Anal. Chem. 1903, 55, 1591-1595
employed in the carbon paste approach. In terms of experimental convenience, flexibility, and quantitative reproducibility, however, the carbon paste CMEls clearly possess attractive properties. This electrode modification procedure is one of the most general approaches available. In principle, virtually any water-insoluble species can be added to the paste mixture; and, although the modifying species may eventually be lost into solution, the initial quantity added can be directly controlled. Thus, extension of the carbon paste CME scheme to numerous other electrocatalytic systems is easily envisioned. Furthermore, regenerakion of new electrode surfaces can be performed in a matter of minutes and with a 5-10% reproducibility that is virtutally the same as that observed for ordinary carbon paste ellectrodes. The development of CMEs possessing these properties is especially important if CMEs are to be employed for routine quantitative measurements. Registry No. TMF'D, 100-22-1;PD, 106-50-3;N M H , 512-68-4; ascorbic acid, 50-81-7; dopamine, 51-61-6.
LITERATURE CITED (1) Cheek, G. T.; Nelson, R. F. Anal. Lett. 1978, 1 1 , 393-402. (2) Oyama, N.; Anson, F. C. J . Am. Chem. SOC.1979, 101,3450-3456. (3) Rublnsteln, I.; Bard, A. J. J . Am. Chem. SOC. 1980, 102, 6641-6642. (4) Cox, J. A,; Majda, M. Anal, Chlm. Acta 1980, 118, 271-276. (5) Slria, J. W.;Baldwin, 13. P. Anal. Lett. 1980, 13, 577-588. (6) Price, J. F.; Baldwin, R. P. Anal. Chem. 1980, 5 2 , 1940-1944.
1591
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54, 1980-1984. a n o n , F. G.;Fombariet, C.; Buda, M. J.; Pujol, J. Anal. Chem. 1981, 5 3 , 1386-1369. Faiat, L; Cheng, H.-Y. Anal. Chem. 1982, 5 4 , 2108-2111. Murray, R. W. Acc. Chem. Res. 1960, 13, 135-144. Tse, D. C.; Kuwana, T. Anal. Chem. 1978, 50, 1315-1318. Degrand, C.; Miller, L. L. J . Am. Chem. Soc. 1980, 102, 5728-5732. Fukui. M.; Kitani, A.; Degrand, C.; Miller, L. L. J . Am. Chem. SOC. 1982. 104. 28-34. (14) Jaegfeldt, H.; Torstensson, A.; Gotton, L.; Johansson, G. Anal. Chem. 1981, 5 3 , 1979-1982. (15) Ravlchandran. K.; Baldwin, R. P. J . flecfroanal. Chem. 1981, 126,
293-300. (16) Kitani, A.; Miller, L. L. J . Am. Chem. SOC. 1981, 103, 3595-3597. (17) Kitani, A.; So, Y.; Mlller, L. L. J . Am. Chem. SOC. 1981, 103, 7636-7641. (16) Adams, R. W. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1969;pp 56-58. (19) Adams, R. W. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1969;pp 356-361. (20) Nicholson, H. S.;Shah, I. Anal. Chem. 1984, 36, 706-723. (21) Daum, P.; Lenhard. J. R.; Rollson, D.; Murray, R. W.J . Am. Chem. SOC. 1980, 102. 4649-4653. (22) Roullier, L.; Waldner, E.; Laviron, E. J . flecfroanal. Chem. 1982, 139,
199-202. (23) Kuwana, T.; French W.G. Anal. Chem. 1964, 3 6 , 241-242. (24) Adams, R. N. Anal. Chem. 1976, 48, 1126A-1136A.
RECEIVED for review March 7,1983. Accepted May 10, 1983. This work was presented in part at the 1983 Pittsburgh Conference on Analytical Chemistry and Applied SpectrosCOPY.
