explicit interferences is expected to be quite high; and second, the bell-shaped coincidence peak would no longer have circular symmetry about the intensity axis. Projections of it on each energylintensity plane would approximate to two normal distributions with standard deviations in the ratio of about ten to one. This asymmetry was thought t o render initial experiments more difficult to interpret since the experiment arrangement would consist of Ge(Li) and sodium iodide detectors without an anticoincidence shield. CONCLUSIONS
The tabulations indicate that Ge(Li) detectors have sufficient resolution to enable most coincidence peaks from many radioisotopes t o be identified without interference from other coincidence peaks. A similar test for fission products has not been made, though the input data is available. N o significant alterations are expected if longer lived isotopes are considered, since in many cases the last isotope in a mass chain can also be produced by an (n, 7 ) reaction on a stable isotope. With the increasing use of Ge(Li) detectors, it is becoming apparent that some revisions are required in the accepted decay schemes. Usually this implies a change of perhaps a few keV in known energy values and perhaps the alteration of accepted intensities
for transitions by a few per cent and the discovery of weak lines not previously observed. However, it is expected that most of the features of present decay schemes will remain and t o that extent the overall picture presented by the computer search is good. It is likely that slight changes in the data will show some interferences from the tests are less interfering, while other new interferences will appear. Other activation modes are also available besides neutron activation, and the input data required for similar tests on these production modes for all isotopes up to bismuth have been prepared. The total number of coincident pairs for each of these modes is comparable with the total in the present test and qualitatively a similar result may be expected from a uniqueness test. ACKNOWLEDGMENT
The authors appreciate the assistance of Mary Caste1 and Alexander Zahradnitsky, summer students at this Institute, who prepared the lists of coincident pairs from the Nuclear Data sheets. RECEIVED for review April 22, 1968. Resubmitted August 20, 1969. Accepted January 7, 1970. Bedford Institute contribution number 115.
Comparison of Oxidative and Reductive Methods for the Microcoulometric Determinations of Sulfur in Hydrocarbons L. D. Wallace’ and D. W. Kohlenberger2 A R C 0 Chemical Co., Anaheim, Calif.
R. J. Joyce, R. T. Moore, M . E. Riddle, and J. A. McNulty Dohrmann Instruments Co., Mountain View, CaliJ
Two microcoulometric methods for the rapid determination of total sulfur in hydrocarbons are compared. In the oxidative method sulfur as SOz is coulometrically titrated with iodine. In the reductive method sulfur as H2S is coulometrically titrated with Ag+. For either method, duplicate determinations require only ten minutes. Precision i s in the order of ~ k 0 . 2ppm or +3% whichever is greater. The oxidative method is simpler to use and is low in nitrogen interference. However, it suffers from chlorine and heavy metals interference and nonstoichiometric conversion of sulfur to SO,. The reductive method is free from chlorine and heavy metal interference, is stoichiometric for most sulfur types, but suffers from nitrogen interference. The availability of both methods will allow determination of total sulfur in nearly every type of hydrocarbon sample encountered in the analytical laboratory. THERECENT emphasis upon low sulfur and sulfur-free products to minimize corrosion and pollution, as well as the need for low sulfur feeds to refining processes using sensitive catalysts, has spurred the investigation of total sulfur measurements in Present address, California State College at Fullerton, 800 N. State College Blvd., Fullerton, Calif. Present address, Hunt Foods Inc., 1645 W. Valencia, Fullerton, Calif.
