5. C. Creason
and R. F. Nelson1 Sacramento State College Sacramento, California 95819
A LOW Cost, Versatile Electrochemical Instrument
I n the past, a number of circuits have been proposed for the construction of rugged, versatile electrochemical instruments. This particular breed of instrument was originated by Booman (1) and DeFord (2, S), and many modifications and improvements have been proposed since that time (e.g., refs. (4-10); many of these instruments are highly sophisticated and require a good deal of expense t o construct. During the last few years, a variety of electronic components have become available which offer a much better cost-performance trade-off than those which were previously obtainable. For example, utility-grade solid state operational amplifiers are available for less than $12.00 at present; this is a several-fold decrease in price from past operational amplifiers, with relatively little sacrifice in performance. Accordingly, a study was undertaken to develop a lon-cost, multipurpose electrochemical instrument, one that would easily be within the price range of almost everyone. The resultant instrument is capable of performing potential step chronoamperometry, linear scan and cyclic voltammetry, cyclic and unidirectional chronopotentiometry, and, when used in conjunction with a commercial power supply-amplifier, serves as a 50-W potentiostat capable of small-scale coulometry. The parts cost, exclusive of the power supply-amplifier, is approximately $400. The power supply-amplifier is an additional $350. I t should be emphasized that no originality is claimed for the basic design of the instrument. The design is essentially that of Hamley (lo), which in turn is based on designs which have appeared in the literature previously (1-3, 6). It should be emphasized that this is not a high-powered research instrument and should not be used as such. If high-performance characteristics are desirable, then one of the more sophisticated instruments mentioned above should be employed. The instrument described herein has been in use in our laboratory for several months performing routine electrochemical studies with great ease and reliability. For everyday electrochemistry, it would seem to be a good compromise between cost and performance. Herein, only the basic schematics of the various sections of the instrument are presented. Layouts of the front panel (positioning of dials, switches, etc.) and the chassis are available upon request, as are more detailed instructions concerning balancing and calibration procedures and operation of the instrument. Basic Design
I n the potentiostat mode, the instrument can provide a pair of fixed potentials for chronoamperometry, each of which can be independently adjusted to a maximum
Figure 1.
Block diagram of instrument in potentiostot mode.
value of 4 V of either polarity. For linear scan voltammetry, a scanner is available which can operate up to 2 V on either side of an arbitrary initial potential, at scan rates of from 0.2 to 100 V/min. Both automatic and manual switching of scan direction at arbitrary limits is available. I n addition, the scanner can be operated independently of the remainder of the instrument, in which case its output is externally available for use as a time base. This is quite useful in performing chronoamperometric and chronopotentiometric experiments, since one does not need a recorder with a built-in time base (this can mean a savings of several hundred dollars as recorders go). Typically sweep rates from 6.0 sec/in. t o 0.12 sec/in. are available (see below). I n the amperostat mode, arbitrary currents up t o 5 mA are available a t potentials between +10 V. For cyclic chronopotentiometry, the relative forward and reverse currents can be arbitrarily selected so that i,,,,,,, is any desired fraction or multiple of i,o.,, *. Automatic switching at any desired switching potential is available. Block diagrams of the instrument in the potentiostat and amperostat mode are shown in Figures 1 and 2, respectively. Performance
To test the usefulness of the instrument in performing chronoamperometry, the switching time required to scan from +4 to -4 V was measured with the aid of a Tektronix 564 storage oscilloscope and found t o be less than 50 msec. This is a far greater potential step than would normally be required, so it is felt this rather slow response time is acceptable. I n a subsequent experiment, the potential change during a chronoamperometric determination of a 'Present address: University of Idaho, Moscow, Idaho 83843. Volume 48, Number 1 1 , November 1971
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cathodicAnodic Relo"
Figure 2.
, : ! , ,
la
M F OIIC lb*~,
Block diagram of instrument in amperortot mode.
