Table 111. Comparison of Material-Balance Values of Free Water Fed in a Test Rig With Values Determined by IITRI Pad Method (Temperature: 75' F; Fuel: JP-4) Free-water determination Free-water IITRI pad method content, Time of feed rate, mg/liter run, min mg/liter 30 30 44 36
12.0 9.0 5.5 3.5
12.3 9.5 5.1 3.6
weight pads since one uses a difference reading on the same pad. Also the condition of the unused pad is satisfactory if its moisture content is equal to or greater than the equilibrium saturation value of the water-saturated fuel. In this work, the pad constants for JP-4 and aviation gasoline were respectfully 1.8 and 2.1 mg of water per Hart unit. A study of the variation in the pad constant in different types of fuel was not made. However, most fuels probably fall within this range. The final evaluation of the Hart pad or IITRI pad method showed that it can be used to determine within 0.5 ppm the actual amount of free water present in a fuel system. This is evident in Table 111. CONCLUSION
In this presentation it is evident that there are two methods for the free-water determination. One is the Karl Fischer pad determination. Because it is a destructive test, it is necessary to use a group of constant weight pads which have been preconditioned to a known moisture condition. The other test is the Hart pad method which must first be calibrated with the Karl Fischer pad determination. It is a nondestructive method which is based on data shown in Figure 1, Hence one may evaluate the pad before and after sampling and obtain the amount of free water in the sample by multiplying the difference of the two readings by the slope of the line in Figure 1. In this method it is not necessary to use constant
A simple, accurate and reliable method suitable for determining free water in fuel has been developed. The accuracy of the method is not affected by a lack of homogeneity or the amount of dissolved water in the fuel, and all the main reasons for the inaccuracies in the Karl Fischer method are eliminated. One disadvantage of the method is that it cannot be used for fuel systems containing salt water as free water. All the additives normally found in fuel, however, do not affect the method. RECEIVED for review January 22, 1968. Accepted February 21, 1968.
Determination of Trace Sulfur in Hydrocarbons by Pyrolysis and Hydrogenation L. L. Farley and R. A. Winkler Chevron Research Co., Richmond, Calif. 94802 A rapid accurate method for the determination of sulfur in the fractional parts per million range by means of pyrolysis and hydrogenation is described. A 2-ml sample is vaporized in a hydrogen stream, pyrolyzed in a hot zone, and passed over a platinum catalyst at 1 2 0 0 O C. The sulfur is converted to H,S, absorbed in a dilute zinc acetate solution, and reacted with N,N-dimethylphenylenediamine to form methylene blue. The spectrophotometric finish covers the range from 0.2 to 5.0 ppm with an accuracy of *lo% of the amount present. The catalyst has an exceptionally long life for the relatively large samples that are used. An analysis requires about 1 hour.
IN A SEARCH for a technique suitable for the determination of fractional part per million sulfur contents of petroleum liquids, no existing method was found which was entirely adequate. The procedures which approached the required sensitivity and precision were those suggested by Schluter ( I ) and Kat0 (2) and their coworkers. Schluter pyrolyzed a 0.2-OS-gram sample in a hydrogen stream and reduced the sulfur over a nickel catalyst at 1200" C. The colorimetric finish used gave a sensitivity of 5 ppm. Kat0 used the same (1) E. P. Schluter, Jr., E. P. Parry, and George Matsuyama, ANAL.CHEM.,32,413 (1960). (2) Motohiko Kato, Iwao Fujishima, and Tsugio Takeuchi, Bunseki Kuguku, 11,178 (1962) [C.A., 57, 155% (1962)l.