the 0-1000 ppm range. The most comprehensive method t o date has been the application of the oxy-hydrogen Wickbold burner ( 1 ) followed by one of two finishes: a colorimetric finish using thorin-methylene blue, or a turbidimetric finish using a photometer t o measure the suspended barium sulfate precipitate. With very careful technique, a precision of 0.3 ppm can be achieved using these methods (2), but the analysis time increases t o about two hours in the low ppm range due t o the 50-100 ml of sample t o be combusted. The X-ray technique (3) gives rapid and useful analysis down t o approximately 50 ppm, but the technique is sample matrix dependent and must be calibrated for each particular sample type. For the low ppm range, it is unsuitable. The microcoulometric method for the detection of sulfur compounds in gas chromatograph effluent was developed by Coulson, et al. (4). Since then, numerous examples of the application of this combined technique have appeared in the (1) R. Wickbold, Angew. Chem., 60, 530 (1957). (2) R. N. Wheatley and R. J. Burger, Hydrocurbon Process. Petrol. ReJiner 47 (lo), 133 (1968). (3) ASTM 2622-67, “Sulfur in Petroleum Products (X-Ray Spectrographic Method),” ASTM Standards, 17, 1019 (1968). (4) D. M. Coulson, L. A. Cavanaugh, J. E. DeVries, and B. Walter, J. Agr. Food Chem., 8, 399 (1960).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
387
-
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200 cc/mkl
REMOYPgLE EXIT TUBE
H 2 300 cc/min
REWCTIVE TUBE
Figure 2. Comparison of pyrolysis tubes REDUCTIVE SULFVI SYSTEM
Figure 1. A comparison of the oxidative and reductive systems for the microcoulometric determination of sulfur literature (5-9). Recently, Drushel (IO) described an improved oxidative pyrolysis tube and experimental conditions for the determination of total sulfur, nitrogen, and chlorine in petroleum by microcoulometry. Farley and Winkler (11) introduced a one-hour reductive method which covers the 0.2 to 5.0 ppm sulfur range. Their method utilizes a 10% platinum on quartzcatalyst at 1200 " C in a hydrogen stream followed by a spectrophotometric finish. The method suffers from catalyst decomposition at the high temperature. In the present paper, the oxidative microcoulometric method for sulfur was extensively investigated using a pyrolysis tube similar to that described by Drushel. Furthermore, a reductive microcoulometric method using a catalyst similar t o that described by Farley and Winkler was developed and investigated. Finally, the relative merits of each of these methods were compared experimentally. The oxidative and the reductive methods that are compared here are suitable for the 0.2 ppm to 1000 ppm range with an analysis time of typically ten minutes per duplicate analysis. Where dilution is possible, higher sulfur concentrations can be readily handled by reducing the sulfur to approximately 50 PPm. I n the case of samples containing heavy residues that would plug the injection syringe, a special technique has been developed which involves room temperature injection into a quartz boat. The boat is subsequently introduced into an 800 " C inlet zone (12). (5) E. M. Fredericks and G. A. Harlow, ANAL.CHEM.36, 263 (1964). (6) R. L. Martin and J. A. Grant, ibid., 37,644 (1965). (7) Ibid., p 649. (8) H. V. Drushel and A. L. Sommers, ibid., 39, 1819 (1967). (9) H. V. Drushel, ibid., 41, 569 (1969). (10) H. V. Drushel, presented the 158th American Chemical Society Meeting, New York, September 7-12, 1969. (1 1) L. L. Farley and R. A. Winkler, ANAL.CHEM.,40,962 (1968). (12) R. T. Moore, R. J. Joyce, and M. E. Riddle, presented at the
Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 8, 1969. 388
In both of these methods the sample is pyrolyzed t o convert the sulfur to a form that can be automatically titrated in a microcoulometric cell which responds only t o the pyrolyzed sulfur and is insensitive to most other pyrolysis products. I n the oxidative method, the sample is combusted in oxygen in an open quartz tube a t 800 "C with a highly efficient burner tip. The sulfur is converted to SOL The SOz is titrated with internally generated iodine in a null-balance microcoulometric cell (Dohrmann Instruments Co., Mountain View, Calif.). I n the reductive method the sample is pyrolyzed in hydrogen at 1150 "C over a 10% platinum on Alundum catalyst. The H,S formed in the pyrolysis is automatically titrated with Ag+ ions in a microcoulometric cell. The reductive method suffers from nitrogen interference in proportion t o the H C N formed in the pyrolysis (13) (approximately 1 % of the nitrogen is converted t o HCN). In the case of the oxidative method, chlorine concentrations above 1000 ppm begin to interfere negatively. Thus, for high chlorine content the reductive method should be used, while for high nitrogen content the oxidative method is superior. The availability of both methods is useful where a wide variety of sample types will be encountered. In both methods the system components are easily regenerated if contaminated, and in particular the platinum o n Alundum catalyst does not suffer from high temperature decomposition, as has been previously reported (11). The oxidative system is simpler to operate, and when the concentration of heavy metals o r chlorine is not excessive, this method is recommended. The strength of the reductive method lies in its nearly stoichiometric conversion of sulfur t o H2S independent of sulfur type or sample matrix and the apparent noninterference from heavy metals or high chlorine content. However, nitrogen does interfere as HCN. EXPERIMENTAL
Description of the Apparatus. A block diagram of the two systems is shown in Figure 1. I n both methods the sample is introduced into the hot (550 "C) inlet, volatilized, and swept by the carrier gas into the pyrolysis zone. I n the reductive (13) P. W. Sherwood, "Synthesis of HCN by Autothermic Reaction," Refining Engineering, Sec. C. 22, Feb. (1969).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
I
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e4+4+@ - 1
COULOMETER
10% NH3 IN N2 LeMe 1
j
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/ I
GENERATOR ANODE
i T 300 PI 'IOMNE CELL
RDJCTIM CELL ( T 400 SI ' U Y D I CELL'
Figure 3.
