Voltammetry as a Model for Teaching Chemical lnstrumentation H. Gunasingham and K. P. Ang National University of Singapore, Kent Ridge, Singapore 051 1 Miniaturization and large-scale integration of circuit elements has led to a dramatic enhancement of the performance-to-cost ratio of electronic devices. Perhaps the hest example of this is the microprocessor. Numerous applications can he cited where, previously, the use of computer techniques would have been ruled out on grounds of cost. The advent of the microprocessor has, however, virtually removed this obstacle. Chemical instrumentation has, inevitahly, henefited immensely from the new microelertronirs technology. There are two imoortant devsloumrnts that have resulted. First. is the trend toward the modular design of instruments. The situation is aooroachine the noint where the desiener no loneer has to he coLerned wkh thk detailed design of ilectronic Zrcuits. Instead, he only has to specify the functional modules required for a particular instrument, and i t will then be only a matter of assembling the appropriate integrated circuit (IC) packages. For example, a device for monitoring a photomultiplier current can he constructed bv connecting- IC modules that perform the functions of current-voltage conversion, amplification, and, perhaps, analog-to-digital conversion. Second is the trend toward integration of the microprocessor as a c o m ~ o n e npart t of the instrument. The microprocessor can be progr&nmed to emulate functions which previously required dedicated analog or digital circuits. Apart from the saving on hardware cost this has the advantage that functions can he changed by simply altering the control program. Another consequence of microprocessor integration is that instruments are becoming more intelligent, with built-in diagnostic and error correction facilities, in addition to automated data processing capabilities. However, the growth of new and increasingly complex instrumentation technology brings about its own problem; namelv. ,. the user finds himself more and more remote from the actual experimvnt, separated hy several layers of hardware and software. Fieure I shows the "onion skin" structure of computer-controlled instruments. In order for the user to interact effectively with the experiment he first has to become familiar with the function of each layer and also the interface between lavers. Clearly, users of modern chemical instruments-namely, chemists-reouire s~ecializedtrainine. Moreover. as the use ofsophisticatt.d equipment requires somr skill in their handling (as well as understandinc ot'ths undrrlvinr . - theorv of operition) the training should incorporate "hands-on"- in-
struction. This dual objective of skills development and theoretical treatment Doses a challenge to teaching chemical instrumenratiun to "ndergraduatehemistry sr;,drnts, rsprcially with the limited time a\,ailable for such a course umithin a typical chemistry curriculum. The authors have found that the success of any course on chemical instrumentation depends, t o a large extent, on the use of teaching models; that is, examples of instrumental techniques to illustrate theoretical principles discussed in lectures, as well as to afford "hands-on" instruction. The following are the criteria, selected by the authors, for a successful teaching model: 1) It must afford an intemated view of instrumentationon the basis ofintbrmntim sxrmcrmn and hnndling.'I'hu*n d~rertnnnlng).
and compnrwn can be madc with other in~rrummrnltrrhniques. 2) It should give historical perspective, in particular, how the development of electronics has affected chemical instrumentation. 3) The use of both analog and digital electronics should be demonstrable. 4) It should he amenable to on-line computer control. 5) I t should show the lntrgratlon of t h e mirropnressor as n componellt part of the instrument. 6 ) The basic instrumrntnl technique shmld he simple and inprpensive encmah for use in t h~ undergraduate latwratory. HI^ever, it sh~uldhe capable u1 :IIultrat:ng m8.w complex t e ~ h niques. The authors have used the technioue of voltammetrv as a model for teaching chemical instrumentation to chemistry undereraduates a t the National Universitv of Sinea~ore. - . The ensuing sections discuss how the technique satisfies the above criteria.
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~
-
user
Figwe 1. The onion-skin structure of computerized insmments.
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Journal of Chemical Education
The Technlaue of Voltammetrv Voltammetry is concerned with the current-potential relationships that arise when a potential waveform is applied to an electrochemical cell. The essential requirement of instrumentation for voltammetry is to control the potential and to monitor the current. At its simplest level, voltammetric measurements are plotted as dc current-potential profiles. These profiles provide information about the processes taking lace a t the working electrode. Refinements in voltammetry have used more com~lexpotential waveforms and more suhtfe current measurements. The importance of voltammetry stems from the growing relevance of electrochemistry to science and industry. Rethat cently, there was a series of articles in THIS JOURNAL highlighted the importance and scope of the subject ( I ) . The Modular View of Instrumentation I t is increasingly heing recognized that instrumental chemical analysis is best described in modular terms on the hasis of chemical transduction followed by signal processing. Analysis is thus an information-processing exercise with the ohiective of extractine useful information about the chemical system heing monitoied (2).The provision of an externally derived ~erturhationwaveform serves to increase the information Eontent of the transduced signal. Thus, instrumen-
, WEOUENCI
PHOTOCURRENT CURRENT VOLTAGE
L W ~5 FILTER
CCIIYE-TCII
ANIVCG-
DIGIT& CC*IMRSCII
nrmt
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Figure 2. Modular description of insbumentation for a) vaitammetv and b) spemOSCopy.