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procedure in a closed system to reduce atmospheric contamination and obtained a sensitivity of 1 ppm. Of the other approaches investigated, none were considered able to measure sulfur in the fractional ppm range. X-ray fluorescence (3) can be extended to the 1- to 5-ppm range, but matrix effects may cause variations. The Wickbold (4) oxyhydrogen burner operates well also in this range but becomes lengthy and loses sensitivity rapidly below 2 ppm. The Raney nickel method (5) is sensitive to 0.1 ppm, but olefins interfere. Microcoulometry has been used (6-8) to determine small amounts of H2S but was not used for this work in an effort to keep the instrumentation as simple as possible. The procedure discussed in this work extends the range to at least 0.2 ppm and is designed to operate as high as 5 ppm with an accuracy of + l o % of the sulfur present throughout this entire range. Appropriate changes in sample size can expand both ends of the sensitivity range. A platinum-on-quartz catalyst was developed and was shown to have a longer life than the previously used nickel catalyst. The injection system T. C . Yao and F. W. Porsche, ANAL.CHEM., 31, 2010 (1959). R. Wickbold, Angew. Chem., 64, 133 (1952). Lawrence Granatelli, ANAL.CHEM.,31, 434 (1959). V. T. Brand and D. A. Keyworth, Ibid.,37,1424 (1965). (7) R. L. Martin and J. A. Grant, Zbid.,37,644 (1965). (8) D. F. Adams, G. A. Jensen, J. P. Steadman, R. K. Koppe, and T. J. Robertson, Zbid.,38, 1094 (1966).
(3) (4) (5) (6)
1
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Figure 1. Apparatus assembly tube is wrapped with insulated resistance wire to give a temperature of at least 350' C in the evaporation zone. The quartz boat is 35 mm long, 12 mm wide, and 8 mm high; the bottom is lined with a piece of platinum gauze. A IO-mm lip is constructed at one end through which two 1.5mm holes are sand drilled. To this lip, and to similar holes drilled in a lip fastened to a quartz-enclosed '/(-inch iron bar, a stiff platinum wire is fastened. The wire should be long enough so that the boat is next to the catalyst when the assembly is moved into the pyrolysis zone with an external magnet. The sample is fed into the evaporation zone by a system which is made up of a 2-ml capacity hypodermic syringe, a 20-gauge 21-inch long needle, and a syringe drive apparatus capable of adding the sample at a rate of 2 m1/30 min.
of this procedure easily accommodates 2-ml samples which permits the analytical range to be extended downward to 0.2 PPm. EXPERIMENTAL
Apparatus. The apparatus used for the hydrogenation of the sample and the absorption of the hydrogen sulfide is shown in detail in Figure 1, and Figure 2 gives the dimensions of the combustion tube. The furnace must be capable of maintaining a temperature of 1200-1240" C . The combustion tube is made of quartz, and the section in the furnace is surrounded by a translucent quartz protector tube which retards the devitrification of the combustion tube. The forward section of the combustion
BRASS W I R E TYGON T U B I N G
2 5 mm
2 mm C A P I L L A R Y T U B I N G BOROSILICATE GLASS
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WATER O U T L E T , FACED TOWARD REAR
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r Y) N
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QUARTZ WATER J A C K E T SCRUBBER TUBE
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QUJRTZ TUBE 26 mm I.D. i
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Figure 2. Combustion tube detail VOL 40, NO. 6, M A Y 1968
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Reagents. PLATINUM CATALYST.The catalyst is prepared from 8-14 mesh quartz either by grinding this material or by purchasing it from a commercial supplier. The sieved quartz is washed with aqua regia to remove traces of metals and then rinsed with distilled water to remove the acid. A solution containing 19.9 grams of H2PtC16.6H20 in 15 ml of distilled water is poured onto 142 grams of the quartz particles in an evaporating dish. The particles are stirred with a glass rod until they are uniformly coated. The dish is placed on a steam plate and, with the aid of an infrared lamp, the catalyst is dehydrated. Occasional gentle stirring of the quartz aids the dehydration and assures a uniform coating. The dried catalyst is then positioned in a combustion tube (Figure 2), the tube is inserted into the furnace, and the apparatus assembled. The cold furnace is then brought to a temperature of 1200" C with a hydrogen flow of 1200 ml/min through the combustion tube. The catalyst is usually ready for use after reduction with hydrogen for 30 min at 1200" C. ISOOCTANE. This material is purified by passing 99 mole isooctane through a silica gel column. The blank on 2 ml of the purified material should be within 10.002 0.d. (5-cm cell) of the apparatus blank when pyrolyzed