Anal. Chem. 1980, 52, 1511-1515
operations can be looked at as functions switching between +1 and -1. Several of these Walsh functions (35) are square waves, and these have been indicated with their respective frequency as w , Zw, etc. We note that these are the very switching functions used in synchronous detection; in fact, both the in-phase and quadrature switching functions are present, as indicated by the subscripts i and q respectively. Some Walsh functions, identified by an asterisk, are not square waves and are not useful to our purposes. Clearly, application of the Hadamard transform accomplishes the task of synchronous detection in a digital way, without the need for physical switches as in a lock-in amplifier. A number of simple “fast” Hadamard transform programs are available in the literature (36-38). ACKNOWLEDGMENT T h e authors are grateful to Tetsuya Osaka for calling our attention to the work of Hayakawa et al., to W. C. Craig for building the high-speed clock and for outfitting our computer with a pause instruction, and to M. Krishnan and C. Chang for technical assistance. LITERATURE CITED (1) Grahame, D. C. J . Am. Chem. Soc. 1941, 6 3 , 1207. (2) MacAleavy. C. Belgian Patent 443003, 1941. (3) Breyer, B.; Gutmann, F. Trans. faraday Soc. 1946, 42, 645. (4) Jessop, G. British Patent 640 768, 1950. (5) Jessop, G. British Patent 776 543, 1957. (6) Evilia, R. F.; Diefenderfer, A . J. Anal. Chem. 1967, 3 9 , 1885. (7) Smith, D. E. Anal. Chem. 1963, 3 5 , 1811. (8) Hayes, S.W.; Reilley. C. N. Anal. Chem. 1965, 3 7 , 1322. (9) de Levie, R.; Husovsky, A. A. J . Electroanal. Chem. 1969, 20, 181. (10) de Levie, R.; Kreuser, J. C. J , Nectroanal. Chem. 1969, 2 7 , 221. (11) Glover, D. E.; Smith, D. E. Anal. Chem. 1972, 44, 1140.
1511
(12) Hueber, B. J.; Smith, D. E. Anal. Chem. 1972, 44, 1179. (13) Creason. S. C.; Smith, D. E. J . Nectroanal. Chem. 1972, 3 6 , A l . (14) Creason, S.C.; Hayes, J. W.; Smith, D. E. J . Electroanal. Chem. 1973, 4 7 , 9. (15) Hayakawa, R.; Wada, Y . I€€ Conf. Public. 1979, 777, 396. (16) de Levie, R.; Thomas, J. W.; Abbey, K. M. J . Electroanal. Chem. 1975, 62, 1 1 1 . (17) Airey, L.; Smales, A. A. Analyst (London) 1950, 75, 287. (18) Hans, W . ; Henne, W.; Meurer, E. Z . Elektrochem. 1954, 5 8 , 836. (19) Grahame, D. C.; Poth, M. A.; Cummings, J. I. ONR Tech. Rept. 7, Dec. 13, 1951. (20) de Levie, R. Anal. Chem. 1960, 52,Aid, this issue. (21) Melik-Gaikazyan, V. 1. Zh. fir. Khim. 1952, 26, 560. (22) Barker, G. C.; Jenkins, I. L. Ana/yst(London) 1952, 77, 685. (23) Barker, G. C. Anal. Chim. Acta 1956, 78, 118. (24) Leikis, D. I.; SevasGanov. E. S.;Knots, L. L. Zh. f i z . Khim. 1964, 38, 1833. (25) de Levie. R. J . Nectroanal. Chem. 1965, 9 . 117. (26) Grahame, D. C. J . Am. Chem. Soc. 1946, 68,301. (27) Gardner, A. W. “Polarography 1964”;Macmillan: London, 1966;p 187. (28) Newman, J. J . Electrochem. Soc. 1970, 117, 198. (29) Smith, G. S. Trans. Faraday Soc. 1951, 47, 83. (30) Bresle, A. Acta Chem. Scand. 1956, 10,942. (31) Schwall, R. J.; Bond, A. M.; Loyd, R. J.; Larsen, J. C.; Smith, D. E. Anal. Chem. 1977, 49, 1797. (32) Mohilner. D. M.; Kreuser, J. C.; Nakadomari. H.; Mohilner, P. R. J . Electrochem. Soc. 1976, 123. 359. (33) Garreau, D.; SavBant, J. M.; Tessier, D. J . Electroanal. Chem. 1979, 703, 321. (34) Miller, B.; Bellavance, M. I.; Bruckenstein, S.Anal. Chem. 1972, 44, 1983. (35) Walsh, J. L. Am. J . Math. 1923, 5 5 , 5. (36) Ulman, L. J. I€€€ Trans. Comput. 1970, C19, 359. (37) Shum, F. Y. Y.; Elliott, A. R.; Brown, W. 0. I€€€ Trans. AU1973, 2 1 , 174. (38) Kunt, M. I€€€ Trans. Comput. 1975, C24, 1120.
