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Anal. Chem. 1981, 53, 1088-1093
Determination of Total Sulfur in Hydrocarbons by Reductive Pyrolysis with Polarographic Detection R. E. Relm" and D. D. Hawn Analyflcal Laboratories, The Dow Chemical Company, Midland, Mlchlgan 48640
Organic sulfur compounds are converted to H2S by use of reductive pyrolysis and detected by use of differential pulse polarography (DPP) or differentlal pulse cathodlc stripping voltammetry (DPCSV). The method Is ideally suited for the determlnatlon of trace sulfur In matrices cqntalnlng hlgh concentrations of chlorine, bromine, or nitrogen. Mean recovery from fortified chlorobenzenes and bromoethane is 104% and 98%, respectively. Nitrogen does not interfere until N/S mole ratio exceeds 500. By use of standard addition analysis, sulfur can be determined In solutions where N/S Is 20000. At 0.7 Fg of S/mL in rnonochlorobenzene RSD Is f11.4%. The dynamic range covers 4 orders of magnitude from a limit of detection of 30 ppb S.
Chemical technology and environmental concern have required the determination of sulfur in a variety of matrices with levels a t or below 1ppm. Although methods are available for the determination of total sulfur at parts-per-million levels, these methods are often complicated and/or time-consuming. Further, certain constituenb in the sample matrix make some methods impossible to apply. The Wickbold method (I, 2) or Raney nickel reduction (3, 4) allows determination of sub-part-per-million concentrations of sulfur; however, large samples are required (50-100 mL) and the total analysis time is 2-3 h. Also, with Raney nickel reduction, oxidized sulfur species (e.g., sulfonic acids) are not quantitatively converted to H2S. Sulfur is also determined by oxidative combustion flask techniques using a variety of finish procedures (5-9). These methods are generally restricted to sulfur concentrations greater than about 100 ppm, and the analysis time is about 2 h. X-ray fluorescence is used to determine sulfur directly in liquids and solids (IO). The determination is rapid but is influenced by the sample matrix and is usually limited to sulfur concentrations >10 ppm. Presently the microcoulometric methods (11-15) are widely accepted in the chemical and petroleum industry for the determination of sulfur a t concentrations less than 10 ppm. These methods have been reviewed by Wallace et al. (11). Both oxidative and reductive pyrolysis have been used for sample decomposition. The oxidative method can detect 0.1 ppm S but chlorine, bromine, heavy metals, aldehydes, and nitrogen (as NO) can interfere. Conversion of sulfur compounds to SO2 is nonstoichiometric. Improvements toward stoichiometry (16) and in precision and detection limits (17) have recently been reported. With careful operator attention the reductive method can detect 1ppm S. Nitrogen and high concentrations of bromine can interfere. In another reductive pyrolysis technique recently reported (18), sulfur compounds are noncatalytically reduced to H2S which is monitored by photometric measurement of the blackening of a moistened lead acetate impregnated paper tape. The method can detect 25 ppb S, is rapid, and is also claimed to be free from nitrogen and chlorine interference. Conversion of sulfur compounds to H2S is nonstoichiometric under the pyrolysis conditions employed. 0003-2700/81/0353-1088$01.25/0
This paper describes a new method for the determination of total sulfur in hydrocarbons based on a combination of catalytic reductive pyrolysis and polarographic detection of HzS. The method offers several inherent advantages for total sulfur determination. With reductive pyrolysis most sulfur forms are quantitatively converted to H2S as compared to oxidative pyrolysis where conversion to SO2 is typically 80-85% (11). Polarographic detection of H2S is specific, since halogens (HCl and HBr) are electrochemically inactive at potentials where sulfide is detected. The effect of nitrogen (as HCN) is minimal. Also polarographic detection is sensitive. By use of differential pulse polarography (DPP), 1ppm S can be detected. For more demanding applications, sulfide can be preconcentrated at a hanging mercury drop electrode and determined using differential pulse cathodic stripping voltammetry (DPCSV). A limit of detection of about 30 ppb S can be achieved.