Comparison of titration cells
Oxidative cell (T-300-P) Iodine cell Electrolyte: 0.05 potassium iodide 0.50% acetic acid 0.06 sodium azide Sensor: Pt' Reference: Pt '-Triiodide Saturated Anode: PtCathode: Pt' Bias Voltage: 160 mV Titration : 21" SO, H20 + SO3 212HGeneration: 21- -,21" 2e-
+ +
+ +
+
Reductive cell (T-400-S) Silver cell Electrolyte: 0.3M ammonium hydroxide 0.1M sodium acetate Sensor: Ago Reference: HgC/HgOSaturated Anode: Ago Cathode: Pt" Bias Voltage: 110 mV Titration: 2AgC S2- -+ Ag2S
+
Generation:
+
Ago -,Agf eAuxiliary gas flow: 10% "3 in N2 at 40 cc/min
method a single hydrogen stream acts as both carrier and reactant gas, while in the oxidative method the sample is swept by an inert argon carrier into the pyrolysis zone where it is combusted by an independent reactant stream (oxygen). Another difference is that a catalyst (10% platinum on Alundum) is used in the reductive method while a n open quartz tube with an efficient burner tip is used for the oxidative combustion. The difference in pyrolysis temperatures (1150 "C cs. 800 "C) results in shortened pyrolysis tube life for the reductive system compared to an indefinite life for the oxidative tube. The life of the reductive tube is extended to three months or more by lowering the pyrolysis zone temperature to 900 "C when not in use. Both methods utilize removable exit tubes in the outlet zones of the pyrolysis furnace. These tubes can be removed and cleaned when inadvertently contaminated by deposits of coke, metals, or inorganic residues which will cause reduced recoveries. Both methods utilize heater tapes on the inlet capillary of the titration cell to prevent moisture condensation from reducing recoveries and increasing tailing of peaks. Both systems include a cell-coulometer-recorder combination whose principle of operation is based on null-balance coulometry. While the coulometer-recorder portion is identical in the two systems, the cells are quite different both chemically and physically. The system can be further automated by the addition of a n electronic digital integrator-printer combination for automatic printout of peak area. In the event several systems are run simultaneously, the output of the digital integrator can be fed directly to a computer for storage and data handling (14). The construction and composition of the quartz pyrolysis tubes are compared in Figure 2 . The reductive pyrolysis tube has only a single gas flow (hydrogen, at 200 cc/min) (14) Humble Oil Company, Baytowrz Briefs, 16 (Nov.), 2 (1968).
Figure 4. Schematic : T-400-S titration cell and coulometer
and utilizes a 10% platinum on Alundum catalyst a t 1150 "C. The oxidative tube utilizes a separate carrier gas (argon, a t 160 cc/min) and reactant gas (oxygen, at 40 cc/min) combined with a highly efficient cross-hatched burner tip to provide maximum oxygen to sample ratio a t the combustion point. In the oxidative method, the sample matrix burns rather brightly a t this burner tip at temperatures generally well in excess of the 800 "C temperature of the pyrolysis zone. It is believed that most of the conversion of the sulfur in the sample to SO2 occurs here. The removable exit tubes described above are shown in place. The geometry of the two titration cells and the difference in the composition of the electrolytes and reference electrodes is compared in Figure 3. The reference electrodes are chosen for stability in the electrolyte media. The electrolyte composition is chosen for proper conductivity, the efficient scrubbing of H2S or Son, and the proper chemical concentrations necessary to ensure regeneration of the depleted titrant ion. Sodium azide is used in the oxidative cell to suppress nitrogen interference while ammonium hydroxide is used in the reductive cell to suppress chlorine interference. To illustrate the operation of the cell-coulometer-recorder portion of the system, a schematic diagram of the interconnections between the reductive titration cell and the microcoulometer is shown in Figure 4. Here the coulometer detects the difference between the preset bias voltage and the voltage output from the sensor-reference electrode pair in the cell. This sensor-reference voltage is a direct function of the concentration of the titrant ion in the cell (Ag+ for the reductive method, I " for the oxidative method) and the coulometer functions to correct any change of this titrant ion concentration by generating or removing titrant ions in the generator loop until the titrant ion returns to such a concentration (the null-balance concentration) where the bias voltage exactly balances the sensor-reference voltage. Hence, this is nullbalance coulometry. Operation of the Oxidative System. A 5- to 10-pl sample is introduced into the 550 "C inlet section of the pyrolysis tube (Figure 1) with a syringe a t a maximum rate of 0.2 p1 per second, volatilized in the inert argon flow of 160 cc/min and swept on to the 800 "C combustion zone where it mixes with an oxygen flow of 40 cc/min at the burner tip (Figure 2). Here the sample burns with a faint to bright glow, depending o n the sample matrix and the sample constituents being converted into the combustion products shown in Table I. Those products are swept into the titration cell where the SO? reacts with iodine. Any SO1 formed in the pyrolysis is not titrated, and accounts for the nonstoichiometric nature of the method. The iodine consumed in the titration is replaced by the generator anode in the cell electrolyte. Two electrons are required for each sulfur atom titrated. Thus, it is merely necessary to measure the total charge (total number of electrons) that flows in the generator circuit. This charge is ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