RECEIVED for review March 7, 1980. Accepted May 5 , 1980. Work supported by the Air Force Office of Scientific Research under grant AFOSR 76-3027.
Gel Permeation Chromatography of Coal-Derived Products with On-Line Infrared Detection R. S. Brown, D. W. Hausler, and
L. T.
Taylor‘
Depatfment of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 1
The compiextty of solvent refined coal (SRC) enables one to gain only negligible structural and chemical information with conventional liquid chromatographic detectors. Infrared spectrometric detection has the potential to readily provide a great deal of information regarding the presence of various chemical functionalities. The high selectivity of liquid chromatographic infrared detection with a conventional single beam spectrometer has been demonstrated in the gel permeation chromatographic separation of a seven-component synthetic mixture. Application of infrared detection to the size separation of hexane sdubie SRC and various fractions of SRC which had previously been separated on a silica column was achieved employing conventional infrared spectrometric detection. Numerous characteristic infrared absorptions were observed in these samples and tentative functionality assignments have been made for the coal-derived fractlons.
Solvent refined coal (SRC) is one of a number of synfuel processes which would allow our coal resources to be better utilized (cleaner power generation, petrochemicals, etc.). T o better understand coal conversion processes and to maximize their utility, analytical techniques are needed to more fully characterize products such as SRC as well at its predecessor coals. In previous work ( I , 2 ) , we have developed with refractive index detection preparative liquid chromatographic (LC) techniques which separate SRC into rough molecular 0003-2700/80/0352-151 l$Ol.OO/O
“sized” fractions (GPC). Analysis of these isolated gel permeation chromatography (GPC) fractions (3)employing I3C and ‘H nuclear magnetic resonance (NMR) resulted in negligible structural and chemical information; yet such measurements provided a clue as to the complexity of GPC fractions to SRC. Liquid chromatographic detection via refractive index and ultraviolet spectrometry of course provides even less information regardii3g speciation. Combination approaches involving SRC such as separation via sue followed by functionality and vice versa have been met with similar results ( 4 ) . A much finer characterization of the total SRC product is desirable. Two options present themselves. The chromatography can be improved and optimized for a better separation, or a more selective detector system can be employed which responds to only specific functionalities. Since heteroatom content and heteroatom distribution in SRC products are important measures of SRC character, an inexpensive highly selective detector which is especially sensitive to various heteroatom functionalities appeared to be the better option. A logical choice in this regard is a variable wavelength infrared detector system since many of the heteroatom containing functionalities (e.g., phenols, ethers, acids, aldehydes, etc.) are infrared (IR) active. Infrared spectrometry is also a well proven method for both qualitative and quantitative organic analysis. A limited number of scattered reports employing conventional single-beam or double-beam IR spectrometry as an LC 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
detector have appeared. These reports have dealt exclusively with the GPC of polymers such as silicones (5),polyethylenes (6), poly(styrene-covinyl stearate) (7), poly(styrene-co-tertbutyl methacrylate) (8)and polystyreneacrylonitrile mixtures (9). Both stop-flow and on-line procedures have been utilized. On-line IR detection for the separation of exceedingly complex mixtures such as coal derived liquids which contain widely varying functionalities in both large and small molecules has not been demonstrated. Conventional IR, however, should lend itself readily as an LC detector for analyses of nonpolymeric materials as well, where the concentration of eluting species is greater than 10 ppm. I t is limited in several respects since it typically allows only for isocratic elution (10) and single wavelength, singlebeam detection unless stop flow techniques are used. Also, most LC solvents are generally poorly suited for IR work over large window ranges. In addition, those solvents that have reasonable IR windows are typically