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Anal. Chem. 1982, 5 4 , 971-974 (5) Yoshlda, T.; Maekawa, Y.; Hlguchl, T.; Kubota, E.; Itagaki, Y.; Yokoyama, S. Bull. Chem. SOC.Jpn. 1881, 5 4 , 1171. (6) Mead, W. L. Anal. Chetm. 1968, 4 0 , 743. (7) . . SCheDDele, S. E.; Grizzle, P. L.; Greenwood, G. J.; Marriott, T. D.; Perrelra,'N. 0. Anal. Chem. 1976, 48, 2105. (8) Yokoyama, S.; Tsuzukl, N.; Kato, T.; Sanada, Y. J. Fuel SOC.Jpn. 1978, 5 7 , 74a.
(9) Hlrsch, D. E.; Hopklns, R. L.; Coleman, H. J.; Cotton, F. 0.;Thompson, C. J. Anal. Chem. 1972, 4 4 , 915.
for review November 2, lggl. Accepted January 25, 1982.
Determination of Sulfur-Containing Species in Solids by Molecular Emission Cavity Analysis Jau-Hwan Tzeng and Qulntus Fernando* Department of Chernlstty, Unlversify of Arizona, Tucson, Arizona 8572 1
Two types of flames, the nltrogen-cooled and the argoncooled hydrogen flame, have been used for the determination of sulfur-contalnlng specles In solids by molecular emlsslon cavity analysis (MECA). The argon-cooled flame has a much greater sensltlvlty than the nltrogen-cooled flame for the determination of Sod2-. I n a solid mlxture containing Sa, s*-, SOa2-, and SO,2", the presence of one or more of these sulfur-contalnlng species can be determlned with the ald of the argon-cooled flame. The nltrogen-cooled flame Is useful in speclal cases, for example, In the determination of the comwhich are present In a ponents of a mixture of Sa and SO-: solld matrix. All these sulfur-contalnlng species can be quantltatlvely deterrnlned In the argon-cooled flame In the concentration range from about 10 to 5000 ppm. The varlatlon from 10% to 20% In the reproduclbillty of these measurements has been attrlbuted to the nonhomogeneltyof the solid materials and the mall sample sizes that had to be used In these determlnatlonr.
A method based on MECA (1) was proposed for the determination of s", Sg, SO3%,and SO?- in solids (2,3). In this method, a solid inample containing one or more of these sulfur-containing species was weighed in a small aluminum cup which wasi placed in a cluartz-lined cavity that was fitted on the end of' a stainless steel rod. A solution containing phosphoric acid and a wetting agent was added to the solid sample and the end of the steel rod was positioned in a relatively cool hydrogen--nitrogen flame. The emission from the molecular sulfur that was produced in the cavity was recorded a t 384 nm as a function of time. The peaks in the emission spectrum were identified with each of the sulfur-containing species in the solid sample, and the peak areas or peak heights were used in conjunction with calibration curve to determine the concentration of each of the sulfur species. In this method, it is evident that there are several variables that must be carefully controlled to obtain the maximum sensitivity and an acceptable reproducibility. The sensitivity of the method is governed by the intensity of the flame emission, which depends on the population of the molecular sulfur species, Sz, in the excited state, and their residence time in the flame. Allhough the mechanism of formation of molecular sulfur from the various sulfur-containing species is not fully understood, it has been established that several characteristics of the flame, in particular its temperature, affect the emission intensity. The excited state Sz molecules that
emit are almost exclusively produced by radical interactions rather than by thermal excitation. If monatomic argon gas is substituted for the diatomic nitrogen gas in the MECA flame, the number of radicals produced by collisional processes in the flame should increase. As a consequence, the number of Sz molecules in the excited state should also increase. We have observed that, under comparable conditions, the emission intensity of Sz molecules produced in a hydrogen-argon h e is greater than that in a hydrogen-nitrogen flame. In this work we have compared the use of these two types of flames in the determination of sulfur-containing compounds by MECA. Additional variables that affect the emission intensity are the concentrations of the components of the wetting solution (H3P04and Triton X-100) and the surface composition of the material that was used in the fabrication of the sample cup. In all our previous work that was reported ( 2 , 3 ) ,small aluminum foil cups, 3 mm in diameter and 2 mm deep, were used, and no special attention was paid to the nature of the aluminum surface. In subsequent experiments, however, we have observed that the emission intensity produced in the cavity is dependent on the previous history of the aluminum foil surface. In the work that is reported below, we have adopted an empirical approach in our attempts to maximize the signa1:noise ratio and to separate the emission peaks that are obtained from solid samples containing sulfur in several oxidation states. The effects of all the variables mentioned above have been studied, and an optimum set of operating conditions has been found for the identification and determination of sulfur-containing molecules and anions in a solid matrix. Attempts were made to use this MECA technique for the determination of sulfur-containing species in coal. EXPERIMENTAL SECTION Equipment. A modified flame emission system which was