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389
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Chemistry of the reductive sulfur cell
Solubility product Precipitation concentration Reaction constant at Ag+ = lo-'* 2Ag+ S + AgSS 10-50 S2- = 10-26 Ag+ CN- 4 AgCN 10-21 CN- = 10-9 Ag+ C1- -+ AgCl 10-10 c1- = 100 Ag+ 2NH3 -+ Ag(NH3)2+ 10-7 Ag+/Ag(NH3)~+ = 10-6 GNH3 = 0.3
+ + + +
Table I. Comparison of Pyrolysis Products Sample Pyrolysis __constituents Oxidative Reductive X (Halogens) Xz, XO, HX HX N NO, NOS "3, (HCN) S SO?, so3 HsS P PzOe, P40lO PH 3 R COS,Hz0 CH4, C2H6
represented as the area under the peak of generator current c's. time on a strip chart recorder. The 0.2 pl/sec injection rate must not be exceeded so as to prevent overloading the pyrolysis capability of the furnace and the formation of coke or other uncombusted hydrocarbon residue. Such products reduce recovery and therefore necessitate a thorough cleaning of the exit end of the combustion tube. Accordingly, an empty quartz exit tube placed in the outlet of the combustion tube facilitates cleaning the exit portion of the tube. These precautions hold true for the reductive sulfur method as well. Operation of the Reductive System. As in the oxidative method (Figure l), the sample is introduced into the 550 "C inlet section of the pyrolysis tube with a standard syringe at a rate of 0.2 pl/sec. The volatilized sample then mixes with a humidified hydrogen flow of 200 cc/min, and is swept into the 1150 "C platinum catalyst zone (Figure 2). The sample is converted to the pyrolysis products indicated in Table I, traverses the 750 "C outlet zone, and is swept through the heated inlet capillary on into the titration cell (Figure 4). Here the H B reacts with the electrolyte and Ag,S precipitates, changing the silver ion concentration in the electrolyte. The sensor-reference pair of electrodes senses this change in concentration and signals the microcoulometer to regenerate silver ions to reestablish the original Ag+ concentration. The 750 "C outlet temperature was selected because reduced recoveries due to dissociation of H2S in the outlet portion of the furnace have been experienced at higher outlet temperatures. A satisfactory method of producing the 10% platinum on Alundum catalyst is the following: sieve 75 grams Norton 390
80
?-/
-
Figure 5.
lMlli
ANALYTICAL CHEMISTRY, VOL. 42, N O . 3, MARCH 1970
0-0
-
0- -0 0--0
Oxygen 40
Argon
100
160 100
160
40
On/Arratio 1:4 1:l
4: 1
SA-203 Alundum Support (10-20 mesh) and digest for a half hour in constant boiling HCI. Wash the digested Alundum with deionized water for a half hour or until no chloride can be detected in wash water. Air dry the washed Alundum, which should now be free of metals, and place 23.5 grams of HrPtC16'6Hz0, 30 ml HzO, and 75 grams of washed Alundum in a crucible and evaporate to a red-brown dryness over a burner o r a hot plate, using constant rotation and stirring to avoid spattering. Pack the pyrolysis tube (Figure 2) and condition at 1000 "C under hydrogen at 300 cc/min for three hours. The chemistry in the reductive sulfur silver cell requires some explanation and sheds light on the operational parameters in the cell. Ordinarily, the slope of the titration curve in the pAg+ = 11 region is extremely steep, thus exaggerating the overall system gain, which leads to instability. If, however, the ammonia concentration in the electrolyte is maintained at 0.3M by the auxiliary flow of 10% N H 3 in N2 (Figures 3 and 4), the system is buffered with respect to Ag+ at the lo-" Ag+ ion concentration normally used. This buffering action causes the knee in the titration curve of Figure 5 at pAg+ = 12 and permits stable operation at this extremely small Ag+ concentration, Stable operation is achieved here due to the formation of the complex Ag(NH&+ which leaves only one out of 106 Ag+ ions free to act as Ag+ ion and thus reduces the effect on the sensitive sensorreference voltage of any Ag+ ions generated by the generating circuit by a factor of a million. This reduces the overall system gain to a stable level. The table shown in Figure 5 illustrates the basis of this effect and shows why chlorides d o not interfere by precipitating Ag+ ions from solution, but why sulfides and cyanides do precipitate. The reason for this selective precipitation is dependent on the solubility product of the species of interest and illustrates the principle that the product of the soluble components of the species must be equal to the solubility product before precipitation can occur. Thus, at a silver ion concentration of lo-'*, the chloride concentration must reach lOOM before precipitation can occur, while if even one sulfur atom enters the electrolyte, precipitation of a silver sulfide molecule will result. RESULTS AND DISCUSSION
