Instrumentation for Cyclic and Step-Function Voltammetry Using

Instrumentation for Cyclic and Step-Function Voltammetry Using Operational Amplifier Switching Modules RICHARD

P. BUCK and ROBERT W. ELDRIDGE

Bell & Howell Research Center, Pasadena, Calif.

b Operational ampliiier (OA) switching modules (comparators or contactors) may serve as ,.he basis of control units used with potentiostats or amperostats for cyclic or step-function A simple, electromevoltammetry. chanical comparator-relay unit and a more complex comparator-based, allelectronic square and triangular wave generator, as cyclic control units, a r e described and compared.

T

HE USEFULNESS of voltammetry, especially when dealing with transient intermediates or unstable products of electrode reactions, can be extended by multiple sweeps of voltage or current, or by step-functional changes in voltage or current. A number of control units based on operational amplifier (OA) switching modules, when used in conjunction with OA-based potentiostats or ampe-ostats, will provide these functions. Applications of OA’s to electroanalytical instrumentation have been described by Booman (d), Kelley, Jones, and Fisher (6, 6), DeFord (4),and more recently by Buck (S), and Alden, Chamoers, and Adams (1). Circuits of the 1)eFord type can be found in articles since 1958 by many other workers. The convenience and versatility of these c i r x i i s have led to their acceptance by nuinerous analytical chemists and other students of electrode processes. The control units d1:scribed here are intended for use with a basic potentiostat-amperostat of the DeFord type. The control units range in complexity from simple relay-bawd devices activated by OA comparators to more sophisticated, all electronic square and triangular wave per erators. I n all cases, flexibility in settings of initial voltages or currents, weep rates, and the extremes of voltage or current excursion, is possible. By deletion or change in external connections, ordinary single sweep voltammetry can be performed.

POTENTIOSTAT-AlHPEROSTAT

Figure 1 is a combination of two arrangements given by DeFord. The upper position of w i t c h 1 (SW-1)

b TO COMPARATOR INPUT OF CONTROL U N I T 1

Figure C:

B: F:

I: INT: SA: RJ: REC:

%v-3

I

R*:

RJ: R4:

Rs to Rm:

R11:

sw-1:

s w-2: 91

I: +t

1.

FROM CONTROL UNIT

I

Potentiostat-amperostat

Control amplifier, high gain adding inverting amplifier Current booster follower Unity gain follower amplifier, 1 OOM input impedance Inverting amplifier Sweep generator, analog integrator with variable input impedance, precision 10 pf. capacitor in feedback loop Scaling amplifier, inverter with gain of -0.1 000 Recorder input, precision 0.1 %voltage divider attenuator Potentiometric recorders, oscilloscope or oscillograph Shown as variable resistor, is series of resistors on 1 pole, 10 position switch (Integrator Time Constant Selector). Values given in Figure 5 1 OK, 0.1 %, 0.5 watt, wire wound 33K, carbon 1 K, 2 watt carbon 1 OOK, 0.1 %, 0.5 watt, wire wound Precision wire wound resistors, 0.1 %, 0.5 watf, from 10 ohms to 1 M on multistep switch 3 pole, 2 position shorting switch 2 pole, 3 position nonrhorting switch (Integrote, Hold, Reset) Working Electrode Auxiliary Electrode Reference Electrod e

provides controlled potential between the working electrode and the reference electrode equal to the sum of the primary voltage (PV) and the sweep generator voltage ( I N T ) . The actual applied voltage across the two active electrodes (working and auxiliary) is greater than this sum by the internal iR drop of the chemical cell. Current through the cell is measured by the voltage drop across the precision measuring resistor, R,. The lower switch position provides constant current between the two active electrodes, and the resulting potential of the working electrode is measured with respect to the reference electrode. The magnitude

of the constant current is given by the sum of the PV and I N T divided by R,. The recorder input (RI)attenuates voltages up to 10 volts to outputs suitable for a IO-mv. full-scale recorder. The small triangles superimposed on larger 08 triangles indicate chopper stabilization of the differential input d.c. amplifiers, typically the Philbrick K2-XA ( 7 ) . Chopper stabilization is achieved by use of either the Philbrick K2-P or K2-PA amplifiers. I n all modules, except the follower amplifier ( F ) , the input signal is fed directly to pin 2 of the chopper amplifier and is also capacitively coupled to pin 2 of the K2-XA. The chopped signal VOL. 35, NO. 12, NOVEMBER 1963