Differential Pulse Polarography and Differential Pulse Anodic Stripping Voltammetry for Determination of Trace bevels of Thallium Llnda
K. Hoeflich,' Robert J. Gale," and Mary L. Good2
Depatiment of Chemistty, Louisiana State University, Baton Rouge, Loulsiana 70803
Slmuttaneous determlnfitlonof TI+ and (CH3)ZTI' Ionic specles by differential pulse polarography (DPP) and differential pulse anodic strlpplng voltammetry (DPASV) is described for various buffered matrices. Detection llmits for TI+ and (CH3)ZTI' were 130 ppb and 250 ppb, respectively, by conventlonal DPP. Corresponding values are 3.2 ppb and 3.4 ppb with DPASV. Pb( I I), Zn( I I ), and Cd( I I ) presence interfered wlth these thallium species identifled by DPP, although EDTA addition prevented Interference of the TI+ peak by these metals. When EDTA is used to remove interferences in DPASV, the thallium species peak ehlfts to a more nogatlve potentlai and the peak current Is decreased such that the portion due to (CH,),TI+ is unaffected, while that due to TI+ Is modified. The electrochemlcal response of TI' ion is unaffected by pH while that for (CH3),TI+ Ion is decreased with Increasing pH. (CH3)ZTI' ion reduces in an lrreverslble $)-electronwave with kinetic parameters a := 0.368 f 0.091 and k , = (4.05 f 0.55) X IO-' cm 8-l for E , = -0.800 V SCE, and a diffuslon coefficient D o = 1.06 X lo-' cm s-l.
The past 20 years have been characterized by an awareness of environmental polluition from a variety of sources. One of the major contributors is energy production, where problems 'Present address: E. I. du Pont de Nemours and Co., Inc., Seaford, DE 19973. Present address: UOP,Inc., Corporate Research Center, Des Plaines, IL 60016. 0003-2700/83/03551591$01.50/0
of concern include the environmental impact of stack gas effluents and residual waste products. Special attention has been given to the quantitation and speciation of heavy metals, because of their emission with coal fly ash and subsequent deposition into natural waters, Certain inorganic compounds are biomethylated to organometallic forms by microbial action in these waters, which complicates their determinations (1-5). Thus, procedures must be designed to determine total heavy metals content and to differentiate the chemical forms present. One of the heavy metals of interest is thallium because of its toxicity. This metal undergoes a series of bioenvironmental transformations and, although it is not present at concentrations as high as certain other heavy metals (e.g., Cu, Cd, Pb), its inorganic compounds are considerably more toxic (6-11). Concentrations of thallium and thallium compounds are likely to be at the parts-per-billion level (12-191, so that any methods chosen should be capable of both quantitative and qualitative determinations at these levels. In addition, for full utility the analytical methods preferably would be adaptable to field use. Electroanalytical methods are obvious choices for speciation studies, and while organothallium compounds have been identified electrochemically, the results are not especially applicable to procedures for routine analyses of low level natural samples (20-29). The present study was an investigation of the electrochemical analysis procedures appropriate to environmental needs. Conventional voltammetric methods, namely, cyclic voltammetry (CV), differential pulse polarography (DPP), and differential pulse anodic stripping voltammetry (DPASV), were used to characterize 0 1983 American Chemical Society
1592
* ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
T 'PA
4
4 0
Q,
C
I
0.0
- 0.8
-0.4
-!.2
- 1.6
E VvsSCE -0.2
- 0.6
-1
.o
'
- 1 .'4
Figure 2. Differential pulse analyses for (CH,),TII; drop time 0.5 s; V = 5 mV s-'.
E v v s SCE
Flgure 1. Typlcal cyclic voltammograms of 1.0 X tow4M (CH3),TiI (lower), Y = 200 mV s-', and 1.0 X lo-' M (CH,),TINO, (upper), Y = 100 mV s-'; HMDE area = 1.85 X lo-* cm2;pH 7 phosphate buffer.
the electrochemical reactions and to assess their analytical capabilities.
