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Instrumentation
Pelxin He James P. Avery Larry R. Faulkner Department 01 Chemistry University of lliinois 1209 West California St. Urbana, 111. 61801
Cybernetic Control ofan Electrochemical Repertoire There has been such diverse growth in the means of approaching problems in analytical chemistry that we now find ourselves in need of new methods for controlling experimental power. The older modes of control that have brought about the growth itself are excellent for ensuring quality within a single format of experimentation, such as differential pulse polarography,
atomic absorption, or reversed-phase liquid chromatography, because they concern details of measurement. For three decades we have witnessed a campaign for precision and speed, in which the goals have been to identify factors affecting these properties of measurement in all sorts of experiments, and to control them hy improved design of hardware, by servo
systems, by analog feedback, or by digital supervision. The era now drawing to a close has been regarded as the “electronic age” of chemical instrumentation, but electronics has really only served as a primary agency of control in an age of measurement. The root of new power in analytical chemistry is variety itself, that is, in having the ability to pursue several
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ANALYTICAL CHEMISTRY. VOL. 54. NO. 12. OCTOBER 1982
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lines of attack on a chemical problem. Advanced concepts of control will be needed as instruments capable of coordinated investigations come to pass. Their advent will very likely alter the practice of analytical chemistry as substantially and as quickly as the campaign for precision and speed did. The era now heginning will be the age of coordination and interpretation, and ita chief agent willbe the computer. In this article we deal with these ideas, but within the specific context of electrochemical instrumentation. The starting point is the premise that most chemical systems of electrochemical interest ought to be examined by several techniques. In analytical applications, particularly in the development of methods, one gains greater command of a sample by examining it by various types of pulse voltammetry, pssihly by ac voltammetry in any of several forms, and perhaps by one or more stripping procedures. The goal, of course, is to find the combination of conditions that leads to optimum sensitivity, precision, and immunity from interference. Optimizing conditions and recognizing interference demand the ability to change conditions, both within the cell and without. Accordingly, flexibility in the format of electrochemical excitation and observation is required. Likewise, in diagnostic applications, different techniques tend to highlight different aspects of a mechanism. For an overview of chemistry, cyclic voltammetry is powerful, but normal and reverse pulse voltammetry, rotatingelectrode methods, or chronmulometry may provide simpler means for obtaining quantitative evaluations of parameters; and bulk coulometry is invaluable for determining n values. Thus we arrive at the concept of repertoire. Power in an electrochemical laboratory arises from an ability to bring a variety of techniques to bear on a problem. With current commercial instrumentation, such power is out of reach in all but the most elaborately equipped laboratories because individual experimental stations must be dedicated to each family of related methods. Alternatively, one can use very complex instruments that can be interconnected in various ways to carry out different types of experiments. The impediments to implementing a repertoire in hardware WE almost prohibitive. In usual practice, therefore, studies are made by the one or two methods that are readily available in the laboratory at the time of investigation. Ideally, one would have a full range of options on a single instrument. There should be a simple, common style for selecting and setting up the experiments, regardleas of experimen1514A
tal mode; and the time needed to switch between modes ought to be negligible. The only realistic means for approaching this ideal is to place the entire repertoire under the charge of a computer. It must have full control of the potentiostat and cell by an automated switching network; it must offer a full range of excitation waveforms and schemes for acquisition of data; and it must poasess a faculty for decision and communication, so that it is able to interact simply and in a “humane” way with the person supervising the investigation. The cybernetic solution to this problem in instrumentation succeeds, as we show b,elow, precisely because it allows simplicity to coexist with variety. Outstandingly simple control is maintained over a great diversity of experimental methods and time scales. We expect to find that this coexistence will change the way in which electrochemical investigations are conceived and carried out.
