Anal. Chem. 1982, 5 4 , 861-864 Norrls, B. J.; Meckstroth, M. L.; Heineman, W. R. Anal. Chem. 1976, 48, 630-632. Anderson, J. 1.. Anal. Chem. 1979, 51, 2312-2315. Hendler, R. W . Anal. Chem.'1977, 49, 1914-1918. Su, C. H.; Heineman, VV. R. Anal. Chem. 1861, 53, 594-598. Margollash, E.; ScheJter,A. "Advances In Protein Chemistry"; Amflnsen, C. B., et ai., Eds.; Academic Press: New York, 1986; Chapter 2, p 21. Haiadjlan, J.; Blanco, P.; Serve, P. A. J. Necfroanal. Chem. 1979. 104, 555-561. Tarasevlch, M. R.; Bogdanovskaya, V. A. Bloelectrochem. Bloeng. 1976, 3 , 589495. Betso, S. R.; Klapper, M. H.; Anderson, L. B. J . Am. Chem. SOC. 1972, 94, 81617-8204. Eddowes. M. J.; Hill, H. A. 0. J. Am. Chem. SOC. I97QV107, 4461-4464, 7113-7114. Lewis, N. S.; Wrlghton, M. S. Science 1961, 211, 944-946. Dryhurst, G. "Electrochernlstry of Biological Molecules"; Academic Press: New York, 1977; Chapter 7.
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(23) Stankovltch, M. T.; Schopfer, L. M.; Massey, V. J . Biol. Chem. 1978, 253,4971-4979. (24) Stankovitch, M. T. Anal. Biochem. 1980, 109, 295-308. (25) Scheller, F.; Strnad, G.; Newmann, B.; Kuhn, M.; Ostrowskl, W. J. Hectroanal. Chem. 1979, 104, 117-122. (26) Friedrlch, W.; Muller, L.; Moritz, M. J . Electroanal. Chem. 1976, 6 9 , 36 1-367. (27) Bourdllion. C.: Bouraeois. J. P.: Thomas, D. J. Am. Chem. SOC. 1980. 102, 4231-4235. (26) Durliat, H.; Comtat, M. J. Electroanal. Chem. 1978, 8 9 , 221-229. (29) De Angells, T. P.; Heineman, W. R. J. Chem. Educ. 1976, 53, 594-597 (30) Baudras, A.; Spyridakls, A. Blochimie 1971, 53, 943-955. (31) Hubbard, A. T.; Anson, F. C. Electroanalytical Chemistry"; Marcel Dekker, New York, 1974; Vol. 4. pp 129-213.
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RECEIVED for review September 14,1981. Accepted December 10, 1981.
Stationary Porous Carbon Electrode in a Stirred Solution Joseph Wang" and Bassam A. Frelha Depatiment of CheinWry, New Mexko State University, Las Cruces, New Mexico 88003
A statlonary porous carbon electrode made of reticulated vitreous carbon (RVC) placed In a stirred solutlon has been evaluated for exhaustive electrolytic and electroanalytical purposes. The ciurrent-time characteristics of a controlledpotential experiment are examined for different solutlon volumes,stlrrlng rates, and electrode areas and geometries. By use of large electrode surface-to-solution volume ratios complete electrolysis occurs In about 13 mln. The electrolysis currents obey the exponential decay (Llngane) law. Sensitive voitammetric metrsurements are Illustrated employing condltlons of a small eloctrode surface-to-solutlon volume ratio. For dopamine the detection llmlt Is near 35 nM using puked,stlrrlng modulatlon voltammetry. The cell is lnexpenslve and easy to fabricate.