EXPERIMENTAL SECTION Chemicals and Solutions. Reagent grade chemicals were used without further purification unless otherwise specified. Inorganic thallium compounds were obtained from Alfa Products (Danvers, MA). Dimethylthallium iodide, (CH&TlI, was synthesized following the procedure described by Gilman and Jones (30). Dimethylthallium nitrate, (CH3)2T1N03,was synthesized from (CH&TlI as described by Goddard (31). Elemental determinations were 6.44% C (6.65% calcd) and 1.67% H (1.67% calcd) and 7.79% C (8.09% calcd), 2.00% H (2.18%calcd), and 4.75% N (4.74% calcd), respectively. Characterizations of these compounds by infrared spectrometry were in satisfactory agreement with the spectral data of Deacon and Green (32). Natural water samples were prepared with lake water from Campus Lake, LSU. The water was acidified with HN03 (concentrationapproximately 1%wt) and stored in a polyethylene bottle for 6 months to settle particulates. Coal fly ash leachates were obtained by treating fly ash from the Clinch River and Amos power plants of the Tennessee Valley Authority, according to the A.S.T.M. procedure (33). Supportingelectrolyteswere prepared from deionized and distilled water. Apparatus. Cyclic voltammetry studies were made with the PAR Model 173 potentiostat/galvanostat, PAR Model 175 universal programmer, and a Houston Instruments Model 2000 X-Y recorder, using conventional three-electrode configuration cells and a PAR Model 9323 HMDE. Pulse techniques were applied with a PAR Model 174-4 polarographic analyzer and Model 174/50 drop timer assembly. DPASV was performed with a thin mercury film plated on a glassy carbon electrode (diameter = 0.32 cm), using the method described by Copeland et al. (25). The electrodewas rotated with a Sargent cone drive stirrer (-lo00 rpm). All measurements were made at the temperature of the laboratory,nominally 25 "C, and potentials are quoted with respect to the saturated calomel electrode. RESULTS AND DISCUSSION Electrochemical Characterization. Firstly, cyclic voltammetry and normal pulse polarography were used to characterize the electrochemical behavior of the organothallium species a t bulk Hg electrodes. These experiments were subsequently made with the nitrate compound because iodide ion adsorption and product waves, at ca. -0.1 V SCE, might interfere with kinetic analyses. Figure 1 illustrates typical voltammograms for (CH,),Tl+ ion reduction/oxidation at the HMDE. On the first sweep, the broad reduction peak at -1.08 V can be assigned to an irreversible three-electron
reduction, as proposed by DiGregorio for dialkylthallium(II1) ions
R2T1++ 3e-
f
2H+
-
2RH
+ T1(Hg)
(1)
where R represents methyl, ethyl, or n-propyl (29). The reversible couple at ca. -0.46 V is due to the oxidation of thallium amalgam and subsequent reduction of inorganic Tl(1) ions. The kinetic parameters, a and K,, for (CH3)2T1+ion reduction were obtained by foot-of-wave analyses, assuming the applicability of Reinmuth's procedure (34). Kinetic parameters found were independent of sweep rates €or 20 mV s-l and 50 mV s-l and had values a = 0.368 f 0.091 and K, = (4.05 f 0.55) X lom3cm s-l, for Ei the initial potential of -0.800 V SCE. Normal pulse polarography (NPP) is commonly used to determine the reversibility, or otherwise, of a current wave. For (CH&Tl+ ion reduction, the relation E vs. In (i - iJ/i resulted in a curved line, and application of the appropriate form of the Ilkovic equation gave a calculated cm s-l. Whereas cyclic diffusion coefficient, D = 1.06 X voltammetry and normal pulse polarography are useful techniques for characterization of the electrochemicalreactions per se, they are not suitable for trace analytical procedures. Analytical Methods. Differential pulse polarography has been used to gather calibration curve data for TlN03,inorganic Tl(1) ion. Test solutions (100 mL) were made by diluting the appropriate amount of 100 ppm TlN03 stock solution with phosphate buffers. Experimental calibrations for the range 250 to 1000 ppb were analyzed for detection