A Working Example We have constructed a cybernetic potentiostat on the basis of these ideas. The design is shown schematically in Figure 1,and a photograph of the entire apparatus is available at the end of this article. The heart of the device is a commercial Intel SBC-80/ 10B single-board computer, which supervises all operations. It operates in a standard Intel Multibns chassis, by which it interacts with additional memory and with three additional, custom-designed boards that carry the potentiostat, hardware for synthesizing waveforms for the potentiostat, circuitry for acquisition of electrochemical responses, apparatus for switching cell and scale connections, a controller for video display, and circuitry for automatic compensation of cell resistance. Details will be provided elsewhere ( I ) . The operator receives information through an integral 5-in. video screen capable of presenting text and 192 X 256 point resolution graphics. Commands are entered through a standard keyboard. Permanent copies of grapbical data are available at high resolution from a digital plotter or at low resolution from a dot-matrix printer. The figures shown in the remainder of this article are photograph of results from the high-resolution plotter. Control of all functions is exercised through software resident in 36 Kbytes of read-only memory (ROM). The remaining 28 Kbytes of memory exist as random-access storage for data and other temporary information. Of course, our major contribution is the development of the software in ROM, and the bulk of the remaining
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
Xepertolre of the lnstrum mu.n w t ~ Dilferentialpulse polarography and voltammetry
Normal p u b powcgraphy and voltammeby
Reverse pulse powography and voltammetry Square wave polarography and voltammetry Tast polarography SlrlPPim N(IIM.
DifferenHalpulse strlpplng Square wave strlpping Linear sweep strlpplng SW-4
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Linear sweep voltammetry Cyclic voltammeby C I-
polarography and voltammetry ’hase9electlve a0 polarography and voltammetry IC
Second-hamnlc ac polarography and voltammetry Y*E*I.I*(U
Chronocoulometry Bulk electrolysis with Coulometry
ilectrocapillary m e s \utomatlc mbasurement and wmpensatbn of reslstanca
discussion concerns its design and operation. Our work is built upon earlier developments by Anson, Bond, Lauer, the Osteryoungs, Perone, Smith, and their co-workers, who laid the foundations of computer-controlled electrochemical instrumentation (2-12). More recently there has been interest in adapting inexpensive microcomputers, including personal computers, to electrochemical purposes ( I >I 7). Our work is, of course, an extension of these earlier efforts by others; however, we have brought something new to the field by full integration of all control circuitry into the apparatus and by our stress on richness in repertoire. Finer details of our software will be covered in a forthcoming pnblication (1).T h e are actually two versions: one for the 8080A microprocessor and the Intel SBC-series boards, the other for a set of boards, based on the 2-80microprocessor, which we developed to operate in the Intel chassis. The instrument contains control circuitry and software that make it fully compatible with the static mercury drop electrode (SMDE) produced
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by EG&G Princeton Applied Researc Corporation. In addition, there are a real-time clock and a set of interfaces to external devices such as printers and control terminals that are of occasional use, but are not of concern here. The potentiostat has a standard adder design and is programmed by the output of a 16-bit DAC. Step and pulse measurements are possible on time scales from a few milliseconds to several seconds; cyclic and linear sweep voltammetry can be done at rates up to 50 Vfs; and ac voltammetry is possible at 10-250 Hz. The full repertoire is shown in Table I. One has access to almost the complete range of electroanalytical methods that can be carried out with a single potentiostat. Many of the methods are effective either in a “polarographic” mode, with a conventional DME or with the SMDE ordered to produce new drops periodically, or in a “voltammetric” mode, for use with conventional stationary electrodes or with the SMDE ordered to hold a single drop for the entire experiment. An important feature of the instrument is a provision for automatic compensation of cell resistance. Through software, the system first makes an actual measurement of the uncompensated resistance in the cell; then it decides the maximum degree of compensation that can be prudently attained by positive feedhack. The decisions are made on the basis of the responses of the actual cell to small pulses in potential. The apparatus has the ability to switch stabilizing elements into the feedback loop, if it regards them as necessary for stability at high degrees of compensation. The potentiostat is never allowed to break into oscillation; nonetheless, full compensation is usually possible. There is also a separate repertoire of display and plotting operations. Results can be examined smoothed or unsmoothed, as averages of multiple runs, as differences between sample and background voltammograms, as first or second derivatives, as magnified views of data in specified potential ranges, or as plots of individual sweep segments in multicycle voltammetry. Any data point can be read out numerically, and one can obtain partial interpretation of results, as slopes and intercepts of Anson plots (18) of cbronocoulometric data, or as peak currents and potentials or wave heights and half-wave potentials for all types of voltammetry. All interpreted results of this type are achieved by handling baselines exactly as an electrochemist would handle them, even for reversal currents in cyclic voltammetry. Many of these modes of display and interpretation are illustrated in this article.
