Thermal Decomposition of Barium Sulfate to Sulfur Dioxide for Mass Spectrometric Analysis Ben D. Holt and A. G. Engelkemeir Chemistry Division, Argonne National Laboratory, Argonne, ill. 60439
THESULFUR COMPOUND most commonly used in mass spectrometric studies of sulfur isotopes is SO*. Sulfates in aqueous solutions are usually precipitated as BaS04 and converted to SOz through a series of chemical reactions. Rafter ( I ) reduced BaS04to BaS in a platinum Gooch crucible, dissolved the Bas, and precipitated AgzS. The Ag,S was filtered, dried, and burned in oxygen to produce SO*. Gavelin et al. ( 2 ) converted BaS04 to the sulfide by heating (750-950 "C) with iron carbonyl and zinc. They treated the sulfide with HCl and conducted the resulting HzS into a cadmium-acetate absorbing solution to precipitate CdS. The CdS was washed, dried at 400 "C, mixed with V205, and heated at 600 "C to produce SOz in a container attached directly to the mass spectrometer. Ricke (3) used a variation of Gavelin's procedure by sealing off the dried mixture of CdS and V205 in an evacuated quartz tube and heating in an oven for 1.5 hours at 1000 "C. The tube was cooled rapidly so that the SOz,which was to be retained in the sealed vessel for subsequent mass spectrometric analysis, represented the equilibrated system at 1000 "C. Thode et al. (4) reduced BaS04to HzS by boiling in a mixture of HI, H3P04, and HCl in a 200-ml flask with a reflux condenser. CdS was precipitated and converted to Ag& which was filtered, washed, and dried. The Ag2S was placed in a quartz boat and burned in Oz at 1350 "C to form
soz.
All three of these procedures are good in that the multiple operations involved in each method reportedly cause no appreciable fractionation of the isotopes. The chief disadvantage, common to all three, is the requisite time and labor, Such preparative procedures could very well consume a major part of the experimental effort in a research program focused on the study of sulfur isotope variations. Examples of such programs are: first, the study of natural variations of sulfur isotopes in well-water sulfates, sea-water sulfates, gypsum, pyrite minerals, sulfide ores of igneous origin, meteoritic sulfides, volcanic sulfur, organic sulfur, native sulfur, sulfurated water, and sedimentary sulfides (2, 4-8); second, the study of the origins of air-borne sulfur (9); and third, the study of atmospheric sulfur as it relates to air pollution (IO). This report describes a method by which BaS04 is rapidly
(1) T. A. Rafter, N . 2.J. Sci. Tech. B, 38, 849 (1957). (2) S. Gavelin, A. Parwell, and R. Ryhage, Econ. Geol., 55, 510 (1960). (3) W. Ricke, 2.Anal. Chem., 199,401 (1964). (4) H. G. Thode, J. Monster, and H. B. Dunford, Geochim. Cosmochim. Acta, 25,159 (1961). (5) H. G. Thode, J. Macnamara, and C. B. Collins, Can. J . Res., 27, 361 (1949). (6) J. Macnamara and H. G. Thode, Phys. Rev., 78. 307 (1950). (7) J. Macnamara, W. Fleming, A. Szaba; and H. G.Thode, Can. J. Res., 30, 73 (1951). (8) T. A. Rafter and Y. Mizutani, Nature, 216, 1000 (1967). (9) G. Ostlund, Tellus, 11,478 (1959). (10) R. Baldwin, L. Cohen, J. Forrest, J. Frizzola, B. Manowitz, L. Newman, C. Scarlett, M. Smith, M. Steinberg, and W. Tucker, Brookhaven National Laboratory Report BNL 50082 (S-70), Upton, N. Y.,Dec. 1967, p 75.