Results Obtained with Oxidative System. The effect on the oxidative sulfur method of varying the furnace temperatures, the azide concentration in the electrolyte, and the ratio
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800
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Figure 7. trolyte
650
Oxidative sulfur analyzer using 0.06% azide elec--
0--0 0- -0
-
Figure 8. Oxidative sulfur system: interference from 5 pl of 10,000 ppm nitrogen (argon = 160 cc/min, 02 = 40 cc/min). Interfering systems tested were diethanolamine in water, triethylamine in methanol and isooctane, and pyridine in methanol and isooctane
Flow rate (ccjmin)
~-
Oxygen 40
Argon
200
50
160
On/Arrat;* 1:4 4:1
of oxygen to argon flow are illustrated in Figures 6 through 9. Figure 6 shows the percentage recovery of the system t o 5 p l of thiophene in a cyclohexane standard at 10.0 ppm sulfur when the furnace temperature as well as the oxygen t o argon flow ratios were varied. These trends are typical for most sulfur types and matrices and show that recoveries are 10% lower in the 800-900 "C region than at 700 "C. Another interesting result was that with lower oxygen t o argon flow ratios, the recovery increased. This result is attributed t o less SOa formation in the more oxygen lean atmosphere. Thermodynamics predicts a shift toward SO2 for increasing temperatures, a result reflected in the positive slope of the response in the 800-900 " C range. The increased response at 700 "C is not understood, especially because in Figure 7 we see that the sample matrix does not contribute any noticeable response over the full temperature range. Thus, from Figures 6 and 7 we conclude that the 1 :40 2 t o Ar flow ratio is preferred because of increased recoveries and negligible matrix effect. From Figures 8 and 9 we find that two other considerations dictate the choice of the 800 "C operating point and the 0.06% sodium azide electrolyte, these being the nitrogen and halogen interferences in the oxidative system. The nitrogen interference is large and negative for the whole temperature range when no azide is added to the cell electrolyte (Figure 8). The 0 . 0 6 z and 0.24% sodium azide additions have equivalent response and show minimum nitrogen interference for the 800-900 " C range. At 800 " C the 0.06% sodium azide electrolyte yields minimum chloride interference (a negative 1 .O ppm apparent sulfur for a 1000 ppm chlorine sample) and is slightly better than the 0.24% sodium azide electrolyte (Figure 9). The chemistry of how the azide ion addition reduces the nitrogen and chloride interference t o such low levels is not well understood, but it is suspected that the azide ion acts o n the interfering species directly and rapidly and thus prevents them from oxidizing the iodide t o iodine. The equations of Feigl (15) for the iodometric chemistry of the azide ion offers an alternate but not very satisfactory explanation which is based o n the catalytic conversion of iodine t o iodide by the azide ion itself. ~ _ _ _ -
-
(15) F. Feigl, "Chemistry of Specific, Selective and Sensitive Reactions," Academic Press, New York, N. Y., 1949.
NaN3 concentration I--I __ O---O
0.06z
A--A
0.24%
,
-
-
-10
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u
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8
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4
CENTER FURNACE T E M P E R A T U R E
1.C)
Figure 9. Oxidative sulfur system: interference from 5 pl of 1000 ppm chlorine from chlorobenzene in cyclohexane (argon = 160 cc/min, O 2 = 40 cc/min)
NaN3 concentration 0---0 A--A
0.06%
0.24%
Results Obtained with Reductive System. The per cent recovery of sulfur in the reductive method proved to be much less sensitive to sulfur type, sample matrix, and instrumental conditions. A comparison of per cent recovery with the oxidative method is given in Table 11. Interference from nitrogen, because of cyanide formation, dictated the high temperature (1 150 "C)and humidification of the reactant gas. Using room temperature humidification of the reactant gas the nitrogen interference (as CN-) is illustrated for the reductive method in Figure 10. Here the percentage of HCN formed is proportional t o the carbon to nitrogen ratio in the sample. Because a water standard containing 1% ammonia gives no interference at all, it is assumed that the nitrogen which does not show up as CN- in the cell has been converted to ammonia. From the data in
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
391
1050
1075
I100
1150
I I25
PYROLYSIS TEMPERATURE
['C)
Figure 10. Reductive system: nitrogen interference: conversion of nitrogen to CNTEA = triethylamine, DEA = diethanolamine
Table 11. Response to Prepared Standards Per cent recovery of sulfur Sulfur compound Oxidative Reductivea Reductiveb In kerosene Thiophene 84 100 100 Di-ri-butyl sulfide 87 100 102 Di-rz-butyl disulfide 84 94 100 Di-t-octyl sulfide 79 88 103 t-Dodecyl mercaptan 86 102 102 Sulfur 73 86 100 In butanol p-Toluene sulfonic acid 75 91 100 Butyl sulfone 79 84 101 Diethyl sulfate 68 80 100 Dimethylsulfoxide 93 100 100 a Standard 10 pl syringe used. b Solidiliquid sample inlet, inlet temperature = 800 "C.