in the same manner as a sample. PHENYL DISULFIDE (CsH& A solution of phenyl disulfide is prepared in purified isooctane so that 2 ml contains 5 pg of sulfur. Two milliliters of this solution are analyzed to establish satisfactory catalyst activity. COLORIMETRIC DETERMINATION. The color development procedure as described by Gustafsson (9) was followed. The color was developed in 50 ml of solution and measured in 5-cm cells. A calibration curve covering the range of 0-10 pg of sulfur was prepared from dilute sodium sulfide solutions per Gustafsson. Dilute solutions of phenyl disulfide in isooctane were used also to construct a calibration curve and gave one which had the same molar absorptivity. This is the preferred method because instrument variables are taken into account. The isooctane solutions were used also as standards; satisfactory operation of the apparatus was assumed if the determined value was within *lo% of the known concentration. Procedure. A 2-ml syringe is filled with sample; the needle is attached and placed in the syringe drive assembly. A separate 2 ml of the sample is weighed to give the sample weight. Five milliliters of zinc acetate solution are pipetted into a 50-ml mixing cylinder which is placed on the apparatus as an absorber and surrounded with an ice water bath. The nitrogen is swept from the combustion tube with hydrogen, the syringe needle is inserted into the sample port, and the sample is added to the sample boat. When all of the sample has been delivered, the sample boat is moved slowly into the furnace with the aid of an external magnet. The boat is allowed to remain in the 1200" C zone for 10 min. Nitrogen is substituted for the hydrogen flow. The combustion tube is swept for 2 min at a flow rate of 900 ml/min. The absorber is removed from the apparatus, while the delivery tube is washed down with a thin stream of demineralized water. The color is developed as noted above. The volume is brought to 35 ml with demineralized water and the contents of the cylinder are mixed. Five milliliters of the N,Ndimethyl-p-phenylenediamine dihydrochloride reagent is added, and the contents again are mixed. One milliliter of the ferric ammonium sulfate solution is added, the cylinder stoppered, and the contents shaken vigorously for 30 sec. The volume is brought to 50 ml with demineralized water, the cylinder stoppered, and the contents shaken vigorously for 30 sec. After 25 min, the absorbance is read at 667 mp in a 5-cm cell. Because rapid removal of the sample boat from the hot zone has resulted in severe devitrification, it should be removed
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(9) Lilly Gustafsson, Talanta, 4, 227 (1960).
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slowly with the aid of the magnet and finally positioned under the sample port. A blank determination of 2 ml of purified isooctane is made before each day's operation. If this is followed by the phenyl disulfide in isooctane standard and satisfactory results are obtained, the apparatus is operating adequately. RESULTS
The accuracy of the procedure was determined on known concentrations of phenyl disulfide in isooctane. Table I shows the results obtained. The procedure has been applied chiefly to petroleum stocks with a boiling range maximum of 700" F. A summary of typical results is shown in Table 11. A comparison is made with the Wickbold (4) procedure in which a large (50-100 gram) sample is burned and the sulfate in the absorber solution titrated with barium perchlorate. The pyrolysis hydrogenation procedure is faster and has greater precision than the Wickbold technique in the low parts per million range. DISCUSSION
Pyrolysis. It was found that to obtain complete and reproducible recovery of the sulfur compounds as hydrogen sulfide, very little deposition of uncombusted hydrocarbon could be allowed in the combustion tube. The variables controlling this deposition are the hydrogen flow rate, the degree of humidification, and the size of the combustion tube. Extreme cases of such deposition can cause plugging of the combustion tube with no visible flow of gas in the absorber, Hydrogen flow rates varying from 150-1200 ml/min were used to hydrogenate isooctane solutions of phenyl disulfide over a 30-minute period at a catalyst temperature of 1200" C. The lower hydrogen flow rates resulted in severe deposition in the catalyst zone, while the higher flow rate of 1200 ml/min showed no deposition in the catalyst zone but included downstream deposition of carbon and hydrocarbons on the cool part of the reaction tube. The downstream deposits contained sulfur. These deposits did not occur when the correct humidification of the hydrogen stream was made. The hydrogen stream is bubbled through water to humidify it. The water is used in this system to aid carbon removal and to minimize its deposition on the quartz surface by the "watergas" reaction.
