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Anal. Chem. 1982, 5 4 , 782-787
concentrations of manganese(I1) ion completely inhibit adenosine deaminase activity at pH 8.0 (17). Figure 8 shows that manganese(I1) ions a t a concentration of 1 mM inhibit about 80% of the adenosine deaminase activity. Because of the complex structure of the tissue slice, it might be expected that this inhibition would require a long incubation period in order to allow sufficient time for the inhibitor to penetrate into and to disperse itself throughout the tissue layer; however, the results presented in Figure 8 suggest that this is not the case since the inhibition occurs in a relatively short period of time. Also shown in Figure 8 is the reversibility of the inhibition with rapid reactivation of the interfering pathway after removal of the inhibitor. As can be seen from Figure 9, a 10 mM concentration of manganese(I1) completely eliminates the interfering response to adenosine and its related compounds. Thus a careful selection of optimum operating pH, buffer constituents, and selective inhibitors results in a tissue electrode for guanine with high selectivity, good lifetime, and other attractive operating characteristics. The combination of such biochemical “tuning” steps with appropriate selection of membrane materials, tissue thicknesses, and immobilization procedures represents the essential elements of the best strategy for electrode optimization available at the present time.
LITERATURE CITED Rechnltz, 0. A. Science 1981, 214, 287-291. Brady, J. E.; Carr, P. W. Anal. Chem. 1980, 52, 980-982. Carr, P. W. Anal. Chem. 1977, 49, 799-802. Hameka, H. F.; Rechnitz, G. A. Anal. Chem. 1981, 53, 1586-1590. Rechnltz, G. A.; Arnold, M. A,; Meyerhoff, M. E. Nature (London) 1979. 278, 466-467. Arnold, M. A.; Rechnitz, G. A. Anal. Chem. 1981, 53, 1837-1842. Nlkolells, D. P.; Papasthopoulos, D. S.; Hadjiioannou, T. P. Anal. Chim. Acta 1981, 126, 43-50. Arnold, M. A.; Rechnitz, G. A. Anal. Chem. 1981, 53, 515-518. Amold, M. A.; Rechnltz, G. A. Anal. Chem. 1980, 52, 1170-1174. IUPAC Analytical Chemistry Division, Pure Appl. Chem. 1978, 48, 127- 132. Albert, A.; Brown, D. J. J. Chem. SOC. 1954, 2060-2071. Ito, S.; Takoaka, T.;Morl, H.; Teruo, A. Clln. Chlm. Acta 1981, 715, 135- 144. Kobos, R. K. I n ”Ion-Selective Electrodes in Analytical Chemistry”; Frelser, H., Ed.; Plenum: New York, 1980; Chapter 1. Gullbauk, G. G.; Smith, R. K.; Montalvo, J. G., Jr. Anal. Chem. 1988, 4 1 , 600-605. Hewitt, E. J.; Nicholas, D. J. D. I n “Metabolic Inhibitors”; Hochster, R. M.. Quastel. J. H., Eds.; Academic Press: New York, 1963; Vol. 11, Chapter 29. Kuriyama, S.; Rechnltz, G. A. Anal. Chlm. Acta 1981, 731, 91-96. Alkawa, T.; Aikawa, Y.; Brady, T. G. Int. J. Blochem. 1980, 12, 493-495.
RECEIVEDfor review November 18,1981. Accepted January 11,1982. We are grateful to the National Science Foundation (Grant No. CHE-8025625) for supporting this research.
Differential Normal Pulse Voltammetry for the Anodic Oxidation of Iron(I I) Timothy R. Brumleve,’ R. A. Osteryoung, and Janet Osteryoung* Department of Chemlstty, State University of New York at Buffalo, Buffalo, New York 14214
Dlfferentlal normal pulse voltammetry has been used to determine klnetlc parameters for the oxidation of Iron( 11) In l M H2S04at a glassy carbon electrode. The data ftt the theory well. Analysk of dependence of peak potential on pulse width produces values of anodlc transfer coefficient of 0.35 and formal rate constant of 1.5 X cm s-’. The enhanced current response over that obtalned In differentlal pulse voltammetry Is shown and reverse dlfferentlal normal pulse techniques are demonstrated.