F solution of N,N-dimethyl-p-anisidine in 1.25 X acetonitrile (MeCN)/O.l F tetraethylammonium perchlorate (TEAP) which should be nil, was monitored. (This oxidation has been characterized as a reversible one-electron process (11, lB).) When the potential was stepped from 0.0 to 0.7 V, a potential which is sufficient to oxidize the compound, a 15 mV overshoot was observed. The potential then decayed to a steady state value uvithin 25 msec. A chronoamperometric determination of a 1.0 X 1 0 P F solution of 5,lO-dihydro-5,lO-dimethylphenazine, DMPZ, in MeCN/O.l F TEAP (a reversible, oneelectron oxidation) yielded a value of 58.7 1% for it'lp/C. The data are presentedin Table 1. To test the usefulness of the instrument in performing linear scan voltammetry, a series of voltammograms for the electrolysis of a 1.25 X 10-3 F solution of N,Ndimethyl-p-anisidine in MeCN/O.l F TEAP was obtained over the scan rate range of 0.2-100 V/min. For scan rates between 2 and 20 V/min, peak currents Jvere measured with the aid of a Hewlett-Packard 7035B X-Y recorder. I n this range, i,/v'/'C, which should be constant, was 27.1 0.4. For scan rates between 30 and lOO.V/min, peak currents were measured with the aid of the aforementioned storage oscilloscope to an
*
*
Toble 1. Chronoamperometric Determination of o 1.0 X 10-8 F Solution of DMPZ in MeCNIO.1 F TEAP
Time (see)
itlh/C @Asec'h-mF-'
Time (see)
illh/C PA-
set'/-mF-'
Toble 2. Linear Scan Voltammetric Determination of a 1.25 X 10-8 F Solution of N,N-Dimethyl-p-Anisidine in MeCNIO.1 F TEAP
Values less than 30 V/min measured by X-Y recorder, values greater than 10 V/rnin measured by oscilloscope. 776
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Journal of Chemical Education
accuracy of about 5%. For this range of scan rates, i,/u"'C varied between 27 and 29. The data are presented inTahle 2. To test the stability of the instrument in performing cyclic voltammetry, half a cycle of a long-period triangle wave which was generated by the scanner was recorded with the aid of the X-Y recorder. With a scan rate of 0.5 V/min, a 45 sec anodic scan was performed, a t the end of which the scan direction was switched to cathodic. At the end of another 45 sec, the scan was terminated. This procedure was repeated twice. I n all portions of each scan, the linearity of the trace is within the width of the ink line, which corresponds to.less than 0.1%. Scan rates in the anodic and cathodic directions are 0.48 and 0.49 V/min, respectively. Reproducibility of the respective rates from one scan t o another is within O.lyoand may be limited by the accuracy of the recorder used. I n a subsequent experiment, three cycles of a triangle wave generated a t 20 V/min with automatic switching at +1.6 V was recorded. The time between successive 0.3 see, while the reproducibility of peaks is 20.9 the switching potentials lies within 0.1% and again may be liroi1,edby the recorder used. A chronopotentiometric determination of a 1.0 X F solution of DMPZ in MeCN/O.l F TEAP yielded a value of 85.5 + 2% for is'12/C for values of 7 between 110 msec and 10.1 see. The data are summarized in Table.3. Table 3. Chronopotentiometric Determination of a 1.0 X
*
10-8 F Solution of DMPZ in MeCNIO.1 F TEAP
Reverse-current chronopotentiometry experiments were also run to test the instrument's capabilities. Reproducible switching potentials (performed electronically) were obtained in all cases and with i,,,,,,,= in the ratio of forward to reverse transition times was 3.0 0.1; with i,,,,,,, = 0.414 in,,,d, the ratio of transition times was 1.0 =t0.05, as expected.