limits according to the method of Osteryoung and Myers (35).Detection limits of T1+ ion added as TIN03 were 88, 40, and 156 ppb for solution pH values of 4, 7, and 10, respectively. The calibration curves for each solution were collinear and had slopes of 2.5 x pA/ppb of T1+. The peak current vs. concentration relationship for Tl(1) ion is essentially pH independent. A similar procedure has been used to determine (CH3),T1N03 by DPP, Figure 2. The peak appearing at -1.06 V was chosen as the analytical peak and evaluated for linearity with trace level concentrations. The peak at -1.37 V may be assigned to a hydrogen wave. However, unlike reduction of inorganic T1+ ion, the current vs. concentration data are strongly pH dependent, with slopes of 5 X lo4, 11 X lo4, and 13 X lo4 pA/ppb (CH3)2T1+for pH values of 4, 7, and 10, respectively. Figure 3 illustrates this pH influence on the peaks of (CH&Tl+ ion. Detection limits for the dialkylthallium(II1) species reduction were 120, 37, and 220 ppb a t pH values of 4, 7, and 10, respectively. When mixtures of organothallium and inorganic thallium compounds are determined by DPP, calibration curves for each compound may be obtained simultaneoualy and the data are linear for both species. Average
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983 * 1193 3d
I
0.E2,ua 3c
Figure 3. Wave form shapes for (CH3),TIN02, from DPP. Parameters are given in Figure 2.
Table I. Sample Lose; with Time for TINO,, (CH,),TlI, and (CH,),TINO, in Cilass Flasks A. TlNO,
Peak Current, A
ppm
Oh
12h
PH 7 Oh 12h
1
2.3 4.2 8.6 12.8
2.3 4.3 8.5 12.9 17.2
2.0 4.2 8.4 12.6 17.0
concn,
2 4 6 8
PH 4
17.0
concn, 1
2 4 6 8
Oh
PH 4 60h
1.19 3.8 8.0
12.2 16.6
2.0
4.0 8.4 12.8 17.0
C. (CH,),TlNO,
concn, ppb
PH 4
Oh
-
48h
250
0.8 3.9
500 750 1000
7.1
61.3
10.3 12.3
101.0
50
pH 10 _.____
01.8 21.7
12.6
Oh
2.1
2.0
4.4 8.6
121.8
13.1
17.1
17.8 X
60h 1.5 2!.9 6.0 8.9 12.1
12h 2.2 4.3 8.7 13.3 17.3
lo-'
PH 7
1.4 2.9 5.8 9.1 12.0
cc
4.3 8.3
Peak Current, A Oh
I
lo-*
_ I _ _ _
B. (CH,),T11 ppm
X
pH 10 60h 1.0 1.0 2.0 2.0 4.0 4.0 6.0 5.9
Oh
8.1
8.0
Peak Current, A x PH 7 Oh 48h
pH 10 Oh
0.7
1.2
2.8 5.2 7.9 10.7
2.4
0.8 1.1
5.2 7.8 9.6
3.1 5.3 6.2
36h 0.6 1.4 2.3 4.6 5.9
calculated detection limits for the 3 pH values were 130 ppb and 250 ppb for TlNO, and (CHJ2TlNO3, respectively; the marked decrease in the detection limit for TlNO, may be caused by increased interference from thle hydrogen wave. No thallium losses or degradation was detected in samples or mixtures stored in glass flasks a t the three pH values, as summarized in Table I. Conventional differential pulse polarography is a useful preliminary procedure to assess the total metals content and distribution in environmental sampleo. Although, insufficiently sensitive for trace analyses below part-per-billion levels, the method is useful for the evaluation of prospective interferences and for the evaluation of masking procedures. The detection limits achieved with DPP were, for the most part, in the 100-200 ppb range with conventional polarographic procedures and parameters. However, environmental levels of thallium compounds would be probably in the lower parts-per-billion or upper parts-per-trillion range, suggesting the need for a preconcentration method, such as DPASV. Because the consequences of mixing organothallium compounds with mercury salts are unknown, the plating of a thin mercury film on GCIS was performed separately from the sample deposition step, rather than in situ film deposition and compound accumulation. Generally, the potentials at which metals reduce is more negative for thin films than for mercury drops. A pertinent
- 0.4
- 0.5
E V vs SCE
-1.2
Flgure 4. DPP of mixture TINO, and (CH3),TIN03 in the presence of Interfering metals and EDTA, acetate buffer pH 7: (-) blank, (---) 4 ppm TINO, and 4 ppm (CH3)2TIN03, (--) 4 ppm Pb2+, Cd2+, Zn2+ added, (-) solution made 0.01 M EDTA; (A peaks) Cu2+,(B peaks) TI', (C peak) Cd2+,(D peak) (CH,),TI+ and Zn2+, (E peak) (CH,),TI+, (F peak) (CH,),TI+, Pb2+, Cd2+, and Zn2+.