The technique of differential normal pulse (DNP) voltammetry with alternating sign of the second (differential) pulse provides a number of advantages over conventional techniques for voltammetric analysis and for determination of electrochemical kinetic parameters. The current-potential response is peak-shaped and symmetrical about the peak. Peak position, height, and width are related simply to kinetic parameters for totally irreversible reactions (1). Also, most of the time the electrode is held at a potential at which product is not formed. This may be contrasted with classical differential pulse voltammetry in which the potential is scanned through the entire range of interest, and therefore product is produced continuouslyas in DC voltammetry. This paper is concerned with the application of DNP voltammetry to the totally irPresent address: Anderson Physics Laboratories, 406 N.Busey Av., Urbana, IL 61801. 0003-2700/82/0354-0782$01.25/0
reversible oxidation of Fe(I1) in 1 M H2S04at a stationary glassy carbon electrode. The analytical utility of this technique is demonstrated, and comparisons with normal pulse (NP) and classical differential pulse (DP) voltammetry point out advantages in background discrimination and control of reaction conditions for solid electrodes. This technique is also applied to the quantitative evaluation of electron-transfer kinetic parameters for Fe(I1). The results are in good agreement with the recent theoretical predictions of Brumleve and Osteryoung (1)for the DNP waveform for totally irreversible systems. The use of a glassy carbon electrode is a particularly stringent test of the technique both for analytical determinations and for extracting kinetic information, since residual (background)currents are usually quite large at these electrodes (2). As a final note we consider a variation of the technique which we term reverse differential normal pulse (RDNP) voltammetry and which is analogous to the reverse pulse (RP) variation of NP voltammetry (3, 4). Application of this technique to the anodic oxidation of Fe(I1) reveals that RDNP in the alternate pulse mode embodies many of the features of cyclic voltammetry (such as peak shaped voltammograms) while retaining the advantages of pulse voltammetry (charging current discrimination and superior control of timing parameters and reaction conditions).
EXPERIMENTAL SECTION The computer-controlled pulse voltammetric instrument has been described previously (5). 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982 -?O@
fi
-41 I
I
2 - !GO
230
430
802 60C 1010
E ( r n V I v s SCE
Flgure 1. Forward ( i 3 ,reverse (i,ev),and alternating pulse (Ai) DNP voltammograms for oxidation of 1 mM Fe(I1) at a stationary glassy carbon electrode. Also shown is a conventional normal pulse voltUpper and lower dotted lines are background scans ammogram (iNP). for i,, and Ai, respectively t , = t , = 50 ms; t , = 1.1 s; A€ = 25 mV; t , = 16.7 ms, AV = IO mV.
The stationary glassy carbon disk working electrode was sealed in a glass tube and was polished to a mirror finish with 0.3 ,um alumina abrasive and a rotating lap. The geometric area of the electrode was 0.27 cm2. A coiled platinum wire was used as the auxiliary electrode. The reference electrode consisted of a conventional SCE/sailt bridge arrangement. The "thirsty glass" junction of the salt bridge was placed as close as possible to the working electrode 1( t,, however, the DNP responses begin to approach the classical DP response ( I , 5), with a corresponding decrease in sensitivity predicted for irreversible cases (1). Figure 5 illustrates an extreme example of this effect for the oxidation of Fe(I1). The top curve is the DNP response shown in Figure 1 (with tl = t , = 50 ms). The lower voltamlmogram was obtained by using a DP waveform similar to that available on commercial in-