Figure 10, the nitrogen interference can be calculated. As an example, a typical hydrocarbon containing 100 ppm nitrogen (TEA in isooctane, 0.01% N) yields 3.6 ppm N as CN(3.6% of the nitrogen is converted t o CN-) which becomes 4.1 ppm sulfur apparent (ppm Sa) according to the formula: ppm Sa
molecular weight of S
'iz X molecular weight of N
=
ppm N(,,cN-) X
=
1.14 ppm N(= CN-)
The factor of is due to the fact that two silver ions are precipitated by the S2- ion compared to only one for the CNion.
Known concn. (ppm)
Oxy-hydrogen Turb. finish Labs reporting B-P Finish Labs reporting Coulometric Oxidative Labs reporting Reductive Labs reporting
392
Unfortunately, the catalyst and process used is identical to that used for the commercial production of H C N (13), and any catalytic process which is 100% efficient for the formation of H2S will probably also tend t o form some CN-. However, recent experiments where the temperature of the humidifier was increased from room temperature to 85 "C have shown a ten-fold reduction in H C N production and render the reductive method much more versatile. Initial results indicate that a t 50 ppm sulfur and 500 ppm nitrogen, the recovery o n the sample is 52.0 ppm sulfur apparent. Thus, a ten-fold excess of nitrogen over sulfur will result in a 4% positive error in the sulfur assay. I n practice the most convenient and safest means of controlling humidity is to use a motor driven syringe. Any desired rate of water flow is available, there is no need to heat trace the gas lines, and the hazard of a heated, pressurized, hydrogen filled water reservoir is eliminated. A water injection rate of 6 ml per hour is equivalent to the quantity of water delivered by 200 cc/min hydrogen humidified at 85 "C. Comparison of Oxidative and Reductive Methods. The accuracy of both procedures for different sulfur types was determined from prepared standards in the 10-100 ppm range (Table 11). For the reductive method, some cases of low recovery (free sulfur, 8 6 7 3 are attributed to hold-up in the syringe rather than pyrolysis efficiency and other cases to the thought that the higher oxidation states of sulfur (diethyl sulfate, 80%) are more difficult to reduce t o H2S. As seen in the third column of Table 11, the low recovery on elemental sulfur was established as syringe hold-up by using the Solid/ Liquid Sample Inlet (12), the operation of which will be discussed later in this section. The precision in the case of standards was +0.2 ppm o r *3.0%, whichever was greater. For the oxidative sulfur method, low recovery is attributed again t o hold-up and t o S02/S03ratios in the pyrolysis, only SOzbeing detected by the cell. By permission of ASTM Sub-committee D-2/D-16, results o n a cooperative study of four sulfur methods are included which show how the present systems compare to other methods on low ppm standards (Table 111). The range of averages is defined as a spread which includes all results reported. Results on petroleum samples from Atlantic Richfield's Watson Refinery are shown in Tables IV through VIII. The percentage recovery for the various sulfur types and boiling ranges normally encountered in refinery samples are shown in Table IV. These data provide a further example of the dependence of the oxidative method on sample matrix and sulfur type. The results in Table V of the determination o f trace sulfur
Table 111. ASTM Cooperative Tests-Average and Range of Averages of Duplicates Dibutyl disulfide Thiophene in toluene in kerosene Thiophene in cyclohexane 12 3 12 3 12 3
11.8 f 1 . 5 6 11.8 1 1 . 5 5
11.1 f0.5 4
10.0 1
3.8 +2 6
3.8 f 2 5
3.0 1 1 . 2 4 2.4 1
12.6*3 9 12.6 1 3 9
3.7 1 1 . 3 9 3.7 1 1 . 3 9
12.3 1 3 6 12.313 6
11.8 1 0 . 3 4 11.8 1
3.1 f 0 . 3 4
12.5 1 12.0 1
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
2.8
1
4.7 4 ~ 2 . 5 6
4.7 1 2 . 5 6
3.4 f 0 . 7 2 2.9 1
Table IV. Standards Used for the Determination of Sulfur in Petroleum Products Oxidative Method B.P. Per cent range, "C Standard recovery Sample type 80-400 Cyclohexane 68.8 Naphtha sulfide Jet fuel & 350-525 Benzyl98.8 stove oil Thiophene Diesel & 400-675 Dibenzyl100 heavier Thiophene Reductive Method Assume 100% recovery of sulfur in plant samples. System performance confirmed before each run on thiophene in cyclohexane standard.
