+
species requiring nl and n l n2 electrons for the first and second waves, respectively, is a special case of Equation 19 in which m = 2 and C+ = 0. The relation applicable to the total transition time is thus 01
aiesp[O(n ,L -t
n)I erfIbYn (n,
+
ItL),~’..l
+
~ 2 ) I L i 2=
D ~ ’ ~ T , O(21)
The ratio of Kquation 21 to Ekjuation 16 writtcu for r1 yields the vsult )?I
Table VI. Exponential Current Chronopotentiometry of Oxygen Air-saturated 1M KC1. Teflon cup mercury 001 electrode. Preset potential, -0.02 volt us. S.8E.
ax 10 -6 25 50 50
P
71
72
0.100 0 I0200 0,0500
9.20 6.85 5.75
6.36 15.80
(n1
+ n.)
x
10-2 346 ._.
356
352 -
8.5F,
See Equation 21.
Avg. 351 zk 4
nl
FADLi2
x
Ratioa
10-2 _181 _ ~
178
1.91 .~ 2.00 1.95 ~
181 -
-
180 zk 1
1.95 zk 0.03
-+-)‘= 11 1
The ratio of the first and second transition times is thus dependent on the current parameter 0 a i d the relative number of electrons for the two waves, and is independent of C Y , concentration, and diffusion coefficient:,. The stepwise reduction of oxygen was used to examine the wefulness of the aboLe equations. Table VI shows some representative results of experiments involving use of different combinations of anc 8. The data show a reasonable cor respondence to the theoretical predictions, and the chronopotentiometric constants for the first and second waves agree well with those obtained in Tablc IV, where the same electrode was employed. APPLlCAT10NS
The precision of concentration measurements utilizing any of the above techniques is comparable with that of the common constant Icurrent method. Because of the increased complexity of the apparatus required for utilization of ((odd”power of time currents, these then
would offer no appreciable advantage in analytical measurements. It is possible, however, that the relatively simple exponential and r = 1 and 2 techniques, because of the sharper transition time breaks obtained, may prove of advantage in the study of irreversible systems. A particular case of interest would be the attainment of constant current density chronopotentiometry a t the dropping mercury electrode. The area of this electrode varies approximately with the two-thirds power of time, and a current-time function varying in the same manner would provide a constant current density, Preliminary experiments have been conducted with this technique and the results of this investigation will be reported in a future communication. LITERATURE CITED
(1) Berzins, T., Delahay, P., J . Am. Chem. SOC.75, 4205 (1953). (2) Bowers, R. C., Ward, G., Wilson, C. M.. DeFord. D. D.. J . Phws. Chem. 65,672 (1961). ’
(3) Douglas
Aircraft Company, E. Segundo, Calif., literature on “Douglas Quadratron,” 1960. (4)pierst, L., “CinBtique d’approche et reactions d’blectrodes irr6versibles,” These d1agr6gation, University of Brussels, 1958. (5) Hoffman, H., Jaenicke, Fs’., Z . Anal.
Chem. 186, 93 (1962). (6) Hurwitz, H., J . Electround Chem. 2 , 328 (1961). (7) Hurwitz, H., Gierst, L., Ibid., 128 (1961). (8) Murray, R. W., Reilley, C. N., Ibid., 3. 64 (1962). (9) ‘Ibid:, p. 182. (10) Reinmuth, W. H., ANAL. CHEY.32, 1509 (1960). (11) Senda, bf., Rev. Polarography Japan 4, 89 (1956). (12) Takemori, Y., Kambara, T., Senda, M.. Tachi,. I.,, J . Phus. Chem. 61, 968 (1957). (13) Testa, A. C., Reinmuth, W. H., ANAL.CHEW33, 1324 (1961).
RECEIVEDfor review July 10, 1963. Accepted August 14, 1963. Division of Analytical Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April, 1963. Work supported in part by Advanced Research Projects Agency Contract SD100.
A Multipurpose Electrochemical Instrument for Control of Potential or Current GEORGE LAUER, HELIAAR SCHLEIN, and R. A. OSTERYOUNG North American Aviation Science Center, 8437 Fallbrook Ave., Canoga Park, Calif.
b A multipurpose instrumental system for control of current or potential in electrochemical studies is described. The instrument is consructed of commercial high-gain operational amplifiers. Switching circuitry for precise voltage detection has been developed and used to carry out measurements between fixed potenti(31levels.
