2136 Anal. Chem. 1982, 54, 2136-2139

Table IV. Heats of Solution of Thiosulphates AH"/kcal mol-'

by calo- from solubility NBS 500 rimetry data a (9) value

NazSzO, t 2.03 2.47 (over range t 2.0 50-80 "C)

Na,S20,.5H2O -11.35 10.44 (over range 10-25 "C)

a Both have positive temperature coefficients of solubility.

~~~ ~~

achieved by using a Soxhlet column filled with molecular sieve (3A) below the condenser. Thus NazSz03.5HZ0 (32 g) in MeOH (100 mL) after 3 h left an amount of soluble thiosulfate corresponding to a 5.0/1 ratio rather than the 3.911 ratio calculated from reactants. The improvement is rather better than indicated, because the vapor phase is impoverished in water content (7). A mole ratio of 3.9/1 in the liquid phase is in equilibrium with a 9.011 ratio in the vapor being dried.

Properties of Anhydrous NazSz03 of Analytical In- terest. Rate of Solution. The fine powder produced from methanol dissolves rapidly enough for solutions to be made up in situ in standard flasks, in marked contrast to potassium iodate solutions.

Heats of Solution. These were determined at 25 "C by injecting gram samples into 200 mL of water and comparing the temperature changes with those caused by injecting po- tassium chloride samples. Magnitudes of heats of solutions were also calculated from slopes of the logarithm of mole fraction solubility against 1/T (K) curves (Table IV). The maximum heat rise in making up a molar solution of NaZSZO3 would be 0.47 "C in comparison with a 2.53 "C cooling for a molar pentahydrate solution.

Rate of Water Uptake. This is the crucial property for analytical use. Previous workers by not defining their pa- rameters have given nonunique figures which could fit a va- riety of conditions. In a series of experiments, samples were exposed to different relative humidities (R.H.) over sulfuric acid solutions at constant temperature. Samples were weighed to the nearest 0.1 mg immediately after removal from the constant humidity chamber. Air in the balance case was ionized with an 241Am a-source to overcome electrostatic charging effects. Moisture uptake was linear for short times, but decreased with time, especially at high R.H. Uptakes were averaged over 21-24 h periods, assuming a linear uptake. In a given cylindrical tube the uptake was independent of weight-upper layers act as desiccant for lower layers-but increased proportionally to the cross-sectional area of the tube. It was also strongly dependent on R.H. above 12% R.H., below which the salt remained anhydrous (Figure 2).

A t a given humidity the moisture uptake can be minimized by using the smallest practical cross section. Thus a cylindrical

E 3 60t / - I .o 0 5

M OISTURE JPTAKE/ rng/hr/cm2

Flgure 2. Effect of humidity on rate of hydration of Na2S20,.

weighing bottle, 7 cm by 2.2 cm diameter, filled with thio- sulfate dehydrated by methanol, would only take up 0.01 % of ita weight of water per hour when exposed to an atmosphere of 50% R.H. at 25 "C, or alternatively it could be left open 10 h before changing weight by f0.1%. This thiosulfate packs by light tamping to 43% of the maximum X-ray density of 2.334 g (8).

Standardization against KIO,. Specimens of anhydrous thiosulfate prepared from methanol were checked against iodine solutions (0.1 M in I) prepared from KIO, (AR) using both starch and amperometric end points (for the latter, the current produced between fixed Pt foil and Ni wire electrodes in a constantly stirred solution was observed). All volumetric ware was calibrated by weight. The Na2S20, and KI03 were equivalent within 0.07%, which is within the limits set by the stated KI03 purity (a buoyancy correction was ignored, since it was less than 0.05% and in any case would further reduce the discrepancy).