tation can be seen as consisting of functional modules strung together, each a part of an infofmation processing chain; ~ n k e takes this view in his data-domains approach (3). In the case of voltammetry, the modular elements of its instrumentation can be easily identified. The perturbation waveform is usuallv of the form of a time vawina " -.notential ramp; transduction of chemical information into electrical information is a consequence of the electrode reaction itself. The resultant faradaic current can be converted into more useful forms tbroueh a series of transformations. each accomplished by adr$cawd IC module. Usually, thelirst stage is current-voltaee conversion. folluwed bs amnlification. The voltage signal g a y then be fukher transformed, for example, into loe form or into dieital form. Finallv. the sianal mav be outpuito a chart recorded or digital plotter. Figure 2a shows the modular view of voltammetry instrumentation. In comparison, Figure 2b shows the modular view of instrumentation for spectroscopy. As can be seen, the electronic instrumentation is essentially similar. Historical The first use of electroanalytical techniques must have been prior to 1864 as eledrogravimetry was, apparently, well known then. However, it was not until after the invention of the polarograph, by Herovsky and Shikata ( 4 ) ,that voltammetry
became established as a technique. (The term voltammetry, as used in this naner . . includes nolaroeranhv.) .. . . The development of instrumentation for voltammetry since Herovskv's invention. ran bt. trarrd alone two lines: first. the method i f generation A d control of the &urbating potential waveform; second, the method of current measurement. In both areas major developments arose because of specific advances in electronics technoloev. The table outlines the e v o k o n of instrumentation for voltammetry in regard to developments in potential control and current measurement. The concurrent advances in the field of electronics are also given. The table begins with Herovsky's polarograph, in which the applied potential is controlled bv a motor-driven potentiometer and its current measured by a galvanometer. The electromechanical nature of the polarograph reflected the state of technology at the time of its development. Herovsky's polarograph was greatly improved by the use of automatic control with the invention of the potentiostat by Hickling in 1942 (5).The potentiostat was based on a servomechanism. With the advent of the operational amplifier the operating characteristics (in particular, speed and accuracy) of the potentiostat was greatly improved. Initially, vacuum tube operational amplifiers (op amps) were used to construct the potentiostat; and this was followed by solid state devices. With the advancement of electronic circuit integration, op amps have continuously improved in performance and this has correspondingly improved the performance of the potentiostat in regard to its speed and accuracy. Apart from the control of potential, op amps are used for the purpose of current measurement bv means of current-to-voltaae conversion. The next development in voltammet~instrumentationwas the use of hvbrid. analoa-dinital circuits. These circuits employed digital osri~~atorLand logic circuitry ior the control of tield effect transistor IFETI switches that, in turn, control the op amp circuits. ~ ~ b rcircuits i d affordedmore subtle control of potential generation and current sampling, enabling techniques such as differential pulse voltammetry. With the advent of the laboratory minicomputer, computerized voltammetry became widely used. The computer replaced analog and digital circuits for the purpose of generating the applied voltage waveform and for the monitoring of the current. Moreover, these functions could be handled by software control. Figure 3 is a schematic diagram of an on-line computer-controlled potentiostat. The figure clearly illustrates the onion-skin structure of computerized instruments decribed earlier.
The Evolution of lnslrumentatlon for Voltammetry Date' 1990'
1980
Advanced software systems AutDmated analysis of voltammetric data VLSl technology microcomputer-integrated as a component pan of instrument. On-line minicomputer
1970
Hybrid analogdigital circuits
1960
lntegraedcircuit, opampbased Solid state I ~ ~ s ~ s I M instrumentation Vacuum tube
1950 1942 1940 1930 1925
Voltage conhol
Technology
Cunent monitoring
Cybernetic instruments with inferential capabilities
Cybernetic inshuments with inferential capabilities
Integrated intelligent conml
Integrated intelligent control
Software generated waveform1 DAClpotenstat Logicdriven FET switches conhoi op amps (pulse) improved speed and accuracy
ADClsoftware control
Integrator connected to potentiostat
Current-voltage conversion
Sampled current
t
t improved speed and accuracy
Servomechanism
Potentiometer cantrob potentiostat which drives threeelectrode cell
Electromechanical(polamgraph)