limited LC solvents. If lower concentrations (on the order of 1 ppm) are to be detected or if multiple wavelengths are to be simultaneously examined, Fourier transform IR spectrometry is required which has short scan times ( - 1 s) and signal averaging along with spectral subtraction capabilities. Recently high performance liquid chromatography-Fourier transform IR spectrometry (HPLC-FTIR) has been demonstrated in the separation of industrial waste water effluent ( I I ) , silicone polymers (IZ), etc. (13,14). The cost of conventional IR vs. FTIR precludes the use of the latter in many laboratory situations. We, therefore, wish to report the application of conventional IR detection employing a low volume flow-through IR transmission cell for the separation of coal derived materials via high performance liquid chromatography in the size exclusion mode. EXPERIMENTAL Samples. A synthetic mixture (1% w/w) containing 7 components thought to model SRC was prepared by dissolving equal weights of squalane (Microtek), benzophenone (Fisher), dibenzofuran (Aldrich),anthracene (Eastman), 2-tert-buty1-4-methyl phenol (Aldrich), aniline (Theta Corp.) and cyclohexanol (Fisher) in chloroform. SRC samples derived from Amax feed stock were obtained from a Southern Services Inc. pilot plant (Wilsonville, Ala.) funded by Electric Power Research Institute and operated by Catalytic h c . Two preliminary separation schemes were employed to obtain simpler fractions of the SRC material for subsequent LC-IR. Scheme 1 consisted of a continuous hexane extraction of SRC (40:l) followed by evaporation of the hexane to yield an “oils” fraction (15). Scheme 2 is a modification of the Sequential Elution Selected Solvent Chromatography (SESC) procedure developed by Farcasiu et al. (16, 17). SRC material is sequentially eluted from silica gel by a pseudostep gradient. Two of these fractions, SESC #3 and #4 (designated polar aromatics and simple phenols, respectively) were chosen for analysis in this feasibility study. Chromatography. Chromatography in the size exclusion mode was carried out with a Waters 6000A reciprocating piston pump. A Varian TSK-1000 (originally packed in THF but for this study equilibrated in CHC13)column (3/s inch X 25 cm) was employed for the separations. The TSK-1000 column had -6000 plates/m as determined by an injection of benzene. The lower than normal plate count was due to bed volume changes arising from the solvent change. Flow rates of 1 mL/min of chloroform were employed. A differential refractometer (LDC Model 1107) served as an auxiliary detector for monitoring the separation. A variable wavelength IR spectrophotometer (Miran-IA, Foxboro Analytical) was chosen as a conventional “on-the-fly” IR detector. It employs a variable filter system to select wavelengths between 2.5 to 14.5 pm with a resolution of 0.05 pm at 3 pm; 0.12 pm at 6 pm and 0.25 pm at 11 pm. The flow cell consisted of either a conventional KBr IR cell adapted with O-rings for a flowing system with a 0.1-mm or a 1-mm spacer or a NaCl p-flow (1.5 mm, 4.5 pL) cell available from Foxboro Analytical. Multiple pathlengths of the flow cell were required
Table I. Elution Volumes of Synthetic Mixture Components elution volume, mL
component squalane benzophenone c yclohexanol di benzofuran
3.92 5.24
6.10 6.28
6.40 6.70 7.04
2-tert-butyl-4-methylphenol
anthracene aniline
0:O
0
2
0.5
4
ELUTION
6
I:O
Kd
8
IO
VOLUME (rnl)
Figure 1. Chromatogram of synthetic mixture with differential refractive index detection
to obtain usable windows in the solvent background which is dependent on the wavelength monitored. R E S U L T S AND D I S C U S S I O N GPC of Model System. A synthetic mixture containing many of the functionalities suspected to reside in SRC was prepared for subsequent size exclusion chromatography. Prior to their separation, individual components were independently chromatographed to determine retention volumes (VR)(see Table I). Figure 1shows the GPC separation of the mixture as monitored by refractive index (RI). The RI trace yields 3 resolved peaks and a shoulder for the 7 component mixture. This is due to the fact that the RI detector responds to both positive and negative RI changes which can and do have a net canceling effect in the chromatograms. However, even if all components were resolved in the RI trace, no information as to the nature of the compound other than its relative molecular size is provided. By employing an IR detector with the same column, chromatograms (Figure 2) are obtained which characterize the functionality of each component in the mixture. The chromatogram produced by detection a t 2.75 pm shows the previously incompletely resolved separation of the “phenol” component and “alcoholic” component now detected free of interferences. Monitoring the 2.95-pm band yields a chromatogram which shows a main peak for the characteristic absorbance of the -NH2 functionality in aniline with a small peak due to “bonded” -OH functionality. The synthetic mixture contains only one ether component (dibenzofuran) and only one peak is observed in the 8.4-pm chromatogram. An identical situation is expected for 5.9-rm (carbonyl) detection, but a weak additional band is observed which suggests that a second component of the mixture weakly absorbs IR radiation in this region. Three components contain aliphatic
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
L d t m
--OH”
275 t 0 0 5 P
‘.LlPHATlC~u
(3636 em-’) I
0
I
I
I
I
I
4
2
ELUTION lOf0
0
I
6
1
8
8
VOLUME
I37 0 5
I
4,2 M W I O Kd
1
IO (mil
I
0
‘
‘
8
4
2
1513
1
1
I
1
1
I
10
0
6
ELUTION VOLUME Irnli 1000 137 4 2 M W
cu
0
0 5
IO
Kd
Figure 2. Gel permeation chromatograms of 7-component synthetic
mixture as detected at various characteristic I R frequencies. Flow cell = 0.1 mm KBr (-OHand -NH chromatogram, NaCl p-flow)
hydrogen detected at 3.4 pm but only two peaks are resolved. The peak a t Kd closest to zero is attributed to squalane, while the other peak is due to both alkylated phenol and alcohol based on independently measured retention times. It appears that 2.75 pm is a more selective wavelength for the detection of alkyl phenols/alcohols than the 3.4-pm band. This demonstrates the ability to select the most useful wavelength for the compound being separated. Five aromatic materials appear in the synthetic mixture; however, the 6.25-pm wavelength detects what appears to be only three components. The largest “sized” fraction is assigned to benzophenone. The middle peak is due to dibenzofuran and the substituted phenol, while aniline and anthracene elute near the totally permeated region. Absorption by CHC13 in the aromatic C-H stretching region (3.2 pm) precluded our monitoring this wavelength. In summary, we have been able to selectively observe all components of the synthetic mixture except in those cases where the chromatography is not sufficient to separate common functionalities. GPC of SRC “Oils” Fraction. Employing the same column and detector characteristics, SRC “oils” (hexane solubles) have been “size” separated. The GPC chromatogram of the “oils” fraction employing differential RI detection provides only the limited information that material is eluting over the entire range (Kd = 0 to 1). Upon examining the chromatograms of the “oils” at various IR detection wavelengths, we see significant absorbances for heteroatom functionalities in what has typically been thought to be a coalderived fraction with little or no heteroatom content (15). Figure 3 illustrates a series of chromatograms describing the GPC separation of SRC “oils” employing sequential 8.2- to 9.2-1m detection. The chromatograms show a shift in response from high (& = 0) to low (Kd = 1) molecular size as the wavelength of detection decreases. The region from 9.2-8.8 pm is characteristic of aliphatic ethers, and absorbances in this region appear to be concentrated in the high molecular “sized” fraction. Absorbances in the 8.6- to 8.2-pm region (lactones and esters) are observed and concentrated in the smaller molecular “sized” region. Further evidence for the presence of low molecular “sized” lactones can be seen by noting Figure 4. Seven IR wavelengths which are characteristic of various “carbonyl-like” environments have been independently monitored (5.3-5.9 pm). Relatively strong absorption is observed for the small “sized” material a t 5.6-5.4 pm, This spectral region is characteristic of lactones and “size-wise” corresponds with the ether chromatogram (Figure 3). These assignments are not conclusive because alicyclic ethers also absorb over the entire 9.2- 8.2-pm region being monitored. The 5.7- to 5.9-pm region represents ketone and ester functionalities which are present in small amount in the larger “sized” fraction. Detection at
L
0
I
t
I
1
I
I
I
I
-
2 4 6 8 ELUTION VOLUME ( m i ) 137
42
MW
05
IO
Kd
1000
0
Figure 3. Gel permeation chromatograms of Amax SRC “oils” with IR detection in the “-C-0-C” region. Flow cell = 0.1 mm KBr
59i012 p m
f
IOpOBAbs.