converted to SO2in one operation. The BaS04, covered with pulverized quartz powder in a fused quartz tube, is heated under vacuum to the softening point of quartz (about 1400 "C). The SOzproduced from the net reaction BaS04 + BaO
+ SO2 +
'12 0 2
is collected in a cold trap; the oxygen is pumped away; the BaO fuses with the silica surroundings. The collected SOzis distilled into a storage bulb for subsequent mass spectrometric analysis. This method of conversion does not assure a uniform oxygen-18 abundance in the SOz,as do procedures in which comparative sulfide samples are oxidized by oxygen from a common source. However, a correction for the small oxygen-18 interference in the mass-spectrometric analysis can be conveniently calculated from measurement of both the 66/64 and the 50/48 mass ratios by means of the formula
6 34S is the per mil isotopic ratio enrichment of sulfur-34 in a sample relative to a standard. 50Rand 66Rare the 50/48 and 66/64 ratios measured by the mass spectrometer. The presence of gaseous impurities that interfere with the measurement of ratios at mass 66 and mass 50 can be ascertained by a preliminary scan of the spectrum. Possible sources of such impurities, if present, are the "background" of the mass spectrometer and/or of the vacuum line in which the SOz was prepared. No inherent source of interfering impurities is associated with the thermal-decomposition preparation of pure BaS04 in a clean quartz environment. EXPERIMENTAL
Apparatus. The essential parts of the apparatus are the quartz reaction chamber and the vacuum line in which the SOz is collected and transferred to the gas-sample bulb. The reaction chamber, shown in Figure 1, is a 90-mm length of 9-mm fused-quartz tubing, closed at one end and attached to the collection line at the other end through a 3/8-in~h Cajon "ultra-torr" union (Cajon Company, Solon, Ohio). It contains the BaS04 sample in a smaller, closed-end, quartz tube (22-mm long by 7-mm 0.d.) which is packed with pulverized-quartz powder and quartz wool. An oxygen-gas, two-burner, cross-fire torch is mounted so that the sharply pointed flames are directed toward the rounded end wall of the reaction chamber. The burner tips of the torch are Purox No. 5 (3-mm orifice) or equivalent. The SO2 collection line consists of a U-tube cold trap, a sample bulb, and a capillary-trap manometer (Figure 2). The manometer [described previously ( I I ) ] is optionally used for checking yields in the conversion of BaS04 to SOz. Stopcocks A , B, and C have 4-mm bores. Trap Tiis made of 10-mm borosilicate tubing. Procedure. Place about 20 mg of BaS04 in the 7-mm inner tube; add a 6-mm layer of pulverized quartz (ground (11) B. D. Holt, ANAL.CHEM., 27, 1500(1955).
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Table I. Yields of Condensable Gas from Thermal Decomposition of Barium Sulfate BaS04, Condensate, Series pmole pmole Yield,
TORCH
QUARTZ INNER TUBE BOROSILICATE GLASS7
YPUARTZ ENVELOPE
A
ART2 POWDER
B
TOP VIEW
21.9 22.1 21.9 23.3 22.5 23.4 22.3 4.5 22.6 43.1 95.9 182 230
21.7 22.2 21.9 23.6 22.4 23.3 22.8 4.5 22.3 43.2 95.8 186 228
Figure 1. Reaction chamber t o a powder with pestle and mortar) atop the BaS04; insert a tuft of quartz wool to confine the powder. Slide the inner tube into the end of the envelope and add another tuft of quartz wool as shown in Figure 1. Attach the envelope to the collection line u a the vacuumtight Cajon union. Evacuate all components of the system, Figure 2, to about Torr. Flame the envelope, trap T I , and the storage bulb to remove absorbed moisture. Slide an aluminum-foil heat shield onto the quartz envelope to protect the O-ring coupling and the nearby stopcocks from the radiant heat of the flames and molten quartz. Immerse trap TI in liquid nitrogen. With the cross-fire torch mounted to focus the two flames on the end wall of the quartz envelope, light the torch and adjust the flames for softening quartz. With properly adjusted flames, the end wall of the envelope begins t o collapse onto the end wall of the inner tube, and the BaS04 begins to melt and decompose. Gas bubbles evolve from the liquified mass and a pronounced increase of pressure in the vacuum line registers on the thermocouple gauge. Continue heating until the decomposition is over, as indicated by a cessation of bubble formation in the sample area and by a decrease in pressure. Move the cross-fire torch 2-3 mm along the axis of the reaction chamber so that the hottest zone approaches the quartz-powder section of the inner tube, If the evolution of more oxygen is evident, heat in this position until gas evolution subsides, and then advance
T O VACUUM HEAT REACTION CHAMBER
CAPILLARY MANOMETER
S o p COLLECTION
99 100 100 101 100 100 102 100 99 100 100 102 99 Std dev 100 =t1
the torch again. If the torch is advanced too rapidly after the beginning of the heating procedure, the walls of the quartz tubes tend to collapse on the quartz powder and the resulting fused mass tends to block passage of the gaseous products from the decomposing BaS04 to the vacuum line. Within ten minutes of the initiation of heating, the pressure Torr, inin the vacuum line ordinarily drops back to dicating that the decomposition of the BaS04 is complete. Turn off the torch; close stopcock A ; rotate stopcock C to isolate the collection line from the vacuum system. Distill the SOz from trap Tit o the storage bulb by removing the liquid nitrogen Dewar from the trap t o the bulb. Close stopcock F and remove the bulb for subsequent mass spectrometric analysis of the SOn. If it is desired to check the yield of SOzfrom the conversion, distill the gas first into trap Tz of the capillary manometer; measure at room temperature; then redistill from i", t o the storage bulb. The dual-range manometer measures 0.030.0 pmoles of SOn when only the capillary section (bounded by stopcocks D and E and by the mercury column) is used; when the hollow plug of stopcock D is included in the volume the manometer measures 0.0-200 bmoles. RESULTS
Table I shows the yields of condensable gas extracted in two series of thermal decompositions of BaS04. Sample weights were about 5 mg in series A, and ranged from about 1 to 50 mg in series B. The yields for the two series averaged 100% with a standard deviation of 1%. Mass spectrometric analyses indicated that the chief impurity was COP, which averaged 1.3 =k 0.2% for series A and 0.5 f 0 . 2 x for series B. The lower COz content of series B may have been due to the use of cleaner quartzware than that used in series A. The series-B quartz was chemically cleaned and thoroughly rinsed with water and acetone before use. Rafter ( I ) attributed COZ contamination of SOz produced by burning A g S in oxygen to carbon residues left on the inner walls of quartz tubing during fabrication of the tubing. By cleaning the tubing in 1 :1 HF, he was able to reduce the CO, content of his SOzto 2 %. The gas produced by the present procedure in series B averaged 99 SOe,0.5 C o n ,and 0.5 other impurities, mainly water and mass-28 (Ne and/or CO). No SOowas detected and only traces of CS:, and COS were observed.