ydration has just come to our attention (10). Note Added in Proof. A patent covering methanol deh-

LITERATURE CITED (1) Tomlinson, H. M.; Ciapetta, T. C. Ind. Eng. Chem., Anal. Ed. 1941,

13, 539. (2) Vogel, A. J. "A Textbook of Quantitatlve Inorganic Analysis", 4th ed.;

Longman: London, 1978. (3) Skoog, D. A.; West, D. M. "Fundamentals of Analytical Chemistry",

4th ed.; Saunders: New York, 1982. (4) MacNevln, W. M.; Kriege, 0. H. Anal. Chem. 1953, 25, 767. (5) Duval, C. "Thermogravlmetric Analysis", 2nd ed.; Elsevier: New York,

1963. (6) Young, S. W.; Burke, W. E. J . Am. Chem. SOC. 1906, 28, 315. (7) Cornell. L. W.; Montana, R. E. Ind. Eng. Chem. 1933, 25, 1331. (8) SBndor, E.; Csordis. L. Acta Crystallogr. 1961, 14, 237. (9) Linke, W. F. "Solubilltles of Inorganic and MetaCOrganlc Compounds",

4th ed.; Amerlcan Chemical Society: Washington, DC, 1965; Vol. 11. (10) Dlmsdale, W. H.; Kendall, J. D.; Axford, A. J. British Patent 737295,

1955.

RECEIVED for review April 14, 1982. Accepted July 14, 1982.

Extraction of Various Forms of Sulfur from Coal and Shale for Stable Sulfur Isotope Analysis

Linda M. Westgate" and Thomas F. Anderson Department of Geology, Universiv of Illinois, 130 1 West Green Street, Natural History Building, Urbana, Illinois 6 180 1

Studies of sulfur isotopes may provide unique information about the timing and mechanism of sulfur incorporation in organic-rich sediments. The sulfur isotope geochemistry of sediments is complicated by the fact that sulfur can exist in

several oxidation states and chemical forms. In coal and shale, for example, sulfur can exist as pyrite (FeSZ), other metallic sulfides (e.g., ZnS, PbS, FeS), sulfates (e.g., FeS04, Fez(S04)3, or CaSO,), organic sulfur, which is not well defined in coal

0003-2700/82/0354-2136$01.25/0 0 1982 American Chernlcal Society

ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982 2137

Coal or Shale

11.0 -+ hand-t icked

rnasslv pyrite 9 I 1.2 I . 1.4 I c

L [fract ion 2)

i (fraction 1) acid-solubls8 ! sulfides as . I sulfate sulfur 1

I 11.7 L -

Residue 1

11.5

11.6

[ f ract ion 31 organic sulfur

I

Flgure 1. Diagram of extraction procedure, 1.0 through 1.9 refer to procedure section in text.

or shale, and elemental sulfur (SO), which usually occurs only in trace amounts (1). Recent studies (2-4) have demonstrated that the 34S/32S ratios of these different forms of sulfur can vary by as much as 30% in a single sample. For this reason, sulfur isotope studies can provide useful information only if the isotopic composition of individual sulfur components is analyzed rather than the total extractable sulfur.

Ideally, one analytical method which provides a complete and quantitative procedure for extracting and analyzing the different forms of sulfur is preferred. The method must (a) extract in sequence each form of sulfur without affecting other forms, (b) maintain the isotopic integrity of each fraction, and (c) convert all extracted forms of sulfur to a common stable substance, such as AgzS, which can subsequently be converted to a gas for mass spectrometric analysis. Unfortunately, no one published method exists which fulfills these criteria. However, by combining modified techniques from several different sources, a complete extraction procedure has been developed (Figure 1). It is the purpose of this paper to present a consolidated and complete account of these techniques in a single procedure for analyzing sulfur isotope ratios in coal and shale.

EXPERIMENTAL SECTION Reagents. ( a ) Cadmium acetate solution: dissolve 62.5 g of

cadmium acetate in 2.5 I, of 3.5 M acetic acid. ( b ) Treated tetrahydrofuran: before use, dry over 3 A molecular sieve. (c) Eschka compound: thoroughly mix 3 parts, by weight, of MgO with 2 parts Na2C03. ( d ) Sulfate reduction solution: prepare a solution containing 1.8 M HI, 1.8 M H3P02, and 6.3 M HCl. Boil this solution for at least 1 h to expel any trace of HzS.