695 crl-l! c
L
-
I
-
( I 887cm-’ )
1
0
1
1
2
1
1
1
1
1
I
I
1
I
I
I
4 6 8 0 2 4 ELUTION VOLUME ( m l )
1000 137 0
05
42 M IO
W
Kd
I
I
6
1000 137
0
0 5
1
I
8
42
MW
10
Kd
Figure 4. Gel permeation chromatograms of Amax SRC “oils”with IR detection in the “C=O” region. Flow cell = NaCl p-flow
2.75-3.10 pm (Figure 5) reveals the presence of a broad distribution of 0-H and/or N-H functionalities over the entire molecular “sized” range. Absorption, however, a t 2.8 pm (characteristic of phenolic and nonbonded 0-H)predominates. The detection of etheral- and carbonyl-containing species in SRC derived “oils” has not been reported to our knowledge. Although their concentration is low, their presence is nevertheless surprising since “oils” are normally considered to be simple aliphatic and aromatic hydrocarbons. Our sample of Amax SRC “oils” has not been protected from the atmosphere; therefore, it is possible that selective oxidation of certain hydrocarbons has occurred. On the other hand, the sensitivity and selectivity of LC-IR may have provided the clearest
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
1514
-C-0-C-
3 I t 0.05p m ( 3 2 2 5 cm-')
REGION
h.
Lsc
3 05 t 0 . 0 5 p m
C = O REGION
M
-
L ' " ' " I
0
2
4
1
0
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1
1
1
1
1
1
1
42
M W
OS
10
Kd
137
0.5
0
42
M.W
1.0
Kd
Figure 5. Gel permeation chromatograms of Amax SRC "oils" in the bonded -OH and -NH- and nonbonded -OH region. Flow cell = 1 mm KBr SESC "4
A
SESC *3 275 * 0 0 5 p m (3636cm-'I 0002Abs
0008 Abs
1
0
1
I
I
I
1
"
I
'
-
2 4 6 6 ELUTION VOLUME (ml)
0
'
"
(3636 cm-')
'
137
42
0
0.5
1.0 K d
"
-
2 4 6 8 ELUTION VOLUME ( m l )
M.W.
1000
"
Flgure 6. Gel permeation chromatograms of #4 with IR detection in the nonbonded -OH
1000
137
0
0.5
42
M.W.
1.0 K d
Amax SRC SESC #3 and region and bonded -OH,
-NH- region. Flow cell = NaCl p-flow ( S E X #3);0.1 mm KBr (SESC #4). Chloroform and THF serve as the mobile phase for SESC #3 and #4,
0 VOLUME
l
l
l
l
1
I
I
I
-
2 4 (ml)
6
1000 137
0
0 5
6
42
M W
10
Kd
Figure 7. Gel permeation chromatograms of SESC #3 with carbonyl and ether detection. Flow cell = p-flow
1
2 4 6 6 1 0 ELUTION VOLUME ( m l ) 1000
l
1000 137
0
1
I
6 8 ELUTION
respectively
picture to date of trace species in the "oils" fraction of a coal-derived product. Naturally, better preserved and more highly documented SRC "oils" must be investigated via this technique if definitive answers are to be obtained regarding the chemistry of coal liquefaction. Suffice i t to say that the LC-conventional IR detection approach appears promising for elucidating the molecular composition of these materials. GPC of SESC Fractions. Extension of this GPC separation-IR detection technique to the size separation of selected crude SRC fractions from a silica gel column was accomplished. We chose to examine the SESC ,*3 and SESC 24 fractions which have been previously designated, based on the elution pattern of model compounds, polar aromatics (nonbasic N,O,S) and simple phenols respectively. Figure 6 compares the "OH-NH-" region for the GPC separation of SESC $3 and $4. From these chromatograms, we can see that low