EXPERIMENTAL SECTION Apparatus. The reductive pyrolysis/polarographic analyzer system consisted of a fixed-rate sample injector, pyrolysis furnace, and polarographic detector as shown schematically in Figure 1. Sample injections of typically 40 pL were made with a 50-pL syringe with a removable stainless steel or platinum needle (3 in. x 24 gauge% The syringe plunger was advanced at a controlled rate, using a Sage Model 351 syringe pump. The pyrolysis furnace was a Dohrmann Model S-300 furnace (Dorhmann/Envirotech, Santa Clara, CA) equipped with three separately controlled heating zones. The furnace-operating temperatures were 700-950 OC for the inlet zone, 1100 OC for the center catalyst zone, and 850 O C for the outlet zone. A standby temperature of 900-950 "C (catalyst zone only) was used overnight to reduce the devitrification rate of the pyrolysis tube. The furnace was equipped with three gas flow meters for regulation of the reactant (humidified hydrogen), auxiliary (hydrogen), and carrier (argon or nitrogen) gas flows. Hydrogen reactant gas (Mathesonprepurifed) was humidified by bubbling at 200 mL/min through a standard 250-mL gas washing tower filled with deionized water before entering the pyrolysis tube through a side arm. The washing tower was enclosed in a housing constructed of 1/4-in.Plexiglas plastic for safety reasons discussed below. The quartz pyrolysis tube was typically of the design shown in Figure 2A. An alternative design with an auxiliary hydrogen purge (Figure 2B) was also used. The center section was packed with granuh 10% platinum on alundum (A120,)catalyst obtained Santa Clara, CA, and held in place atinum screen. The pyrolysis tube required replacement every 3 to 4 weeks, since the weight of the catalyst (-60 g) caused the center section to bow. In later studies, the wall thickness of the tube (design A) was increased to 3-4 mm which significantly decreased devitrification problems and extended the tube lifetime to at least 2 months. After each week of operation the pyrolysis tube was purged first with argon for 1h to remove hydrogen and then with oxygen for 2 h to oxidize coke formed on the catalyst and tube walls. The system was then purged overnight with argon at 50-1oQ mL/min before the hydrogen flow was reestablished. The catalyst was then reconditioned 1 to 2 h under hydrogen at 1100 "C prior to use. Caution: Failure to adequately remove oxygen can result in a flashback. No flashbacks occurred when the system was purged overnight with argon. 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
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12/5 Female
5 mm
II IH
I
I
Figure 1. Schematic dlagram of analysis system: (W) worklng electrode; (R) reference electrode; (A) auxiliary electrode. A,,
18/9 Female wall)
7/8" OD 13-4 mm wall)
0 Ring Groove End View
Figure 2.
Quartz pyrolyslis tubes.
The outlet of the pyrolysis tube was connected via an 18/9 female ball joint to a three-way plug valve (Fluorocarbon SPC 124-3B, Fluorocarbon, Anaheim, CA) of all Teflon construction operated by a pneumatic actuator (Whitey MS-131 DA, Whitey Co., OaMand, CA). The valve directed gas flow from the pyrolysis tube to a vent or to a small gas washing tower containing 20 mL of deionized water or acid solution as shown in Figure 1. The actuator was driven in either direction by argon at 25 psi, the argon flow direction being controlled remotely by a four-way electrically driven solenoid. The four-way solenoid was in turn activated by a switched 110-V ac outlet using a Chrontrol DL-4 relay-equipped timer (Lindberg Enterprises,Inc., San Diego, CA). A second relay was used to activate the electrolysis controller for the analysis. With this configuration, the instrument was sufficiently automated to allow unattended operation through the entire analysis cycle. The detector consisted of an EG&G Princeton Applied Research (PARC) Model 303 static mercury drop electrode (SMDE), controlled by a PARC ]Model 174A potentiostat. For cathodic stripping experiments a PARC Model 315 automated electrolysis controller was also used. The porous glass frit of the Model 303 SMDE reference compartment was replaced by a microporous polyethylene frit obtained from PARC. The frit hiad a higher leak-rate than porous glass but was less susceptible to chemical attack by the basic electrolyte. The Ag/AgCl reference electrode filling solution was 3 M LiCl saturated with silver ion. The Model 303 polarographic cell was modified by the addition of a side-arm bubbler and a drain as shown in Figure 3. The upper end of the bubbler was fitted with a 12/5 female ball joint and the end of the bubbler which protruded into the cell was drawn to a capillary 0.2-4.3 mm in diameter. The drain allowed easy rinsing and refilling of the cell after each analysis. The side-arm bubbler and all connecting lines were heated by use of heater tapes to prevent water vapor condensation and subsequent sulfide loss. Chemicals. The supporting electrolyte was aqueous 0.20 M LiOH-0.02% EDTA (w/w). The solution was prepared from Baker Analyzed Reagent LiOH.H20 and polarographic grade EDTA (ethylenediaminetetraaceticacid, Southwestern Analytical Chemicals). All solvents were of spectrophotometric grade obtained from Burdick and Jackson (Muskegon, MI). Procedure. A typical analysis cycle required about 15 min. The detector cell was rinsed once with 0.1 M HNOBand twice with deionized water and filled with 10 mL of electrolyte,which
Figure 3.