,,.,*
The Circuit Building Blocks The Current and Voltage Foflowers and Control Amplifier
A schematic diagram of that portion of the instrument consisting of the current and voltage followers and control amplifiers is presented in Figure 3. I n the amperostat mode, the current follower serves to provide arbitrary reference voltages which are applied to the input of the current follower through an external resistance decade box. With the 1 mohm resistor switched into the feedback loop, the output of the current follower can be varied between 0 and *10 V. The sign depends on the position of relay wiper RYla, while the magnitude depends on the setting of whichever potentiometer, R1 or Rz,is active. I n practice, to perform cyclic chronopotentiometry, one potentiometer is adjusted to full scale and the other is adjusted to a value appropriate for the relative reverse current desired. The absolute magnitude of both the forward and reverse currents is determined by the set-
Figure 3. Current ond voltage followerr and control ompliRer. 1. Blonk porition of rwikh SJ is tert porition. The current follower and control omplifierr ore connected in o gain of - 1 0 configuration with grounded input and Rooting outputs for o f f M null odjurtments. The voltoge follower remains in a follower conflgurdion with grounded input ond floating output. 2. See Toble 5 for major ports identiflcotion. 3. Retistonce valuer are in ohmr, kilo-ohms IKI, or megohmr (MI, or indicated. W, 1% tolerance. 4. Fixed resistors are 5. Rehy RYl is in cothodic position, relay RY2 is in hold position.
'I2
ting of the external resistance decade box. Note that the minimum external resistance that may he used is 2000 ohm. This corresponds to an input current t o the control amplifier of 5 mA (provided one of the potentiometers, R, or R2,is set to full scale), which is the maximum rated output current of the control amplifier and therefore the maximum cell current available. However, in no case may the external resistance used be less than the cell resistance if one of the potentiometers, R1or Ryis set to full scale. I n the potentiostat mode, the current follower provides a 10-V output for cell currents of 10 and 5 mA (the extremes available) when resistances of 1 mohm and 2000 ohm, respectively, are switched into its feedback loop. A listing of the full scale current corresponding t o each of the remaining feedback resistors is given in Table 6. These provide various current scales for the Y-input of an X-Y recorder. The control amplifier circuit is straightforward. However, it should be noted that the lead from t,he reference electrode to the voltage follower is shielded as a precaution against noise pickup. The Sconner and Initial Potential Generofor
A schematic diagram of the scanner and initial potential generator is shown in Figure 4. When the scanner is in the scan mode, it generates a steadily changing voltage which is applied to the control amplifier input across the series combination of a 90,000 ohm fixed resistor and 20,000 ohm variable resistor. Provision of the 20,000 ohm variable resistor allows any one scan rate to be adjusted to exactly its design value. The magnitude of the scanner input, and hence the rate of change of scanner output voltage, depends on the setting of switches Snand Ss. Scan rates from 1 to 500 V/min are available (these are reduced by a factor of 5 at the control amplifier). The scan direction depends on the position of relay viper RYlb. Wit,h a positive input, the scanner output will swing toward its negative limit and vice-versa. If switches Ss and ST are both in the ungrounded position, the scanner input
will he periodically switched (provided the comparator is operative and relay RY2, the hold-operate relay, is energized) and the scanner output will he a triangle wave. If either Se or S1 is in the grounded position, the scanner will cease to operate when the anodic or cathodic limit, respectively, is reached and will maintain that output voltage. Any slight drift in the scanner output at that point can be corrected by adjustment of variable resistor R1. I n the time base mode, the scanner input is connected to a negative source at a point in the circuit ahead of relay wiper RYlb so that the scan direction is not dependent on the state of that relay. The scanner output is disconnected from the control amplifier input circuit and connected t o an externally accessible terminal. Using a recorder with a full scale sensitivity of 10 V, scan rates of from 0.0167 to 8.33 in./sec are available. Integral values of scan rates are available through the use of a recorder with variable full scale deflection. Alternat,ively, integral values of scan rates of from 0.01 to 6 in./sec are availahle for a recorder with a full scale sensitivity of 10 V, provided RTis tied to a -9-V source (a voltage divider across the negative power supply), rather t,han a - 15-V source. Either of two initial potentials may he applied to the control amplifier input depending on the setting of switch S8. Alternat,ively, neither is applied if switch S s is off. The magnitude of the input to the cont.rol amplifier depends on the setting of whichever potentiometer, Ra or R4,is active, and on the setting of switch So. Each initial potential can be positive or negative and can he adjusted t,ofrom 0 t o 4 V. A terminal is provided which allows application of a n externally supplied voltage t o the control amplifier input across a 20,000-ohm resistor. By this means an external signal generator can be used to program the reference electrode-working electrode potential for techniques such as rapid sweep cyclic voltammetry. The Comporofor
A schematic diagram of the comparator circuit is presented in Figure 5. The reference voltage is applied to the comparator input through switch SIS. The magnitude of the reference voltage depends on the settings of & and whichever potentiometer, Rs or RE,is active. The sign of the
Figure 4.
