edited by
GALEN W. EWlNG Seton Hall University South Orange, New Jersey 07079
XCVI. A Practical PotentiostatCoulometer for the Student Laboratory and for Routine Research Use M. van Swaay
Kansas State University, Manhattan, 66506
lntroductlon The current interest in electrochemical processes in non-aqueous solvents has created a need for coulometric equipment with high voltage compliance, so that reasonable current levels can be maintained even in high-resistance solutions. Although commercial equipment meeting these needs is available, its cost has generally restricted its use to a dedicated research environment. To meet our own need for a simple instrument, we have developed a low-cost potentiostat/coulometer which can maintain an electrolysis current of up to 100 mA at cell voltages as high as 100 V. The number of eaulombs passed is displayed digitally in sign-plus-magnitude format with a resolution of 0.01 coulomb. so that a nrecision of 1%can hemsintained for samnles vawine" from -to 10-3 equivalent. For convenience in student laboratories, operator interaction has been reduced to an absolute minimum. Only three controls are provided: astandbyloperate switch, an integrator reset switch, and a direct-reading electrode potential control. Two such instruments have nroved their reliahilitv for over a war in ou; Isboratorv.. ~-~~~ ,~~ during which time they have met virtually all our research and teaching needs. Parta cost for each instrument is less than $250.
~.
Dr. van Swaay was born in The Haeue in 1930. received his underRrahuate education at Leyden Lnivenity in the Netherlands, and earned s Ph.D. degree in physical rhemistry at ~rineetonUniversity in 1956. upon return to the Netherlands he did ptdactoral work at Leyden.and later joined the staff at the'rechnical Cniversity at Eindhoven. In 1963 Dr. van Swaay joined the Faculty a t Kansas State University, where he now holds the rank of Associate Professor of Chemistw. Over the veata. his interests have changed from ,~~ physical to analytical chemistry, then to instrumentation, and from there to laboratory computer operations. Dr. van Swaay has served as an electronic consultant, and has recently been conducting aeries of workshops across the country in the use of microprocessors. The present paper is the third article Dr. van Swaay has written for this Series; earlier articles appeared in September 1969 (The Control of Temperature) and in June 1973 (Introduction to Microelectronics). ~~
~~
~~
~.~~~~
~~-
~~~~~
~
.
he much higher than the 1&12V available from common amplifiers. A simple highvoltage stage consisting of transistors Q1-$8 makes possible a voltage compliance of 1CQV. Pairs of transistors Q1Q2 and Q6Q7 ate used for the pass elements rather than single devices of higher rating, in the interest of economy. Output current is split evenly by virtue of the emitter resistors R14R15 and R16R17. The high-voltage stage contains current-limiting circuitry for reasons that will bediscussed later in connection with theintegrating circuitry. Zener diodes ZD1 and ZD2 will remove base drive from transistors QlQ2 or Q6Q7 11the voltage drop across atnsmg resutors R14Rl5 or R16R17 exceeds 6V, prowdrng a sharp current lrm~tof 100 mA et the auxiliary electrode.
+
Figure 1. Simplifieddiagram of potentlostat section. AI:ICLB007: A2. A3:741C: 01. Q2. Q5:MPS U60; Q3MJE721: Q4. 08. 07:MPS U10: 08: MJE711: f and j: cannectlanslo +15Vand -15V. For me sake of clarity, trimpots, protection diodes, logic clamps etc. have been excluded from all dlaorams. A comnlste set of schematics. etch oatto Cover the cost of copylng and handling.
