Focus
New Electroanalytical Pulse Techniques The everyday application of electrochemistry in analytical laboratories has increased tremendously in the past 15 years. This increase is largely due to the availability of instrumentation that has made it possible to perform voltammetry, particularly the powerful pulse voltammetric techniques, conveniently and inexpensively. The operational amplifier-based instruments introduced in the late 1960s by EG&G Princeton Applied Research Corporation (PARC) represented a big improvement over earlier instruments with pulse polarographic capabilities. The first commercial pulse polarograph, for instance, was a vacuum tube-based instrument marketed by Southern Analytical Ltd. in the early 1960s. According to Robert Osteryoung of the State University of New York at Buffalo, "It looked like a large NMR and cost around $25,000." EG&G PARC's Model 174, on the other hand, was originally priced at around $2000, complete with drop knocker, when introduced in the early 1970s. It was and still is a best-seller. Historically, electroanalytical chemistry had most often involved what is now referred to as DC polarography (DCP). With the availability of instruments capable of performing normal pulse polarography (NPP) and differential pulse polarography (DPP), the pulse techniques took over for most practical laboratory analyses and even for many research applications. In the past few years, still other pulse techniques have been under development. Some of these new pulse techniques are even more powerful than N P P and DPP for certain applications, though most of them are not yet accessible on commercial instruments. At the recent Pittsburgh Conference in Atlantic City, Robert A. Osteryoung discussed the relationships between some of these new pulse techniques and some of the electroanalytical techniques in common use today. Osteryoung's presentation, "Pulse Voltammetry— Today and Tomorrow," coauthored by Janet G. Osteryoung and John J. O'Dea, was part of a symposium on Electroanalytical Methods and Mate-
rials on the Horizon, arranged by J. T. Maloy and J. F. Jackovitz. Symposium participants can be seen in photo, this page. This article is based largely on material presented in Osteryoung's talk. Classical Techniques DC Polarography. The contribution of DCP to instrumental analysis is held in such high esteem that J. Heyrovsky won the Nobel Prize in chemistry in the late 1950s for his development of the technique. The DC voltage waveform and a typical current response are shown in Figure la. The DC voltage input is a linearly increasing ramp. For a reduction as depicted, the initial potential is selected so the reduction of interest does not yet take place. The potential is then scanned cathodically through the reduction wave. The term "polarography" refers specifically to electrochemistry at a dropping mercury electrode (DME). Electrochemical techniques performed at other electrodes, such as the solid electrodes that have become so popular of late, are referred to as "voltam-
metry." DC polarography is always performed at the DME, and the fluctuating pattern of the DCP current response in Figure la is a direct reflection of current variation during the lifetime of each Hg drop. Tast Polarography. In Tast (current-sampled) polarography (Figure lb), the fluctuating pattern of the DCP current wave is eliminated by sampling the current near the end of the life of each drop. The sensitivity of Tast polarography is of course equal to that of DCP. Cyclic Voltammetry. Cyclic voltammetry (CV) is commonly performed at solid and stationary electrodes. The one-cycle CV potential waveform shown in Figure lc perturbs the electrochemical system under study so as to create a reduction wave on the forward scan. Some oxidative current is observed on the reverse scan as the reduction product that accumulated around the stationary electrode in the first half of the cycle is oxidized. CV is important for the study of redox rates and mechanism^and is accessible to one extent or another on commercially available instruments.