I
OUT I-esearch in the electrochemical area, we found it necessary to construct a versatile, general purpose instrumental system for the control of potential or current. In the design of thc instrument a system of high-gain orerational ampliN CARRYING
fiers was constructed. As our primary interest lay in fast controlled-potential sweeps, with simultaneous integration of the current, a switching and triggering system for control purposes was also designed and constructed. Figure 1 shows a general schematic of the system and a parts list is given in Table I. All amplifiers shown are manufactured by G. Philbrick Associates. They were chosen for high gain, high-gain bandwidth product, and stability, consistent with our requirements. For potential measurement applications, performance had to be sacrificed for high input impedance. The instrument itself is not novel in
concept. The basic circuits are essentially those of De Ford; Booman; Kelley, Jones, and Fisher; and Alden, Chamber, and Adams (1-3, 6, 8). In addition, many of the circuits may be found in the manufacturers’ literature. The switching circuitry and the applications, on the other hand, are considered novel. ilmplifier #l is used for actual control of the current or potential. The booster is for power amplification, in those cases where more than 20 ma. are drawn from amplifier #1. SW 1 removes all power from the electrodes and keeps amplifier #l in a standby, nonsaturated condition by feedback from a VO1. 35, NO. 12, NOVEMBER 1963
1789
dummy resistor. While SW 1 removes the power from the electrodes, it does not remove the potential-measuring circuitry, thus allowing relaxation phenomena to be observed. The potential between reference and controlled electrodes is measured by the differential electrometer comprised of amplifiers #3, #4, and ~ 5 .The circuitry is identical to that given in the Philbrick specification sheet (11). The reason for the use of three amplifiers is the high input impedance, and the high common-mode rejection offered by the circuit. The current is measured by monitoring the potential drop across the precision resistor placed between the controlled electrode and ground. The measurement is accomplished with amplifier #6, which can be easily modified to give a voltage gain for measurement of extremely small currents. This modification consists of placing a reqistor ( R J between the output and the negative input, and another (Rz)between the latter terminal and ground. The voltage amplification is then given by Eout = E,, (1 RI/R2). Stable gains up to lOOX may be achieved wing the P2 amplifier. It should be noted, though, that increasing the amplification decreases the frequency response of the amplifier. In those cases where unity gain is saticfactory, amplifier #6 could be omitted and the current measured a t the output of amplifier #3 in the electrometer. The potential from amplifier #5 serves as the feedback potential in the control loop. This
+
Table 1.
Symbol SWl
sw2
sw4 sw5 sw7 SW8
sw9 sw 10 sw 11 sw 12
RLY 1 RLY 2 RLY 3 RLY 4 RLY 5 RLY 6 AMP 6 AMP 2 A M P 3, 4, 5 AMP 6 AMP 7
Booster
R1 R7, 8, 9, 10, 13 R14, 15, 16, 17
S W 2 shown in controlled potential mode
potential is also fed to an X-Y oscilloscope or an X-Y recorder. SW 2 switches this feedback from amplifier 85 to #6, thereby making the current the controlled parameter. This switch also tranqfers the current path out of amplifier #1to a switching circuit which allows current reversal for chronopotentiometry. If amplifier 96 is set a t some gain other than unity, current control is more sensitive, increasing any
Descriptive Parts List
Type 3PDT 3PDT SPDT SPDT SPDT Push button, normally open Push button, normally closed SPDT (Hg switching useful) SPDT (Hg switching useful) SPST Clare Type HGA1003 Clare Type HGA1003 Clare Type HGS1048 Clare Type HGA1003 Clare Type HGA1003 Clare Type HGS1048 UPA-2 UPA-2 P-2 P-2
UPA-2
K2-X or suitable Pastoriza electronics model
R11 R12
Bo 1-4 P1
P2 P3
c1 c2
1790
1 0-Tu& 10K linear potentiometer 0.25y0 Linearity 10-Turn lOIi linear potentiometer 0,25Y0 Linearity IO-Turn 1OK linear potentiometer 0.25y0 Linearity 0.1 Plug-in capacitor 0.1% Plug-in capacitor
ANALYTICAL
CHEMISTRY
Function Master Controlled i or E Bias anodic or cathodic Bias anodic or cathodic Ramp anodic or cathodic Initiate sweep Reset sweep Bias on or off Bias on or off Manual sweep "hold" Current reversal Chrono--"dummy or cell" Automatic sweep halt Sweep control Analog integral control Analog integral hold Control Sweep generator Potential measurement Current measurement Analog integration Dummy Adding network Potential meamrement network Integral RC constant Sweep RC constant Bias voltages Bias Bias Sweep speed Sweep RC constant Integral RC constant
instability in the overall control loop. This instability causes oscillations which, if not damped, drive the amplifiers into saturation. Damping can be accomplished by placing a low-leakage capacitor between the positive input of amplifier #1 and ground, and another between input and output of amplifier #la The values of these capacitors must be kept small (less than 0.001 Bf.) for good frequency response of the whole system. The control circuitry for amplifier #1 was designed in the configuration of an analog adding circuit using 50,000ohm resistors. This configuration has the advantage of presenhg a low-impedance source to the input of amplifier #I which reduces the noise, and allows various control potentials to be simultaneously applied. The two potentiometer circuits apply constant potentials which add linearly. Amplifier #2, wired as an integrator, serves to generate a voltage ramp which is fed into the adding circuit of amplifier #l. Addition of another control potential can easily be accomplished by placing another 50 K resistor a t the summing point of amplifier #1 and feeding the potential across this resistor. Thus, for example, a small alternating voltage may be superimposed on a slowly varying potential for a.c. polarography. It should be noted that all switches as shown must be free of contact bounce. It has been found advantageous to use mercury switches or mercury contact relays for those cases where a "clean" vdtage step is required.