'
,
Circuit Description The cell control circuitry is shown in Figwe 1;it is designed along conventional lines, but contains two features of special interest. Because cell current may be as large as 1CQ mA, the current through the working electrode is snlit bv resistors R1 and R2 in such a wav that 1sof the~ total onlv . ~ ~ .~ -~ ,current needs to b; handled by the current sensing a&lifier~2. The voltage drop across the splitting resistors must be accounted for in the cell control loop; this is done at the summing junction of control amplifier A3, so that Vmr - V,,*. + V.a = 0,or V,, = V,, - V,d. The second feature of interest is the high-voltage stage controlled by amplifier A3. To maintain the full current capability even when non-aqueous solvents are used, the cell driving voltage will have to ~
~
~~
~
~
~~
Figure 2. Voltage-to-frequency convener and counter contnrl circuitry. A4:ICLB007C: A5. A& 741C: AG1, AG2:IH5012 F1, F2. F37474; GI. 0274132 (Schmln bigger); all other gates:7400. The output of current sensing amplifier A2 is made available externally as acell current monitor, and also serves as input to a voltage-to-frequency converter acting as charge integrator. The voltage-to-frequency converter (Fie. . 2) consists of intemnt,or A4.limit (Continued onpoge AS)
-
Volume 55, Number 1, January 1978 1 A7
Chemical instrumentation detectors A5 and A6, charge pump switches AG1 and AG2, and associated Logic. A negative signal applied t o A4 via resistor R20, corresponding to the passage of cathodic cell current. will cause the outnut of A4 to rise. Whm the switching level of limit detector A i is exceeded, A> will enable yareGI toestablish a feedhaek path around flrpflop F1, causing it t o switch states every time i t receives a pulse from the clwk oscillator. The output from F 1 controls charge pump switch AG1, which will therefore inject boxcars of charm into inteerator A4. The Leneth of each .. Iwrcar is equal to the time betwern adjacent clock pulses, and the height oiearh hurcar is controlled hy resistor W l . Thc ~ i g ngrf the charge pulses is such that the integrator output will he brought back toward zero volts. Because flipflop F1 is edge-triggered, gate AG1 can turn on or off only a t the leading edge of a clock pulse. This guarantees that no fractional charge pulses can be applied to the integrator. As soon as the integrator output falls below the switching levelof A5, the loop around F1 will he disabled, and further charge pulses will beinhibited. Iftheoutput of A4 falls below the switching level of A5 while a charge pulse is being routed to the integrator, that pulse will end a t its scheduled time without truncation. Anodic currents are handled in a symmetrical manner by switch AG2 and flipflop F2 under control of limit detector A6; their detailed operation need not be described here. Proper functioning of the converter re~~~~~
~~
~
-~
~
A8 1 Journal of Chemlcal Education
quires that a single charge pulsecannot drive the integrator from one switching limit to the other. In addition, thecurrent thmugh AGI or AG2 during a charge pulse must be a t least twice as large as the maximum signal current through R20. As long as those conditions are met, the circuit will generate charge pulses of the nroner nolaritv a n demand. so that the inteirairr , i t p u t 411d w q s be brought bark between the lcvelsset by the limit drtectors AS and A 6 The time integral uf the cell current can then he obtained by signed summation of the output pulses from F1 and
..
lection and attendant oooortunitv far readout and set-up mistakes. In keeping with our desire to limit operating controls to the ahsulute minimum, the charge pulse count is accumulated in a bidirecuund sign-and.mawitude counter. The control circuitr, for this counter is included in Figure 2; the circuit for the counter module is shown in Figure 3. The instrument may be ~
~
.".The maximum pump rate possible with the P1
circuit components shown in Figure 2 is 100 WA.average (200 &A pump current a t 50% duty cycle). Again, with the components shown, a signal current of 100 MAinto the integrator corresponds to a cell current of 200 mA. The cell current is limited to 100 mA maximum by the limiting circuitry in the high-voltage stage, so that the voltage-tofrequency converter cannot be driven into saturation. Hecause charge pulses are generated on demand, the number of pulses generated per coulomb of cell current is inversely proportional to the length of each charge pulse. Thus control of the clock frequency provides a simple means to calibrate the response of the integrator. In our instrument the clock tuns a t about 40 sec-'. The actual frequency is adjusted so that each pulse represents 0.01 coulomb. Pulses are counted with a 4-digit counter to give a full-scale capacity of 99.99 coulomb. This allows one t o retain 1% resolution for samples ranging from 10Wtto equivalent, without any need for range se-
Figure 3. Counter and display logic. COUNT cirCuiIs:74192: DECODE:7447;display:DR2000series: sign and overflow indicator:OR2020:. OR oates: ~7432:N A ~ garep: D 7401; inveners.7405 dmoes: IN914 Drivers 101the 9 gnenoavartlaw ndlcalors conset of me two logic tbnclionr in parallel. ~
-~
used for both cathodic and anodic experiments without concern for the direction of cell current. For studies of reversibilitv. an ~, experiment may be allowed w pruceed in one direction, after which it may be reversed by adjustment of the cell control voltage, without any need to zero or change the integrator setting. The integrator output remams valid (Continued on page A101 ~~
~
~~~~~~
~~
~
~~
~~
~
~~
~~~
~
.
