ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
interfere with the Na' determination. T h e 90% rise time of the electrode obtained by adding 1 mL of 1 M solution of NaCl to 50 mL of a M NaCl is about 5 s, significantly longer than the rise time of the electronic equipment with a source resistance of 10" Q. The double barreled electrodes (tip diameter 2 pm) described here have a resistance of about 10'O R. Since the speed of mixing in the sample solution after the stepwise addition of NaCl solution is rather critical, the actual response time of the sensor will be considerably smaller than indicated. Indeed, 90% response times of 0.2 s were reported for the same electrode (22). The lifetime of the microelectrode in contact with aqueous electrolytes is several weeks and the drift is 51 mV/day. T h e microelectrode described here is designed for intracellular use. A first series of intracellular measurements has been successfully conducted (22); in analogy to observations on other microelectrodes based on electrically neutral carriers (13), no interference due to the presence of proteins was detected. Since the extracellular ion activities differ sharply from the intracellular ones, extracellular measurements with this electrode are prone to interference due to the rather poor Naf/Ca2+-selectivity.It is a fair assumption that a useful Na+ microelectrode on the basis of electrically neutral ligands can be developed for extracellular work, inasmuch as corresponding macroelectrodes with KNaCaPot as low as have been described (9,23). Because of their small selectivity with respect to A? (KNaKPot = 5 X lo-' (9,23))!their application will be limited to extracellular environments. The same might hold for the monensin-based electrode (see Table I (8)).
LITERATURE CITED (1) M. Lavallk, 0. F. Schanne, and N. C. Hebert, Ed., ''Glass microelectrodes'', Wiley & Sons, Inc., New York, London, Sydney, Toronto, 1969.
353
H. J. Berman and N. C. Hebert, Ed., "Ion Selective Microelectrodes", Plenum Press, New York, London, 1974. M. Kessler, L. C. Clark, Jr., D. W. Lubbers, I . A. Silver, and W. Simon, Ed., "Ion and Enzyme Electrodes in Biology and Medicine", Urban and Schwarzenberg, Munich, Berlin, Vienna, 1976. G. Eisenman, Ed., "Glass Electrodes for Hydrogen and Other Cations", M. Dekker Inc., New York. 1967. R . C. Thomas, in ref. 3, p 141. R. N. Khuri, in ref. 3, p 123. J. L. Walker, Jr.. Anal. Chem., 43 (3), 89A (1971). R. P. Craig and C. Nicholson, Science, 194, 725 (1976). M. Guggi, M. Oehme, E. Pretsch, and W. Simon, Helv. Chim. Acta, 58, 2417 (1976). H. D. Lux and E. Neher, Exp. Brain Res., 17, 190 (1973). G. G. Guilbauit, R. A. Durst, M. S. Frant, H. Freiser, E. H. Hansen, T. S. Light, E. Pungor, G. Rechnitz, N. M. Rice, T. J. Rohm, W. Simon, and J. D. R. Thomas, "IUPAC Information Bulletin, No. l " , 1978, p 70. P. C. Meier. D. Ammann, H. F. Osswald, and W. Simon, Med. Progr. Techno/., 5, 1 (1977). P. C. Meier, D. Ammann, W. E. Morf, and W. Simon, in "Medical and Biological Applications of Electrochemical Devices", J. Koryta, Ed., John Wiley & Sons, Ltd., Chichester, 1978 (in press). K. Cammann, "Das Arbeiten mit ionenseiektiven Eiektroden", Springer Verlag, Berlin, Heidelberg, New York, 1977. M. Oehme and W. Simon, Anal. Chim. Acta, 86, 21 (1976). M. Oehme, M. Kessler, and W. Simon, Cbimia, 30, 204 (1976). R . C. Thomas, W. Simon, and M. Oehme, Nature (London), 258, 754 (1975). W. E. Morf and W. Simon, in "Ion-Selective Electrodes in Analytical Chemistry", H. Freiser, Ed., Plenum Publishing Co., New York, 1978 (in press). T. Zeuthen, J . Mernbr Biol., 39, 185 (1978). J. A. Coles and M. Tsacopoulos, J . Physiol., 270, 12P (1977). S. W. de Laat, W. Wouters, M. M. Marques da Silva Pimenta Guarda, and M. A. de Silva Guarda, Exp. Cell Res., 91, 15 (1975). S. Levy. Experimental Ophthalmology Laboratory. Geneva, Switzerland, private communication, 1978. D. Ammann, R. Bissig, Z.Cimerman, U. Fiedler, M. Guggi, W. E. Morf, M. Oehme, H. Osswald, E. Pretsch, and W. Simon, in ref. 3, p 22. W. Simon, D. Ammann, M. Oehme, and W. E. Morf, Ann. N. Y . Acad. Sci., 307, 52 (1978).
