Operational amplifier circuits for controlled potential cyclic voltammetry. II

JOURNAL

152

OF

ELECTROANAL+TTCAL

CHEMISTRY

OPERATIONAL AMPLIFIER CIRCUITS FOR POTENTIAL CYCLIC VOLTAMMETRY. JOHN

R.

Depadmmt

XLDEN,*

JAMES

of Chemistry,

Q_ CHAMBERS

University (Received

AND

of Kansas.

lMay

zznd,

CONTROLLED II

RALPH

L.awmce.

N_ Kan.

ADAMS (U.S.A.)

rg6z)

We wish to describe as briefly as is possible several practical, inexpensive, operational amplifier (OA) circuits that are particularly useful in single sweep and cyclic Specific adaptations of OA’s to electroanavoltann-netry at stationary electrodes. lytical instrumentation were made some time ago by BOOMAN and coworkers1 and among others. Highly finished polarographic instruments have been DEFORD* of circuit designed by KELLY and coworkers 3.4. No claim is made for originality design in the instruments described herein. They are functional, devoid of multipurpose appendages, and specifically oriented towards solid electrode applications. Their reliability has been proven by continuous operation for several years in a variety of modem electroanalytical operations_ The units function satisfactorily with aqueous or non-aqueous electrochemical systems. The instruments consist of the basic O-4 potentiostat coupled with a cyclic voltage scanning input. Two forms of cyclic scanner are described, a mechanical or motor driven unit, and an OA integrator network which is more versatile. We are convinced of the utility of cyclic voltammetry and, since any cyclic unit can be used for single sweep operation, we see no purpose in building single sweep polarographs as such. A.

OPEX4TIONAL

AMPLIFIER

POTENTIOSTAT

AND

POWER

SUPPLY

Figure I shows the heart of the instrument, the potentiostat. The “E Amplifier” maintains the desired potential between the reference (Ref.) and controlled (Cont.) electrodes_ The “I Amplifier” maintains the controlled electrode at amplifier ground potential while providing an output voltage proportional to the current. The recorder output signal (current measurement) is designed for either 5 or IO mV span recorders_ The voltage between the yellow and green scanning -potential jacks is the same polarity and magnitude as the voltage applied between the controlled electrode (green binding post) and reference electrode (yellow binding post). The potentiostat can be used with fixed input voltage for recording current-time curves at. constant potential in linear diffusion studies. Alternately, by connecting a resistor between Cont. and Ref., _a constant &rent becomes available between can be carried out. Ref- and the auxiliary elecfrsde -(Azcx) ; i.e., chronopotentiometry * Electronics

Associate,

-Department

of Chemistry.

J_ R. ALDEN,

J-

Q.

CHAMBERS,

R.

A.

ADAMS

=53

For these cases where the input voltage is constant, we have found it desirable to stabilize the E amplifier with the Kz-PA unit (it is desirable to have the 1 amplifier stabilized with its K2-P unit at all times). When, however, the potentiostat is coupled with the scanning unit to carry cut controlled potential voltammetry, the K2-PA in the E amplifier’should be-replaced

YESI- H

‘OPERATE

rJ-____, Aur.electrode

I

1 t I

I

10K f

I

AMP

ContralIed electrode

Fig. I_ Operational amplifier potentiostat: RAL, amplifier balance adjusts, coarse I M, fine ro K; TEST, test jacks for amplifier balancing; CR, current range switch with following 1% precision resistors and corresponding full scale current ranges: RI. IO M. T ,uA; R z. 5 M, 2 PA; R 3. 2 3X, 5 PA; R 4. 1.42s M. 7 pA; R 5. 1.0 M, IO PA; R 6, 667 K, 15 PA; R 7. 500 K. 20 p-q; R 8, 333 K. 30 PA; R g, 200 XC. 50 pA; R IO, 133 ICI. 75 PA; R xr. IOO II;. IOO pA; R 12. 30 K, op200 PA: R 13. 20 K , goo p-4; R 14, IO. K, I mA; E AMP, voltage amplifier, Philbrick K2-XA, tional K2-PA and associated balance network; I AMP. current amplifier, Kz-XA. K-ZP and associated balance network; REC, recorder output for 5 or IO mV recorders; 5 + , 5-way binding posts; 0. banana jacks.

by the dummy plug which shorts pins I and 6 of the K2-PA socket. With the Kz-PA removed, the input impedance from whatever type scanning unit is used may be quite large (so-xoo klR) and the sweep frequency can also be quite high. If the K2-PA is not removed, the source impedance from the scanner must be less than about 500 SL to avoid introducing 60 cycle ripple caused by the chopper stabilizer. While there are many other ways to circumvent this problem, the simple removal of the- Kz-PA for scanning operations has been found quite satisfactory. (Clearly if one plans to do only potential sweep operations, the KC+-PA is’ unnecessary and could be left completely out of the circuit_) The errors in unstabilized- operation should be less than 2-3 mV for the normal time intervals used in scanning. In-the test position; the amplifiers aire balanced by adjusting the individual balairce pots to approximate zero voltage at the test jack-positions. In the test ~&&ion, J.