example is tho T1+ ion reduction which occurs at -0.45 V on DMEs or HMDE's, whereas the reduction potential is -0.67 V for a mercury thin film electrode (MTFE). DPASV peaks appear at an intermediate potential and also vary within the same standard addition sequence when the film is thick or beaded, All DPASV studies were performed in pH 7 phosphate buffers using the method of standard addition, except as noted. The chosen electrolysis potential for TlN03was -0.9 V. This voltage has been selected by several authors (24-28) and allows a complete electrolysis of the thallium salt and, at the same time, does not cause the organothallium compounds to be plated as elemental thallium. The DPASV curves for 100-500 ppb TlN03, 3 min deposition time, gave a calibration curve with a slope of 1.2 X lo-, HA/ppb T1+. The calculated detection limit of 3.2 ppb for these data could certainly be improved considerably by technique-large area electrodes, better rotation control, longer preconcentration cycles, etc. The stripping peak potential was -0.665 V for Tl+/Tl(O). A similar procedure was used to analyze (CH3)zTlNO, at an electrolysis potential of -1.2 V. Stripping scan cycles, however, were started at -0.9 V, as previously. A straight line calibration with a slope of 6.6 X pA/ppb (CH3)2Tl+is possible and a similar detection limit calculated, 3.4 ppb. Next, the utility of the procedure for mixtures was studied with 20 ppb of each TlN03 and (CH3)2T1N03.No Tl(0) stripping peak could be detected when (CH3)2T1N03 was electrolyzed alone at -0.9 V but the determination of TI+ in mixtures was consistent with previous calibrations. Electrolysis at -1.2 V provided a stripping peak which was the sum of T1+ and (CH3)zT1+ions. Thus, selective potential determinations are feasible for inorganic T1+ion and organometallic compounds of this type. Interference Study. Environmentalsamples undoubtedly would be mixtures of metals, including those that interfere with DPP T1 determination, such as Pb(I1) (El!z= 4 4 3 V), Cd(I1) (Elp = -0.63 VI, and Zn(I1) (Ellz = -1.02 V). Test determinations with these metals were undertaken in acetate buffer to assess interferences. Insoluble phosphate formation usually eliminates interference in the phosphate buffer. Results from a test solution containing 4 ppm of mixtures of TlNO,, (CH&TlN03, Pb(II), Cd(II), and Zn(I1) are shown in Figure 4. As is readily seen the peaks for Pb(I1) and Zn(1I) are at the same potentials as those for T1' and (CH3)zT1+,and that for Cd(1l) would clearly interfere with the Tl+ peak at higher Cd(I1) concentrations in DPP. The most obvious method for removal of these interfering cations is EDTA
1594
* ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
-de
-0:7
- 0:b
L
- 0:s
E V vs SCE
Flgure 5. DPASV of mixture of TINO, and (CH3),TIN03, with and without EDTA, acetate buffer pH 7: (-) blank and 50 ppb (CH,),TINO,, -0.9 V; (.) 50 ppb (CH,),TIN03, -1.2 V; (---) 50 ppb (CH3)2TIN03and 50 ppb TINO,, -0.9 V; (.-.-) 50 ppb (CH,),TINO, and 50 ppb TINO,, -0.9 V, 0.01 M EDTA: (.-.-) 50 ppb (CH,),TIN03 and 50 ppb TINO,, -1.2 V, 0.01 M EDTA.