(1.0) Massive Pyrite Removal. Before the first chemical extraction of sulfur from a sample, massive pyrite, i.e., that which is visible to the eye, was handpicked from a 5-20-g sample and stored for later conversion to AgzS (section 1.7). The sample was then ground to pass a 230 mesh (62.5 bm) sieve and dried at 60 "C to constant weight.

(1.1) Extraction of Acid-Soluble Sulfides. The chemical extraction of acid-soluble sulfides and sulfates presented here was moditied from ASTM metbiod D2492 (5). A freshly ground, sieved, and dried coal sample wafi weighed in a reaction vessel (item 4, Figure 2a). For each gram of coal, 10 mL of 4.8 M HCl was added. The vessel was attached to the extraction apparatus (Figure 2a) and heated with constant stirring to just below boiling for at least 1 h. Moisture-free N2 gas was flushed at about 0.5 L/min at all times through the extraction apparatus. The acid-soluble sulfides (fraction 1, Figure 1) were released as HzS(g) and precipitated as CdS(s) in the cadmium acetate solution.

Q

Flgure 2. Extraction apparatus: (a) (1) stirring plate, (2), heating mantle, (3) stirring bar, (4) reaction vessel, 500-mL three-neck round-bottom flask, (5) ground glass stopper, (6) condenser, (7) 125 mL of water, (8) 250 mL of cadmium acetate solution, (9) 250 mL of cadmium acetate solution. (b) The same apparatus is used as shown in Figure 2a, except item 5 is replaced by the illustrated 250-mL pressure-equalizing separatory funnel, 2b.

(1.2) After the reaction was completed, CdS was quantitatively converted to AgzS by adding 0.05 M AgN03 The AgzS precipitate was fiitered through a Teflon filter (-15 km), rinsed wth ",OH, and washed thoroughly with distilled water until the filtrate reached pH 7 . The filtrate was discarded, and the AgzS was dried at 60 "C to constant weight and stored for combustion to SOz (section 1.9).

(1.3) The residual solids in the reaction vessel (residue 1) were filtered and rinsed with 35 mL of 0.5 M HCl. Both the filtrate and rinse were retained for the determination of sulfate sulfur (fraction 2). The solids were washed further with distilled water until the new filtrate reached pH 4. This filtrate was discarded and the residue was dried at 60 "C to constant weight.

(1.4) Extraction of Sulfate Sulfur. To the filtrate containing sulfate sulfur (fraction 2), 10-20 mL of saturated bromine water was added; and the solution was made slightly acidic with con- centrated HC1. To expel liberated bromine, we boiled the solution until it was colorless. (The solution must be below pH 4. This can be tested by using methyl red.) Then 10-20 mL of 0.5 M BaClz was added, and the solution was maintained just below boiling for at least 4 h to allow precipitation of BaS04. The BaSO, was then filtered through an ashless filter and washed thoroughly with hot water. After, the filter and precipitate were placed in a weighed crucible, the paper was carefully ashed with a burner. When most of the paper was consumed, the crucible and contents were placed in a muffle furnace and heated to 925 "C for ap- proximately 2 h. After cooling, the BaSO, was stored for con- version to AgzS (section 1.8).

(1.5) Extraction of Disseminated Pyrite. A 3-5-g sample of residue 1 from the acid extraction (section 1.3) was placed in a reaction vessel and attached to the extraction apparatus (Figure 2b). Carefully, 7.5 g of lithium aluminum hydride (LAH) was added to residue 1 (1.5 g of LAH is sufficient to reduce up to 200 mg of pyrite). A gentle flow (0.5 L/min) of Nz gas was started.