Sconner and initial potentiol circuit.
Blonk porition of witch Sn i s test porition. Circuit is identical to tert position of the control omplifler in Figure 3. 2. SeeTable 5 for major ports identincation. 3. Resistonce values are in ohmr, kilo-ohm., or megohmr, or indicated. W, 10% tolerance; others are ' 1 9 4. Fixed rerbton marked are Voriable reristor. marked ore 2 W, 20%. W, 1%. 5. Relay RYI i s in cathodic porition, relay RYZ lr in hold podtion.
1.
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'II
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Volume 48, Number 1 1 , November 1971
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POWER SUPPLY
Figure 5.
1. 2.
3. 4. 5.
oosuF
Comparator circuit.
Blank po9ition of switch Sa is test porition. Circuit is identical to test position of the control amplifier in Figure 3. See Table 5 for major ports identification. Rerirtonce valuer are in ohms, or kilo-ohms or indicated. Fixed resistors ore 'IzW, 1 %tolerance. Relay RYI is in the cathodic position.
reference voltage is determined by the positions of relay wiper RYlc and whichever limit-polarity switch, SIX or &a, is active. Switch SI2,the voltage limit multiplier switch, is ganged with switch SIo,the initial potential multiplier switch, in the initial potential generator circuit. The output of the voltage follower is applied to the comparator input to provide a sense voltage for automatic switching. Manual switching of the comparator is available through the use of momentary contact switches Sls and SIB. The signal applied to the comparator input by the action of either of these switches is sufficient to over-ride any input signal from the voltage fall~~wer.Thus, manual switching is always possible. The comparator output is applied to the base of the switching transistor which energizes relay RY1, the cathodic-anodic relay, if the output is positive. The transistor base is protected from voltage-transient damage by a 0.1 ,LFcapacitor connected between it and ground. A Zener diode is connected across t,he coil of relay RY1 to clip any transient which might result when the state of the relay is changed. A pair of diodes, tied cathode to anode in parallel, is connected across the comparator input to prevent damage from application of excessive voltages. The Power Supply
A schematic diagram of the power supply is presented in Figure 6. The circuit of the *15-V supply is very similar to a circuit described in National Semiconductor Corp. Application Notes (15, 14). The salient characteristics of the suvvlv .. . are listed in Table 4 without comment. The -34-V supply is a simple bridge-rectifier circuit, which is satisfactory for the operation of relay RYI. A 6.3-V transformer orovides voltaee for o~eration of ' relay RY2 and the various indicator liihts. Toble 4.
Characteristics of the Instrument
*I5 V
Power
SUDD~V( 1 3. 14) Parameter Load regulation Line regulation Maximum continuous output current Temperature stability
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Vahm 0.1% 0.05'%/V 1A 0.3% far ambient temperatures between 0 and 70°C
Journal of Chemical Education
Figure 6. 1. 2.
3. 4.
Power supply,
See Table 5 for major parts identiflfication. Resistmcevduer ore in ohms or kilo-ohms, or indicated. Fixed resistors marked ore 2 W, 10%; others ore W, 5yotol. erance. Variobie reritton are 2 W, 20%. 4.7 pF input and 1 PF output capacitors must b e low inductance solid
'I1
*
tantalum type.
Table 5.
Designation
Maior Parts
Identification ZEL-1 operational amplifiers with external 50,000 ohm offset-adjust potentiometer), Zeltex Inc., Concord, Calif. 10 UF Pvranol Ca~acitor.General Electric Cb. 21120 Zener diode, International Rectifier Corp. MDA 920-4 bridge rectifier, Motorola Semiconductors, Phoenix, Aris. Ferrite beads, Amidon Associates, Los Angeles, Calif. 35009-2 10-turn potentiometers with H492-3 dials, Bourns Ine., Riverside, rrw v......