The application of bulk (exhaustive) controlled-potential electrolysis has greatly increased during the past decade. This technique is particularly useful for electrosynthetic purposes, analytical measurements (e.g., coulometry), or removal of solution components for purification and separation purposes High ratio of the electrode surface area to solution volume and an effective miass transport are utilized for shortening the bulk electrolysis times. Solid electrodes in forms of foil cylinders or wire gauzes, usually made of platinum, as well as mercury pools, are usually employed for achieving large surface areas ( I ) . Effective mass transport is achieved by a stirred solution (employing magnetic (2) or ultrasonic (3) stirring) or rotated cells (4). Unfortunately, some of these large electrodes and modes of mass transport are expensive and/or require highly specialized equipment. In this paper we describe the behavior and the use of a stationary porous carbon ring electrode placed in a stirred solution. The electrode is made of reticulated vitreous carbon (RVC), a relatively new and promising electrode material (5). It is an open pore *foam" material, possessing many hydrodynamic, electrochemical, and mechanical properties. In most of its electrochemical applications RVC has been utilized as an working electrode for use in a variety of flowing streams (e.g., ref 6-8). Its only static (batch) applications are as rotated porous electrode (9) and an optically transparent electrode
(IO). The present design is a logical extension of these designs, together with those of bulk electrolysis cells described earlier. RVC is unique among the electrodes that have been employed for batch electrolysis due to its special characteristics: high surface area (about 66 cmn/cm3)and open pore structure (void volume of 97%) provide the desired high current efficiency (i.e., short electrolysis) when incorporated with high-speed stirring; RVC is extremely inexpensive (compared to platinum) and it is easily machined and mounted in various cell configurations. Besides the large (250 cm2) ring electrode evalluated for exhaustive electrolysis applications, a smaller stationary RVC electrode, with small bulk depletion, is examined for sensitive voltammetric measurements. Combining the high analytical currents of the porous electrode with effective discrimination against the high background currents gives detection limits a t the nanomolar concentration level. The characteristics of these electrodes are elucidated in the following work. EXPERIMENTAL SECTION Apparatus. A Pyrex glass cell (6.2 cm diameter, 6.5 cm high) with a three-hole Plexiglas cover was employed (Figure 1). The salt bridges of the reference electrode (Ag/AgCl, Model RE-1, Bioanalytical Systems, West Lafayette, IN) and the counterelectrode (a Pt foil immersed in 0.1 M KNOB)were supported in two of the holes in the cover. The working electrode in the bulk electrolysis studies was a RVC ring (6.0 cm o.d., 4.5 cm i.d., 3 mm thick, 250 cm2 surface area) placed on the cell bottom (Figure lb). Sensitive voltammetric studies were carried out using a thin (2 mm) RVC plate (9 mm X 6 mm, 7.12 cm2surface area) located on the cell bottom (Figure IC).Electrical contact to thr RVC electrodes was made by pressure to one end of a short glassy carbon rod (1.5 mm diameter) that was introduced to the cell through a hole in its wall. A graphite-filled epoxy (Grade RX, Dylon, Cleveland, OH) provided the electrical conductivity between the glassy carbon rod and the RVC electrodes and held the RVC electrodes in the cell bottom. The cell was placed on a magnetic stirrer (Sargent-WelchModel 76490) and a 2.2-cm long stirring bar was placed in the center of the cell bottom. The three electrodeswere connected to a Princeton Applied Research Model 364 polarographic analyzer, the output of which was displayed on a Houston Omniscribe strip-chart recorder. Reagents. Chemicals and reagents used have been described in detail ( I I ) , except as noted. A M stock solution of chlorpromazine hydrochloride (Sigma Chemical Co.) was made
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Flgure 1. Cells with stationary RVC electrode in a stirred solution: (a) cell assembly for bulk electrolysis, (b) top view of the same cell, (c) top view of the cell employed for voltammetric measurements; (A) RVC electrode, (B) reference electrode, (C) counterelectrode, (D) stirring bar, (E) glassy carbon lead to the working electrode.
up fresh each day. Aliquots of the stock solution were added to the supporting electrolyte to give the desired concentration. Procedure. For bulk electrolytic studies at the RVC ring the following procedure was employed. The supporting electrolyte solution was introduced into the cell and the desired working potential in the mass transport limited (plateau) region was applied. After a waiting period of 2-3 min during which the carbon surface transient currents were allowed to decay the system was ready for the experiment. The solution stirring was started and the current-time response for the blank solution was recorded. When a steady-state current was achieved 10 pM of the analyte was added and the current-time curve for ita electrolysis was recorded. Voltammetric measurements were performed at the small RVC plate electrode (Figure IC)with a sample solution volume of 150 mL. Linear scan voltammograms were recorded at scan rate of 50 mV/s and stirring speed of 500 rpm. Pulsed-stirring experiments (12) were done by switching manually between low and high stirring speeds about 30 s after the working potential was applied. Pulsed-stirringvoltammograms were developed pointwise by making 100-mV changes in the applied potential and waiting about 30 s before applying the stirring pulse. For anodic stripping M mercury stock solution and experiments, 9 mL of a 1 X 141 mL of the 0.1 M KNOBsolution were introdked to the cell, and a stripping procedure, as described in ref 11,was performed.