Flgure 6. DNP (A) and RDNP (B and C) voltammetric responses for Fe(I1)with application of the waveforms shown. The current, A;, is the difference of the currents sampled at times indicated by filled circles (0).All conditions are as in Figure 1 except: (6) t , = 1.1 s, A€ =-25 mV, E , = 1.1 V; (C) as in B but t , = I s; t , = 2.1 s. strumentation. The waveforms and current sampling schemes are indicated adjacent to each curve. The classical DP waveform, unlike the present DNP waveform, does not provide a delay time at the initial potential between successive pulses. Rather, the base staircase (or linear ramp) carries the electrode into potential regions where product is formed continuously. This is a trivial distinction for polarographic experiments in which the electrode surface and the boundary conditions are renewed before each pulse. For an irreversible reaction at a solid electrode, however, there is severe depletion of the reactant at the electrode surface; this depletion is far greater than that which would occur for a reversible system (1, 5). The DP waveform at a solid electrode is in a sense analogous to a DNP waveform with tl increasing as El increases. In Figure 5,85 s have elapsed from the beginning of the DP experiment to the time at which the peak potential is reached. The situation is further complicated by the fact that reactant diffusion may be complicated by natural convection at these long times (13, 14). Similar effects have been described in previous studies of DP voltammetry at stationary electrodes (15-17). The general conclusion of these studies is that optimum sensitivity and reduction of depletion effects for irreversible systems is best accomplished by shortening the time between DP pulses to the point at which it is equal to the DP pulse width (this is not possible with most commercial instrumentation). The waveform then becomes that of square wave voltammetry (8, 16-1 9). Thus in comparison to DP voltammetry, the DNP waveform with short total pulse time provides superior control of reaction and diffusion conditions, while still providing an easily interpretable, peak-shaped voltammogram. For the choice of timing parameters illustrated in Figure 5, the Ai peak current is about 7.5 times that of the classical DP peak current for the oxidation of Fe(I1). Reverse Differential Normal Pulse Voltammetry. Finally, we consider a variant of the DNP waveform which we term reverse differential normal pulse (RDNP) voltammetry. The RDNP waveform is a logical extension of the DNP technique and is analogous to the reverse pulse (RP) variant of NP voltammetry (3, 4 ) . Figure 6 illustrates the general features of the RDNP waveform (B) in the alternating pulse mode and the resulting
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
voltammetric response for a solution containing Fe(I1). For reference the DNP voltammogram is shown as well (curve A). As in ordinary RP voltammetry, in waveform B pulsed potentials are applied starting from Ef, where Ef is the final potential of the DNP voltammogram. As seen in (B), the voltammogram may be continued profitably past the initial potential, Ei, of the DNP voltammogram (A). Taken together the resulting voltammograms (A and R) bear a strong resemblance to the cyclic voltammetricresponse of an irreversible system (8,20,21). The ratio of the cathodic peak current (curve B) to the anodic peak current (curve A) is 1.06, while the average of the anodic and cathodic peak potentials, 0.683 and 0.187 V, respectively, is 0.435 V, which is almost exactly the measured formal potential (0.431 V). The characteristics of curve B can be used to obtain kinetic parameters for the reduction of product using the theory above for DNP provided that the concentration profile of product is flat on the time scale tl t,. As shown above, for the DNP voltammogram (curve A) a flat concentration profile of reactant is achieved (to within 95%)for ti/(tl + t,) 2 11. The generation of a flat concentration profile of product in the RDNP voltammogram (Curve B) requires a larger time ratio, tf/(tl + tp), for in this case diffusion removes rather than supplies the species of interest (Fe(I1) in case A, Fe(II1) in case B). An estimate of the time required can be obtained from the results for double potential step chronoamperometry (22). We consider a first step from potential Ei to potential Ef (Figure 6), time duration t f at potential Ef, and a second step back to Ei. The currents measured at equal times after pulse application (e.g., i, a t tl + t, and i, at t f + t , t P )are equal in magnitude if time tf is sufficiently large to generate a flat concentration profile of product at the time scale tl + t,. The exact relationship is