RECEIVED
for review August 28, 1978. Accepted November
9,1978.
Reticulated Vitreous Carbon Flow-Through Electrodes A. N. Strohl and D. J. Curran" Department of Chemistry, GRC Tower I, University of Massachusetts, Amherst, Massachusetts 0 1003
Flow-through electrodes have been fabricated from Reticulated Vitreous Carbon (RVC). This material has low electrical resistance, large surface area, and a physically continuous structure. The resistance to solution flow is low and flow rates up to 25 mL/min were easily attained. Solutions in the concentration range 1 to 1000 pM produced currents from about 1 to 1000 PA, respectively, using flow rates from 0.5 to 25 mL/min. Current-voltage curves were generated using these electrodes. At constant potential, the analyte solution was pumped through the electrode at a constant rate, thereby generating a steady-state current. The linearity and slope of calibration curves (steady-state current vs. concentration) were found to depend on flow rate. Higher sensitivities were obtained at higher flow rates and coulometric conversions occurred at the lowest flow rates. Ferricyanide ion and ascorbic acid were used as test systems, and ascorbic acid in vitamin C tablets was determined with results that were in excellent agreement with a titrimetric check method.
Electrodes based on various forms of carbon have received increasing attention in recent years for use in flowing streams. In general, three configurations have been used which are described by the position of the electrode in relation to the 0003-2700/79/035 1-0353$01 O O / O
flowing stream: flow-by, where the electrode surface is parallel to the direction of flow; flow-onto. where the surface is normal to the direction of flow; and flow-through. Pungor and coworkers have developed a silicone rubber based graphite flow-by electrode (1-3) and have summarized earlier work. The same electrode has been used for turbulent hydrodynamic voltammetry in a flow-onto mode (4-6). Carbon paste flow-by electrodes have been used by Kissinger and others in electrochemical cells for liquid chromatography detectors ( 7 ) . Curran and co-workers ( 8 ) have used a pair of carbon-wax electrodes for the same purpose. Two types of flow-through electrodes can be distinguished, depending upon whether the electrode is simply a hollow cylinder or is a porous matrix of some sort through which the solution flows. SVith respect t o the former, the work of Blaedel with tubular electrodes should be mentioned (9). An excellent discussion of the latter is found in the paper by Johnson and Larochelle where a packed electrode is used as a coulometric detector for liquid chromatography ( I O ) . Various forms of porous matrix flow-through carbon electrodes have been used previously. A review of electrolytic chromatography by Fujinaga and Kihara ( 1 1 ) describes the work of these authors with columns containing glassy carbon grains, which served as the electrode material. Crushed graphite was used in a short column electrode by Blaedel and Strohl (12). Finely divided 1979 American Chemical Society
354
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
graphite chips or flakes were used by Bennion and Newman t o construct a porous flow-through cathode for t h e removal of copper in a flowing stream (13). Ostrovidov made porous carbon electrodes by hot pressing techniques using a polyethylene binder (14). These electrodes had significant resistance t o flow, and a considerable pressure (0.2-0.3 atm) was required to produce a flow rate of 0.2 mL/min. In later work, porous carbon electrodes were formed from blocks of commercial pyrographite, which were used for the determination of dichromate and ferric ions (15, 16). T h e electrodes were cylinders 0.9 cm in diameter