2138 ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982

Table I. Examples of Sulfur IsotoRic Determinations on Coals and Shales

location repeatability of (Herrin Coal)

extraction procedure 11, B (Herrin Coal)

RK, T (Herrin Coal)

0 3 , S1B

(Anna Shale) c2

Shale) C2 (Energy

repeatability of Ag,S (Herrin Coal) combustion procedure C 2 , 2 A

(Herrin Coal) C2, SAT

(Herrin Coal) C2, SAM

(Herrin Coal) 11, T

(Herrin Coal) RK, M

(Herrin Coal) RK, B

(Herrin Coal) 0 3 , S1M

(Herrin Coal) 0 3 , S2M

(Anna Shale) c2

sulfur forms in shale

(Energy Shale) C2

sulfur forma

os

os

DP

os

DP

DP

MP

os

DP

os

DP

os

os

os

os

os

DP

os DP

os MP

w t % S

0.55 0.60 2.71 2.32 1.25 1.23 1.50 1.58 1.63 1.56 5.84 5.87 5.84 5.66

2.26

3.17

2.68

0.21

0.70

2.55

2.43

2.07

0.89

1.60 f 0.03

0.96 5.85 f 0.02

0.21

s("SCDT),b O i m

+ 5.92 + 5.60 -7.41 -7.83 + 8.78 t 8.64 +9.14 +8.61

-15.28 -15.14 -14.68 -14.21 -14.21 -13.51 + 16.91 + 16.73

-0.25 -0.34 -3.32 -3.29 -5.38 -5.42

+17.86 + 17.75

+7.23 +7.17 -6.37 -6.24 -3.11 -3.11 + 4.49 + 4.48 +4.17 +4.14

-15.21 +- 0.07

-9.14 -14.30 i 0.22

-3.73 -2.76

mean dev, "I,

i0.16

0.21

0.07

0.26

0.07

0.32 (lu)

0.09

0.04

0.02

0.02

0.06

0.03

0.06

0.00

0.00

0.02

a DP, disseminated pyrite; OS, organic sulfur; MP, massive pyrite. Sulfur isotope ratios are reported as permil deviations from Canyon Diablo Triolite (CDT) in the usual S terminology: S ( % ) = [('S/BS)mple - (34S/32S)CDT]/("S/32S)CDT X 1000, '/-. All sulfur isotope measurements were done on a MAT 250 isotope ratio mass spectrometer with the source and inlet system at 90 "C. The S(=S) values are corrected for instrument biasing and for contributions of I8O to mass 66.

Then, 75 mL of treated tetrahydrofuran (THF) was added to the pressure-equalizing funnel and slowly added to the LAH-coal mixture (50 mL of THF for every gram of LAH was used). With constant stirring, this mixture was heated to 85 "C for 45 min.

After the mixture was heated, the traps were disconnected from the condenser and the heating mantle was replaced with an ice bath. The condenser was reconnected to the traps and 75 mL of distilled water, same amount as THF used, was added to the pressure-equalizing funnel and DROPWISE added to the mixture. The intensity of the reaction governed the rate at which the water was added. Next, and equal volume of concentrated HC1 was placed in the pressure funnel and slowly added to the mixture.

The ice bath was then replaced with the heating mantle and the mixture was heated to 90 " C until there was no further release of H2S (at least 2 h). The CdS precipitate was conirerted to Ag2S as indicated in section 1.2. The LAH-coal residue (residue 2) was filtered, washed with distilled water, and dried to constant weight at 60 "C. Residue 2 was used for extraction of organic sulfur.

(1.6) Extraction of Organic Sulfur. Organic sulfur was determined by using a total sulfur analysis modified from ASTM method D3177 (6). A 1-3-g sample of residue 2 from the LAH reduction (section 1.5) was weighed and thoroughly mixed with 3 parts of Eschka compound (w/w). The Eschka-coal mixture was placed in a crucible and covered with more Eschka compound so that no coal remained exposed. The crucible was then placed in a cold muffle furnace which was slowly heated to 800 "C. This temperature was maintained for 1.5 h. The sample was allowed to cool slowly to room temperature in the furnace.