e

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enters the K2-XA at pin 1. In the F, the input signal is capacitively coupled to pin 1 of the K2-XL4and the chopped signal enters through pin 2. Biasing of each module to remove the initial offset between pins 1 and 2 of the K2-XA is accomplished by the “Preferred” method of G. A. Philbrick Researches, Inc. ( 7 ) . Biasing of pin 2 by approximately - 1.3 volts is required by most modules; although, in the F case and a few others, pin 1 is biased positively. This method uses power from the -300-volt supply connected through a 1RI resistor and a 10K carbon potentiometer to ground. The wiper of the potentiometer is connected to pin 2 through a 2M resistor to limit grid current. With a resibtive feedback, the bias is adjusted to give zero output within k 0 . 1 mv. when the input is connected to a high quality ground. Each signal ground and chopper ground, as well as power supply ground, should be independently attached to a single common point to avoid ground loops. It is particularly important that electrolytic cell currents which flow to ground should not be run through reference ground lines. This will cause the reference signal ground to be different from true ground with the result that comparator circuits will respond to a false nonzero input sum. The crossover point may then deviate from ground by as much as a few millivolts. The power supply uced is the G. A. Philbrick Researches, Inc. Model 10013. Current available from the potentiostat-amperostat is +20 to 25 ma. using the current booster follower ( B ) Philbrick K2-Bl or SK2-Bl. Without the B, =t4ma. are available. If higher currents are required, transistorized F’s of greater current capabilities can be used. One such circuit is available from the authors.

TO INTEGRATOR OR FROM POTENTIOSTAT-

AMPEROSTAT

I

RELAY

Figure PV:

SV: Relay:

NEC: R1

to R4:

2. Control Unit I

Primary voltage supply, chopper stabilized d.c. amplifier with Standard Weston Cell in feedback loop followed by precision scaling amplifier for establishing voltages to 0.1 my. while providing current up to about 1 ma. Available range is * 10 volts Secondary voltage supply, see text Double pole, double throw Neon comparator amplifier, see text 1 OOK, 0.5 watt, 0.1 %, wire wound

cuit shown in Figure 2, with the switch in the upper position. The extremes of the voltage sweep are set by the two S V s , and the slope of the ramp is determined by the PV used and the integration rate. The S A in the control unit is set for minus unity gain. If it is desired to produce a positivegoing sweep rate different from the negative-going sweep rate or vice versa, the gain of S A is adjusted by varying its feedback resistance t o provide the desired input voltage to the integrator. If a triangular wave which does not cross zero or a n initial voltage other than zero is required, a constant bias can be added at the input of the control amplifier (C). A simple batteryoperated Helipot voltage divider or a second PV could be used. ,4n alternate application requires grounding of the input to the N E C which is normally connected to the

General Characteristics. Control Unit I consists of a chopper-stabilized polarity-sensing neon comparator amplifier ( N E C ) , PV, scaling amplifier ( S A ) , secondary voltage source ( S V ) , and relay, as shown in Figure 2 . The NEC is a simple bistable amplifier whose output voltage is practically independent of input level and is determined by the neon bulb breakdown voltage -+’io volts. The sign of the output is the negative of the net input voltage. The N E C is used t o activate a relay. There are several possible ways of using the control unit with the potentiostat. The normal and intended way is thc switching of a ramp voltage at two limit potentials to produce a triangular voltage sweep. This is automatically accomplished with the cir1830

ANALYTICAL CHEMISTRY

I

n W

8.2K

Ne-2

CONTROL UNIT I-NEON COMPARATOR AMPLIFIER

reference electrode. I n this case the variable voltage seen at the N E C is proportional to the current in R,. Kow the applied voltage sweep will reverse when the limiting current reaches some predetermined high value set by SV. Step-functional voltages can be applied by removing the integrator and its S A and introducing the output of the relay a t one of the summing inputs of the C. Switching of inputs will occur when the cell current reaches some desired level. When the control unit is used nith the amperostat, current scans and step-functional current applications can be performed depending on whether or not the integrator is used or is bypassed. With the inputs to NBC, shown in Figure 2 , the potential of the working electrode determines the switching of current or direction of current scan in the cell. The unit switches the direction of the current when the working electrode potential us. the reference electrode reaches a preset value a t one extreme, either positive or negative. The working electrode potential is then driven the other way toward a second preset voltage value which determines the switching point a t the other extreme. For equal and opposite currents, the S A between P V and the relay is used as a simple inverter. Otherwise currents of opposite sign, but unequal in absolute magnitude, are produced. The specific application of this technique is cyclic chronopotentiometry, which we are investigating. Control Unit I permits settings of reference voltages from SV which are accurate to 0.2 mv. The A’EC can sense a voltage crossover nithin + I mv. of the set value. The output of NEC is scaled down to provide 48, 24, 12, and 6 volts to operate typical relays.