Sample number
Table V. Trace Sulfur in Light and Middle Distillates (ppm) Coulometric Oxy-hydrogen thorin finish Oxidative Reductive
90301 90302 90303 90304 90305 90306 90311 90312 9033 1
13 9 3 2 1 3 2 8 < 100"
13.5 13.2 0.42 0.18 9.55 0.1 8.03 19.6 38.5
10.6 11.5 0.90 0.70 12.4 0.20 10.2 23.8 47
Determined by X-ray emission. Table VI. Normal Sulfur in Light and Middle Distillates (Wt %)
a
Sample number
X-ray emission
90307 90308 90309 90310 90314 903 15 903 16 90322 90323 90325 90326 90327 90328 90329 90330
0,75. 1 . 14a 1.43 1.51 0.03 0.06 1.15 0.40 0.12 0.76 1.30 1.40 0.07 0.42 0.04
Coulometric Oxidative Reductive 0.66 0.29 1.21 1.04 0.031 0.054 1.06 0.40 0.11 0.74 1.16 1.31 0.069 0.45 0.040
Direct syringe injection Trace sulfur in L& M distillates Normal sulfur in L& M distillates Normal sulfur in residual fuel oils Quartz boat injection Normal sulfur in residual fuel oils
Precision of Reductive Method Per cent of measured value 2 u of 2 u of single measuremean of u of ment duplicate method f3.4
f7.8
f5.5
11.2
12.6
11.8
58.2
f20.1
114.2
12.0
f4.5
3~3.2
in light and middle distillates indicate a good t o fair comparison between all methods. However, because of the consistent agreement between the oxidative and reductive methods, the coulometric data are considered more reliable at the part per million level. Agreement is excellent among all methods in Table VI except for samples 90308, 90309, and 90310. I n these samples the oxidative results were significantly lower than those of the other methods. Further investigation showed that these samples contained several hundred parts per million of either vanadium or nickel. Later it was discovered that the presence of heavy metals causes severe reduction in the per cent recovery of sulfur in the oxidative method. This is probably due t o metal catalyzed conversion of SO2 t o SOain the pyrolysis tube. In Table VII, the case of residual fuel oils a t approximately 1 sulfur, it can be seen that the agreement was not good using standard syringe techniques. The agreement was acceptable, however, when a total displacement syringe (Precision Sampling Corp., Baton Rouge, La.) was used-the whole sample being in the needle and the needle itself inserted at a rate which yielded the 0.2 pl/sec injection rate required. During the course of the needle injection, the internal plunger traversed the needle itself and pushed all residue into the hot inlet. While the mean of duplicates using such a technique was in good agreement with the standard quartz tube method, the relative standard deviation was quite large (8.2z).These same fuel oil samples were run using a Solid/Liquid Inlet System on the Microcoulometric Titrating System (12). I n the technique the sample is injected with a normal 10-p1 syringe into a quartz boat in a hydrogen at-
Determined by ASTM D1555. Table VII.
Sample number
Quartz tube 90313 0.39d 903 17 1.41 903 18 1.40 90319 1.42 90320 1.35 90321 1.53 90324e 0.132 * Standard 10-pl syringe used. 10-pl total displacement syringe used. Solid/liquid sample inlet with quartz boat. e
0.805 1.05 1.48 1.42 0.0316 0.057 1.16 0.424 0.124 0.83 1.34 1.42 0.07 0.478 0.051
Table VIII.
Normal Sulfur in Residual Fuel Oils (Wt %) CoulometricOxidative Reductivea Reductive* 0.13 0.72 0.64 0.67 0.68 0.56 0.11
0.232 0.65 0.97 0.89 0.86 0.802 0.132
0.338 1,436 2.131 1.425 1.495 1.520 0.132
Reductivec 1.35 1.36 1.23
Oxygen bomb method. Lube oil.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
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mosphere. The boat is subsequently pushed slowly into a n 800 “C inlet. The results of this latter technique on the fuel oil samples exhibited much greater precision (+2,0% relative standard deviation) and good accuracy, so that for routine analysis the latter procedure is considered the more reliable. T h e precision of the reductive sulfur technique for industrial petroleum samples is summarized in Table VIII. All samples with sulfur greater than 500 ppm were diluted in toluene to approximately 50 ppm for the coulometric data shown here. The number of duplicate analyses per man-day
is on the order of twenty (40 determinations), exclusive of dilution time. ACKNOWLEDGMENT The,authors thank Dean Hoggan of A R C 0 Chemical Co., Anaheim, Calif., for permission to use the ASTM results presented and for valuable help in coordinating this study.
RECEIVED for review September 22, 1969. Accepted January 9, 1970.