EZectroamzZ.

Chem..

5 (1963)

152-157

AMPLIFIER

154 the

amplifiers

used

are connected

CIRCUITS FOR CYCLIC .VOLTAMMETRY with

a gain-of

ioo,

so an ordinary

multimeter

can

be

for balancing.

An excellent by BAIRD.

discussion

of

OAP’s-

is contained

_in the .Gew

instrumentation-

.teut

The power supply for the potentiostat (which also powers. the triangular -wave generator of Fig. 4) can be a Philbrick R-100 B, which will power at least 3 potentidstats or Grnilar devices. Alternately, one can build a power supply similar to the Philbrick R-100 A, which is no longer commercially availableB. CYCLIC SCANNING UNIT with sweep rates-of The utility of cyclic volt axnrnetry using pen and ink recording ca_ o-z--IO V/rnin for inirestigating complex organic electrode reactions at solid electrodes has been discussed6. The cyclic scanning v+tage is a relatively slowly varying isosceles triangle which czn be conveniently generated either by motor driven slide wires

or electronic

integrator

networks-

ON- OFF

I

X

RY coil

X

-l

.

NC 0

+> I

RY

0

ONC I

c NC 0 ’

T

ON0

NO

1 i I I

X-

MS1
NC

!!<

NO MS2

NO

(NO)

’

To

micro&itches

: I I Lead

screw

assembly VS

r

To potentiostat

Fig_ z. Elec&mech~caJ txianghla.r_wave generator:_Jk. 8 _wa.tt; Bristol‘S30-SD;. I is V. .&I cyclesf :IO rev./&.

-

0

reversible, sytichroxious motqr. typi&l. -RY.. relay._~ tqpical; Guardian -El-c

G-.Sj4qq, _II~_V, 60 cyqles. _DPDT;c~MS;. and MS?. n&rq$~titchs, .typi+zHBT-I-; coupling~t_~ scanning helipot, hand kdju+ablti for.sweep_ratesl-VS,.voltage sx*~p e a-tied ; H:‘I o tirn.. 0.:I Ojo'liae'drie h&Fti_ot-br equivalent _

Gear-train, uni‘t, -n&I

Figure 2‘ shows -the ~‘elec&l&n&tioik of the -motor- driven unit. ‘The syrichror nous motor eves a Io_tu&r helipot which ~rovi&s~th~Swe~p voltage. The:‘motor is coupled -through variable gear- ratios (hand- adjustment). td a- lead screw assembly (Servomechanisn& Inc.) which then enga&s i-nicroL~ switches at both ends -of -its travel. --The- micro switches and- latching relay provide the motor reversing action: The micro switches are adjustgble -for v~arying the sweep. width (voltage range). Figure 3 is a-diagram of the mechanical layout of the scanner. Surprisingly little-lag DE

JTS -IX

8

8

Fig. 3- Mechanical layout of motor driven triangular wave scanner: DB, dr&ed board, -typical “Servoboard”, Servo Corp. of America (masonite Pegboard having 3/16 in. holes on I/Z in. centers. available from- Country Workshop, Newark, N. J_, can be used and all adjustable shaft hang-. etc. fit this Pegboard well) ; SH. shaft hangers; MS, micro switches, adjustable positions; H. helipot; HMB,. helipot mounting block; M. motor; MMB, motor mounting block; GS. gear shift, MMB adjusted manually along gear ti; ISA, lead screw assembly, typical 0.5 in. travel per rev. (Servomechanisms, Inc.) ; S. 1/4 iiz_ diam. stainless steel shaft; JTS, Jones ter&inal strip; All gears were common brass spur gears (Boston Gear, Co.); All batteries. wiring, etc. not shown is below DB or on dial plates at the side. exists in the reversal and the unit functions -very well for moderate sweep rates. Sweep. rates are very simply gdjusted by-the total span voltage applied to 4he h&pot with intermediate adjustments via the gear -couplings. Sweep. rates between o._%-V and g V/mm have-been used successfully_ Undoubtedly many other -electromechai , nical triangular wave SCanning devices can be de&red.2. OA

-in+grator.