470K

T

E UT

Figure 3.

Neon feedback comparator amplifier

1 OOK resistors, 0.1 70,0.5 watt, wire wound. Other resistors with values shown a r e 10% carbon, 0.5 watt 2 2 K , 25 watt, wire wound R1: 1.5K, 25 watt, wire wound R2: 10K, 25 watt, wire wound Rs:

Cl: Cp:

D1,Dp: SW-1: S W-2:

0.1 pf., 200-volt, mica 0.25 pf., 600-volt, molded tubular 500 volts, silicon diodes, 81 N1695 1 pole, 2 position shorting switch 1 pole, 4 position shorting switch

‘l’he rise time (10 to 90% of maxiniuni voltage) of the entire system is 180 microseconds. Thus, stabilized square output voltages are easily obtained up to the chopper frequency 60 c.p.s., providing the relay response is not limiting. The monostable 24-volt Sigma Relay, 8000-ohm coil, operating a t 2.3 ma. from a normally closed position, gives satisfactory waves when switching once per second. Above this frequency, the square waves develop unequal “half” periods with longer residence time in the closed position. Csing a bistable, mercury-wetted Clare Relay (Manufacturer’s #HG2A 1004) equal “half” period square waves up to 50 C.P.S. are achieved. Above this frequency, periods become irregular. Oscillations from the NEC output, used t o activate the relay, can be picked up in the signal being switched if the source impedance is high. Dual Secondary Power Supply. This unit consists of two parallel voltage dividers with F’s in the output of each, as shown in Figure 4. The P’s eliminate loading errors and interdependence of the two potentiometer settings. The dividers may be set separately so that each will give an output voltage in the range * l O O ~ o of the range voltage. The available range

*Ei

Ne-51 h

U

Figure

4.

Secondary voltage supply

745K, 0.1 70,1 / 2 watt, wire wound 145K, 0.1 70,1 watt, wire wound 25K, 0.1 %, 5 watt, wire wound 2.OK, l.O%, 1 watt, wire wound 2SK, 1.0%, 1 watt, wire wound 0.1 Nf., 100 volts, mica Balance potentiometer, 1 K carbon 0-1 OK Dekapot potentiometers 2 pole, 4 position shorting switch

u

Currents up t’o 20 ma. are available from the YVEC to a:tivate the relay coil. Response of the control unit is limited by the relay. General applications require switching as infrequently as once in 500 seconds to 100 times per second. Mercury-wetted relays are suitable in this range. Circuit and Performance. The S E C circuit is sh0.m in Figure 3. Voltages, whose sum .vi11 cross zero, are fed into the differenh.1 d.c. amplifier K2-XA through IOOK resistors. The resistor, in series with the neon bulb, limits the feedback current for rapid response. The K2-P amplifier is placed conventionally between the negative arid positive inputs of :he K2-XL4. The second amplifier combination scales the final outliut value to set fractions of the neon breakdown voltage. Since the latter value changes with the neon bulb history, the output voltages are only nominal values. Switch 2 (SW-2) provides sdection of output voltages. The B, SK2-B1, provides currents up to 20 ma. To obtain equal currents for both poshive and negative output voltages, pins 2 and 6 are biased Ly the HP method recommended by 0. A . Philbrick :12esearches, Inc., shown in Figure 3. Diode-controlled outputs are available for use with monostable relays. The iiecond neon bulb warns against amplifier overload. The zero switch, SW-1, substitutes a lOOK feedback resistor in place of the neon bulb. The mair input is grounded and t’he first stage ‘iutput voltage is monitored with a IO-mv. full-scale meter. The offset bias potentiometer is adjusted to give an output reading of zero within 1 0 . 1 niv. The S A is theii

zeroed by measuring the output voltage after the SK2-Bl mhile adjusting the bias potentiometer of the second K2-

x,4,

EOUT

Figure 5. Stabilized triangular and square wave generator with integrator timing Biased diode feedback detail, see Figure 7 legend

Ri-RI4:

Rts: R16:

R17:

1 OOK, 0.1 %, 0.5 watt, wire wound 1 OOK, 1 0-turn, Helipot

39K, 1 watt, carbon 12K, 1 watt, carbon

1 OK Trimpot Rlr 1 OK, 1 0-turn, Helipot Rlo and Shown os variable resistor, is series of resistors on 1 pole, 10 posiS W-3: tion switch (Integrator Time Constant Selector). Resistors used were 0.1 70, 0.5-watt wire wound 1 OOK, 1 OOK, 300K, 500K, 1 M, 3M, 5M; 1 %, 2 watt, carbon, 1 OM, 30M. S W-1 2 pole, 3 position nonshorting switch SW-2 1 pole, 10 position shorting switch Rzl: 1 K, 2 watts Symmetric zener diode, 5.9 to 6.5 volts, Hoffman #N822,7.5 mo., Z: Rls:

C:

maximum 10 pf. polystyrene capacitor

VOL. 35, NO. 12, NOVEMBER 1963

1831

t3?0 V

Eout =

when

6 6 EOUT

Figure 6.