RESULTS AND DISCUSSION Depletion Studies at the RVC Ring. The current-time relationship in a controlled-potential electrolysis (for reactions which are limited by the rate of mass transport and free of chemical complications) is given by the Lingane equation (13): where i is the current a t time t and io is the initial current. k is the electrolysis rate constant and it is given by k = DA/V6 (2) where D is the diffusion coefficient of the electroactive species, A is the electrode area, V is the solution volume, and 6 is the thickness of the Nernst diffusion layer. Figure 2 shows a current-time response for the addition of 10 KM ferrocyanide to the stirred supporting electrolyte solution. The current decays exponentially with time and rapidly approaches a constant value as the electrolysis proceeds to completion. The noise level, caused mainly by stirring, is relatively very low as compared to the large faradaic current for species at the micromolar concentration level.
0
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Figure 2. Typical current-time response for the oxidation of 10 pM K,Fe(CN), In 0.1 M phosphate buffer (pH 7.4): solution volume, 40 mL; stirring speed, 500 rpm; applied potential, +0.90 V. 500
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Flgure 3. log current vs. time plots for different solution volumes ((A) 40 (a) and 70 (b) mL, stirring speed 500 rpm) and different stirring speeds ((B) 825 (a) and 380 (b) rpm, solution volume, 50 mL), with 10 pM K,Fe(CN)6 in 0.1 M phosphate buffer. Sample and potential are as in Flgure 2. Number on each plot represents the value of the slope.
In general, as the solution volume decreases or as the stirring speed increases, the electrolysis rate constant increases and the electrolysis time decreases (eq 1and 2). Figure 3 examines the effects of these parameters. Highly linear plots of log i vs. t are obtained, indicating that the electrolysis current obeys eq 1 (nonlinear plots were obtained with specialized cells for rapid electrolysis (e.g., ref 4 and 8)). The slopes of these plots (Le., the rate constants) range from 0.0038 s-l (50 mL solution, 380 rpm stirring speed) to 0.0084 s-l (30 mL solution, 700 rpm stirring speed (not shown)). Space limitation in the present cell and vortex formation restrict the use of smaller volumes and their incorporation with higher stirring speeds. The
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
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Flgure 5. Pulsed-stirrlng response for 4.5 pM chlorpromazine (A) and 1.0 yM dopamine (6): pulsed-stirring between 0 rpm for 90 s and 500 rpm for 30 s; supportlng electrolytes, 0.1 M phosphate buffer; apphed potentials, i-0.9 Y (A) and 4-0.8V (B). Flgure 4. Linear scan voltammograms obtalned after successive standard additions of dopamine to a 0.1 M phosphate buffer (pH 7.4) solution: (A) blank solution, (B-D) successive concentration increments of 10 p M dopaminie; scan rate, 50 mV/s; stirring rate, 500 rpm.
increased rate constant a t higher stirring speeds is due to thinner diffusion layers inside the pores of the RVC (eq 2); the high void volume of the RVC promotes circulation of the solution inside the electrode. The solution motion is, probably, not uniform inside the entire volume of the RVC (with faster convection in regions closer to the magnetic bar). For this reason, nonuniform thickness of the diffusion layer is expected. Accordingly, electrodes with similar surface area but different geometries may result with different rate constants. By substituting the highest rate constant obtained in the t99,9w = 6.9/k relationship ( I ) , a 99.9% completion of electrolysis would require 820 s or 13 min. Typically exhaustive electrolyses (employing conventional cells) are slower than this, requiring 30-60 min (I). The RVC cell is not as fast when compared to specialized cells (e.g., rotated cell (4) or ultrasonic stirring (3) with rate constants of 0.05-0.1 s-l), but its price, construction, and operation mode are much cheaper and simpler (the large ring electrode costs about $0.50). Higher electrolysis rate constants may be obtained by incorporating the RVC electrode with more effective mass transport (e.g., rotated electrode or cell) and smaller sample volumes; this possibility is presently under investigation. Voltammetric Measurements. In addition to controlled-potential electrolytic studies at the large RVC ring electrode, experiments were undertaken to examine the feasibility of utilizing a stationary RVC electrode in a stirred solution for sensitive voltammetric measurements. For this purpose a much smaller RVC electrode (6.34 cm2,Figure IC) and a large (150 mL) solution volume (i.e., small A/V conditions) were employed to minimize bulk depletion effects. During the times used for voltammetric measurements (1-1 0 min) a small bulk depletion (0.8-7%, respectively) occurs, employing stirring speed of 500 rpm. The 45-min experiment results in about 30% depletion of the bulk concentration. Figure 4 showgi repreaentative linear scan hydrodynamic voltammograms (under conditions of continuous stirring), obtained after successive standard additions of dopamine, each addition effecting a 10 pM increase in the concentration. Well-defined waves and plateau regions are obtained, and the limiting currents (corrected for the background) are proportional to the dopamine concentration. The anodic potential limit of the background current (A) is around +1.0 V, as expected for carbon electrodes. The noise level at the plateau region is larger than a t the preceding potential region. The plateau noise slightly increases with the analyte concentration, indicating that this may be due to fluctuations in the convective transport i[i.e., small variations in the stirring speed). On the basis of a signal-to-noise ratio of 2 (in the plateau N