and 1-2 cm long. No information on their physical properties was provided. It is stated t h a t the lowest concentration of Fe(II1) which could be determined was 7 p p b (16). We report here on the use of Reticulated Vitreous Carbon, RVC (trademark) as an electrode material for studies in flowing streams. It is available from Chemotronics International, Inc., Ann Arbor, Mich. T h e only previous reports of its use as a n electrode are that of Norvell and Mamantov (17)who used a thin slice of RVC as a n optically transparent electrode, and that of Butcher, Chambers, and Pagni who used a stationary RVC anode for the coulometric electrolysis of uranocene in THF (18). Tentorio and Casolo-Ginelli have reported a study of t h e mass transfer characteristics of a reticulated copper electrode made by electroless deposition of copper on polyurethane foams (19). In the present work, current-voltage curves were obtained with electrodes of very large surface area using the reduction of ferricyanide and the oxidation of ascorbic acid as examples. Determinations by steady-state voltammetry were investigated and the conditions necessary t o achieve coulometric conversion are presented. Ascorbic acid was determined in vitamin C tablets. EXPERIMENTAL Chemicals. All chemicals were reagent grade. Ascorbic acid and potassium ferricyanide were dried at 110 "C for 1 h. Distilled water was redistilled from alkaline permanganate. Solutions. Stock solutions of ascorbic acid and potassium ferricyanide were several millimolar. Boiled distilled water containing 0.5 g/L Na2EDTA.2H20was used in preparation of all ascorbic acid solutions to prevent chemical oxidation (20). Iodate was used to assay the ascorbic acid stock solution. The hydroquinone method of Simon and Zgka (21)was used to potentiometrically analyze the ferricyanide stock solution. Dilutions for electrochemical analyses were made with supporting electrolyte which was a 0.025 M phosphate buffer (pH 6.82), 0.1 M in potassium chloride. Vitamin C tablets were obtained from commercial sources and the stock solutions were approximately 3 mM in ascorbic acid. Assays and dilutions followed the treatment for ascorbic acid. Equipment. Either a Princeton Applied Research Corp. Model 170 Electrochemical System or a PAR l'i3/376 System with a Houston Model 2000 Omnigraphic X-Y Recorder was used. A Hewlett-Packard Model 3439A Digital Voltmeter or a Textronix DM 501 Digital Multimeter was available for digital readouts of currents. Most current-voltage curves were obtained without the use of positive feedback IR compensation. A Cole-Parmer Masterflex Console Pump with a ~ 7 0 1 3pump head maintained constant flow through the RVC Electrode. The flow rate was checked volumetrically. Silicone rubber tubing, 0.8-mm i.d., was used for all fluid connections. A Perkin-Elmer Model 202 Spectrophotometer was used. Electrode Construction. RVC-A, a high conductivity form of RVC, obtained from Chemotronics International, Inc., Ann Arbor, Mich., was used for all experiments. Figure 1 shows the electrode construction. A cylinder (A) cut from a RVC-A block with a cork borer and a piece of glass tubing (D) were encased in heat-shrink plastic tubing (B). A spectrochemical grade graphite rod sidearm (C) was fitted through the plastic tubing to make a press-fit electrical contact with the RVC cylinder. All joints and any possible locations for leaks were sealed and strengthened by a glass outer shell filled with epoxy cement (E). The electrode used for all determinations except the ferricyanide
rD'1 7
Figure 1. RVC Electrode. (A) RVC cylinder, (B) heat-shrink tubing, (C) graphite rod sidearm, (D) glass tube, (E) glass and epoxy support I
A
\I
I1
Figure 2.
Schematic diagram of equipment for steady-state electrolysis.