molecular size material in SESC $4 has a strong absorbance in the phenolic-nonbonded -OH region (2.80-2.85 pm) which would be expected of simple phenols. The bonded -OH, -NH- region (3.0-3.05 pm) shows maximum absorbance a t high molecular size. In SESC 4r3 chromatograms, bonded -OH, -NH- functionality is also contained in high molecular size material while little preference is shown for phenolic, nonbonded -OH absorbances as far as molecular size is concerned. It also should be noted that the absorbances in this region are much less intense for SESC f 3 than for SESC #4 which gives credence to the simple phenol designation for SESC r4. Some caution is required in making this interpretation because SESC #4 separations were performed with tetrahydrofuran eluent rather than CHCl,. SESC #4 has limited solubility in CHC13and the lack of suitable "windows" in THF precludes monitoring of ether/carbonyl wavelengths. The results described here regarding hydroxylic and amine detection complement our work concerning functional group NMR (18). The NMR analysis via trifluoroa~etylation-~~F technique demonstrates the presence and concentration of phenolic and alcoholic functionalities in SESC it3 and #4; whereas, the GPC-IR method provides evidence for both bonded and nonbonded OH as well as the size distribution for molecules containing this type functionality. Figure 7 shows that some of the remaining heteroatom content in SESC 13 is carbonyl and ether species but with different distributions than were observed in SRC "oils" (Figures 3 and 4). The carbonyl absorbances characteristic of ketones, aldehydes, and esters is more intense at moderate to high molecular sizes, and there is an absence of the low molecular size spike (attributable to lactones) seen previously in the "oils" chromatograms. The ether region chromatogram (9.0-8.9 pm) mimics the chromatograms obtained by monitoring the aliphatic C-H region (-3.4 pm) rather than the phenyl region (-6.25 pm) which would suggest that aliphatic ethers are present rather than aromatic ethers.
ACKNOWLEDGMENT We thank Harry Dorn for his advice and comments concerning this work. Technical assistance provided by E. Denyszyn and J. Hellgeth is acknowledged. LITERATURE CITED
.
(1) . , Coleman W M Wooton. D L , Dorn. H. C , Taylor, L T Anal Chem. 1977, 4 9 , 533-537. (2) Hausler, D. W.; McNair, H. M.; Taylor, L. T. J . Chromtogr. Sci. 1979, 17 , 617-623 _-_ (3) Wooton, D. L.; Coleman, W. M.; Glass, T. E.; Dorn, H. C.; Taylor, L. T. Adv. Chem. Ser. 1978, 170, 37-53. (4) Welsh, D. J.; Hellgeth. J. W.; Glass, T. E.; Dorn, H. C.; Taylor, L. T. ACS Symp. Sew 1978, 7 7 , 274-293.
Anal. Chem. 1980, 52, 1515-1518
(5) Rodriquez, F.; Kulakowski, R. A.; Clark, 0. K. I n d . Eng. Chem., Prod.
Res. Dev. 1966, 5 , 118-121. Ross, J. H.: Shank, R. L. Adv. Chem. Ser. 1973, No. 125, 108-116. Mirabella, F. M.; Johnson, J. F.; Barrall, E . M. Am. Lab. 1975 (IO), 65-74. (8) Polym. Sci. 1975, 19, . . Dawkins. J. V.: Hemming. M. J . ADD/. .. 3107-3118. (9) Bartick, E. G. J . Chromatogr. Sci. 1979, 17, 336-339. (IO) Ettre, L. S. J . Chromatogr. Sci. 1978, 16, 396-417. (11) Shafer, K. H.; Lucas, S. V.; Jakobsen. R. J J . Chromatogr. Sci. 1979, 17. 464-470. (12) Vidrine, D. W. J . Chromatogr. Sci. 1979, 17, 477-482. (13) Kuekl, D.; Griffiths, P. R. J . Chromatogr. Sci. 1979, 17, 471-476. (14) Vidrine, D. W.; Mattson, D. R. Appl. Spectrosc. 1978, 32, 502-506.
1515
(15) Bockrath, B. C.;Schroeder, K. T.; Steffgen, F. W . Anal. Chem. 1979, 51. 1168-1172. (16) FarcaskT M. Fuel 1977, 56, 9-14. (17) Hausler, D. W.; Hellgeth, J. W.; Taylor, L. T.;Borst, J.: Cooley, W . 8. Fuel, in press. (18) Ghss, T.E.;Dorn, H. C.; Taylor, L. T.;Manheim. A ; Sleevi, P. S. Anal. Chem. 1980, 82, 1135.