Participants in a symposium on Electroanalytical Methods and Materials on the Horizon at the 1982 Pittsburgh Conference included (left to right) R. M. Wightman, J. T. Maloy, L. R. Faulkner, R. A. Osteryoung, W. R. Heineman, and F. C. Anson
698 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
0003-2700/82/0351 -698 A$01.00/0 © 1982 American Chemical Society
••••M
Focus
Typical Current-Potential Response
Potential Waveform Normal and Differential Pulse With both NP and DP, one voltage pulse is applied for each drop of Hg when a DME is used, although both techniques can also be used at solid electrodes or at a static Hg drop elec trode (SMDE) such as the EG&G PARC Model 303. Normal Pulse. In N P P , explained Osteryoung at the conference, "we sit at a constant potential and apply pulses of successively increasing am plitude to successive drops from the D M E " (Figure Id). The current is sampled in the last few milliseconds of the pulse. The current-potential re sponse is similar in shape to that in Tast polarography. In fact, Geoffrey Barker, who originally developed many of the pulse techniques, decided to name the technique "normal" pulse precisely because the shape of the waveform was the normal shape one would get with the DC techniques. N P P is a more sensitive technique than DCP, however (see Table I). It is available on all commercial polarographic analyzers.
Table 1. Typical Sensitivity and Detection Limit for DCP, NPP, and DPP Sensitivity (μΑ/πιΜ)
E(init) Time-
Time-
Detection limit (M)
DCP
5
NPP
30
ίο-6
DPP
20
10~7
10
Θ
y
-5
Differential Pulse. In DPP, pulses of equal amplitude are superimposed on a linear potential ramp (Figure le). The current is sampled just before the pulse is applied and just before the end of the pulse. These currents are "differenced," and when the differ ence current is plotted vs. potential a peak-shaped current response curve is generated. As seen in Table I, the sensitivity of differential pulse lies between that of DCP and that of N P P , but the detec tion limit of DPP is better than the detection limit attainable with the more sensitive N P P . This is because DPP discriminates more effectively against charging currents. In voltammetry the current that flows across the electrode-solution in terface arises from two sources. What we wish to observe in electroanalytical chemistry is the faradaic current, the current arising from the reduction or oxidation of species in solution. How ever, a capacitance current also flows
I
0
A
.
θ Time-
Θ
θ
TimeFigure 1. Potential waveforms and typical current-potential responses for selected electroanalytical techniques The potential waveform axes are potential (£) with respect to a reference electrode vs. time. The cur rent response axes are current (/) vs. potential (£) with respect to a reference electrode. The positive and negative signs on the axes are not meant to represent absolute values, but indicate the sign con vention for current and the scan direction for potential. Reduction current is considered to be positive current. The potential is scanned from more positive to more negative potentials when a reduction is taking place, except in the case of RPP. Reductions are depicted in all the scans shown here, except
;•: >
Focus Typical Current-Potential Response
Potential Waveform
(e)
φ
θ
Ε
1
E(init)
ο'
mmmÊai^0^^
^
β
β
β
β
θ
Time-
(f) E(init)
Θ Time-
150
Δι Ι(ηΑ) 0 ^ ^
\
'rev\
'for ^r
-100 Time-
-300
Ε (mV) vsSCE
-600
Timethat some oxidation current is also observed for some of the techniques, (a) DCP. (b) Tast polarography. (c) CV; arrows on current response curve indicate direction of scan, (d) NPP; current is sampled at the end of each pulse, as indicated by blue dots, (e) DPP; current is sampled just before each pulse and near the end of each pulse, as indicated by blue dots, (f) RPP; current sampled as in NPP, at end of each pulse, (g) DNP; inset shows parameters of potential waveform. Current is sampled at the end of each pulse, as indicated by blue dots; current response shows forward, reverse, and difference DNP currents for the experimental reduction of 9.9 μΜ Pb + 2 in 0.5 M CH3COOH/0.5 M CH3COONa. (h) SWV; current is sampled at points 1 and 2. Forward, reverse, and difference currents are shown. For further informa tion, see text