addition, which would not be expected to alter the peak positions of the thallium compounds. To accomplish this, 1 mL of 0.4 M EDTA solution was added and the consequence is shown in Figure 4. The peaks for Pb(I1) and Cd(I1) were shifted to more negative potentials so that they no longer interfered with the T1+ peak, which remained at -0.45 V; however, along with the peak for Zn(I1) they now interfered with the (CH3)iW peak at --1.1 V. The effect of adding EDTA to a solution (pW 7) to be analyzed by DPASV is shown in Figure 5. The peaks obtained were shifted to slightly more negative potentials and the peak current decreased. However, the difference between the peak for T1+ ion alone and the mixture was equal to that in the absence of EDTA, which indicates that the portion due to (CHJ2T1NO3 is unaffected by EDTA, while the same cannot be said for T1+ion. No mention of possible synergistic problems has been made earlier (26-28) and this effect requires further study and quantitation to determine its cause (36). Analyses in Environmental Matrices. Utility of DPASV for trace thallium analyses and speciation in “pristine” systems has been demonstrated, so several procedures have been followed to repeat these analyses in less defined conditions, such as lake water and coal fly ash leachates. The lake water was expected to contain contaminant metals as well as various forms of biota. In each case, a 25-mL sample of water was added to 25-mL of phosphate buffer to provide a final pH -6. The lake water indicated the presence of Pb(I1) and Cu(I1) but no Tl(1). Figure 6 illustrates the results of the addition of T1N03. The addition of T1+ ion caused an increase in the Pb(I1) peak and it may be postulated that these ions could be involved in competing equilibria with the humic acid present in the lake water samples. The calculated detection limit for T1+ ion in this matrix was 3.7 ppb, and while this value corresponds to those for other aqueous systems, the slope was 0.91 pA/ppb Tl+ as opposed to an average of 0.65 pA/ppb T1+ for other curves. Again, these discrepancies are probably caused by some undefined component in the lake water. Dimethylthallium nitrate was likewise analyzed in lake water/phosphate buffer. The same increase in the lead peak was noted. In this case, the calculated detection limit was 3.4 ppb, and, again, the slope of the standard additions curve was 0.45 &/ppb (CH3)2Tl+as compared to 0.34 &/ppb (CH&Tl+ for the other matrices. A more thorough comparison of de-
\
‘
-0:5
-0!7
E VvsSCE
Flgure 6, DPASV of TINO, in lake water, 50/50 (v/v) lake water/ phosphate buffer, pH 5,20 to 110 p p b (-) blank, Pb2+peak appears; (.-.-) 20 ppb; (..-..-) 40 ppb; (.) 50 ppb; (.-*-) 80 ppb; [---) 100 ppb.
I
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
1
-0.1
E V v s SCE
Figure 7. DPASV of mixture of TINO, and (CH,),TINO, in coal fly ash leachate, 20 ppb; 50150 (v/v) CFAL and acetate buffer, pH 6: (-) blank, Pb2+ and Cu2+ peaks and 20 ppb (CH3),TINOB, -0.9 V; (.-.-) 20 ppb (CH3),TIN03, -1.2 V; (.) 20 ppb (CH3),TIN0, and 20 ppb TINO, and 20 ppb TINO,, -1.2 V.
