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S. A. Wilson and J. H. Weber, Anal. Len., 10, 75 (1977). S. A. Wilson and J. H. Weber, Cbem. Geol., 19, 285 (1977). D. C. Grahame, J . A m . Cbem. Soc., 7 6 , 4819 (1954). E. Gileadi, S. D. Argade, and J. O'M. Bockris, J . Pbys. Cbem., 70, 2044
(25) 0. L. Kabanova and S. M. Beniaminova, ,J. Anal. Cbem. USSR(€ng/. Trans/.), 16, 94 (1971). (26) E . M. Roizenblat and G. N. Veretina, J . Anal. Cbem. USSR, 19, 2043 (1974).
(1966). (22) . . L. I. LiDchinskava. M. S. Zakharov. and V. V. Pnev. J. Anal. Chem. USSR (Engl.' Trans/.j,23, 855 (1978). (23) S. M. Beniarninova and 0. L. Kabanova, J . Anal. Cbem. USSR(€ngl. Trans/.),20, 55 (1975). (24) M. Kopanica and F. Vydra, J . Hecfroanal. Cbem., 31, 175 (1971).
RECEIVED for review November 28, 1978. Accepted March 19, 1979. This work was partially supported by National Science Foundation Grant OCE 77-08390.
(18) (19) (20) (21)
Flow Electrolysis on a Reticulated Vitreous Carbon Electrode W. J. Blaedel" and Joseph Wang Department of Chemistry, University of Wisconsin- Madison, Madison, Wisconsin 53706
A flow-through electrolytic cell which contains a working electrode composed of a variable number of Reticulated Vitreous Carbon (RVC) disks is described. Well defined current-potential curves are reported for various numbers of disks. The dependence of the limiting current and the limiting degree of conversion on the flow rate and on the number of disks employed has been investigated. The completeness of the electrolytic process is as hgh as 92 YO.The limiting current is linear with respect to the concentration of electroactive material. Stopped-flow voltammetry at a RVC electrode permits discrimination against nonconvective background currents, and allows a limit of detection around 1 nM ferrocyanide.
T h e interest in continuous electrolysis conducted on porous electrodes with flowing solutions has been growing recently. T h e high ratio of t h e porous electrode surface to solution volume was exploited for various purposes such as the analysis of flowing streams ( I ) , preparative electrolysis (21, and removal of metal ion contaminants from aqueous media ( 3 ) . Various geometric shapes and materials have been employed as porous electrodes to obtain high conversion. These include columns packed with small particles as platinum chips ( I ) , sintered silver disks ( 4 ) ,graphite powder or granules ( 5 , 6 ) , amalgamated chips of nickel ( 7 ) , or parallel screens of gold ( 8 ) , platinum ( 9 ) , and carbon ( 3 ) . In this paper we describe the electrochemical characteristics and the analytical exploitation of a flow-through cell employing a Reticulated Vitreous Carbon (RVC) electrode. RVC is a new open-pore material with a honeycomb (foam) structure (Chemotronics International, Inc., Ann Arbor, Mich). Specifications a n d characteristics are available along with several suggested non-electrochemical applications ( 1 0 ) . However, it appears t o be well suited as an electrode material for flow-through cells. I t combines t h e hydrodynamic advantages of high surface area, high void volume (as high as 97 7'0 ), low resistance t o fluid flow, and self-supporting rigidity (10). Electroanalytical advantages are wide operating voltage range, chemical inertness, high electrical conductivity, and relatively reproducible performance ( 1I ) . RVC is available in several porosity grades from 10 to 100 pores per inch (ppi), and it may easily be machined into various geometric shapes as disks, tubes, or rings. T h e application of porous electrodes for t h e analysis of flowing streams has been limited because of the high background currents that accompany the high analytical currents. 0003-2700/79/0351-0799$01.OO/O
Presently, the signal-to-background ratio of porous electrodes is less than t h a t of other amperometric detectors which electrolyze only a small fraction of t h e electroactive species (12, 13). This is due t o the geometry required t o achieve a high conversion yield. As the electrode length is increased to improve the yield, each increment of electrode length contributes proportionately less to the electrolysis of t h e species, but approximately the same to the background current owing to medium decomposition. These high background currents may be compensated by employing differential current measurements, such as t h e stopped-flow or t h e pulsed-flow techniques, originally employed a t the open tubular electrode (14, 15). These techniques were mainly designed t o compensate for nonconvective background currents. Such a combination of high analytical currents with the discrimination against high background currents would permit the analysis of very low concentrations of electroactive species in flowing streams.
