A Simple and Inexpensive Function Generator and a
Four-Electrode Cell for Cyclic Voltammetry F. N. Albahadily and Hwaclo A. Mottola Oklahoma State University, Stillwater, OK 74078 A revival in the teaching of electroanalytical chemistry has occurred during the past 10 years as the appearance of recent textbooks on the suhiect testifies (1-3). Among the many electrochemical techniques useful in chemical ailalvsis, linear w e e p voltammetry in its cyclic form wyclic v o tammrtry) ha.; received special attention and its fundamentals as well as its practical incorporation to the teaching of electrochemical techniques in analytical chemistry have been recently presented in the pages of THIS JOURNAL (4-6). One feature of electroanalvtical techniques is the . attractive . fact that the instrumentation needed is comparatively inex~ensive:even so~histicated.state-of-the-art. instrumentation is competitively priced when compared with instruments a t the same level of develo~mentin other analvtical areas, such as spectroscopy. Popularization of this technique a t the undergraduate level is, however, better accomplished with less sophisticated units, and the possibility that the students/instructor build the necessary units inexpensively and in a simple manner is an additional attraction. In this note, which can he considered supplemental to the articles recently published in THIS JOURNAL on cyclic voltammetry (4-61, we describe the construction and performance of an inex~ensivesienal eenerator and a fonr-electrode electrochenkcal cell for use in voltammetric experiments. Assembly of the instrumental devices described here is within the reach of limited budgets since in the design we have limited the number of components to a minimum of inexpensive and readily available units without sacrificing the needed features normally found in commercial units. Typically, highly symmetrical triangular signals can be generated a t scan rates from 10-3V.s-I t o lo4V.s-' by selection of the time constant for the inteerator Dart of the circuit. Ascending and descending potential ramps can he initiated and terminated a t anv desired neeative or ~ o s i t i v potential e with respect to the reference. ~ o t multiple c and single trianeular eeneration is Dossihle. This note also describes the construction and performance of a four-electrode electrochemical cell that can he used to illustrate elimination (or minimization) of background currents due to electrochemical reactions by species other than that of interest which appear at the same potential (or very close) as the working electrode reaction for the species to be studied.
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Clrcult-Related Conslderatlons Triangle Wave Generator Figure 1 gives the diagram for the signal generator. Switches S1 and Sz permit choosing a multicycle operation (free run) or a single cycle operation (single shot). Switching S1and Szto the midway position disables pushbutton switch P S and connects the noninverting input of the multivihrator, OAl, directly to the output of the integrator, OA2, setting conditions for a free run operation. The multivihrator has two metastable states of +13 V and -13 V, and the amplifier is continuously switching between these limits generating a square-wave ourpuf of 26 V of amplitude. Such potential is sipplied hy the integrator operating a t frequencies dictated by the selection of resistor R1 and capacitor C. When OAl has just switched to its +13 V output, OA2 will integrate the
Flgure 1. Circuit diagram for triangle-wave generation. OAl and OA2 are pA 741Coperational amplifiers (Texas lnslruments), RI = 1 mR: R2 = 4.7 k% R. = 10 k n : P, = 20-kR. 70-turn potentiometer: P2 = 5-kn. 10-turn patentiorneter: PI = 50-kR, 10-turn potentiometer: St and S2 are three-point rotary switches: PS is a pushbunon switch: C = 10 pF: 0, and D ~ a r eIN4149 silicon signal diodes.