Biased diode comparator

Left hand drawing is simplifled version of Figure 7 for convenience in discussion, see text Right hand drawing shows transfer function.

voltages are 1, 5, or 25 volts. The desired fractions are set to 0.2 mv. on two 10K Dekapot dials. The midpoint of the Dekapots is made to read zero volts output by adjusting the 1 K balancing potentiometer. An alternate control unit uses a biased diode comparator (BDC)as a polarity sensing bistable amplifier with variable voltage output and current capabilities for operating sensitive relays. This comparator is discussed below in connection with all eiectronic square and triangular wave generators. All functions available in Control Unit I are possible; and in addition, the output voltage to the relay is continuously variable.

about zero volts and are continuously variable from zero to A 2 volts with the zener-controlled attenuator when taken a t Elout, or may be k 2 0 volts when taken a t the output of SA. Triangular waves are generated a t the integrator output and the amplitudes are equal to the trigger level. This circuit uses a BDC with zenerregulated attenuator output as shown in Figures 6 and 7. The output voltage of the BDC assumes one of two voltages depending on the polarity of the input sum and opposite sign to the input sum. The output voltages are approximately

300 volts

is positive, or R'

Eout

6

2

--x f Rs

X 300 volts

when E!. is negative. The absolute values of R, and R'&, R f and R'f are determined by the requirements of E,, constancy and current available from the OA. The feedback resistors, R , and R ' f , must be small compared with I?,, if the output is to be independent of E:,,. On the other hand, during operation, current flows through R f or R'f to hold the summing points, SIor SZ, near ground potential. If R f or R'f is smaller than about 15K, the OA becomes overloaded. Thus, the output voltage curve as a function of input level, shown in Figure 6, depends slightly on the input. For the circuit in Figure 7 , the dependence is about '/z volt per volt input. Equal currents for equal and opposite output voltages are achieved by connecting pin 6 of the K2-XA through a 150K resistor to pin 3. A booster follower may be used after the KZ-XI, but within the feedback loop. Using the circuit of Figure 7, the nonlimiting region near E,, = 0 has a width = t 2 mv. maximum before the output voltage of *60 volts is reached.

CONTROL UNIT II-FREE-RUNNING SQUARE AND TRIANGULAR W A V E GENERATOR

Square waves can be produced using OA switching modules without the use of relays. By incorporating flexibility in output voltage and period control, the resulting square voltage waves can be connected directly to the summing input of the potentiostatamperostat, C, to give step-functional voltages or currents a t the cell. Alternatively, the square wave output may be integrated using the integrator in Figure 1, or an integrator built into the square wave generator, to produce a triangular wave a t the input of the C. Free-Running, Chopper-Stabilized Square and Triangular Wave Generator with Integrator Timing. The basic unit is an astable multivibrator with a trigger potential in the positive feedback, and a variable switching potential from an integrator in the negative feedback as shown in Figure 5. The switching voltage is controlled by the integrator and the S A which feeds the integrator. Period control is achieved by varying either the S A gain or the time constant of the integrator, or both. Since the present integrator has a series of fixed time constants, complete flexibility in period selecttion mainly depends on the SA. The square wave voltages are centered 1832

ANALYTICAL CHEMISTRY

EOUT

Figure 7. Biased diode comparator 1OOK resistors, 0.1 %, 0.5 watt, wire wound 240K, 1%, 2 wott, wire wound 1OOK, wire-wound Potentiometer Ra, Rs: 1 K Heiipot R,, Rs; 100K, 1%, 2 watt, wire wound 39K, 1 O%, 1 watt, corbon Re: 12K, lo%, 1 watt, carbon R1* R11: 1 OK Trimpot Rlr 1 OK, 10-turn, Helipot Rlr: 150K, 1 O%, 2 watt, carbon Rl,i 1 OK, wire-wound, potentiometer Ct: 0.1 pf. 200 volts, mica S W-1: 2 pale, 2 position shorting switch Z: Symmetric zener diode, Hoffman #IN822, 5.9 to 6.5 volts, 7.5 ma. maximum R f , Rzl Rs, R,:

D1*":

Da, D4;

500 volts, silicon diodes, #IN1 695

Within this narrow region the output voltage is given by the El. X amplifier gain (open loop). If the input resistors, Ria, are increased to lM, the width of the linear response region is over *100 mv. If the input resistors are decreased to 50K, the region of linear response is diminished below rt2 mv. The width of’ the nonlimiting region depends upon ,,he desired output voltage and is lees than +2 mv. for output voltages less than A60 volts. The lack of flatnear in the output voltage with respect to input variation can be eliminated or avoided by two methods; both have heen used. One is to use a zener-controlled output with potentiometer vdtage selection; or, to feed the BDC with equal and opposite voltages from a previous comparator. The latter may be a simple amplifier with either a neon bulb or double-anode zener diodes in the feedback. To zero the BDC, the diode feedback is switched out, SW-1; and a IOOK resistor takes its place. With the input grounded, the bissing potentiometor is adjusted to bring the direct output to zero. The comparator balancing potentiometer is set as follows. With the normal comparator feedback on, adjust coarse and fine tontrols to minimum settings. With alternate equal and opposite voltage3 from PV impressed on the coniparator, adjust the balance potentiometer to give equal and opposite output readings on a VTVM or a voltmeter protected by an F. Output voltage symmetry, at the zener-regulated divider, is accomplished

by achieving current symmetry from the BDC through use of the 150K resistor between the output and pin 3 of the K2-XA amplifier, and adjustment of the voltage output of the BDC with a fine potentiometer to pass proper current through the zener. This adjustment equalizes the plus and minus zener voltages which otherwise deviate slightly because of inequality in the positive or negative currents. After the BDC is balanced according to the procedures above, the output level is set to maximum output k60 volts. This setting is suitable for driving the zener-controlled output with enough current to give good regulation of the output voltages. With an arbitrary but precise input voltage to the BDC, say f1.000 volt from the PV supply, measure the zener-regulated output to 0.1 mv. With an equal and opposite input, measure the opposite output voltage. Adjust the coarse BDC output control potentiometer t o give equal and opposite zener-controlled output voltages for equal and opposite input voltages. The symmetric output of the divider is fed through a n F t o guard against overloading of the divider since the S A and the inverting amplifier (I) would draw too much current. The output of F is inverted and used as the trigger voltage a t one input of the BDC. Timing is accomplished by scaling the trigger voltage upward and driving a precision integrator, whose output is fed back to the remaining BDC input. The gain of the S A can be set by the feedback resistor selected with SW-2 and the potentiometer Rls. The gain is continuously variable from almost zero

to 10. Since the integrator time constants are fixed, periods are selected by the product of the integrator time constant and the ratio of trigger level to scaled voltage. When fully warmed (30 minutes) , the generator is capable of producing usable square and triangular waves from 500 seconds/cycle t o 100 cycles/second. Rise times are on the order of 400 microseconds per volt output a t the square wave. The amplitude of the triangular wave will equal the trigger level and can be no greater than k 2 . 0 volts. T o achieve this amplitude a t frequencies greater than 5 cycles/ second, the gain of the S A must be increased by switching into the feedback 1M resistors, or by decreasing the input resistance of the integrator. However, care must be taken t o avoid overloading the SA. Alternate all-electronic circuits using RC period control or fewer OA’s are available from the authors. LITERATURE CITED

(1) Alden, J. R., Chambers, J. Q.,Adams, R. N., J . Elcctroanal. Chern. 5 , 152

(1963).

(2) Booman, G. L., ANAL.CHEM.2 9 , 213 (1057).

(3)’BUik, R. P., Proceedings of AID Symposium, Instrument Society of America, Charleeton, W. Va., May 1962. (4) DeFord, D. D., 133rd ?leetinq, -4C’S, San Francisco, Calif., April 1958. ( 5 ) Kelley, M T., Jones, H. C., Fisher, D. J., ANAL.CHEM.31, 1475 (1959). (6) [bid., 32, 1262 (1960). (7) Philbrick, G. A. Researches, IAC.,127 Clarendon St., Boston 16, LIass. Amplifier descriptions and characteriatics available at this address. RECEIVEDfor review June 13, 1963. Accepted Auguet 26, 1963. Division of Analytical Chemiatry, 141th 1Ieeting ACS, Lon Angel-, Calif., March 1963

END OF SYMPOSIUM

VOL 35, NO. 12, NOVEMBER 1963

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