Solids from the crucible were placed in a beaker, and 100-150 mL of hot water was added. The beaker was covered with a watch glass and heated at a moderate temperature for 30 min, stirring occasionally. This solution was then filtered, znd the solids were washed with hot water. The fitrate (fraction 3), containing organic sulfur derivatives, was retained and treated as in section 1.4. The solids were discarded.

(1.7) Massive Pyrite Conversion to Ag,S. The handpicked massive pyrite (section 1.0) was ground to pass a 230 mesh (62.5

ANALYTICAL CHEMISTRY, VOL. 54. NO. 12. OCTOBER 1982 2139

discussion of the LAH reduction method in comparison to other oxidation and reduction methods in determining the forms of sulfur in coal has been presented hy Kuhn et al. (12). To teat the specificity of LAH for disseminated pyrite in coal, we added a spike of dihenzothiophene (DBT), a likely (but not the only) organic sulfur component of coal (13), containing 0.229 g of sulfur to a coal sample containing 0.017 g of pyritic sulfur and reacted the sample with LAH as in section 1.5. A 0.018-g portion of pyritic sulfur was recovered fmm the spiked sample demonstrating that, a t most, only 0.4% of the DBT sulfur was liberated by the LAH procedure. We did not perform experiments to check the quantitative yield of pyritic sulfur by LAH extraction. However, Price and Shieh (3) report that more than 94% of the disseminated pyrite is recovered from <230 mesh coal powder by LAH treatment. These results suggest that crass contamination of pyrite and organic sulfur is negligible (<5%) in our extraction procedure.

The repeatability of the sulfur extraction procedure is il- lustrated by the replicate analyses shown in Table I. We have to date completed three duplicate extractions of diaseminated pyrite and of organic sulfur from our samples. Average de- viations from the mean sulfur isotope ratio of replicate ex- tractions (f0.07 to 0.33%) are slightly higher than those on replicate combustions of Ag& samples ( i O . 0 0 to 0.09%). This comparison indicates that the chemical extraction procedure does not introduce signiiicant uncertainty in the measurement of sulfur isotope ratios.

The procedure has also been applied with good results to the extraction and isotopic analysis of sulfur in shales overlying coal seams. Some initial results on sulfur in shales are given in Table I. Since the organic matter content of shales is much lower than that of coal, one must use a larger initial sample to ensure a t least 5 mg of sulfur in the organic extract, which is the detection limit of this procedure. In conclusion, the composite of methods we describe provides a direct and quantitative procedure for extracting different fo rm of sulfur from coal and shale for the measurement of sulfur isotope variations.

ACKNOWLEDGMENT The authors express their thanks to F. T. Price for his

technical advice.

LITERATURE CITED (11 Rms, 0. W. Rep. Inwst. In. state Geol. Smv. low, No, 220. (2) Smiih. J. W.: Batts, B. 0. oecchim. Mmmhbm. Acfa 1974, 38. ..,. .**

(b)

HV

3

5

I

3 4

Figun 3. SO, combustion line: (a) (1) quartz sample boat, (2) quartz combustion tube. (3) quartz wool plug; (b) (1) open end combustion furnace on rollers, (2) same as 3a, (3) 87844 Threaded Ace thread connectors and bushing, (4) cold trap, (5) 6 m m glass sample tube; (e) 88194, Ace Glass high vacuum TeRon stopcock; LV. low vacuum; HV. high vacuum; VG. vacuum gauge; MN. manometer.

pm) sieve. An appropriate sample was weighed in a reaction vesnel and treated as described in section 1.5.

(1.8) Conversion of BaS04 to &a. This methd is modified from Thode et al. (7). A sample of BaSO, was weighed in a reaction vessel and attached to the extraction apparatus (Figure 2a). For every 200 mg of BaSO,, at least 100 mL of sulfate reduction solution was added. The reaction vessel was heated to 95 OC until the reaction was complete. CdS(s) was converted to Ag#(s) as described in section 1.2.