+
+
-ic/ia = 1 - Z / ( t , + t,)/(tf + e,
+ tP)
(4)
for a diffusion-controlled reaction (22). Thus -ic/ia 2 0.95 if tf/(tl + tP) L 400. In the present case of voltammogram B we have not one double pulse but a sequence of double pulses. Thus the effective time for which product has been generated when the scan has reached potential El is not t f ,but rather approximately 2t& - EA/AV, where A V is the value by which the potential El is incremented on alternate double pulses. For the conditions of curve B, t f ) E l -Efl/AV(t, + tP)L 200 for IEl - EfI 2 180 mV. Thus a flat concentration profile of product is established well before the cathodic peak potential, EP,,, is reached. However, because the ratio tf/(tl + tP) is much less than 400, one might expect depletion to occur when the pulse application causes appreciable reduction of product (Le., for El 5 Ep,J The fact that -AiP,JAip,, 1for curves A and B suggests that depletion does not affect the currents significantly. The peak widths in curves A and B of Figure 6 are 243 and 253 mV, respectively. The value of a, can be calculated from the width of the cathodic peak or from the difference in peak potentials, E,, - EP,,.Either procedure yields a, = 0.35. Thus a, a, # 1, and the electrode process does not follow the simple Butler-Volmer relation. This kind of behavior is common in cases involving two consecutive one-electron transfers (23). In this case of single electron transfer it may be that a coupled chemical step complicates the reaction mechanism. It has been observed previously that the reduction of Fe(1I) at mercury electrodes involves a dehydration step (23).As noted above the dependence of the width of the anodic peak on pulse width is also a clear indication of some mechanistic complication. Further discussion of the mechanism is beyond the scope of this paper. Part C of Figure 6 illustrates a variation of the RDNP waveform in which reaction conditions during the electro-
-
+
generation step are more carefully controlled than in the waveform of B. This is accomplished (as in the DNP experiment) by spending a large proportion of the time at potential Ei where the material present in bulk solution does not undergo faradaic reaction. The potential i s then stepped to potential Ef for time tf, followed by a double pulse in the direction of potential Ei, as in B. This procedure simulates restoration of the boundary conditionsas in the polarographic experiment. Voltammagram C (dotted line) of Figure 6 is the response of the Fe(I1) system to this waveform. The response is similar to voltammogram B except for the decrease of the cathodic peak height at 0.186 V and the appearance of an additional small cathodic peak at about the same potential as the anodic peak (0.683 V). The ratio of the two cathodic peak currents is about 7 to 1. An analogous RP scan (i.e., currents sampled at the end of tl using waveform C) generates two waves with half-wave potentials at about the same points as the peak potentials of voltammogram C, with about the same 7 to 1 ratio of wave height. The voltammogram C appears to be roughly proportional to the derivative of the RP scan. Oldham and Parry (3)have reported similar effect.9 for RP polarography at the DME for irreversible electrode reactions, although their work did not continue the reverse scans into the limiting-current region for the reverse process. Even if the scan of curve C is assumed to be proportional to the derivative of the RP scan, the ratio of the two cathodic peak heights is not easily correlated with theory since the RP wave shapes will depend upon the kinetics of the reaction. The situation in Figure 6C is further complicated by the short