across the interface. Since the working electrode is held at a potential differ ent from that of the solution during an experiment, the electrode-solution in terface acts like a capacitor. When a new Hg drop grows, effectively chang ing the surface area of the interface, or when the potential on the electrode is changed, a current must flow to charge or discharge the capacitor. This cur rent is nonfaradaic, since it flows in the absence of an accompanying redox process. The charging currents detect ed in DCP and NPP are of approxi mately equal magnitude, but are one order of magnitude less for DPP, lead ing to lower detection limits. DPP is available on all modern commercial instruments. New Pulse Techniques Reverse Pulse. Though a couple of papers on reverse pulse polarography (RPP) had been published earlier, Janet Osteryoung and Emilia KirowaEisner revived the technique in a re cent paper in ANALYTICAL CHEMIS TRY (1980,52, 62-66). In a recent in terview, Janet Osteryoung explained the technique with an example: "Con sider a situation where we had Fe(III) in solution. With NPP, we sit at a po tential where Fe(III) is not reduced and we pulse to increasingly more neg ative potentials. In RPP, we would pick a very negative potential where Fe(III) is reduced at a diffusion-con trolled rate (Figure If). Then we would pulse to successively more posi tive potentials and look at the electro chemistry corresponding to the oxida tion of Fe(II) that had been generated at the electrode at the initial poten tial, between pulses." Note in Figure If that the current response is similar to that in NPP, ex cept that it is shifted toward the oxi dation current region on the current axis (toward more negative currents). Also note that due to the reverse na ture of the RPP potential waveform, the current is displayed in the oppo site sense from the usual convention, with the scan proceeding from more negative potentials at the left to more positive potentials at the right. The current in RPP is sampled at the end of the pulse, as in NPP. RPP is of great value for determining mecha nisms of electrochemical reactions, and may be of value for some special ized analytical applications, such as the determination of compounds (halogenated organics, for example) diffi cult to determine with other electro chemical techniques. Differential Normal Pulse. Dif ferential normal pulse (DNP) is a hy-
Focus Irreversible
SW, 100 Hz
80
/
\
ϋ 40
DP, 50 ms
1
100
50
-50 Potential (mV)
1
-100
-35l!5atefc—_
-150
-200
Figure 2. Theoretical SWV and DPP scans for the determination of an irreversible analyte Conditions as in Table II
brid of the differential pulse and nor mal pulse waveforms. Figure lg shows DNP in the alternating pulse mode. A conventional DNP scan would consist of only the odd or only the even "dou ble pulses." In DNP (alternating pulse mode) the experiment begins at an initial po tential where no faradaic current flows. The first double pulse consists of a potential pulse to potential E\, followed by a small differential pulse, AE, to potential E2. The potential is reset to the initial potential at t2. The
next double pulse consists of a step to E\, followed by a —AE pulse to E'2. Each consecutive pair of double pulses is similar, except that Ε ι is increased linearly as the scan progresses. On a DME or SMDE, a new drop is formed at t2 so each double pulse is applied to a fresh Hg surface. The current response from DNP (alternating pulse mode) may be plot ted as the forward current (from cur rent sampling at level E2 on alternate pulses), the reverse current (from cur rent sampling at level E2), or the dif
ference current (called Ai). The differ ence current is peak-shaped, as in DPP, and, as will be seen in the next section, the DNP (alternate pulse mode) current response is very similar to the forward, reverse, and difference currents obtained with square wave voltammetry. In addition, the forward and reverse currents, when plotted to gether, resemble a CV current re sponse (Figure lc). D N P retains the usual advantages of pulse techniques: high current sen sitivity (due primarily to short pulse widths) and excellent discrimination against charging currents. "We think DNP detection limits will be better than for D P P , at solid electrodes and for irreversible electrochemical sys tems," explains Robert Osteryoung. Osteryoung's group is currently per forming research designed to investi gate the predicted advantages of DNP quantitatively. Experimental evidence is already available indicating that DNP (alternating pulse mode) can be an order of magnitude more sensitive than DPP for irreversible systems and approximately of equal sensitivity for reversible systems. Despite its advan tages, however, DNP is not at present accessible on commercial instruments, For more information on DNP, see Anal. Chem. 1981,53,702-6. S q u a r e Wave. "From the analytical point of view," says Robert Oster young, "square wave voltammetry is the technique most likely to dominate pulse voltammetry in the future." The square wave voltammetric (SWV) waveform and current response are shown in Figure lh. At a DME, a drop of Hg is permitted to grow for time £