(A) Analyte reservoir, (B) pump, (C) RVC electrode, (D) platinum electrode, (E) SCE reference electrode, (F) downstream reservoir, (G) runover collector, (H) potentiostat, (I) recorder, (J) digital voltmeter
work was 1.0 cm in diameter and 10.8 cm long, having a calculakd volume of 7.8 mL and an electrode surface area of 515 cm2, The electrode used for ferricyanide determinations was 0.5 cm in diameter and 10.8 cm long, having a calculated volume of 1.9 mL and a surface area of 125 cm2. The resistance of these electrodes measured end-to-end in air was 11.0 and 16.3 R for the 7.8- and 1.9-mL volume electrodes, respectively. For the larger electrode, the resistance between the graphite rod and one end of the RVC was 12.4 R and 5.2 R to the other end. Similar resistances for the smaller electrode were 12.9 and 5.4 R. These numbers demonstrate that the contact resistance between the graphite rod and the RVC is no more than a few ohms. The major contributor to any electrode resistance effects is the electrolyte resistance which was about 700 R for the 7.8-mL electrode. Electrode Operation. The solution to be analyzed passed from the analyte reservoir (A) (Figure 2) through the pump (B) to the upper portion of the electrode (C). A layer of solution was kept over the top of the electrode to ensure that it was totally filled with liquid. This was initially back-pumped from the down-stream reservoir. Thus the entire surface area of the electrode could be available. The other end of the electrode dipped into the downstream reservoir (F)containing P t counter and SCE reference electrodes (D and E). During analysis, the pumped solution flowed through the electrode into the downstream reservoir and overflowed from it into the runover collector ( G ) , so a constant volume of liquid was maintained in the downstream reservoir. In the amperometric mode each solution was pumped through the electrode until a current plateau was achieved. Before the next solution was introduced for analysis, the analyte solution reservoir and pump system were washed and flushed with several milliliters of the test solution. At fast flow rates, i t was necessary t o turn off the pump to add wash and sample solutions. During all analyses the potentiostat (H) applied a constant potential to the RVC electrode, and the currents were monitored by recorder and digital readout (I and J ) .
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979 120
Table I. Physical Properties-Nominal Values,u RVC-A-100
355
L
100 -
bulk void volume 90% pores per linear inch 100 surface area (cm7/cm3) 66 bulk resistivity (ohm-cm) 0.23 Bulletin 1 7 6 R 2 k , Chemotronics International, Inc., Ann Arbor, Mich. 48104.
80
-
(I
40
Table 11. R V C Anodic Limiting Reactionu DME.
phosphate buffer, pH 7
id ( P A )
at E , -1.2 V
air sat’d 0:removed 21-h electrolysis,b RVC at 0.8 V 0 : removed 19-h electrolysis,b RVC at 1 . 0 V Potentials vs. SCE. Under N,.
03
0.2
0.1
L 1
z 0o /
5.7 0.2 0.8
I1 0.2
0.0
0.1
-0.2
-0.1
-0.3
-0.4
E vs. SCE IVI
Figure 4. Current-voltage curve of 7.8 X M potassium ferricyanide. Electrode volume, 7.8 mL; flow rate, 0.83 mL/min; scan rate, 1 mV/s; no IR compensation. Electrolyte; 0.1 M KCI, 0.025 M phosphate buffer (pH 6.82). (A) Scan uncorrected for background, (8) scan of background
0.2 5.5
KCI
U
//
E 2
H
/
1-
L 0.4
- 0.1
0.2
o
1
-0.4
-0.2
E,
V
YS
-0.6
-0.8
-1:o
J
SCE
lo3 M potassium ferricyanide corrected for background. Flow rate, 8.0 mL/min; scan rate, 2 mV/s; positive feedback IR compensation used; supporting electrolyte: 0.1 M KCI, 0.025 M phosphate buffer (pH 6.82) Figure 5. Current-voltage curve of 1.6 X
1
0.7
,
0.6
Ll
1
0.5
0.4
0.3 0.2 E vs. S C E , V
0.1
0.0
-0.1
Figure 3. Background current-voltage curve. Electrolyte 0.1 M KCI; electrode volume, 7.8 mL; flow rate, 8 mL/min; scan rate, 2 mV/s
RESULTS AND DISCUSSION Table I lists physical properties of the RVC-A material. The low bulk resistivity and high surface area are attractive properties for a flow-through electrode. In contrast to materials used in packed or fluidized bed electrodes, RCV is mechanically continuous and therefore does not rely on physical packing for electrical contact throughout the matrix. In addition, the pressure drop across the material is extremely small in a flowing stream. Both gravity flow and high flow rate systems are easily implemented. Except for the time required for epoxy cement to harden, electrodes (Figure 1) can be fabricated in less than one hour. Electrode Potential Limits. Figure 3 shows a residual current-voltage curve obtained in 0.1 M KCl. The background current in this