TRAPTI
W
DISCUSSION
STORAGE BULB
Sample Size. Although 20 mg of barium sulfate, equivalent to about 2 cc of sulfur dioxide (STP), is a convenient size for the procedure, samples up to 50 mg can be quanti-
Figure 2. Vacuum line for collection and measurement of SO2 1452
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Experiment 1 2
3 4 5 6
7a
a
Table XI. Results of Thermal Decomposition of Barium Sulfate by Induction Heating Principal components Heating Yield of time, Max. of gas, condensate min temp., "C Container Tin capsule in graphite 18 80 1600 crucible c02, cs2 Graphite capsule in 8 1500 60 graphite crucible con, cs2 Platinum cup in graphite 14 1450 40 crucible co2,cs2 1 1125 40 Tungsten crucible SO2 Platinum cup in tungsten 60 14 1500 crucible so2 Platinum cup in platinum 60 46 1400 crucible SO2 Quartz cup in platinum 85 80 1400 crucible so2 81 70 1330 SO2 21 a 70 1400
7b 7c 7d 7e 7f 7g Condensate was not analyzed for composition.
z
1350
1275 1345 1370
tatively converted, as indicated by the results of series B in Table I. More care must be taken in heating the larger samples t o avoid loss of BaSOa (with powdered quartz and wool) from the inner tube because of rapid build-up of gas pressure. No difficulty is encountered, however, if the cross fire of the torch is positioned to heat only the tip of the reaction tube initially, and is then moved by 2-mm increments toward the sample as the end wall of the quartz tube thickens and as the BaS04 melts and decomposes. Preliminary Tests. The reported procedure evolved from preliminary tests with thermal decompositions by induction heating. Induction heating was initially selected because of the higher temperature range that was potentially available as compared to that of resistance furnaces. The results of these preliminary tests are listed in Table 11. I n experiments 7a-7g, BaS04 was decomposed in quartz cups contained in a n inductively-heated platinum crucible, Tests were made under varied experimental conditions, the best of which yielded 80-90x of the expected SOz(7a, 7b, and 7g). The unusually low yield of 7c corresponded with the formation of a solidified bubble of molten BaS04 at the bottom of the quartz cup. Apparently the radiant and convective heat losses from this solidified shell were such that a temperature adequate for appreciable decomposition was not reached, although the surrounding surfaces were at about 1400 "C. I n experiments 7d-7f, which yielded only about half the expected gas, the BaS04 was mixed with pulverized-quartz
60 100 180 30
49 40 55 89
SO2 in condensate
a
a
so2
z 0 0
4 34 95 96 98 96
... ...
a
99
powder t o prevent formation of the fused bubble observed in experiment 7c. One or both of two conditions may have caused the indicated low yields: insufficient heat, due to insulation by the quartz powder; and loss of sample by evaporation over the long heating periods (1-3 hours) required for completion of the decomposition. Accumulation of a white, water-insoluble deposit on the walls of the reaction chamber during these experiments indicated partial evaporation of BaS04. In experiment 7g, the BaS04 was not mixed with quartz powder, but was covered with a 6-mm layer of powder. I n all three of the experiments which gave high yields (7a, 7b, and 7g) the Bas04 was in good contact with the quartz wall during heating, and, in all three, the quartz wall was severely etched by interaction with the residue (BaO). These observations led to the notion that the experimental condition to be optimized for the conversion of BaS04 t o SOz was intimate contact between the BaS04 and a SiOn surface at the highest conveniently-attained temperature. The simple achievement of this condition and its success are described in this report. ACKNOWLEDGMENT
The authors are grateful t o C. M. Stevens and D. C. Stewart for stimulating interest in this work.
RECEIVED for review April 16, 1970. Accepted July 17, 1970. Based on work performed under the auspices of the U.S. Atomic Energy Commission.
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