURE CITED
(1) Am. Soc. Testing Materials “Standards for Petroleum Products and Lubricants,” p. 1337, Sovember 1949. ( 2 ) Ibad., D-1266, “Procedure for C o y ; bustion of Liquified Petroleum Gases, p. 655, October 1960. (3) Ibid., Appendix I, “Method of Test for Trace Quantities of Sulfur,” p. 658, October 1960. (4) Fritz, J. S., Yamamura, S. S., A4N.4L. CHEM.27, 1461 (1955). (5) Granatelli, L., Ibzd., 27, 266 (1955). (6) Houghton, S . IT’., Ibzd., 29, 1513 (Oct. 1957). (7) Hudy, J. A., ITair, R. D., I b a d , 27, 802 (1955). (8) Wickbold, R., Angew. Chem. 69, 530 (1957). RECEIVED for review January 29, 1962. Accepted May 15, 1962.
A N e w Type of Rotating Disk Electrode SIR: The theoretical treatment of mass tr:tnsfer in the rotating disk system ,specifies a lamina, of infinitesimal thickness and of infinite diameter, rotating with constant angular velocity in a fluid of infinite volume (IO). This .~oiiiinuiiic:tt.ioiiis concerned with the practical realization of the first two requirement’s: tlic second two are easily satisfied (6). Si1-c.r :tiid I