(1.9) Ag,S Combustion to SO,. AU samples, in the form of Agg, were combustd to SOI using a modified method described by Fritz et al. (8). The SO, p rodud w&s uaed for isotopic analyse? in a ratio mas^ spectrometer. For combustion, 25 mg of Ag,S was mixed thoroughly with 50 mg of CuO and placed in a quartz boat (illustrated in Figure 3a). The boat was placed in a quartz tube and attached to the SO, combustion line (Figure 3b). The com- bustion line was evacuated and the sample combusted at 950 OC for 8 min. Any water generated during combustion was separated by a -70 OC isopropyl alcohol/dry ice slurry, and any CO, gen- erated was removed with an ethanol/liquid nitrogen slurry maintained at a temperature below -125 'C. The purified SO, was measured with a mercury manometer and stored in 6-mm glass break-tubes for isotopic analysis.

Caution: Extreme care should he taken when handling LAH powder. LAH can react explosively with water.

RESULTS AND DISCUSSION We have used this procedure over the past 2 years in an

investigation of the sulfur isotopic composition of coals and overlying sediments in the Illinois Basin (4). In our analyses of coal and shale, fresh samples were taken from working faces of mines and were analyzed as soon thereafter as possible. A minimum delay between collection and analysis would limit the extent of pyrite oxidation in moist air and the effects of kinetic isotope fractionation in that oxidation. If immediate analysis was not practical, samples were sealed in an airtight container with a sulfur-free desiccant. Sulfate sulfur deter- minations were not performed routinely due to the low sulfate content of the coals we analyzed (9). Nevertheless, the acid extraction procedure was performed to remove any carbonate and other acid-soluble components from the matrix.

The reduction of pyrite using LAH was first described by Smith et al. (IO) following Gaylord's (11) report that LAH would not attack organic constituents in shale. A complete

,<,-,"". (3) Price. F. T.: Shbh. Y. N. E m . oeol. 1979, 74, 1445-1461. (4) Westgate. L. M.; Anderson, T. F. Annu. Meem W. SOC. Am.,

Ah.* 40.4 1" c10 -111. ."_.. ,"."._.

(5) "Annual W Of Standards": American Sod* for TeSnng and Male- rials: Philadelphia. PA. 1977; ASTM Stand. 02492-77, Part 26. pp 399-196 _ _ _ _ _ _.

(6) "Annual Bwk of Standarw American Socbty for Tesnng Matwlais: Philadelphia. PA. 1975: ASTM Stand. 03177-75, Part 26, pp 383-389.

(71 Thcde, H. G.; MoMter. J.; Dunfad, H. 8. osochm. CosmochNn. Acta 1961. 25. 159-174.

(8) Fritz. P.: Dummie. R. J.: Nowbkl. V. K. Anal. chem. 1974, 46. 164-166.

(91 Glurkoler, H. J.; S k m . J. A. WC. I n . state oeol. Suv. 1966. 432. (10) Smm. J. W.: Young. N. 0.; LBwIw. D. L. Anal. m. lD64, 36.

CICl-liJJ I .-~-_-.

(1 1) Gaykd. N. 0. '"Redwtbn Mlh Cmnpkx Metal Ky&W Imersdace.

(12) Kuhn, J. K.: Kohbnberger. L. E.: S h i v . N. F. Envkar. oeol. No&

(13) Attar, A. Am. Inst. Chem. EN.. A b @ . 1980. 7Oc, T-106.

New YO&. 1956 p 1024.

(I l l . Sa te oeol. S w . ) 1973. 66.

RECEIVED for review May 3,1982. Accepted July 13,1982. This material is based upon work supported by the Office of Surface Mining, Department of the Interior under Grant No. G5115171. Any opinions, findings, and conclusions or r eo ommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Office of Surface Mining, Department of Interior.