Our first interest in using the system lay in single controlled-potentialsweeps, with rapid determination of the integral of the current between two sharplydefined potential limits. [For details of this technique and experimental details see (9, IO).] Figure 2 shows the requirements set for this type of experiment. Since the experiment was conducted on an expanding mercury drop,
“r
PA - E vs S
90
75
1.0
5mM Cd Clp SUPPORTING E L I - C T R O L Y T E SWEEP H A T E ! i S V / S E C
I N Na Clod
A
/
,’
COUNTER CONTROL GATE
60
45
gineered Electonics, Inc., Santa Ana, Calif.) which accepts only positivegoing pulses. In the experiments, the first detector is set to fire at -0.5 volt and the second at -0.75 volt. (Note that these voltages are with respect to the S.C.E.) When the first trigger fires, the output of the flip-flop goes from 0 to -11 volts, activating the counter; this output returns to 0 volt and deactivates the counter when the second trigger fires. During the return of the potential to its initial value, the triggers switch at their respective set potentials, but the flip-flop remains inactive as both pulses from the triggers are negativegoing, and are therefore rejected. Thus the output of the voltage-to-freyuency converter is sampled only during the preset potential interval of the cathodic sweep. For anodic sweeps the inverse outputs of the triggers must be used. This is easily accomplished by the %lope” switch as shown. Because of the method used to bias the detectors they may be fired a t any voltage from +6 to -6 volts, including zero. I t is important, though, when the flip-flop is used, that the opening pulse come before the closing pulse, or the circuit will seem inoperative. The triggers fire a t the set potential with a rise time of 10 pseconds and an error of i1 mv. in the firing potential. Because of their great sensitivity it is important that the input voltage be noise-free since a voltage transient of 10-pseconds duration will fire the system. If a 1000-ohm measuring resistor is used with the currentmeasuring amplifier set at a gain of 10, and if the voltage-to-frequency converter is set a t full scale of 0.1 volt, then one pulse is equivalent to 10-lo coulomb.
CE
.6
-IIV
S W E E P HALT RELAY VOLTAGI:
ov
30
I - IIV I5
Figure 2. Current, tirne, and voltage relationships between sweep, measuring, and switching circuits Upper scale of abscixo is volts
VI.
S.C.E., lower scale is time, seconds, from initiation of sweep
it was necessary to remove the drop using a solenoid-operated hammer. Activating the solenoid initiated a precise time delay of about 10 seconds, a t which time the pot3ntial sweep was triggered. (See below.) The resulting current peaks and then drops off. The integral of the current between the indicated potential limits was the quantity of interest. Two methods of obtaining this quantity are available, one based on standard analog computer practice, the other ,t digital method. The digital method, though more costly, offered great3r resolution and greater speed in readout. The output of the current-measuring amplifier was fed into a voltage-tofrequency converter 1 Model 240, Vidar Instruments, Palo Alto, Calif.). This instrument gives a frequency output which is a linear function of the input voltage with a lineuity better than 0.25%. The full-scale frequency is 100 kc. with the full scale adjustable from 0.1 to 1000 volts. The frequency output is fed into a counter (Computer Measurements Corp., Type 226B) which had been modified sc that it would accept input signals oily when a -11volt signal was applied. This modification consists of adding a wire from the output of the RST flip-flop (see below) to the “auto” side of the gating switch. The counter has one mode in which no time base gate is present. The -11volt signal from the flip-flop “opens” the gate for counting A voltage-sensitive trigger with a fast rise time was developed to control the counter so that it would accept signals from the voltage-to-frequency converter only during the preset potential interval. The trigger, shown in Figure 3, consists essentially of two Type 1103 plug-in units manufactured by the Engineered
Electronics Corp. of Santa Ana, Calif. The resistors and the potentiometer apply a reference voltage which is compared with the input voltage from the potential-measuring amplifiers. Depending on whether the input is greater than or less than the reference voltage, the output a t pin 8a is -3 or -8 volts. This voltage is directly coupled into another 1103 plug-in which serves as a pulse-shaping device. The two outputs a t pins 7b and 8b give voltage levels of -3 and -8 volts and switch to the opposite level when the output a t pin 8a switches. Figure 4 shows three such triggers arranged for the experiment described. The output of the first two triggers is fed into a standard RST flip-flop (En-
2 2K
0 , 15K3 IN
@IOK
@
2 2K
2 2K
:
-
4
220
3
OOUT
2 2K