D Figure 2. Electrocapillarycurves recorded at the SMDE Trlangle~are for a solution Or 5 X lo-‘ M AaDs in 0.1 M m.Circles are for 0.1 M mU0, wilts$ A O S
All experiments are set up and run in a common format. Once the operation mode is selected, a series of queries is made about the details of the experiment. Only essential information is requested. After the setup is complete, a separate command orders the experiment to be run; then the results are displayed. With appropriate commands, they can then be analyzed, replotted, or the experiment can be repeated. Some of the parameters can be changed, or a new mode of operation can he invoked. Some Results An example of the versatility of the system is shown in Figures 2 and 3, which contain a sequence of plots from a set of diagnostic experiments on a deaerated solution of 2.6-anthraquinone disulfonic acid (AQDS) at 5 X M in 0.1 M “03. The working electrode was the SMDE, modified slightly to permit automatic recording of electrocapillary curves like those shown in Figure 2. The pair of curves available there shows immediately that the compound of interest, or the products of its reaction at the electrode, are adsorbed at potentials more positive than -0.7 V vs. Ag/AgCI, whereas at more negative potentials these species are not adsorbed. The slow-sweep cyclic voltammogram in Figure 3a shows that AQDS is reduced between +0.1 and -0.3 V in a pattern suggesting that the process involves chemical reversibility, but has complex dynamics. The pair of spikes near 0.0 V strongly indicates that adsorbed AQDS is reduced there to an adsorbed product, and the broader peak near -0.2 V indicates the reduc-
1318 A * ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
tion of diffusing AQDS in solution. Taking Figures 2 and 3a together, it is clear that oxidized AQDS is surface bound at potentials more positive than 0.0 V, and that the reduced form is surface bound between 0.0 and -0.7 V. Beyond -0.7 V the reduced form desorbs and exists near the electrode only in solution. Figure 3b is a fastsweep cyclic voltammogram that confirms the assignments of the features in Figure 3a by the much stronger sweep-rate dependence of the peaks at 0.0 V by comparison to the peak at -0.2 to -0.3 V. The bump at -0.7 V in the negative sweep is probably caused by a change in capacitance accompanying the desorption indicated in Figure 2. An important strength of this instrumental approach is the ease with which data can be taken over a wide range of time scales, without the artificial limits often imposed by recording equipment. Figure 3c is the chronocoulometric response to steps from +0.2 to -1.0 V and back to +0.2 V. This technique is especially powerful for quantifying the amounts of species bound to surfaces. However, it is inconvenient to use without a computer, because data are examined by plots on square-root-oftime axes (Anson plots) like those shown in Figure 3d for the data in Figure 3c (18).The difference in intercepts of plots for the first and second parts of the experiments (upper and lower lines in Figure 3d)provides, essentially, the difference in amounts of oxidized and reduced forms that are surface bound at the initial and firststep potentials, respectively. From the electrocapillary curves in Figure 2, we have already learned that the reduced
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jay for more information on these and other Shimadzu instruments. SHIMADZU SCIENTIFIC INSTRUMENTS, INC. 9147 Red Bnnch Road, Columbia. Md. 21C45. U.SA Phone. lYI1l997-1227 Telex:087959
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form is not adsorbed beyond -0.7 V, hence the choice of -1.0 V for the step potential in chronocoulometry allows us to attribute the difference in intercepts in Figure 3d wholly to the amount of surface-bound AQDS at the initial potential In Figure 3e, one sees an ac polarogram of the AQDS system. Prominent 1318A
features are the peak for the surfacebound couple near 0.0 V and a broad maximum between -0.6 and -0.9 V, probably associated with capacitive changes induced by desorption of the reduced species. Figure 3f contains a closer view of the desorption peak in the same set of data. This sequence illustrates the rapid
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
diagimsis of behavior that one can carry out when it is possible to examine different aspects of a system by optimal methods. The real-time clock readings on the plots in Figures 2 and 3 show that all of the measurements were made and plotted in about a 20min span. The plotting of high-precision graphs required 14 min of this pe-
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