Flame Emission Spectrometry with Repetitive Optical Scanning in the Derivative Mode W. Snelleman,’ T. C. Rains, K. W. Yee,* H. D. Cook,2and 0. Menis Analytical Chemistry Diuision, National Bureau af Standards, Washingtori, D. C.
A flame emission spectrometer using a rapid repetitive scan of a narrow wavelen th region has been developed. By this method o f wavelength scanning the second derivative of the output intensity is measured. The use of this approach to minimizing spectral interference in matrices and the use of microsamples greatly enhance the potentialities for flame emission spectrometry, and minimize the need for a monochromator of high resolving power. A quartz plate, made to vibrate at 145 Hz, is mounted behind the entrance slit of the monochromator. The ac amplifier is synchronized with the oscillations of the quartz plate. When the amplifier is tuned to twice the frequency of vibration, the second derivative of the spectrum is obtained. This permits the measurement of weak line spectra nested in or on a broad band or continuum. It is demonstrated that spectral interference due to CaOH bands and/or a continuum are minimized in the measurement of barium. The elimination of interferences from bands and flame structure led to an improvement in detection limits of alkali and alkaline earth elements in the presence of many matrix ions. An analysis can be performed with 50 pl of solution which makes it applicable to biochemical and air pollution studies.
THEMEASUREMENT of the radiant intensity of an atomic line which is located on a bandhead arising from the concomitant or flame gases is of concern to the flame emission spectroscopist. Frequently, an atomic line cannot be used because of interferences from some overlapping band structure. Two typical examples are the CaOH bands (5430-6220 A), which interfere with the Na 5890 A and Ba 5536 A lines, and the MgOH bands (3600-4000 A) which contribute to high background for the Fe 3720 A and Ru 3727 A lines. Also, the O H band system, covering the region of 2800 to 4000 A, limits the measurement of the radiant intensity of many atomic lines in flame emission spectrometry. These interferences have discouraged many workers in the field of flame emission. However, Buell ( I ) in using a high resolution monochromator has shown that many atomic lines in this region can be used.
A technique which permits the measurement of weak line spectra nested in or on a broad band or a continuum resulted in a n improvement in detection limits. In addition, microliter samples can conveniently be taken for analyses. The newly designed optical system permits a rapid repetitive scan of a narrow wavelength region. This scan is synchronized with the ac amplifier. With the ac amplifier tuned to the same frequency as the vibrating quartz plate, the signal produced is the first derivative of the emission intensity with respect to wavelength, At twice the frequency, the second derivative is obtained, This mode of operation permits the measurement of weak line spectra without the interference from background radiation or broad band spectra. When operating at high frequencies (145 Hz), the apparent signal-tonoise ratio is improved by eliminating the low frequency flicker noise of the flame. This technique was applied to the determination of lithium in 50 p1 of solution containing high concentrations of sodium, and to the determination of barium in a calcium matrix. Derivative spectroscopy has been described by several authors (2-4). Giese and French (5) used it to detect low intensity bands by measuring the first derivative of the transmission curve with respect t o wavelength. Under certain conditions, a recording of the first or second derivative of an absorption spectrum can give increased resolution over a normal spectrum. In magnetic resonance spectroscopy the first derivative and occasionally the second derivative are recorded to increase resolution of the spectrum (6). There are various ways for obtaining the derivative of absorption lines or bands with respect to wavelength. Balslev (7) vibrated the exit slit of his spectrometer and synchronously detected the signal at the vibrating frequency while Gilgore et al. used optical wobblers (8). Snelleman ( 9 ) was able to in-
Present address, Fysisch Laboratorium, Rijks-Universiteit Utrecht, The Netherlands. Measurement Engineering Division, National Bureau of Standards, Washington, D. C. “Flame Emission and Atomic Absorption Spectrometery,” J. A . Dean and T. C. Rains, Eds., Dekker, New York, N. Y . , 1969.
(1) B. E. Buell in
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
( 2 ) A. Perregaux and G. Ascarelli, Appl. Opt., 7 , 2031 (1968). (3) F. R. Stauffer and H. Sakai, ibid.,7,61 (1968). 32,370 (1960). (4) E. C. Olson and C. D. Alway, ANAL.CHEM., ( 5 ) A. T. Giese and C. S. French, Appl. Spectrosc., 9, 78 (1955). (6) C. H. Townes and A. L. Schwawlow, “Microwave Spectroscopy,” McGraw-Hill, New York, N. Y., 1955, Chapters 14 and 17. ( 7 ) I. Balslev, Phys. Rec., 143, 636 (1966). (8) A. Gilgore. P. J. Stoller, and A. Fowler, Reu. Sci.Instrum., 38, ’1535 (1967). (9) W . Snelleman, Spectrochem. Acta, 23B, 403 (1968).