Polarographic detector cell.
was then deoxygenated with Matheson prepurified nitrogen for 4 min. The sample syringe was filled with 40 pL of solution, the plunger withdrawn to 45 pL, and the sample (40 pL) injected into the furnace tube at a controlled rate of 0.20 or 0.25 pL/s. The resultant pyrolysis gases were directed into the polarographic cell. After the syringe was emptied, the furnace was purged an additional 40 s to completely displace H2Sfrom the furnace tube. Gases from the pyrolysis tube were then diverted to the vent and the contents of the detector cell analyzed following an additional 1 min nitrogen purge. Typical parameters for analysis by use of differential pulse polarography for detection were as follows: Ei = -0.30 V; scan rate, 5 mV/s; drop time, 1.0 s; pulse height, 100 mV; drop size, large; and current range 2-20 p A. By use of differential pulse cathodic stripping for detection, analytical parameters were as follows: Ed, = -0.30 V; deposition time, 60 s, stirred; equilibration time, 30 s, unstirred; scan rate, 5 mV/s; drop time, 1.0 s; pulse height, 100 mV; drop size, large. Unless otherwise noted these conditions were used for all data reported here.
RESULTS AND DISCUSSION Principles of Detection. Sulfur is determined by using a combination of reductive pyrolysis and polarography. The technique of reductive pyrolysis has been described by Wallace e t al. (11) and Cedergren et al. (19).Sulfur compounds are converted to hydrogen sulfide (H,S) according to
R-s
+
H ~ S+ C H + ~c
(1)
using catalyzed high-temperature pyrolysis. The balance of the sample matrix is ideally converted to carbon, methane, and water. Halogens are converted to their respective acid halides while nitrogen species are converted to nitrogen, ammonia, and hydrogen cyanide. The polarographic determination of sulfide has been well documented (20-22). For data reported here (sulfide concentration M), the mechanism of the reaction at the mercury (Hg) working electrode has been established as the oxidation of mercury to mercuric ion followed by the formation of an insoluble layer of mercuric sulfide (23) Hg Hg2+ 2e(2)
Hg2++ HS-
-+ -+ OH-
HgS + HzO
(3)
Response Characteristics. Typical response and sensitivity of the method are illustrated in Figure 4, which shows the measured current for a methanol blank and standards of 0.22 pg of S/mL and 2.2 pg of S/mL using DPCSV detection. With an injection volume of 40 pL, a limit of detection (LOD) of 0.033 pg of S/mL was calculated at 30, where u is the estimated standard deviation for triplicate blank determinations. Further improvement in the LOD should be possible with additional optimization of detection conditions, e.g., by decreasing the cell volume, by increasing the deposition time, qnd by using multiple or larger sample injection volumes. The mode of sulfide detection (DPCSV or DPP) is determined by the concentration of sulfur in the sample. For
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
0.1 PA
-.50
-.30
-.70
-90
-1.10-.30
-50
Potential ( V I
ti \ -.W
-.70
-1.10
Potential ( V I
Fbwe 4. Typical response using differential pulse cathodic stripping voltammetry as the detection mode: (A) methanol blank; (B) 0.22 pg of S/mL; (C) 2.2 pg of S/mL; E,, = -0.30 V; injection volume, 40 &; test compound, thiophene. 2.5
r
/j
0.10
0 0
5
10
15
20
25
30
35
40
45
50
Sulfur Concentration IMicrograms Per Milliliter1
I
1
l
I
I
I
I
I
0
20
40
60
80
100
1M
140
I
1w
I
180
(
200
Sulfide Concentration ICelll (Nanograms Per Milliliter)
Flgure 5. Comparison of detection mode sensitivity: (A) differential pulse cathodic Stripping voltammetry; deposkion time (stirred), 30 s; equiabratbn time (unsthed), 30 s; Ed, = -0.40 V; (B) differential pulse polarography, Injection volume, 40 pL.
concentrations >5 pg of S/mL, DPP is preferred. It has adequate sensitivity and requires less time per determination than DPCSV. For concentrations