RYI
JM4 110-41 mercury-wetted-contact relay, Potter-Brumfield Co. RYg KHS17A11-6AC relav.. Potter-Brumfield Co. SJ-Sr, 40-S11,SnrS2, Series T-300 shorting switches, International Rectifier Corp. T.-T. F40X transformer. TI is modified bv splitting the secbndary winding the center-tap connection, Triad Corp., Huntington, Ind. F14X transformer, Triad Corp., Kontington, Ind. LM-300 voltage regulator modules, National Semiconductor Corp., Santa Clara, Calif. 2N303,5 transistor, National Semiconductor Corp., Santa Clara Calif. 2N290.5 transistor, National Selhicondoctor Corp., Santa Clara, Calif.
.
The Remaining Circuitry
A schematic diagram of the indicator-light and relay RY2 (the hold-operate relay) energize circuit is presented in Figure 7 without comment. Parts Description and Switch Functions
A list of major parts and a summary of switch functions are presented in Tables 5 and 6, respectively.
Toble 6. Switch 1
2
Figure 7. Indicator lightr and ~e1.y major parts identi0cation.
circuit.
See Toble 5 for
Appendix To function as a caulometer, the instrument can be used in canjnnction with a. Harrison Model 6824A power supply-amplifier (manufactured by Hewlett-Packsrd), which has an output capability of 4 ~ 5 0V a t 1 A, for relatively large-scale electrolyses. I n use, the power supply-amplifier is programmed as an operational amplifier and is switched into the circuit in place of the control amplifier. The working electrode is simultilneously switched from the current follower input to ground, since the follower amplifier current capability is only 1 5 mA. In this configuration, the instrument can be used for electrolysis of dilute solutions (approximately 1 0 - T ) a t an arbitrarypotential which is maintained constant t o within + I 0 mV. If one wishes to determine n-values for the electrochemical oxidation or reduction of compounds in solution, the cell current can be monitored by measuring the voltage drop across a, resistor in series with the mxilixry electrode lead. A convenient value far coulometric studies is 10.363 ohms (obtained from Precision Resistor Co., Hillsdale, N. J.), since this is a. power of ten multiple of the reciprocal of the Faradrry (15). One can determine the amount of current passed by recording the current-time curve on a stripchart recorder and integrating under the curve or by digital presentation (16). The circuit modifications necessary t o carry out coulometry as described above are shown in Figure 8. We have employed the instrument for coulometric work and have found it t o be quite satisfactory in determining n-values. We hhrwe used the digital coulometry setup according t o Bard (16), with the slight modification of using an extra. counter for gating in order t o follow more closely the decay of the current being passed. For the oxidation of 1.0 X lo-* F DMPZ in MeCNIO.1 F TEAP t o the cation radical, an n-value of 1.0 was obtained; electrolysis of this solution bsck t o the parent eompound gave the same number of counts (and hence the same amount of current) to within 1-2yo. Experiments on other systems involving chemical reactions associated with the electron transfer process hrwe yielded reproducible (although not always interpretable!) results. F Although i t functions quite well in dilute solutions and lower), there me problems a t higher concentrations, particularly in nonaqueous solutions. There is a limit to the iR drop the potentiostat can sustain, and we have found that the use of a divider for the auxiliary compartment such as a Vycor glass, combined with the high solution resistance of acetonitrile and other nonaqueous solvents and high cell currents tends t o drive the power supply-amplifier t o saturation. This, of course, is not s. good thing, so i t is advisable t o use a fine or ultrafine sintered glass frit in place of Vycor and work with solvents of low resiqtance if one wishes to electrolyze larger amounts of msterial. If the instrument is t o be used in coulometric applicsi tions for which relatively low power is required-law cancentration aqueous salt solutions, for example-then less expensive alternatives to the Harrison power supply-amplifier aye wsilxhle. One snch (whieh was not tried, hut whieh uses a well-known teehnique) is to add a Zeltex Model l54C booster amplifier to the control amplifier using the manufacturer's recommended circuit (16). With such an arrangement, cell currents up t o 200 mA are available a t working-auxiliary electrode voltages of & I 0 V (much too low for nonaqueous work). The cost of the added components in this case would be approximately $40.