~
~~
Chemical Instrumentation -- -even if the integral crasses zero after a current reversal. The ~trategyfollowed in the design is as follows. If, and only if, the contents of the counter are zero, the next charge pulse, be it from F1 or F2, will cause the counter to count up. In addition, if that charge pulse arrives from F1, it sets sign flipflop F3, via gate G3, and therehy defines the PLUS condition. Similarly, if a pulse from F2 arrives when the counter contents are zero, F3 is cleared via gate G4, thereby establishing the MINUS condition. If the counter does not contain zero, the state of the sign flipflop controls the routing of pulses from F1 and F2 to the UP.L and DN.L inputs to the counter: the PLUS condition directs pulses from F 1 to the UP.L input via gate G8, and pulses from F2 will be routed to the DN.L input via gate G10. If the MINUS condition is established, pulses from F1 will go the DN.L input via gate G9, and pulses from F2 will be routed to the UP.L input via gate GI. The end result is that cathodic currents will be integrated and displayed as a positive number of coulombs, in keeping with common electrochemical practice. The sign and magnitude of the output display will remain consistent with this convention, even if the integral of the cell current passes through zero a t some time during an experiment. The counter and display circuitry consists of a 74192 counter chain, 7447 decoder1 drivers, and Numitrone digit display tubes. Recent developments in counter and display
devices have broadened the choice for such functions enurmously, hut we find the Numitmn displays attractive for the present application, because they can be read comfortably from a distance and are tolerant to a wide range of amhient light conditions. The ripple-blanking capability of the 7441 decaderldriver circuits simplifies the detection of the zero condition of the counter: if leading zeroesare suppressed in the display, the suppression signal can be used to signal the zero condition of three ofthe four digits. Thus only the least significant digit needs to be separately decoded to generate the ZERO simal reauired for the direction control circuitry at the input to the counter. Some additional circuitry is required to implement the sign display and a lamp test capability; these are also shown in the diagram. Carry-out from the counter sets an overflow flipflop, which turns on an overflow indicator and blanks the normal display. On overflow..the ootentiostat oortion continues to function normally,suthat rheinstrument may be used for generation of largeamounts of electrolysis product, e.g. for organic synthesis. Because the display is blanked out on overflow, the lamp test feature cannot be used when the counter has overflowed. For this reason. the RESET and LAMP TEST functions are con~rolledhy a single 3.position momentary actiun switch. The LAW' TEST function does not affect the contents of the counter, and may be used at any time during an experiment. To allow the use of the ripple-blanking terminals of the 7447 decoderdriver circuits for both zero suppression and overtlow blanking, open-collector logic must
-
~~
~~~
.
~
~~.
~~~~
~
be used at those points. Similarly, diodes are required to create the WIRED-OR functions for the lamp test of the sign and overflow indicators.