described in detail in an earlier publication (2) was used for all the emission measurement. The burner assembly included an inlet for premixed hydrogen, the coolant gas (Nzor Ar), and air. The modular flame emission system consisted of a scanning monochromator coupled to a photomultiplier detector with the output displayed on a fast response strip chart recorder. The MECA sample introduction device was the same as that used in all previous experiments (2, 3). Reagents. Solid standards containing one or more sulfur species were prepared from Analytical Reagent grade chemicals: elemental sulfur as sublimed sulfur (Ss); sulfite as NazSO,; sulfate as Na$304;sulfide as CuS (Ultrapure 99.998%) or PbS (Ultrapure 99.999+%). A range of concentration of the solid sulfur containing species was obtained by the laborious process of serial dilution
0003-2700/82/0354-0971$01.25/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
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to the sample and the sample holder rotated into a fixed position in the flame. The emission intensity at 384 nm was recorded as a function of time ( t , s).
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RESULTS AND DISCUSSIONS The emission spectrum of the Hz/Nz/air flame at the optimized gas flow rates of 0.61, 2.4, and 0.69 L min-I, respectively, and the emission spectrum of the Hz/Ar/air flame at the optimized flow rates of 0.78, 1.6 and 0.27 L min-' show the same spectral bands (Figure 1). The only significant difference is the OH emission band a t about 306 nm which is quite intense in the Hz/Ar/air flame but relatively weak in the H2/Nz/air flame. The emission intensity of molecular sulfur in the argon-cooled flame should, therefore, be much greater than that in the nitrogen-cooled flame, if the principal mechanism for the promotionof S2molecules from the ground state to an excited state involved OH radicals and all other variables such as sample composition and wetting solution component were kept constant. The emission spectrum of molecular sulfur (S2) superimposed on the background is shown in Figure 2. The band at 384 nm is free of interferences from the background and can be used to monitor the intensity of the molecular sulfur emission. The values oft,, the time a t which the characteristic S2 emission maximum appeared in the cavity when positioned in the Hz/Ar/air flame, were recorded for sulfide, elemental sulfur, sullite, and sulfate. If H3P04was not added to the solid sample, sodium sulfite and elemental sulfur, S8,gave emission peaks with the same t, value (2 9). Copper(I1) sulfide and lead sulfide gave two peaks, a very weak peak at t, = 2 s and a very strong peak at t, > 30 s, but no peak was observed for sodium sulfate. The results were quite different when the emission spectra were recorded after the addition of H3P04 solution to the solid samples; sodium sulfite gave three peaks, a very strong peak at t, = 4 s and two extremely weak peaks at t, = 12 and 17 s; elemental sulfur gave one peak a t t, = 12 s; copper(I1) sulfide or lead sulfide gave two peaks, a moderatly strong peak at t, 12 s and a very strong peak at t, > 40 e; sodium sulfate gave a single strong emission peak a t t, = 17 s. The relative intensities of the peaks in the emission spectra of sulfur-containing solids in the H2/Ar/air flame in the presence of the H3P04 solution and in the absence of the solution are shown in Figure 3, and the t, values together with the detection limits (signahoise = 2:l) for the various species in the H2/Ar/air flame are collected in Table I. The corresponding values obtained in the H2/N2 flame
II nm
Figure 1. (A) Emission spectrum of the H,/N,/air flame at flow rates of 0.61, 2.4, and 0.69 L min-'. (B) Emission spectrum of the H,/Ar/air flame at flow rates of 0.78, 1.6, and 0.27 L min-'. The emissions at 367 nm and above are from the external iliumlnatlon in the laboratory and are independent of flame composition. 381
106
135
311
311
nm
Figure 2. Emission spectrum of S2 in the H,/Ar/air flame. The Intense band at 437 nm which is crosshatched arises from the external iliumination in the laboratory. The S2 emisslon and the external illumination contribute to the diagonally shaded bands at 405 and 365 nm. The S2 emission spectrum was produced by pure Na,SO, at a scan rate of 20 A/s. The full width at half-maximum of the peak at 384 nm was 4 nm. with SiOz in polystyrene vials with the aid of a Wig-L-Bug as described previously (2). Analysis. The solid sample was weighed in a small aluminum cup which was fitted snugly into a quartz cup at the end of a stainless steel rod. About 13 pL of a mixture of 0.075 M H3P04 and Triton X-100 (13 pL/100 mL of 0.075 M H,PO,) was applied E
d
E
D
C
G
b-
01
_.