tection limits and curve slopes for this method will be made later in this work. Mixtures of TlN03and (CH3);I’lNO3 could be successfullyidentified in this matrix, reinforcing the conclusion that DPASV is a useful technique for quantitation of inorganic and organothallium compounds. Coal fly ash leachates from two sources were similarly diluted with phosphate buffer and analyzed by the DPASV procedure. When analyzed before the addition of any T1 compound, the only metals noted were Pb and Cu. Interestingly, the Clinch River fly ash appeared to have substantially more Pb(I1) than that produced a t Amos. Figure 7 illustrates the analysis of the leachate sample for a mixture 20 ppb TWO3and (CHJ2TlNO3. Calibration for T1+ion alone confirmed a linear relation, a detection limit of 3.2 ppb and slope of 0.65 pA/ppb T1+,as for deionized water analyses. The detection limit for (CH&T1NO3 under similar conditions was 5.3 ppb, but no attempt was made to mask interference from Pb(I1) and it is likely that results could be improved with EDTA addition. Table I1 contains a summary of the calculated detection limits and standard additions curve slopes obtained by using the DPASV procedure. The main discrepancies are the slope values for analyses in lake water and, as stated above, the reason for this is not clear. However, the analysis of thallium compounds is not apparently hindered and may be accomplished in the presence of interfering metals or after their masking. The pH values of these matrices were between 6 and 7, and, as was seen in DPP studies, the elec-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
Table 11. Detection Limits (DL) and Standards Addition Slopes for DPASV Analyses compound
TlNO,
matrixa deionized water/PO,,1ak:e water/PO,,-
DL , ppb 3.2
3.7 CF'AL/Ac3.2 (CH,),TINO, deionized water/POa3- 3.4 lake water/POe33.4 CF'AL/Ac5.3 a Abbreviations: Plot-, phosphate buffer, pH acetate buffer, pH 7; CFAL, coal fly ash leachate (AMOS).
slope 0.65 0.91 0.65 0.31 0.45 0.19 7; Ac-,
trochemical response of T1+ compounds is unaffected by pH while that for (CHJ2T1+is decreased with increasing pH value. From this study it is apparent that differential pulse anodic stripping voltammetry is the electroanalytical method of choice for the analysis and speciation study of thallium compounds in the environment. Whenever the electrolysis potentials of particular metal species are sufficiently different, their selective preconcentratiom is straightforwardand parbper-billion and part-per-trillion detection limits are achievable. Environmental matrices may not affect the procedures appreciably, although the presence of compounds such as humic acid may require certain adjustiments in the method. The electroanalysis of (CH3),T1+is unaffected by EDTA, although there is a decrease in the T1+peak height upon this addition. However, as long as the same concentration of EDTA is used in each solution, this effect is unimportant. The analysis of any sample for these compounds should be done in a buffer system, and comparison of the data obtained may only be made with the pH of the sample considered. Registry No. T1, 7440-28-0; (CH3)2T1+,16785-98-1;water, 7732-18-5.
LITERATURE CITED Agnes, G.; Bendle, S.;Hill, H. A. 0.; Williams, F. R.; Wllllams, R. J. P. J. Chem. SOC.D 1971, 850-851. Huber, F.; Schmldt, W.; Kirchmann, H. I n "Organometals and Organometalloids: Occurrenice and Fate In the Environment"; Brlnckman, F. E., Bellama, J. M., Eids.; American Chemical Society; Washlngton, DC, 1978; ACS Symposia Series 82. Pratt, J. M.; Risdale, S.C.; Kennedy, F. S.; Agnes, G.; Hill, H. A. 0.; Williams, R. J. P. Blochlm. Blophys. Acta 1971, 252,207-211. Wood, J. W.; Kennedy, F. S.;Rosen, C. Q. Nature (London) 1968, 220, 173-174.
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RECEIVED for review December 13, 1982. Accepted by May 2, 1983* We are grateful for the support of EPA funding, Contract No. CR-807579-01-0.