THEORY The theory of flow electrolysis with a limiting current on the porous electrode was developed by Sioda (16) and was confirmed by experimental results for various electrode shapes (9, 16). T h e local limiting current (iJ a t an element of area of the porous electrode is given by:
il = j n F C q " In Equation 1,j is a proportionality constant, n is the number of electrons transfered per molecule, F is the value of Faraday, C is the local concentration of t h e electroactive species, a n d q is the specific flow rate, Le., the volume flow through a unit of cross-sectional area of t h e electrode. T h e exponent a depends on the flow regime and on the shape and size of the electrode. Substituting t h e distribution of t h e concentration throughout the electrode and integrating over t h e total electrode volume leads to the total limiting current (II)(16):
II = n F C o u [ l - exp(-jstz'-"u"-'L)]
(2)
where Co is t h e initial concentration of t h e electroactive species, s is the specific surface of the porous electrode, a is the cross-sectional area of the electrode, u is the volume flow rate, and L is t h e electrode length. From t h e magnitude of the limiting current, t h e degree of conversion R can be calculated according to:
R = I1/nFCou
(3)
By comparison of Equations 2 and 3, one obtains:
R = 1 - exp(-jsul-crua-ll) tZ 1979 American Chemical
Society
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ANALYTICAL CHEMISTRY, VOL 51, NO 7, JUNE 1979 A
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I/ Figure 1. Flow-through cell with RVC electrode and reference electrode A . Sample solution inlet. B Sample solution outlet (Tygon) C Lead to reference electrode. D. O-ring. E. RVC electrode, one disk shown F. Cation-exchange membranes. G. Reference solution inlet H Reference solution outlet J. Lead to working electrode
Differential current measurements between high and low flow rates may be described by the following equation, derived from Equation 3
A I I = 11,1 - I1,* = nFC,(ulR1 - u2R2)
(5)
T h e subscripts 1 and 2 fixed t o the symbol for any parameter designate values of that parameter a t t h e high a n d low flow rates, respectively. In the case of the stopped-flow technique, the second term in Equation 5 is equal t o zero:
AI, = nFCovR
(stopped-flow)
(6)
T h e direct proportionality between t h e current difference
and t h e bulk concentration of t h e electroactive species is the basis of their quantitative determination. EXPERIMENTAL Apparatus. The cell design is shown schematically in Figure 1. T h e body consisted of two 2-inch diameter Plexiglas blocks (Rohm and Haas Co., Philadelphia, Pa.),one 1.365 inch thick and the other 1 inch thick, held together with four stainless steel bolts (not shown). The reference silver-silver chloride electrode (SSCE) was coiled around a core of Plexiglas (0.408 inch long and 0.5-inch diameter), extending from the lower block into a cavity (1.072-inch diameter, 0.450 inch deep) in the upper block. The working electrode was composed of uniform RYC disks (0.096 inch thick and 0.218-inch diameter). One to four disks were placed into the flow channel (0.219-inchdiameter, 0.875inch long) in the upper block. T h e disks were held in place by a snug fit. Unless otherwise stated, all data were obtained with the 100-ppi disks. The working electrode lead consisted of a thin glassy carbon rod pressed into the edge of the RVC disk, and bonded with conducting epoxy (Epotek 410E and 430, Epoxy Technology, Watertown, Mass.) to a copper wire that led to the outside of the block. The silver wire of the reference electrode was also epoxied to a copper leadout wire. Bolting the two blocks together exerted pressure on five cation-exchange membrane washers (each 10 mils thick, 0.219-inch i.d., 1.068-inch o.d., Nafion XR-170, E. I. du Pont de Nemours & Co., Wilmington, Del.) located in the cavity of the upper block. This provided a leak-free electrolytic bridging path between the working and the reference electrodes. Teflon tubing (1.5-mm i.d., 3-mm o.d., Pennsylvania Fluorocarbon Co., Clifton Heights, Pa.) was used for the sample solution inlet and for the reference solution inlet and outlet. The Teflon tubing was fitted to the flow channels of the cell through luer fittings. The Teflon tubing that served as the sample solution inlet was enclosed in an aluminum tube, to reduce diffusion of
:I 2
1 04
~
I
1
0 -04 APPLIED VOLTAGE, V
J
-0.0