scan contains a charging component which, of course, can be reduced considerably if data points are obtained manually a t fixed potentials. The anodic limit is determined by the process responsible for the rise in current in the vicinity of 0.7 V. Experiments conducted in a pH 7 phosphate buffer without any chloride ion present showed that oxygen was produced anodically. The oxygen limiting current a t -1.2 V vs. SCE was monitored with a DME after prolonged electrolysis a t the RVC anode. Results are shown in Table 11. Very little oxygen is produced a t 0.8 V, but more substantial amounts are produced a t 1.0 V. Oxygen is generated a t a considerable overpotential. The anodic limit was examined in solutions of various acidities in the absence of chloride ion and it shifted in the positive direction with increasing acidity as would be predicted by the oxygen reaction. T h e cathodic limiting reaction depends on the solution pH. There does not appear
to be a substantial hydrogen overpotential, and further work in this area will be reported. The Ferricyanide-Ferrocyanide Test System. Figure 4 shows a current-voltage curve for the reduction of ferricyanide (curve A) along with the residual current curve (B). Because of the large surface area of the electrode and convective mass transport, a large current signal is produced with fairly dilute solutions. If the background (curve B) is subtracted from curve A shown in Figure 4, the corrected curve shows a maximum. Similarly, maxima are shown by the current-voltage curves in Figure 8 for the ascorbic acid system. Fujinaga and Kihara have developed equations for the current-voltage curve in packed column electrodes ( 1 1 ) . Their results predict a peak current with fast scan rates and slow flow rates and a limiting current plateau with slow scan rates and fast flow rates. On this basis, our curves should not show peaks. T h e cell geometry is far from ideal and all of these curves were generated without the use of positive feedback IR compensation. Figure 5 shows a current-voltage curve for 1.6 mM ferricyanide which is corrected for background current and where both the background curve and the total current curve were obtained using IR compensation. If no IR compensation is used or the compensation control is incorrectly set, apparent maxima are obtained for corrected curves when the background current is changing rapidly. I t was particularly troublesome to make the IR compensation adjustment correctly for concentrations in the 10-pM concentration region since the divider ratio for the compensation potentiometer is very small. Expansion of the resolution of the voltage divider would improve this situation. Further evidence was obtained which demonstrated that the maxima are only apparent. A current-voltage curve for 1.6 m M ferricyanide was generated under conditions where the cor-
356
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
- 300
Y' /"
~
' ?
, i
15
.n
C"
FLOW R O T E (ml/minj Figure 6. Background current as a function of flow rate. Solution; 0.1 M KCI, 0.025 M phosphate buffer (pH 6.82), potential, -0.2 V vs. SCE: electrode volume, 1.9 m L
VOLTS VS SCE
Figure 8. Current-voltage curves of ascorbic acid. Electrode volume 7.8 mL; flow rate, 8 mLlmin; no IR compensation. (A) 1.6 X M, corrected for background; (E) 0.8 X M, corrected for background: (C) background, 0.1 M KCI, 0.25 M phosphate buffer (pH 6.82)
TIME
Figure 7. Tracing of the recorder readout of steady-state current as M ferricyanide; potential, a function of flow rate. Solution: 3.9 X -0.2 V vs. SCE; electrode volume, 1.9 m L
rected curve showed a maxima a t about 4 . 5 0 V. Holding the potential constant a t this value, the effluent from the electrode was collected, being careful t o avoid diluting it downstream. This experiment was then repeated a t -1.05 V where the current was apparently lower than at the maximum. Analysis of the two effluent solutions by spectrophotometry at 420 nm (only ferricyanide absorbs a t this wavelength) showed that the absorbance of ferricyanide was lower by about 0.14 absorbance unit in the solution obtained a t -0.5 V, as would be expected for a current-voltage curve showing a slightly rising plateau. Ferricyanide ion is thermodynamically capable of reaction with hydrogen gas but no evidence could be found spectrophotometrically that this reaction occurs when the gas is bubbled into a solution of ferricyanide. The conclusion is that these maxima are apparent and arise from uncompensated IR drop. From Figure 4,a control potential of -0.2 V vs. SCE was chosen for further experiments. It