delay time ( t i = 2.1 s), which violates the rule of thumb that ti 2 10(tf + tl + t,). Satisfying this criterion would lead to escessively long experiment times. Further theoretical work is clearly needed before triple pulse waveforms like those of Figure 6C find utility in the quantitative study of electrochemical kinetics. The superiority of the DNP waveform over conventional DP voltammetry at solid electrodes, whether applied for mechanistic or analytical purposes, is clear. Furthermore, DNP and RDNP techniques described here provide distinct advantages over cyclic voltammetry in control of reaction conditions and in discriminationagainst background currents. They have advantages over conventional N P and RP voltammetry in discrimination against background current and in the ease with which the peak-shaped response can be analyzed. In particular, the RDNP techniques appear worth developing as general approaches to the study of reaction products, especially at solid electrodes. ACKNOWLEDGMENT The authors thank John J. O’Dea for technical assistance and helpful discussions. LITERATURE CITED Brumleve, T. R.; Osteryoung, Janet Anal. Chem. 1981, 5 3 , 988-991. Galus, 2.; Adams, R. N, J . f h y s . Chem. 1963, 67, 866-871. Oldham, K. B.; Parry, E. P. Anal. Chem. 1970, 42, 229-233. Osteryoung, Janet: Klrowa-Elsner, E. Anal. Chem. 1980, 52,62-66. Brurnleve, T. R.; O’Dea, J. J.; Osteryoung, R. A.; Osteryoung, Janet Anal. Chem. 1981, 53, 702-706, (6) Myers, Davld J.; Osteryoung, R. A.; Osteryoung. Janet Anal. Cl7em. 1974, 4 6 , 2089-2092. (7) Oldham, K. B.; Parry, E. P. Anal. Chem. 1966, 3 6 , 867-872. (8) O’Dea, J. J.; Osteryoung. R. A.; Osteryoung, Janet Anal. Chem. 1961, 53,695-70 1. (9) Savitzky, A,; Golay, M. J. E. Anal. Chem. 1964, 36,1627-1639. ( I O ) Adams, A. N. “Electrochemistry at Solid Electrodes”, Marcel Dekker, New Vork, 1989: pp 143-182, 156-157, 220-222. (1 1) Tamamushi, R. “Kinetic Parameters of Electrode Reactions of Metallic Compounds”; Butterworths: London, 1975. (12) Wijnen, M. D.; Smit, W. M. R e d . Trav. Chlm. Pays-Bas 1960, 79, 289-312. (13) Laltinen, H. A,; Kolthoff, I . M. J . fhys. Chem. 1941, 45, 1061-1079. (14) Laitinen, H. A,; Kolthoff, I . M. J . Am. Chem. SOC. 1839, 61, 3344-3349. (15) Burrows, K. C.; Brlndle, M. P.; Hughes, M . C. Anal. Chem. 1977, 4 9 , 1459-1461. (1) (2) (3) (4) (5)
Anal. Chem. 1982, 54, 787-789 (16) (17) (18) (19) (20) (21) (22)
787
(23) Delahay, P. “Double Layer and Electrode Kinetics”; Interscience: New York, 1965; pp. 178-180, 216.
Rlfkln, S. C.; Evans, D. H. Anal. Chem. 1976, 48, 2174-2180. Keller, H. E.; Osteryouiig, R. A. Anal. Chem. 1971, 43, 342-348. Rarnaiey, L.; Krause. M. S., Jr. Anal. Chem. W69, 47, 1362-1365. Krause, M. S., Jr.; Rarnialey, L. Anal. Cbem. 1969, 4 1 , 1365-1369. Nlcholson, R. S. Anal. Cbem. 1965, 37, 1351-1355. Nicholson, R. S.; Shah, I. Anal. Chem. 1964, 36,706-734. Olleadl, E.; Kirowa-Eisner, E.; Penclner. J. Interfacial EiectroChemistry”; Addison-Wesley: Reading, MA, 1975; p 423.
RECEIVED for review August 11, 1981. Accepted December 17,1981. This work was supported in part by the Science Foundation under Grant No. CHE 7917543.
Voltammetric Ion-Selective Electrode for Chromium(V1) James A. Cox,” Paweb! J. Kulesza,
and Mweru A. Mbugwa
Department of Chemlstry and Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 6290 1
A sensor was deslgned In which a thln-layer electrolysis cell Is separated from aqueous samples by an anion exchange membrane. A steady state Is establlshed between Donnan dlalysls of Cr(V1) acrosis the membrane and controlled potentlal electrolysis of that Ion at a Pt electrode with a surface modlfled by Iodine adsoription. The steady-state electrolysls current Is proportlonal to the Cr(V1) concentration In the sample over the range 6 X lod to 2 X M CrZO7’-. The detection llmlt Is 2 X 10”’ M Cr,072-. The response Is Independent of sample pH over the range 3-8 when a 1 M HCI electrolyte Is used. Catllons, dissolved oxygen, and anlons that are not electroactlvie at the Pt/I Indicator electrode at 0.228 V vs. Ag/AgCI do not Interfere.