portion of voltage passing through potentiometer PI. Since 0 A 2 has a ncgative feedhack and the signal applied to its inverting input is positive, the result is a negatively increasing (down) signal ramp. This signal will he the output of the eenerator circuit until the outout l the se. ~. o t e n t i areaches Iected minimum limit. At such a mommt. 0 A l will witch to its -13 V outout. The ort ti on of the neaative volraae .. .~asainr: uow throughi', will b i integrated by O A and ~ the result u,ifi be a ~ositivelsincr~nsine -cuu) . sianal - until the output of the integrator reaches its maximum potential when thk multivihrator will switch again hack to the +13 V output. This alternation will continue as long as the power is on. Switching S, and S2to the up or down position permits the selection o f a single triangle output (single-shotbperation). Having hoth switches in up position (as shown in Fig. 1) causes diode Dl to allow only a positive signal to pass through and the multivihrator will show positive saturation. The output of the integrator will decay down until i t reaches its minimum potential value, when it will latch a t such a ootential. The level of the minimum ~ o t e n t i a is l dictated mainly by the setting of potentiometer P Q but it is also affected bv the settine of potentiometer PI. Potentiometer PI should be set first to seiect the amplitude of the triangle, then Pn can he used to shift the triangle UP and down in the voltage scale. Final adjustment using P; may he needed. Operating in the sinale shot mode, momentary closing of p&hbutt& P S will apply a negative potential to the vibrator noninverting input, changing its output to -13 V. The integrator will integrate the negative applied signal until its maximum allowable output is reached. Then DI will open aeain allowine,. the ~ositive-eoine . .. .. sienal .. to nass to OAl and itsoutput will be a negati\.rly going signal until it reaches the orieinal W ~ t hhoth switches in the down ~ositiun - uotential. . the operation is similar, with diode Dz open only tonegativegoing signals. Volume 63 Number 3 March 1986
27 1
Potentiostat Figure 2 illustrates the circuit used t o maintain the potential between the working electrode and the reference electrode constant. The potential is furnished by an external circuit through OA3 acting as a controller. I t remlates the current necessary to com&saw for the curreni prnduced by the electruchernical reaction at the surface of the working electrode. The output current from the working electrode 6 converted into a measurable voltage by OA5. The reference electrode is connected throueh a unit-eain voltaee follower. OA4, to isolate the electrodeand minimize the &rent flow through it. Instrumentation Amplifier A combination of two current followers with gain and a difference amplifier was used to assemble a differentialmode amplifier with hieh - input . imoedance actine as instrumentation amplifier (Fig. 2). m he-two inverting inputs of OA5 and OA6 were connected to the two working electrodes of the four-electrode cell described later. The two input signals are amplified bv the current followers and their diffeience is ampiified hiamplifier OA7. The Four-Electrode Cell Figure 3 shows details of the four-electrode cell centered around a 30-mL beaker. Two holes (2 mm in diameter) were made directlv across from each other about 25 mm from the bottomof the beaker. Two Teflon blocks, machined to fit the circumference of the beaker. were attached (bv usine oaraffin) across the holes in the sides of the beaker, eacyhlock about 2 X 2 X 2 cm in dimensions and each cut into two halves. The upper half of the block was pierced with a central hole of 49-mm2 area. A corresponding hole of the same area hut only 6 mm in depth was cut in the lower part of the block. As shown in Figure 3 these openings permit the assembly of the working electrodes, each made of a copper sheet connected to thecircuit viaa copper wire. On top of the copper sheet a few drops of m e r c ~ r ~ p r o v i d eadcloser contact between the copper sheet and aplatinum foil (the actual workine surface) of about 0.5 cm2 in area. The unoer Dart of n was fastened with screws to the iow& part, the ~ e f i o block sandwiching the platinum foil-He-Cu electrode assemblv. Contact between-the beaker contents and each electroie reservoir (of about 300 wL in volume) was provided by a perforation in the reservoir that was aligned with one of the holes in the beaker wall. T o minimize diffusion of solution between the working electrode compartments and the central container (beaker), small Teflon tubes filled with glass wool were inserted in the channels connecting them.