was of interest to determine the background current a t this potential as a function of the flow rate, and the results are shown in Figure 6. On the whole, the background current is independent of flow rate, a result similar to that found by Jordan and co-worker for a conical microelectrode in flowing streams (22). Above 3 mL/min. the scatter in the data is probably experimental error; while below 3 mL/min, i t appears that the background current is somewhat lower. If we assume a representative background current of 1.60 PA, the background current density is 13 nA/cm2 based on the manufacturer's area specifications. Actual recorder tracings of steady-state currents with 3.9 pM ferricyanide are shown in Figure 7 for a series of flow rates. Initially the analyte reservoir was filled with supporting electrolyte which was used as a wash solution. During the wash, the flow rate was adjusted and back-pumping was used to refill the electrode if necessary. The small perturbations in the base line are due to changing the flow rate, momentarily turning the pump off, or back-pumping. On addition of ferricyanide, a steady-state current was achieved in less than 60 s. For purposes of this figure, supporting electrolyte was
D '
d,,
4iOC
8'.00
li.00
I S 00
FL3H R R T E .
T . o c
'8
24
on
zs.ao,iDao
tlL/lIN
Figure 9. Current output and percent conversion of ascorbic acid as a function of flow rate. Solution 2 08 X 10-5 M ascorbic acid; potential, 0 4 V vs. SCE; electrode volume, 7.8 mL, electrolyte: 0.1 M KCI, 0.025 M phosphate buffer (pH 6.82) (x) current, (+) percent conversion
added between each analysis run. However, when constructing a working curve at a constant flow rate, the wash solution can also be the next standard solution t o be determined. Ascorbic Acid System. Figure 8 shows current-voltage curves for the oxidation of ascorbic acid. T h e steady-state current a t each data point was obtained a t constant flow and potential. Curve A is that of 16 p M ascorbic acid corrected for the background current (curve C) and curve B is the corrected curve for 8 FM ascorbic acid. In contrast to current-voltage curves generated at microelectrodes, these curves involve a substantial change in the bulk concentration of the electroactive species. For example, for curve B, we calculate that 84% of the ascorbic acid is oxidized a t the steady-state a t 0.6 V based on the flow rate, the electrode volume, and the magnitude of the current. Similarly, for the ferricyanide curve shown in Figure 4,the percent conversion is about 100% a t a potential of -0.26 V. Steady-state currents are achieved because of the continual replenishment of the electrode by the flowing stream. The shape of the current-voltage curve is irreversible and the curve is shifted anodically along the voltage axis with increasing concentration as shown in Figure 8. The influence of flow rate for 2.08 X M ascorbic acid is shown in Figure 9. As the flow rate increases, the current rises rapidly a t first, but then more slowly. On the one hand, increasing flow tends to shift the curve anodically. However, because the residence time in the electrode is shorter a t higher flow rates, the percent conversion decreases as is shown in Figure 9, and the amount reacted is smaller. Working curves for the ascorbic acid system are shown in Figure 10 for three different flow rates. Each data point was corrected for the background current. Curves A and B were obtained a t a control potential of 0.6 V vs. SCE which was chosen on the basis of the current-voltage curves shown in
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
flow rates for more dilute solutions. T h e utility of the electrode in the analysis of real samples is illustrated by the determination of ascorbic acid in vitamin C tablets. The work was done early in the study and a control potential of 0.4 V vs. SCE was used. A calibration curve was constructed a t a flow rate of about 7 mL/min. Vitamin C tablets were dissolved in deaerated aqueous buffer containing EDTA as a preservative. Dilutions were made so that the concentrations used for determination were in the linear region of the working curve. The results are compared in Table I11 with those obtained by the KIO, titration method. T h e agreement is excellent. T h e solutions determined with the flow-through electrode were 1000 times more dilute than those determined by titration. However, even with EDTA present, micromolar ascorbic acid solutions were not stable on standing. This problem was minimized by using the standard and unknown solutions as promptly as possible.