Acknowledgment
Ackno~vledgment is made to t,he Nat,ional Science Grant Yo. GP-8941, for support of this ~vork,and to Rodney Hand for technical a s s i s t a n c e . Special thanks are also due to Drs. R. X. Adams and D. E. Srnit,hfor their support and encouragement. Foundation,
Switch Functions
Designation
Positionsz
Cyclic chronopotentiometry Cvclin rhrnnonotentiometrv
End Csthodic, cycle End Anodic. cvcle
4 5
Seen rate multiply Scan rate
6 7
Cyclic voltammetry Cyclic voltammetry Initial potential Initial potent,ial Initial potential multiply Scanner Vnlt,aee limit m o l t i ~ l v Anodic voltage lim(t " Cathodic voltage limit Comparator Anodic Cathodic Initial potential (A) Initial potential (B) Power Operate Hold Potentiostat-normal operation Full scale current, ma
8 9
10 11
12
23 24
~
-~
"test, cyclic ehf& nopotentiometry 1, 3, 5, 7, 10 0.2. 0.5. ?, 1.5. 2. 3.. 4, 5, 6, k, 10 cycle, end cathodic cycle, end anodic A, B on, off 0.5, 1, 2, 4& Scen, test, time base 0.5. 1. 2. 4( poiitih, 'negative positive, negative on, test momentary on, off momentary on, off positiue, negative positive, negative on, off momentary on, off momentary . off, . on
.
normal-potentiostat 0.01, 0.02, 0.05, 0.1, 0.2. 0.5, 1. 2, 5
Itdiciaed rrasition corresvonds to position in appropriate schematic diagram. a Ganged with switch Sn. a Ganged with switch SLo.
TO
El*
Figwe 8. Control amplifier circuit modification to incorporate 50-W wnplifier. Resistance value. are in kilo.ohmr. Resistors ore 2 W 10%
tolerance.
Literature Cited (1) BOOMAN.G.L..A n d . Chcm., 29, 213 (1Q571. (2) D ~ F o n o D. , D., Aanl. Chem., 32, 31R (1960). (3) DeFono, D.D . . Division of Analytical Chemistry, 133rd meeting ACS. San Francisco. Calif.. Aoril 1958. (4) H n n n ~ n . .I . E., S T E P ~ E N F.~13.. . A N D PEOHAOER, R . E..Anal. Chcm.. 34, 1036 (1962). (5) U ~ o ~ n x o uIFr. ~ ~Ln. , A , N DSxnm.I., Anal. Chcm.. 35, 1778 (19631. (6) L m r n . G.,SCHLEIN, H.. A N D O S T E R Y O U N R.~ ,A,, Anal. Chem., 35, 1789 (1963). (71 ROOMAN,G.L.,mnHocnnoon. W. B.,Anal. Chem.. 35,1793 (1963). (8) G o o ~ s n r A. , L.,hxo SAWYER, D . T., A n d . Chem., 39, 411 (1967). (9) D n u ~ u n s G., ~ , Rosrw, hl.. A N D ELVINQ, P. J., Anal. Chim. A d o . 42, 143 (19681. J. G.,A N D HAWLEI,M. D., J . Elecfioonol. Chem., 21, 365 (101 LATYLEBB. (1969). (11) NELSON, R.,Ph.D. Thesis. University of Kansas,1966. (12) . . B~conr.J.. NELSON.R. F.. A N D AOAMS.R. N.. Submitted for publioation. (13) Application Note AN-1. Manolilhie Voltape Regulator, National Semieondoctor Corp.. SantaCI&r&,Calif., 1967. (14) Speeificalion sheet, LIM-SO0 Volluge Regulrrtor, National Semieonduotor Corp., Santh Cihra, Calif.. 1968. (15) B ~ n n A. . J., AND SOLON, E., A n d . Chem., 34,1181 (19621. (16) Ooerstional Am~lifiersand Com~utinc Elements Catalog., Zeltex.
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