LL
'IIII/II//L/1111I/
s. Figure 3. Emission spectra of sulfur-containing solids in the H,/Ar/air flame: (A) 300 ppm of sulfur as PbS; (B) 600 ppm of sulfur as Na,SO,; (C) 600 ppm of elemental sulfur; (D) 40 ppm of sulfur as Na,SO,; (E) 20 ppm of elemental sulfur; (F) 40 ppm of sulfur as Na,SO,; (G)600 ppm of sulfur as CuS. A, B, C, and D were obtained after the addition of 13 pL of the wetting solution containing H,PO,. No wetting solution was added to solids in E, F, and G. Ail solid samples were prepared in SO2. 30
60
15
45
15
A5
15
E5
'5
45
15
45
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
Table I. Comparison of MECA Characteristics of Solid Sulfur-Containing Compounds tm, s
compd
s*
H,/Ar/air flame no H,PO, H,PO, added added
PbS
2 2
cus
35 2
Na,SO,
40 2
12(9)" 12 45(16)" 15 60 4 (17)a
-H,/N,
flame
H,PO, added
H,PO, added
4 5
20-22 27
7
20-22
5
1-5
no
12 17
17 (5)" > 35 Na,SO, The quantities in parentheses are the detection limits in nanograms of sulfur/milligrams of sample. a
are included in Table I for purposes of comparison. It must be emphasized that the t , values are dependent on a number of variables, the most important of which are the flame composition and temperature, the surface composition of the sample cup, the position of the sample holder in the flame and its rate of heating, and the composition of the solid matrix that contains the sulfur species. All these variables have to be rigidly controlled in order to obtain reproducible t, values. The results obtained from a series of simple experiments emphasized the importance of the sample cup material, especially in quantitative determinations of sulfur-containing compounds by MECA. No sulfate emission peak was observed in the Hz/Ar/air flame after the injection of 5 MLof a solution of 4 X M HzS04and 5 MLof the wetting solution into a sample cup made of copper. If a platinum cup was used, the sulfate emission peak was observed, but its intensity was enhanced by a factor of 2 when an aluminum sample cup was used. If the sample cup that was made from bright aluminum foil was first heated in the argon-cooled flame and then employed for the analysis, the intensity of the sulfate emission peak was enhanced by an additional factor of about 4. The surface of the aluminum cup, therefore, plays an important role in the promotion of the molecular sulfur emission. In the nitrogen-cooled flame the S2 emission peak from SO:- appears first (t, < 5 s) and is followed by the peaks from Sz- and S8 which are difficult to resolve (20 s Ct, < 27 9); the emission peak from SO:- appears at t, > 35 s. The sequence in which the emission peaks appear in the argon-cooled flame is different. The emission peaks from Sa, S03z-, and S2-, all appear at t, < 15 s and overlap to a significant extent; a strong well-resolved peak from S042-appears at t, = 17 s, and a broad peak, which is the second emission peak from Sz-, appears at t, > 40 s. The argon-cooled flame, therefore, is ideally suited for the determination of the sulfate anion in solid matrices. The low detection limit for S042-is attributable to the higher temperature and increased production of OH radicals in the argon-cooled flame and to a decrease in the quenching of SZ*, the S2 species in the excited state. The anions S2-and SQ32-give a multipeaked response in this flame (Figure 3 and Table I). The f i s t molecular emission peak from S2- ( t , = 12 s) arises from the HzS that is formed in the presence of the acid solution, and the second broad emission peak ( t , > 40 s) probably arises from a polymeric sulfur species that is formed in the vapor phase. The three-peak sequence for SO3'- can be explained as follows: the first high intensity peak (t, = 4 s) arises from the SOzgas that is initially produced in the cavity. Subsequent reactions in the reducing flame can form species such as SH and HzS which can give rise to the very weak emission peak at t , = 12 s. The third