F
E
K
K 2
3 0
1-
357
I
ACKNOWLEDGMENT Table 111. Determination of Vitamin C in Tablets source
X Y Z a Titration method. 1:lOOO.
M X
103
2.84a 2.86 2.78
M X
io3
2.83b 2.87 2.78
RVC method, sample diluted
Figure 8. The curvature shown by the working curves is due mainly to the shift of the current-voltage curve with concentration and flow rate. At the lower flow rate, the curve is linear to a higher concentration. These curves are most useful in the linear portion, and very large currents are obtained for concentrations in the micromolar region. While the utility of curves A and B is clear, their basis is empirical. At a given flow rate, F,, the maximum possible steady-state current would correspond to complete conversion of the electroactive material. Curves A and B do not achieve this maximum current, and thereby the maximum slope, because the flow rates are too high. Curve C illustrates a case where 100% conversion has been achieved. The slope of curve C is 2.16 A/bM, and is in satisfactory agreement with the theoretical slope of 2.30 A/pM calculated from Faraday's law. Thus, it is possible to achieve working curves linear out to higher concentrations by working at low flow rates to obtain 100% conversion. However, maximum sensitivity is obtained a t higher flow rates. For example, the slope of curve A in the linear region is 22 pA/pM. It is probably better to use higher
The authors thank L. Rubin and the Princeton Applied Research Corporation for the loan of a 173/376 Electrochemical System.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22)
E. Pungor, 2s. FehBr, and G. Nagy. Anal. Chim. Acta, 51, 417 (1970). G. Nagy, Zs. FehBr, and E. Pungor, Anal. Chim. Acta, 52, 47 (1970). Zs. FehBr and E. Pungor, Anal. Chim. Acta, 71, 425 (1974). M. Varadi and E. Pungor, Anal. Chim. Acta, 80, 31 (1975). M. Varadi, M. Gratzl, and E. Pungor, Anal. Chim. Acta, 83, 1 (1976). M. Varadi, and E. Pungor, Anal. Chim. Acta, 84, 351 (1977). P. T. Kissinger, Anal. Chem., 49, 447A (1977). R. J. Fenn, S. Siggia, and D. J. Curran, Anal. Chem., 50, 1067 (1978). W. J. Biaedel and G. W . Schieffer, Anal. Chem.. 46, 1564 (1974). D. C. Johnson and J. Larochelle, Talanta, 20, 959 (1973). T. Fujinaga and S. Kihara, Crit. Rev. Anal. Chem., 7, 223 (1977). W . J. Blaedel and J. H. Strohl, Anal. Chem., 36, 1245 (1964). D. N. Bennion and J. Newman, J . Appl. Electrochem., 2, 113 (1972). E. A. Ostrovidov, Russ. J . Prakt. Chem.. 43, 1502 (1970). E. A. Ostrovidov and P. F. Veselovskii, Zavod. Lab., 40, 1247 (1974). E. A. Ostrovidov and P. F. Veselovskii, Russ. J . Anal. Chem., 30, 156 (1975). V. E. Norvell and G. Mamantov, Anal. Chem., 49, 1470 (1977). J. A. Butcher, Jr., J. Q.Chambers, and R. M. Pagni, J . Am. Chem. Soc.. 100, 1012 (1978). A. Tentorio and U. Casolo-Ginelli, J , Appl. Electrochem., 8, 195 (1978). L. Erdey and G. Svehla, "Ascorbinometric Titrations", Akadgrniai KaidB, Budapest, 1972, pp 21-31. V. Simon and J. Z$a, Collecf. Czech. Chem. Commun., 21, 327 (1956). J. Jordan, R. A. Javick. and W. E. Ranz, J . Am. Chem. Soc., 80, 3846 (1958).
RECEILFD for review September 29, 1978. Accepted December 11, 1978. Presented in part at the 29th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, February 1978.