Donnan dialysis has been demonstrated to be a useful means of enrichment of ions from aqueous samples and of matrix normalization ((1-3) and citations therein). A simple modification of the Dorinan dialysis experiment yields a voltammetric ion-selective electrode. In the basic experiment an ion exchange membrane separates the sample solution from a receiver electrolyte (stripping solution). The test ions transported from the sample into the receiver during a prescribed dialysis time 81-18 subsequently quantified. That concentration is directly proportional to the initial concentration of the test ion in the sample under a proper experimental design. If the receiver solution is used as the electrolyte of a thinlayer three-electrode voltammetric cell and the test ions are continuously electrolyzedl during the dialysis, the assembly constitutes a voltammetric ion-selective electrode (4). The selectivity is a result of the exclusion of coions (ions with the same charge sign as the ion exchange sites) by the ion exchange membrane and the use of appropriate electrolysis conditions. The concentration of the test ion is determined by relating the steady-state electrolysis current to sample concentration with a calibration curve. The sensors are thus similar to the voltammetric sensors for dissolved gases such as the Clark oxygen electrode (5). The difference is that an ion exchange membrane rather than a neutral membrane is used. Donnan dialysis rather than diffusion is therefore the transport mechanism. This allows extension of the sensors to the determination of ionic species in general. Our initial report was on the design of a voltammetric ion-selective electrode for nitrate (4). That sensor did not yield a steady-state current. The problem was later traced to loss of activity of the ZrOClz catalyst for nitrate reduction a t a mercury electrode. The present study utilizes the reduction of Cr(VI) at a platinum electrode onto which iodine is adsorbed. That surface was shown to be uneful for the electrolysis of Cr(V1) 0003-2700/62/0354-0787$01.25/0
in a study on coulometric flow-through electrodes (6). It has been suggested that a chemisorbed layer of monoatomic iodine is formed on the Pt surface that prevents H and 0 electrodeposition over the normal potential range of Pt (7,8).Adsorbed iodide is not electroactive in the range 1.1-0.1 V vs. SCE (8). Chemisorption of other species, such as Cr(II1) or Cr(V1) in our study, would be decreased or prevented which would lead to reproducible electrolysis currents. At the modified surface, background currents are significantly lowered and remain essentially constant (9). EXPERIMENTAL SECTION The voltammetric ion-selective electrode design is shown in Figure 1. The electrolysis chamber contains Pt indicator, Pt counter, and Ag/AgCl reference electrodes. The Pt and Ag surfaces were machined flush with the epoxy (TorrSeal, Varian Associates) plug; that surface was indented relative to the cylindrical glass body of the assembly to accommodate the electrolyte. The volume of the resulting electrolysis cell is about 0.1 mL. A P-1025 anion exchange membrane (RAI Research Corp., Hauppauge, NY)was placed over the open end of the Plexiglass cylinder and held in place by Teflon tape and a small plastic hose clamp. Prior to mounting the membrane, the Pt indicator electrode was cleaned and coated with iodine and the Ag/AgCl electrode was generated. The Pt indicator electrode was cleaned by first polishing with Fisher GamalO.1 pm alumina on a Gamal cloth with distilled water as the lubricant. The assembly was placed in 1 M H2S04,and the indicator electrode was cycled between 1.4 V and -0.2 V vs. SCE about 50-100 times (8). The surface was reduced at 0.4 V vs. SCE until the current decayed to a constant, low value (IO). That decay took 2-3 min. The cleaning procedure is by far the most important step in obtaining good results with the reported electrode. The assembly was transferred into a deaerated 2 mM NaI solution that was freshly prepared. After a few minutes, the modified Pt/I electrode i s ready for use. As long as the potential range of about 0.1-1.0 V vs. Ag/AgCl is not exceeded,the electrode is stable indefinitely. The system is stored in 0.1-1.0 M HC1when not in use. Only after modification of the indicator electrode is the Ag/AgCl reference generated. Assembly of the voltammetric ion-selective electrode was completed by filling the thin-layer electrolyte reservoir wiith 1 M HCl and attaching the anion exchange membrane. Between experimenb it is not necessary to remove the membrane. Because Donnan exclusion breaks down when high ionic strength solutions are in contact with both sides of the membrane, dipping the sensor into 1M HC1 for a few minutes regenerates the electrolyte and removes the electrolysis products. The indicator is held at 0.8 V vs. Ag/AgCl during this time. A PAR Model 174A polarograph (Princeton Applied Research, Princeton, NJ) was used for the measurements. In fact, a simple three-electrode polarograph or potentiostat could be used as the analytical measurement is made on the time scale of minutes under controlled potential conditions. The experiments that were performed to characterize the Donnan dialysis of Cr(V1) species used previously described cells 0 1982 American Chemical Society