Figure 2. Current-voltage curves for 12.5 pM K,Fe(CN),. Flow rate,
1.4 mL/min. 0.1 M phosphate buffer, pH 7.4. Applied vottage measured vs. SSCE in 0.1 M KCI. For clarity, original data points are shown only for t h e two-disk electrode air into the flowing solution. For the sample solution outlet, the Plexiglas block was drilled to receive Tygon tubing, press fitted, and sealed with cyclohexanone. The sample solution was stored in a 250-mL Nalgene beaker, fitted with a Plexiglas cover containing two holes: one for the sample inlet and another for a deaeration tube that delivered a stream of nitrogen. Solution flow from the beaker through the cell proceeded by siphoning. The high void volume of RVC permitted flow rates up to 10 mL/min, for a hydraulic head of about a foot through the 4-disk electrode. After exit from the cell, the sample solution passed through a calibrated rotameter (no. 9143, Fischer and Porter, Warminster, Pa.) and a Teflon needle valve to control flow rate. Deaerated 0.1 M KC1 saturated with AgCl was flushed a t about 0.1 mL/min through the cavity containing the Ag-AgC1 reference electrode. All voltages in this paper are given with respect to this reference electrode. Current-voltage measurements were made with a Sargent Model XV Polarograph. The resistances through the working electrode were measured between the reference electrode and a copper wire inserted into the flow channel on opposite sides of the working electrode, with a conductance bridge (Model RCM 15B1, A. H. Thomas Co., Philadelphia, Pa). In systems containing the supporting electrolyte, the resistances were 450 and 1600 0 , respectively, for 1and 4 disks. Such resistances caused negligible distortion in current-potential curves like those of Figure 2. For amperometric measurements at applied voltages in the plateau region, larger ohmic drops up to 0.1 V were observed, but they did not affect the yield, which was independent of applied voltage in the plateau region. Measurement Procedures. Small gas bubbles inadvertently trapped in the RVC pores were removed before pretreatment by applying suction at the sample solution outlet with a plastic syringe. The RVC was pretreated electrochemically while passing the deaerated blank solution through it. The pretreatment consisted of cycling the applied voltage between +1.0 and -1.0 V for two cycles, allowing 10 min a t each applied voltage. After pretreatment the steady-state current-voltage curve of the blank solution was taken by stepping the voltage manually in small increments and permitting the transient currents to decay a t each voltage until steady state was reached. The current-voltage curve of the sample solution was taken in the same way.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JLiNE 1979
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I i
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I
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,#
/*e'
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4-DISK
I'
-
Q
07
3
0
I-DISK
20 -
1
I 10 0.5
I
1
I
2 FLOW RATE, ml/MIN
I 5
IO
Figure 3. Logarithmic dependence of the limiting current upon volume flow rate 25 pM K,Fe(CN), in 0 1 M phosphate buffer (pH 7 4) Constant applied voltage, -0.6 V Number on each curve represents the power dependence value, N Dotted line represents calculated values for 100 YO reduction efficiency
O01 2 I
-
Data for the dependence of current on flow rate and concentration were obtained in the diffusion limited region, at a constant applied voltage, where the current is independent of voltage. All data presented were corrected for background. Stopped-flow voltammetry was performed by turning a Teflon stopcock on and off that was inserted between the rotameter and the needle valve. Reagents. Analytical reagent grade chemicals were used without further purification. All solutions were prepared from deionized water (17 MR, Culligan Cartridge LVater Trehtment Systems, C d i g a n International, Northbrook, Ill.). The supporting electrolyte was 0.1 M phosphate buffer (pH '7.4) prepared from a 1:4 mixture of KH2P0, and K,HPO,. K3Fe(CNl6and K,Fe(CN)g3H2Owere used to prepare 5 mM stock solutions of each in the phosphate buffer. The ferricyanide solution was prepared weekly, and the ferrocyanide solution was prepared daily just before use. The stock solutions were stored in the dark. All studies were made by adding to the supporting electrolyte, aliquots of the stock solution t o give the desired concentration.