The integrator

tri&gzda+

network

wave

generator

is- more versatile,

providing,

span width,

J_. ElectroanaZ_~

CAem:

sweep rate ad~ 5 (1063)

_152--15‘7

AMPLIFIER

156

CIIRCUITS

FOR

CYCLIC

VOLTAMMETRY

justment, etc., by mere dial changes. Figure 4. details the- circuit.;The scanner is coupled to the potentiostat via the yellow and green output &kiGnals indicated in the upper right comer of Fig. 4. Sweep rates up to- cu. 15 :V/ikin are available. The various panel adjustments are indicated in the legend of-Fig_ 4_ The push button controls initiate scan and allow manual reversal when desired. In adjusting the IOO K helipots for scan limits, the “+ limit” (cathodic limit) should not be allowed to become negative with respect to the “limit” setting. RI

R2

10,uF I I

RJ

ZOOV

R4 R5

E

R6

2K

1K

1K

1%

e a

E

A7

46K 1%

$

RB RS RIO Rl

. 1

--

__----

--A--_

Cathodic *

Limit

1

operate Fig. 4. OA Triangular wave generator: SI. SPST toggle switch: and starting conditions are preset on relays RYr and RYz, etc.; RYr relays; RYI for direction of sweep, when LI on, sweep is cathodic going; rela_v, when L.2 is on, sweep is operative; RI-RII. sweep rate resistors, all Rz. 402K; R3. 268K; R4. 2o1K; Rs. r61K; R6. 107K; R7, 80.6K; RS, 46K; RII, 40_33K; 2.2, output to 5 mV recorder.

_-3mv

to initiate sweep after RYz, 6.3 V as 3PDT RY;?. sweep operating I%, I watt: RI. 804K; 64-5K; RQ. 54I-C; RIO,

The + 150 V supply for the cycling relays is not shown on Fig. 4. This power supply is a single half-wave rectifier with capacitor filter (50 pF) using a Stancor PA 8421 power transformer or equivalent_ A standard 150-500 mA silicon diode serves as the rectifier. Since this supply is completely non-critical it is not included in the circuit. In the output bias circuit (voltage source for the 20 K, IO-turn potentiometer in the upper right of Fig. 4), a voltage of 1.000 V should be adjusted between points (A-B) and (B-C). While there _is no question that the ah-electronic scanner h-k greater versatility, the breadboard style electiomechanical device is more easily constructed and used by workers who ark _beginning to use the apparatus. Excellent‘ results- have been obtained with bothscanning units. /_

EZectroanaZ.,

Chem,.

5 (1963)

152~157

J_ R.

ALDEN,

C.

JT- Q_ CHAMBERS,

RECOWING

R_ A.

157

ADAMS

EQUIPMENT

X-Y recording .is by far the. most satisfactory means of recording- cyclic polarograms. If conventional Z+&-tin-& or strip. chart recor&ng is used, one must- f&ld the paper over. to compare forward and- reverse sCans (while inconvenient, there is nothing ir&erently wrong with such a procedure)_ If only single sweep operation is used, then X-time recbrding is satisfactory_ Nominal pen response of at least' I-see should be available on the reco;der_ A -variety of X-Y recorders are commercially available_ The Moseley Model 3 (which also has an X-time provision) has proven very

sat&factory. ACKNOWLEDGEMENTS

This work AT(II-I)-686

was supported by the and by the Air Force

.Atomic through

Energy Commission through Air Force Office of Scientific

contract Research.

REFERENCES 1 2 3 4 5 6

G. L. BoonrAN, Amal. I%v?z., zg (1957) 221; 31 (1959) IOD_ D. DEFORD, Absf~ucts, 133rd A.C.S. Meeti*zg, San Fvamzisco. rg58_ Anat. Chem.. 31 (1959) 1475~ IX_ T_ KELLY. H_ C--JONES AND D. J_ FISHER, M.-T. KELLY, D. J_ FISHER -45~ H. C. JOXE.S, ibid., 32 (1960) 1262~ E_ J_ BAIR, IxCrodrrclion to ChemicaE Instrztmemtation, McGraw-Hill Book Co., G. GALUS, H. Y. LEE AND R_ N. ADAMS, J_ Electroanal. Chem., 5 (1963) 17~ J_ Electroamal.

Chem.,

N-Y.,

5 (1963)

1962.

152-157