Deposits were found in the hydrogenations with a 1200 ml/min hydrogen flow at humidifier water temperatures of 25 " C and 50' C. A boiling water reservoir removed the deposits, but water condensed in excessive amounts in the sample injection zone. An effective compromise was a 90" C water reservoir temperature. Schluter (2), using a nickel catalyst, found that 80" C f 20" C humidification temperatures were necessary. The nickel-on-quartz catalyst as used by Schluter (2) proved to have a very short life in our apparatus. Twelve successful runs were made, but after this a gradual decline in sulfur recovery was experienced until no conversion was evident in the 19th run. This short life is attributed to the much larger sample size that we use. Thus, much more carbon monoxide is produced which reacts with the catalyst to form volatile nickel compounds. This volatility was evidenced by a dark deposit on the combustion tube downstream from the catalyst after the lack of conversion was experienced. The deposit showed a high nickel content, and
the nickel content of the catalyst had dropped from 2.0 to 0.6%. Schluter (2) reported the use of platinum as a catalyst but gave no details. We made preparations of 0.6, 1.0, and 5.0% platinum-on-quartz granules. The 5 preparation was selected over the lower percentages because of longer catalyst life. With it several hundred runs have been made, and the chief reason for its loss of activity is that the quartz base devitrifies allowing the platinum particles to fall to the bottom of the combustion tube where adequate contact is not possible. Theoretically, a solid platinum catalyst with adequate surface area should last indefinitely on the type of samples which are being analyzed. A high Hz-to-sample gas volume ratio (1.8 liters H2/gram sample) is necessary to complete successful hydrogenations. A quartz combustion tube of 24-mm 0.d. by 125 cm in length is satisfactory both from the standpoint of allowing proper ratio of hydrogen-to-sample gas volume and being long enough so that the sample injection port is not too close to the furnace. The distance of the sample injection port to the hot zone of the furnace is important because it is necessary to inject and volatilize the sample slowly and evenly for complete pyrolysis. If the sample port is too close to the hot zone, the radiant heat from the furnace causes pulsing flow rather than even flow. Nitrogen is used to flush the combustion tube before the hydrogen is admitted, as a standby gas for overnight operation, and as a safety precaution to prevent mixtures of hydrogen and air which could cause an explosion. To flush the hydrogen from the tube after a pyrolysis, a flow rate of 900 ml/min is used for a period of at least 2 min. This gives six sweep volumes of nitrogen. Methylene Blue Synthesis. The hydrogen sulfide content of the absorbing solution is determined by reacting it with N,N-dimethylphenylenediamine and ferric ion as described by Gustafsson (9). In this reaction, methylene blue is synthesized. This is one of the more sensitive means of determining sulfide ion and has been used for some years. It has been well studied in the literature, and it appears to be a totally reproducible technique. One microgram of sulfur in 50 ml of solution can be easily determined. However, one of the disadvantages of this reaction is that it is not stoichiometric. The degree of nonstoichiometry is difficult to determine because of the lack of definition of the true molar absorptivity of methylene blue. Gustafsson (9) reports a figure of 66.7 2 0.5 as the amount of sulfide which is converted to methylene blue. However, the reported molar absorptivity is 6.3 X l o 4liters mole-’ cm-’, based on aqueous solutions of methylene blue. Bergmann and O’Konski (10) list molar absorptivities, as reported in the literature, ranging from 3.9 X l o 4to 9.5 X lo4. The 9.5 value was obtained by
The procedure described above is sensitive enough to determine 0.2 ppm of sulfur. It has been used on a wide variety of petroleum stocks with end points of less than 350” C. Heavier stocks and organic and inorganic solids have been analyzed also down to the 2-ppm level. The analysis of these latter materials is not as sensitive because of the smaller sample that must be used. If future demands require it, the sensitivity of the procedure for hydrocarbons can be improved easily by using larger samples and maintaining the 1 m1/15 min feed rate. Also, the cell path length can be increased from 5 to 10 cm for increased sensitivity.
(10) K. Bergmann and C . T. O’Konski, J. Plzys. Chem., 67, 2169 (1363).
RECEIVED for review November 17, 1967. Accepted January 25, 1968.
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Table I. Analysis of Solutions of Known Sulfur Content Number Sulfur, ppm of Standard runs Added Determined deviation 9 12
0.50 2.50
0.49 2.41
+0.05 b0.23
Table 11. Analysis of Petroleum Hydrocarbons Sulfur. oom Wickbold Sample proposed Procedure procedure 1 2 3 4 5 6 7 8 9 10 11