RESULTS AND DISCUSSION Dependence of the Current upon Applied Voltage, Flow Rate, and Number of RVC Disks. Figure 2 shows t h e current-voltage curves for the reduction of 12.5 i.tM K,Fe(CN),, for various numbers of disks forming the working electrode. T h e curves were taken pointwise by making 50-my changes in applied voltage and waiting for the current to reach steady state. Steady-state currents were corrected for background, obtained in blank supporting electrolyte. Well defined waves and potential-independent transport-limited current regions were obtainable for all t h e curves. Figure 3 presents the logarithmic dependence of the limiting reduction current of K,Fe(CN), upon volume flow rate for various numbers of disks, and for flow rates ranging between 0.57 and 6.55 mL/min. In general, as the flow rate decreases or as t h e electrode length increases, t h e residence time and conversion increase, and so does cy. In the limit, as quantitative transformation occurs according t o Faraday's law, N a p proaches unity. Figure 3 also reveals a transition t h a t occurs at a flow rate around 1.9 mL/min, indicating perhaps a change in the nature of t h e flow regime. Such transitions were also observed by Sioda (16). Figure 4 shows t h e dependence of t h e limiting degree of conversion upon the volume flow rate for 1, 2 , a n d 4 disks forming t h e working electrode. R is calculated from the limiting currents presented in Figure 3 according to Equation 3. As the residence time of an element of solution in the RVC
0
0
1
2
3 4 5 FLOW RATE, inI/MIN
6
7
Figure 4. Dependence of the limiting degree of conversion upon volume flow rate. Data taken from Figure 3 "1-
NUMBER
OF
DISKS, h
Figure 5. Dependence of log (1 - R ) / n 7 - c r ~ "upon -' number of disks. Data taken from Figure 3. Number on each plot represents flow rate
in mL/min
Rvalues taken from Figure
41,
and
(Y
values from Figure
3
electrode increases, and in accord with Equation 4, R increases as t h e flow rate decreases and as the length increases. T h e maximum conversion noted is 92 Yc,which could be increased by increasing the electrode length and/or decreasing the flow rate. I n the case of a porous electrode built of parallel disks, the length of the electrode is proportional to the number of disks forming the working electrode. For verification of Equation 4, a plot of log (1 - R ) / U ~ - ~ vs. U "the - ~ number of disks should result in a straight line. Such plots are presented in Figure 5 for three different flow rates, and straight lines are obtained as expected. T h e dependences of the limiting current and of the limiting degree of conversion upon volume flow rate were also obtained for a 60-ppi RVC electrode composed of two disks (see Figure 6). The logarithmic dependence of the limiting current upon the flow rates yields a straight line with a slope of 0.323 over a 10-fold range of flow rates. Thir, cube-root dependence corresponds to laminar flow, which has also been observed for flow-through gold micromesh electrodes (81, platinum wire and grid electrodes ( 1 6 ) , and for the open tubular electrode
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
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0 C
BLANK
5th 7.5 MIN
10th 15 MIN
15th 22.5 MlN
20th
30 MIN
Figure 8. Sensitivity and precision obtained for successive stopped-flow measurements over a 30-min time period. Conditions: 0.10 pM K,Fe(CN), in 0.1 M phosphate buffer (pH 7.4). RVC fourdisk electrode (100 ppi). Solution flow rates: 4.9 (on) and 0 (off) mL/min. Constant applied voltage, +0.6 V 5c
1
0.5
I I
1 2 FLOW RATE,
I
lo
5 ml/MIN
Figure 6. Dependences of the limiting current and of the limiting degree of conversion upon volume flow rate. Conditions: As in Figure 3, for a 60-ppi electrode composed of two disks Y
12c
Figure 7. Dependence of current upon ferricyanide concentration. Flow rate, 1.45 rnL/rnin. Constant applied vottage, -0.5 V. For clarity, original data points are shown only for the four-disk electrode
(14). R values (dotted line in Figure 6) range between 0.047 t o 0.234 for the various flow rates. For the range of flow rates studied (0.6 to 6.5 mL/min), the ratio of the limiting currents for the 100- and 60-ppi electrodes ranged from 3.1 to 4.2. The R ratio shows similar variations (Equation 3). T h e ratio of the surface areas was only 1.74 (as calculated from t h e manufacturer's specifications ( I O ) ) , confirming t h a t the limiting current was not simply proportional to total surface area of the electrode (see Equation 2).