region) and of the slope of the background current (A) a detection limit around 1 pM dopamine is obtained. The detectability may be greatly improved by compensating the large background current that accompanies the large analytical current a t the porous electrode. This may be accomplished by the pulsed-stirring modulation voltammetry, that has been recently suggested utilizing a stationary planar disk electrode (12).The feasibility of combining this technique with a stationary porous electrode is discussed below. Figure 5 illustrates the sensitivity obtained employing the pulsedstirring technique at a stationary porous electrode. It is a reproduction of a chart record for chlorpromazine (A) arid dopamine (B) at micromolar concentration levels. On the basis of a signal-to-noise ratio of 2 the detection limits for chlarpromazine and dopamine are around 65 and 35 nM, respectively. Compared to a value of around 0.24 pM that was estimated for the stationary planar disk electrode (I2),the better detectability obtained at the porous electrode is due to greatly increased signal level with little increase in the noise level. Six concentration increments from 1to 6 pM of added dopamine yield a highly linear plot (with a slope of 6.5 pA pM-l) between the pulsed-stirring current amplitude and dopamine concentration (conditions as in Figure 5B, except that longer cycling times were employed for increased coiicentrations, as discussed below). The detectability and sensitivity obtained herein are comparable with those of the hydrodynamically modulated rotating disk electrode (9,I.I) which requires more complex and expensive instrumentation. The pulsed-stirring response times vary with the depolarizer and its concentration. While 10 s (stirring “on”) and 90 s (stirring “off“) are required for achieving current steady states with 4.5 pM chlorpromazine and 1pM dopamine (Figure 5 ) , it takes about 7 s (“on”) and 45 s (“off“) and 60 s (“on”) and 90 s (“off“) in the cases of 10 pM ferrocyanide and dopamine, respectively (not shown). The mechanism of this phenomenon (which was observed a t other convective RVC electrodes (15)) has not been elucidated. Shorter response times and lower sensitivity are obtained when pulsing the stirring between two speeds rather than the on/off operation. The precision of results was estimated by 12 repeated pulsed-stirring rneasurements of 11 pM ferrocyanide (conditions: 0 rpm for 25 s, 500 rpm for 15 s, +0.9 V applied potential, 0.1 M phosphate buffer). The mean current amplitude found was 22.7 pA with a range of 22.1-23.5 pA. Thie relative standard deviation found over the complete series was 2.2 % . Well-defined sigmoidal pulsed-stirring voltammograms were obtained for dopamine and chlorpromazine at the micromolar concentration level, with half-wave potentials of +0.27 V and +0.66 V, respectively (not shown). We have also examined the feasibility of performing sensitive trace metal stripping analysis employing the stationary RVC electrode in a stirred solution. RVC was found recently
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to be a suitable substrate for a mercury film electrode, as applied to stripping analysis (9, 16). However, the results obtained with the present configuration (defined cadmium and lead peaks only at the M concentration level, employing 2-min deposition, 500 rpm stirring speed, and the differential pulse stripping mode (not shown)) are inferior when compared with those of previous studies at in situ plated thin mercury film electrodes. This may be attributed to the increased background current and nonuniform mercury plating due to the rough surface of the graphite-epoxy that provides the electrical contact to the RVC.
(2) Lingane, J. J. Anal. Chim. Acta 1948,2, 584. (3) Bard, A. J. Anal. Chem. 1963, 35, 1125. (4) Clem, R. G. Anal. Chem. 1971,43, 1853. (5) Wang, J. Nectrochlm. Acta 1981,26, 1721. (6) Strohl, A. N.; Curran, 0.J. Anal. Chem. 1979, 51. 353. (7) Blaedel, W. J.; Wang, J. Anal. Chern. 1979, 57, 799. (8) Strohl, A. N.; Curran, D.J. Anal. Chem. 1979, 51, 1050. (9) Blaedel, W. J.; Wang, J. Anal. Chem. 1980,52,76. (IO) Nowell, V. E.; Mamantov, G. Anal. Chern. 1977,49,1470. (11) Wang, J. Anal. Chem. 1981, 53, 2280. (12) Wang, J. Anal. Chlm. Acta 1981, 129, 253. (13) Lingane, J. J. J. Am. Chem. SOC.1945, 67, 1916. (14) Mlller, 6.: Bruckensteln, S. Anal. Chem. 1974,46. 2026. (15) Blaedel, W. J.; Wang, J. Anal. Chem. 1980,52. 1697. (16) Blaedel, W. J.: Wang, J. Anal. Chem. 1979, 51, 1724.