RECEIVED for review December 10,1979. Resubmitted March 28, 1980. Accepted May 8, 1980. The financial assistance of Department of Energy Grant EF-77-G-01-2696 and the Commonwealth of Virginia is gratefully appreciated.
Voltammetric Methods for Determination of Metal Binding by Fulvic Acid S. A. Wilson,’ T. C. Huth,* R. E. Arndt, and R. K. Skogerboe” Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
The use of anodic stripping voltammetry (ASV) and differential pulse polarography (DPP) for the measurement of the concentrations of aquo ions in the presence of fulvic acid, and the subsequent use of these data for estimation of the metal-fuivic acid Conditional stability constants, have been evaluated. The results of such measurements for Cd, Cu, Ni, Pb, and Zn, combined with those reported by others, are generally indicative of the adsorption of fuivic acld on the mercury electrodes used, accompanied by the probable formation of metal complexes with the adsorbed fuivic acid so that the stability constants estimated do not appear to be strictly representative of the degree of complexation occurring in the bulk solution. Comparisons are made with stability constants measured by others using other methods which clrcumstantialiy support this conclusion. Therefore, it is suggested that the use of ASV and DPP for studying metal binding by fuivic acid be carefully evaluated for each metal of interest.
The potential importance of fulvic acid (FA) as a complexing agent for metal ions in natural waters has been emphasized in recent reviews by Gamble and Schnitzer (1) and by Reuter and Perdue (2). The ability of naturally occurring fulvic acids to complex metal ions, thereby changing their effective chemical forms and potential interactions with other entities associated with aquatic systems, has led to the development of numerous research efforts focusing on defining the characteristics of fulvic acid and the determination of the conditional equilibrium constants which would permit the thermodynamic modeling of metal complexation. Several analytical approaches have been used as means of differentiating between the various chemical forms of trace metals in aqueous systems (speciation) so that the metal-FA complexation chemistry can be investigated. Some of these have been discussed in the reviews cited above ( I , 2 ) . The most Present address: U.S. Geological Survey, 5293 Ward Road, Arvada, Colo. 80002. ‘Present address: Department of Chemistry, University of Arizona, Tucson, Ariz. 85724. 0003-2700/80/0352-1515$01 .OO/O
widely used methods include potentiometric titrations (3-6), dialysis (7), ion selective electrode (ISE) techniques (8-12), amperometric titrations (I3-18), and voltammetric techniques (9,19-23). The reports by some authors (19,20,24,25) that amperometric titrations may be subject to large errors when compounds are present which adsorb on the electrode, coupled with the fact that voltammetric techniques allow measurements without major modifications of the composition of the test solutions, seems to have led many investigators to rely on the voltammetric techniques (22, 23). The use of polarographic or anodic stripping voltammetric methods for measuring the binding of metals by organic ligands usually involves titration of the ligand with metal ion, or vice versa, under conditions held constant. If the redox potentials of the metal ions shift according to the ion-to-ligand concentration ratio, the investigator may use the shifts to determine stability constants and coordination numbers of the complexes by the Lingane method (26,27). Alternatively, reductions in the limiting currents of the metal ions in the presence of increasing amounts of ligand may be used to estimate the stability constants. In general, the latter requires that the complexed metal ions should not be electroactive and should not contribute to the faradaic current measured at the potentials characteristic of the aquo ions. Complexes which do contribute to the faradaic current at the relevant potentials may be operationally defined as electrochemically “labile”. Similarly, the reduction of the aquo ions a t the working electrode, as in anodic stripping voltammetry (ASV), should not result in a shift in the aquo ion-metal complex equilibrium in favor of complex dissociation. Complexes which do undergo dissociation during the measurement period may be defined as kinetically “labile”. The degree of dissociation, and its concomitant effect on the estimation of the aquo ion concentrations and the stability constants, will accordingly depend on the time scale of the electrochemical measurement. Finally, the presence of the ligand should not affect the diffusion coefficients of the aauo ions in solution nor should the ligand adsorb onto the woriing electrode where it may change the ratesof the redox reactions being monitored 01 it may complex metal ions a t the electrode surface. Failure to satisfy these essential criteria may lead C 1980 American Chemical Society