Analytical Application: Sensitivity and Precision. Figure 7 presents the dependence of the steady-state limiting current upon ferricyanide concentration for various numbers of disks forming t h e working electrode. Linearity between current and concentration, expected from Equation 2, is shown u p t o 8.25 pM, which was the highest concentration studied. Sensitivity is remarkably high, corresponding t o 0.7, 1.3, and 1.6 pA/pM for the 1-,2-, and 4-disk electrodes, respectively. However, this high sensitivity is accompanied by high background currents due t o the high surface area, and these currents are troublesome when measuring very low concentrations of electroactive species. Overall, the detectability of low concentrations is not significantly improved compared to t h a t obtained with other types of low-yield amperometric detectors. T h e signal-to-background ratio may be greatly
improved by differential current measurements which discriminate against the nonconvective component of t h e background currents. This component has been reported as the major contributor to the steady-state background currents of glassy carbon over most of the operating voltage range (17). Figure 8 is a reproduction of a stopped-flow chart record for 0.10 pM ferrocyanide in 0.1 M phosphate (pH 7.4). The blank stopped-flow current (shown on the left) amounted t o 22 nA, corresponding t o a concentration around 11 nM. T h e compensated steady-state nonconvective component of t h e blank current is about 680 nA. The noise level is only around 1 nA. Based on a signal-to-noise ratio of 1, t h e limit of detection for ferrocyanide would be less than l nM. Compared t o a value of around 20 n M t h a t was estimated for the open tubular electrode ( I @ , the better detectability obtained with the porous electrode is due to the greatly increased signal level with little increase in the noise level. For the 20 stopped-flow measurements represented in Figure 8, the relative standard deviation of the differential current measurement was 0.98%. The high sensitivity and low background of the stopped-flow procedure indicate great analytical promise. However, t h e long cycling time (about a minute) of the stopped-flow procedure precludes its use in important applications like chromatographic detection and flow injection analysis. For such applications, pulsed flow procedures (15, 19) have permitted the achievement of subsecond cycling times.
ACKNOWLEDGMENT T h e assistance of R.Schmelzer in the machining of the cell is highly appreciated.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
Johnson, D. C.; Larochelie, J. Talanta 1973, 20,959. Sioda, R. E. J . fhys. Chem. 1968, 72,2322. Yaniv. D.; Ariel, M. J . Electroanal. Chem. 1977, 79,159. Kenkel, J. V.; Bard, A . J. J . Electroanal. Chem. 1974, 5 4 , 47. Blaedel, W.J.; Strohl, J. H. Anal. Chem. 1964, 36, 1245. Sioda. R. E. Electrochim. Acta 1968, 13, 1559. Roe, D. K. Anal. Chem. 1964, 3 6 , 2371. Blaedel, W. J.; Boyer, S. L. Anal. Chem. 1973, 45, 258. Sioda, R. E. J . Electroanal. Chem. 1972, 34, 411. "Reticulated Vitreous Carbon (RVC)", Chemotronics International; Ann Arbor, Mich. 48104, 1976. Zittel. H. E.; Miller, F. J. Anal. Chem. 1965, 37,200. Kissinger, P. T. Anal. Chem. 1977, 49, 447A. Buchta. R. C.; Papa, L. J. J . Chromatog. Sci. 1976, 14, 213. Blaedel, W. J.; Boyer, S. L. Anal. Chem. 1971, 4 3 , 1538. Blaedel, W. J.; Iverson. D. J. Anal. Chem. 1977, 4 9 , 1563. Sioda, R . E. Electrochim. Acta 1970, 15, 783. Blaedel, W.J.; Jenkins, R. A. Anal. Chem. 1975, 47, 1337. Blaedel, W. J.; Yim. 2. Anal. Chem. 1978, 5 0 , 1722. Yim, 2. Work in progress, Department of Chemistry, University of Wisconsin-Madison.
RECEIVED for review January 16, 1979. Accepted March 12, 1979. This work was funded in part by t h e University Sea Grant Program under a grant from the Office of Sea Grant, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, and by the State of Wisconsin.