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peak at t , = 17 s, which is also very weak, can be attributed to the small amount of SO:- that is present in the solid Na2S03. The common sulfur anions and elemental sulfur in solid matrices can be identified by MECA with the aid of the H,/Ar/air flame. If the H3P04 solution is not added to the solid sample, the only sulfur species that can be unequivocally identified (at t, > 30 s) is the sulfide ion, but there is no particular advantage in not adding the H3P04solution because each of the species in a four-component mixture can be readily identified after the addition of the H3P04solution to the solid sample; elemental sulfur and S032-give overlapping emission peaks at t, < 15 s, the S042-peaks appears at t, = 17 s, and the presence of Sz- is determined by the peak that appears at t, > 30 s. The SO:- in a fresh solid sample is oxidized to S042-by the addition of hydrogen peroxide solution; upon repeating the analysis, the appearance of a strong peak at t,, = 12 s confiims the presence of elemental sulfur in the sample and the enhancement of the emission intensity a t t, = 17 SI indicates that S032-was also present in the solid sample. For the qualitative analysis of sulfur-containing solids by MECA, the argon-cooled flame is much more sensitive than the nitrogen-cooled flame and is especially useful for the determination of S042-in a variety of solids which do not have to be dissolved. The peaks that arise from elemental sulfur and Sz- overlap in the nitrogen-cooled flame but are well separated in the argon-cooled flame. An advantage of the nitrogen-cooled flame, however, is that the peaks from S032and elemental sulfur are well resolved. Elemental sulfur, S2-, and S042- could be quantitatively determined from the linear relationships that were obtained between the emission intensities and the weight of sulfur present in the standard solid samples. Plots of log (emission intensity) vs. log (grams of sulfur) were linear for S8 from 10 to 600 ppm, for Sz- from 16 to 2500 ppm, and for SO:- from 5 to 5000 ppm. The slopes of these log-log calibration curves were 1.5, 1.1, and 1.5 for Sa, Sz-, and SO:-, respectively. The calibration curves for S032-,however, were curved and were of limited use for the quantitative determination of SO:- in solids by MECA with the argon-cooled flame. This curvature was caused by the increased loss of SOz, upon addition of the H3P04 solution, with increasing amount of NaZSO3.Between 10 and 30 determinations were carried out for each sulfurcontaining species in the concentration ranges given above The reproducibility of these measurements varied between 10 and 20%. The main reason for this poor reproducibility is the nonhomogeneity of the small samples (1-2 mg) of the, solid standards that were prepared by serial dilution with solid SiOz and analyzed in the MECA sample cup. Additional. reasons for the poor reproducibility are the variable surfaces composition of the aluminum sample cup and the traces of sulfate that are present in the SiOz that was used for the preparation of the standards. The only method that is available for the quantitative determination of Sa, S", S032-,and SO-: at parts-per-million levels in milligram quantities of solid samples is MECA. The method is simple and rapid and, despite its shortcomings, can be employed to determine each or all of these sulfur-containing species in solid samples by the judicious use of the nitrogen-cooled flame and the argon-cooled flame. For example, SO3'- in a solid sample is best determined by measurement of the peak area at t, I 5 s in a nitrogen-cooled flame and trace levels of S042- in solids are readily determined by measurement of the peak area at t, = 17 s in an argon-cooled flame. The determination of total sulfur and the various sulfur forms that are found in coal is an important but formidable analyical problem. The recommended method that has gained
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
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Flgure 4. MECA spectrum of a sample of coal (Department of Energy No. K-95007): (A) H,/Ar/air flame: (B) H,/N,/alr flame. The wetting solution containing H,PO, was added to the solid in A and in B.