- he glass wool was cleaned by boiling i t in concentrated HCI for about 15 min and washine thorouehlv with distilled water until a negative acid of thewashings was obtained. Operation of the four-electrode cell involves two e x ~ e r i menial steps. During the first one, both working elect,rode compartments and the central beaker are filled with the supporting electrolyte solution and the potential is scanned between the preset limits. The current produced a t the two working elecrrodes ~houldbe identical and cancel each other in the output of the read-out device.Small differencescan be expected,-however, because of differences in the areas of the two working electrodes. These small differences can he minimized with the help of potentiometer Pq. In the second experimental step, t h e supporting electrolyte is removed from one of the comnartments containine one of the workine electrodes and replaced by the sample solution. Under th; condition, the signal entering OA5 will consist of the current due to the electroactive species in the sample plus the supporting electrolyte residual current. The signal fed to OA6 will correspond to the supporting electrolyte only. Since OA7 reiects common-mode signals, the signal output corresponding to the supporting el&olyte is s;ppressed and the 272
Journal of Chemical Education
Figure 2. Circuit diagram for potentlostatand four electrode cell. OA3 and OA4 are eA 741C omrational am~llfiers(Texas lnsbumentd: OA5.. OA6.. and OA7 a1eLF 3 5 3 . ~ < Op~ratlonal ~ imp111&r (National~ernobndbctor):R,snd RS= 1 k5?.Re = 80 k% R,aMlR8 = I 0 kO.ReanoR,o = 500 krl: P, = 100 kQ, 70turn potentiometer;RE. reference e8ectrode CE, counter elenrods: WE1 and WE2, working elecbodes. ~
COUNTER ELECTRODE
REFEREWE ELECTRODE
TEFLON TUBE
Figure 3. Four elecbde cell
one corresponding to electroactive species in the sample is amplified. The design could be simplified by having one of the working electrodes immersed in the central beaker and eliminating theneed for one of the Teflon blocks. The design presented here is preferred because suppression of the supporting electrolyte signal is better achieved i t 1) both working electrodes are comparable in size and construction, and 2) the electroactive species in the sample is (are) prevented from diffusion to the suppressing working electrode. Clrcults Performance Satisfactory symmetry of the triangular signal was observed. The ratio of areas under the two halves of the signal was always between 0.98 and 1.00. The system proved to he stable for long periods of operation and to give reproducible results in day-to-day operation. Well-characterized systems (e.g., Fe(CN)s-3/Fe(CN)6-4 and Cu(I)/Cu(II)) were experimentally tested and the generated cyclic voltammogram data and characteristics reproduced the available information in the literature. Performance of the Four-Electrode Cell As stated earlier, the four-electrode cell is particularly useful for the elimination of interfering background cur-
Figure 4.-Cyclic voltammDgram of background (supporting elemolyte 1.00 1.00 M H2S04)obtained with s four elemode (two working elecM KC1 trodes) cell. Sweep rate: 20 mV.s-'. Potential versus SCE. WHC Cycllc voltammogram of the same solution and under the same experimental condllions obtained with a thre~electrodecell (single wohing electrode).
+
rent(s). To illustrate such a situation we have chosen the redox system comprised by the redox couple tris(l.10-phenanthroline)iron(II)/tris(l,l0-phenanthroline)iron(III), the ferroin/ferriin system, in 1.00 M KC1 1.00 M H2S04 a8 supporting electrolyte. Figure 4 shows the cyclic voltammogram of the supporting electrolyte solution with a threeelectrode cell. The background current is the result of the current-potential curve for platinum in HzSOa (8). Figure 4 aho shows the current-potential curve when the four-electrode cell is used; the minimization of the background current is clearly seen. Figure 5 shows the voltammogram for the ferroinlferriin svstem with the three- and with the fourelectrode systems. The solutions used for these experiments were uurged from oxygen by bubbling nitrogen gas for about 15mfn and the systemwas kept air-free during the measurements by flowing nitrogen gas inside a 2-liter, capped polyethylene bottle with bottom cut off and with a lateral hole for nitrogen introduction. This bottle covered the entire cell
+
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Figure 5. Cyclic voltammogram for a 1.25 X 10.' Msolutionot terroln in 1.00 MI(C1 1 00 MH2S0. solutlon -cunsnt outpul Ootainw wllh four-elsmode cment output obta'ned with a thr-lectrode cell. Swwp rate: cell. 20 m V C 1 . Ail potentials measured versus the SCE.
+
assembly. As can be seen in Figure 5, the use of a fourelectrode cell minimizes the background contribution and enhances the details of the voltammogram for the electrochemical species of interest. Acknowledgment
This contribution resulted as part of Project No. 84-063 supported by the US-Spain Joint Committee for Scientific and Technological cooperation. Literature Clted (1) Bard,A.
J.; Faulha
&"".
(7) Bruntlett, C. S.Current SepomLiow 1983,5,21. (8) Fried, I. "The Chcmiam of Eledrade Proees@ea"; Academb London. 1913: p 139.
Volume 63
Number 3
March 1986
273