LITERATURE CITED (1) Bard, A. J.; Faulkner, L. R. “Electrochemical Methods, Fundamentals and Appllcations”; Wiley: New York, 1980; Chapter 10.
RECEIVED for review
November 20, lg81*Accepted February
16, 1982.
Mechanisms of Vaporization of Vanadium Pentaoxide from Vitreous Carbon and Tantalum Furnaces by Combined Atomic AbsorptionIMass Spectrometry D. L. Styrls” and J. H. Kaye Pacific North west Laboratory, P.0. Box 999, Richland, Washington 99352
High-temperaturevaporlratlon of vanadium pentaoxlde (V,O,) samples from vitreous carbon and tantalum furnaces that are heated reslstively in ultrahigh vacuum is lnvestlgated with the slmultaneous atomic absorptlon/mass spectrometric technique. The advantage of thls method Is that if allows the observation of lntermedlates In the vapor phase. The correlated experimental resuits are related to thermodynamically feaslbie reactlons which are consistent with the appearances of these Intermediates and the temperatures at whlch they appear. The atomlzatlon mechanlsms that are indicated by these reactions are developed.
Fundamental understanding of furnace atomic absorption spectroscopy ( U S ) depends largely on what is known of the mechanisms which control atomization processes that take place within the furnace. Surprisingly few workers have investigated these mechanisms, which are responsible for the signal amplitudes and shapes on which AAS analyses are based. Some of the earliest investigations were made by Aggett and Sprott ( 1 ) and by Campbell and Ottaway (2) in 1974 by correlating appearance temperatures and temperatures a t which reductions of metal oxides by carbon are thermodynamically favorable. Aggett and Sprott compared their results from a carbon rod with results from a tantalum strip. They concluded that (i) reduction from a graphite atomizer occurred for oxides of Co, Fe, Ni, and Sn but not for the other 12 elements investigated and (ii) formation of gaseous atoms from the reduced metals (Co and Fe) is controlled by the vapor pressure of those metals. On the other hand Campbell and Ottaway concluded that the majority of the 27 elements they investigated formed gaseous metal atoms by reduction of the oxides in a condensed phase in a carbon furnace. Eklund and Holcombe (3) have recently shown by differential thermal analysis and X-ray photoelectron spectroscopy that a metal oxide such as CuO can be reduced by graphite at a temperature that is lower than the appearance temperature, which implies that the vapor pressure of the
analyte can indeed be controlling the appearance temperature. Sturgeon, Chakrabarti and Langford ( 4 ) used a thermodynamic approach (assuming analyte solid-gas phase equilibrium) combined with a kinetic approach whereby the atomization process is characterized by a rate constant. Activation energies involved in the primary atomization process were determined from the resulting Arrhenius type equation and were related to dissociation energies and heat of atomization of the metal. Johnson, Sharp, West, and Dagnall (5) used a Boltzmann distribution to describe a vaporization rate characterized by the heat of vaporization of the metal or dissociation energy of the metal-oxide bond. Their resulting model of atomization from a carbon filament atomizer worked well for molybdenum but it did not appear to be applicable to metals which vaporize a t lower temperatures. Sturgeon, e t al. ( 4 ) , point out that it may not always be valid for reductions by tantalum to be neglected as was done by Aggett and Sprott (1)in analyzing their tantalum strip data. At high temperatures the Ta20, surface layer may not be sufficiently stable to prevent reduction from occurring. The results of the tracer work of Maessen, Balke, and Massee (6) to investigate thermochemical reactions in graphite furnaces led to the conclusion that reactions with graphite furnaces are so complex that atomization behaviors of arbitrary compositions of analytes cannot be predicted reliably. Most of the above approaches attempt to resolve the problem by thermodynamic or kinetic considerations. The concern regarding validity of these methods is that thermodynamic equilibrium or isothermal conditions are not achieved during the heating pulse. Techniques which will allow direct observations of intermediate phases and interferents, or the effects thereof, must be used to obtain additional data. Eklund and Holcombe (7, 8), for example, have monitored the absorbance under steady-state and transient conditions at various hieghk above a graphite filament atomizer. They showed that (i) oxidation of the analyte can occur in the gas phase via reactions with the thermal decomposition products of the interferent and (ii) gas-phase oxidation of the analyte can be depressed by a preferential binding of oxygen with the interferent in the
0003-2700/82/0354-0864$0 1.251’0 0 1982 American Chemical Society