wide acceptance for routine laboratory use is Walker and Hartner’s method (4). In this method the SO:-, which is usually less than 0.05%, is determined as BaS04 after the coal is extracted with dilute HC1. The pyritic sulfur, mainly found as FeSz, in the residue from the HC1 extraction is dissolved in dilute HNO,, and the iron in solution is determined by permanganate titration. The pyritic sulfur is calculated from this iron determination and the organic sulfur is calculated as the difference between the total sulfur, determined by the ASTM method (5),and the sum of the sulfate sulfur and the pyritic sulfur. A sample of coal (K-95007) was obtained from the Department of Energy and the following analytical data were provided for this sample: total sulfur 3.28%; organic sulfur 0.86%; pyritic sulfur 2.40%;sulfate sulfur 0.02%. The MECA emission spectra in the argon-cooled and in the nitrogen-cooled hydrogen flame are shown in Figure 4. The emission peaks occur in three regions in the argon-cooled flame: (a) t, < 10 8, (b) 10 s C t, C 15 s, and (c) 15 s < t, C 25 8. The molecular sulfur emission peaks, in the region, t, < 10 s, arise from organic sulfur species that are present in coal. If the coal is exhaustively extracted with CHC13, and the MECA spectrum of the residue is obtained, the peaks in region (a9 are eliminated and the peak intensities in region (b) are diminished considerably. This indicates that some of the organic sulfur species contribute to the emission peaks in the region 10 s < t , < 15 s. The main contribution to the emission spectrum in this region is from the pyritic sulfur. The Walker and
Hartner method for the determination of sulfate sulfur in coal is based on its extraction with dilute HCl. This dissolves the sulfate sulfur and leaves the pyritic and organic sulfur in the insoluble residue. Analysis of this residue by MECA indicated that some organic sulfur was also dissolved in the extraction step with dilute HC1 because the emission peak intensities in region (a) were considerably reduced. Whether some of these solubilized organic sulfur containing species are eventually converted into is not known. It is important to identify all the soluble sulfur containing species in the dilute HC1 extract before any valid claims are made about the Walker and Hartner method for the determination of sulfate sulfur in coal. It was observed that the intensity of the peak from sulfate sulfur in region (c) (15 s C t, C 25 s) was considerably lower in the argon-cooled flame than in the nitrogen-cooled flame (Figure 4). This is a direct contradiction of the previous experimental results which showed that the argon-cooled flame is much more sensitive than the nitrogen-cooled flame for the determination of Sod2-in solids. A plausible reason for the reduced sensitivity in the argon-cooled flame is that in solid matrices, such 89 coal, which contain a large amount of carbon, reductive processes occur in the MECA cavity with the formation of species containing sulfur in lower oxidation states. The formation of CH radicals that have molecular emissions between 380 and 400 nm may also interfere with MECA determination of sulfur in coal (6). The results of our preliminary experiments with coal have shown that it is not possible to determine the various forms of sulfur in this complex matrix by employing the usual MECA procedures. Successive extraction steps must be carried out with organic solvents, dilute acids, and mixtures of acids. All sulfur species that are in the residue and that are in solution must be identified after each extraction step. In addition to the many variables in the MECA technique, the flame temperature will have to be controlled to minimize reductive processes that can occur in the presence of carbon. We are attempting to solve many of these difficult experimental problems that have been encountered in the determination of sulfur-containing species in coal by MECA. LITERATURE CITED (1) Burguera, M.; Bogdanskl. S. L.; Townshend. A., CRC Crlt. Rev. Anal. Chem. 1080, IO, 185-248. (2) Schubert, S. A.; Clayton, J. W.; Fernando, Q. Anal. Chem. 1070, 51, 1297. (3) Schubert, S. A.; Clayton, J. W.; Fernando,Q. Anal. Chem. 1080, 52, 963. (4) Walker, F. E.; Hartner, F. E. Int. Clrc.-US., Bur. Mines 1088, No. 8301 ( 5 ) Annu. Book ASTM Stand. 1080, Part 28 (Standard Test Methods for Total Sulfur in the Analysls of Samples of Coal and Coke, D 3177-75), 400-406. (6) Patterson, P. L.; Howe, R. L.; Abu-Shumays, A. Anal. Chem. 1078, 50, 339. I
RECEIVED for review December 14,1981. Accepted February 16, 1982.