Construction Commercial power supplies were used for the 15V and 5V power distribution. The high voltage supply is not critical, a standard center-tap transformer circuit was used, with a Zener-controlled pre-regulator (transistom Q3 and Q8 in Fig. 1). Althaughregulation of the high voltage supply is not strictly neeessary, it tends to reduce the amount of linefrequency noise traveling around the instrument. The analog control circuitry and the counter input logic are mounted on a 4 X 4-in. "collage" hoard; a size designed to hold 16 standard logic packages, with a 22-contact edge connector, will provide ample space. The high-voltage circuitry is mounted on a pair of etched 4 X 4-in. boards with 15-pin edge connectors, The boards are etched with identical patterns, and are arranged so that each board holds one positive and one negative pass transistor. Because positive and negative pass elements cannot be active at the same time, the pair on each board may share a single heat sink, which also cools one of the pre-regulators. With this arrangement, the heat load on a single board will not be more than 7 watts under any condition, including a short connection from t h e o u t ~ uterminal t to ground. The counter and display driver circuitry is mounted on a sandwich structure of two circuit boards. The bottom board holds the drivers and display tube sockets, and the top hoard, which fits behind the display tubes, holds the counter circuitry. This sandwich arrangement is part of a package developed in our laboratory, which can be used as a display module with optional counters andlor
lat,.h.a .-.-..
All power supplies and aubassembliea fit together in a cabinet of 8 X 16 X 9 in. In sddition to the display window, the front panel contains a line switch and pilot light, an operatelstandby switch, a lamp testlreeet switch, and a switch and potentiometer to select the cell potential. The potentiometer is a three-turn model, calibrated to read directly in volts. Leads for the three electrodes as well as for a ground connection are terminated with a 4-pin connector which is plugged into a front panel socket. Total parts cost for the instrument is less than $250.
Performance Table 1illustrates the performance of the instrument. To avoid contributions from chemical effects, the measurements were made on a simulated cell as shown in Figure 4. In addition to resistors R, and R..II, which simulate the dc cell characteristics, the Of circuit contains resistors R, and R. these, R,. is the parallel combination of RI and Rz in Figure 1, and R, represents any additional resistance in series with the working electrode, such as uncompensated cell resistance and connector contact resistance. (Continued on page A12J
.
A10 1 Journal of Chemlcal Education
Chemical Instrumentation
Pertormance data on Simulated Cell shown in Flgure 4 Count 1 W sec.
R,. 30
30
1000
1000
--
300
Figwe 4. Simulated cell used to obtain the data in the tabla. Hewlen-Packard model 3476A rnultimeters were used to monitor cell current and voltage.
300
.,.,
The instrument design allows for the but not for R,. At high presence of R currents, even smallvalues of R, can significantly alter the imposed cell potential, but such effects will always be in the "safe" direction in eoulometric work. For the data of Table 1,the instrument effectively controlled R,, but the "cell the potential across R,i* ootential" was monitored across R,II .... only. . The data imply s cunraet resistnnce of 0 1-03 ohm, w h i m IS not unrrnwnable for thesimple alligator clips used.
.~
10K
10K
+
A12 1 Journal of Chemical Education
2.45
8.2
3.25 3.25 3.27 8.11 8.1 1 7.93 8.04 3.15 3.14 3.13 3.15 7.78 7.95 7.92 7.94 0.32 0.32 0.32 0.32 0.82 0.82 0.82 0.82 0.33 0.32 0.32 0.32 0.81 0.82 0.81 0.81
C~~ntl~o~lornb
1.00 1.00 1.01 1.00 1.00 1.00' 1.00 1.01 100 1.00 1.00 1.00' 1.00 1.00 1.00 100 1.00 1.00 100 1.00 1.00 100 1.00 1.00 1.00' 1.00 1.00 1.00 1.00 1.00 0.99
M B a s ~ r ~ mwere m ~ made rim an anernsting sequence of anodic and cathodic settings. Lines mama with an asterisk demonstrate the enest of contact resistsnce in the woming elestrods lead.