MotMdriven potentiometer direetly controls WD-Blectrode call
Galvanometer
1920 Perlod when tednologl became well developed. 'Pmbned.
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Finally, the most recent development in voltammetry instrumentation is the integration of the microprocessor as a component part of the instrumentation. Thus, instrumentation for voltammetry seems to have benefited from every major advancement in electronics. Whereas, in the past, improvements were to speed, precision and sensitivity, the use of the computer will engender completely new methodologies. In this context future developments would probably he in the software rather than the hardware. Analog - and Digltal - Instrumentation The control, transmission, and transformation of electronic information can he done in andoe (continuous) form or dieital (discrete binary) form. ~nformationcan also he translated from analog to digital form and vice versa. I t is clear that most modern instruments will be hybrid analog-digital devices, because the experimental world is essentially analog whereas information is more easily handled in digital form. For example, a digital pH meter is easier t o use than an analog one, because the computer handles information in that form. In regard to the analog domain, what students essentially have to learn about is the use of op amps, hoth in feedback control, and for signal transformation. In voltammetry instrumentation, op amps are used for hoth functions. The control function entails maintaining the potential across the working and reference electrode equal to an externally derived potential. This is the role of the potentiostat mentioned earlier. The operational amplifier performs its control function by treating the electrochemical cell as a part of the feedback looo. Signal transforkation operations performed with operational amplifiers include amolification. current-to-voltage conversion, dil'ferentiation, intsgrntion, and filtering. Conversion from the analor-to-dieimland dind-to-analor domains are discussed with aiiew Gdescrihing'the function; aspects of devices used for this purpose and their effect on data quality. In the purely digital domain lectures include description of digital devices such as logic gates and flip flops.
Use of Computers in Voltammetry Voltammetrv was perhaps one of the earliest techniaues to t ~ romputerizrd. e Co"side;ahle work was done by ~ m i i h(fi), Ostervouna . - .(71... and Prronr (. 8. )in the earlv '70's usine minicomputers. The function of the mini was for generating the voltace waveform and for recording the current response. The the signal by, for excomputer was used to further amole. dieital filtering and Fourier transformation. ~ o ; kin the 1980'srhas shifted toward the development of microcom~uter-controlledvoltammetrv. Several reDorts have appeared-in the literature where kinexpensive personal Work on computer has been interfaced to a potentiostat (9). the integration of the microcomputer as a component part of the instrumentation is also well advanced. Already, a few "smart" instruments have appeared in the market. The future of computer-aided voltammetry clearly lies in software. A major area of activity will be the development of entirely new methodologies for automating the analysis of voltammetric data. For example, Soong and Maloy have described a transform technique which affords a sensitivity that compares favorably with c&ventionnl pulse techniques (10). We appear to be gradually moving tuward a new generation oF"ryhernetic instrumt!nts" having inierential and derision making capahilities in closed-loop interaction with rxperimental control functions. He et al. (1 . 1.i h a w commented on this development in their recent description of a "cybernetic ~otentiostat"which enables the cuordination of several vol&metric techniques from a single workstation. Although this work reoresents a maior advancement, i t is a lonn way from a truly cybernetic ins&nent. The main limitationis software that can actuallv interoret voltammetric data and then make decisions based on this. However, some progress has been made in this area. Harrison and Small have reported the use of computers for fitting data into a library of mechanisms (12). Pattern recognition techniques have also been successfully applied to cla&fying voltamketric data (13,14). Future work will undoubtedly draw heavily from the artificial intelligence area. If presenttrends continue, we could expect thatUcybernetic instruments" would become a reality by the end of this decade. Laboratory Instruction One of the attractive features of instrumentation for voltammetry is that a working system can be made for a few hundred dollars. Coupled with an inexpensive personal comouter. a oowerful com~uter-aidedwork station can be devLloped fo; the undergriduate laboratory. Students can thus he afforded intensive "hands-on" instruction. Literature Cited (11 "state-ofithe-~rtsymposium: ~ i e ~ o e h e m i s t r y ,J." C H e x
(1983).
Enuc.,MI, 258-340
(2) Liteanu, C., and Ri~a.1.. A n d Chem., 51.1986 (1979). (3) Enke,C.G.,Anol. Chem.,l3.59A(1971). (4) Herouaky, J.,andShikata,M.,Rm.TIoY.Chim..44,495(19251. (5) Hick1ing.A.. T ~ MForndoySoc.3827 . (1942). (6) Smith, D. E.. in "Camputera in Chemistry and lnstrumsntation,"Vol. 2, (Editors: Matlaon. J. S., Mark, H. 6..and MseDondd, H.C.).MmlDek!er, NeaYork, 1972,
Fioure 3. The structure of comouterized voltammetrv where the WmDutw is integrated as a component part of the instrument. The analag control and transformtion part, hybrM malagdigital elemem, and the wmputer system can be clearly identified,
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Journal of Chemical Education
(131 R d i n g , J. F.,Anol.Chem.55,1713 (19831. (141 Schachterle, S. D., and Perone S. P., A n d C k m . 53.1612
(19811.
