Versatile Solid-state Potentiostat and Amperostat Alvin L). Goolsby and Donald T. Sawyer Department of Chetnisti*y,Unicersity of California, Riverside, Calif. 92.502
USE OF OPERATIONAL s mplifiers has permitted the development, during the past decade, of a number of reliable high quality electrochemical instruments (1-4). The early work of DeFord (5)in 1958, elthough never published, has provided the basis for much of this instrumentation, and has served as a model for the effective use of operational amplifiers. Because a n upgraded version of DeFord’s “Universal” electrochemical instrument has not been developed, this has led to the design and constrL ction of a versatile potentiostat and amperostat based on solid-state operational amplifiers. The development of this instrumentation also has been motivated by the deficiencies of commercial instruments and by the limited versatility of most specialized designs in the literature (6-9). The resulting instrument is capable of performing a wide range of electrochemical functions merely by adjusting a few controls, is light and ccmpact, and is easily maintained at its optimum performance, Its versatility and ease of operation make the instrument ideally suited for the multiple needs of electrochemical research. This multipurpose instrument is believed to be a reasonable compromise between the highly specialized instrumental designs (6-9), and those describing complex and expensive instruments (IO,11). DESIGN OF INSTRUMENT
The basic circuit diagram for the entire instrument is presented in Figure 1. The symbols are either identified in the legend or represent operational components that are discussed in detail in the fcllowing paragraphs. Follower Amplifiers (Fl and F2). These are both stabilized noninverting amplifiers with a voltage gain of unity, each incorporating a Philbrick P-25A differential operational amplifier. Inverting Amplifier ( I ) . This is an amplifier with a gain of minus one, incorporating a Philbrick P-65A differential operational amplifier. An additional inverter independent of the main circuit also has been added, using a Philbrick P-75 A U amplifier. Access to this is external, with the intent of having an adding inve1ter for electrical measurements and specialized applications. The inverters employ 100-kohm precision resistors (10.1 %) for the input and feedback resistors. Capacitors e.re provided that can be switched in parallel with the feedba:k resistor to provide damping. This is a variable reCurrent Measuring R.esistor (I?,& sistor composed of a 12-position switch and 12 precision (1) G. L. Booman and \Y. B. Holbrook, ANAL.CHEM.,37, 795
(1965). (2) G. Johansson, Swtisk. Keh. Rdskr, 77, 76 (1965). (3) V. R . Loodma, P. K . Loog, U. V. Palm, V. E. Past, and V. A. Reeben, ZIT. Fiz.Khim.,38, 1374 (1964). (4) Operational Amplifiers Symposium, ANAL. CHEM.,35, 1770-
1833 (1963). ( 5 ) D. D. DeFord, 133rd Meeting, ACS, San Francisco, Calif., 1958. (6) A. J. Bard, ANAL.CHEM. 38, 88R (1966). (7) D. N. Hume, Ibid.,p. ;!61R. (8) D. K . Roe, Ibid.,p. 4tlR. (9) J. J. Stock, Ibid.,p. 452R. (10) Symposium on Elecl roanalytical Instrumentation, Ibid., pp.
1106-48 (1966). (1 1) E. R . Brown, T. G. RlcCord, D. E. Smith, and D. D. DeFord, Ibid.,p. 1130.
Figure 1. Basic instrument circuit diagram, controlled potential mode R,-14 K, w.w., 0.1 %, 0.5 W; Rb, Re-1 K, W.W., O.l%, O S W; Samain function switch (controlled current-controlled potential); sbsecondary function switch (integrate current-sweep voltage); S,sweep direction (- or +)
resistors (&0.1%) to provide a total resistance from 20 ohms to lo6 ohms. Control Amplifier (C). This is basically a n inverting amplifier with an external feedback loop such that the amplifier can control current or potential in the closed-loop by gain variation; the circuit is illustrated in Figure 2a. The control amplifier is based on a Philbrick P-65A differential operational amplifier and incorporates two boosters, either of which may be switched into the external feedback loop. One is a Philbrick P-66A booster follower, capable of currents as high as *lo0 ma a t 1 1 0 volts, and the other is a Philbrick 6154 booster amplifier capable of = t l 0 ma a t i 1 0 0 volts. The unit uses a Daven SPDT shorting switch t o change the operational mode from internal to external (“offon”). It also incorporates a (+15)-0-(-15) volt d.c. meter wired before the boosters, which constantly indicates the state of control of the amplifier. A five-position switch provides for insertion of damping capacitances from 0 to 0.1 kf. Additional connectors are located externally for access to the control amplifier inputs. D. C. Voltage Source (VS). This unit gives an accurate, continuously adjustable voltage from 0 to 1.2000 volts, or from 0 to 10.000 volts i 3 mv, for biasing electrode potentials in controlled potential studies, or for controlling current levels in chronopotentiometric studies. As Figure 26 indicates, it incorporates a Philbrick P-65A differential operational amplifier wired as an inverter, with 115.000 volts from the power supply being switched through 150 kohms or 15 kohms to the input of the P-65A, and a precision rheostat (10.03%) continuously variable from 0 to 12 kohms working as the feedback resistor (Dekastat, Model DS1265). Damping capacitances from 0 to 0.1 pf are provided. Integrator-Sweep Generator (ZNT). This component, which allows either integration of the current flow in coulometric studies or generation of a scan potential for voltammetric VOL. 39, NO. 3, MARCH 1967
41 1
IN
Rl
IN IN
studies, is illustrated by Figure 2c. It utilizes an RC time constant arrangement with a IO-pf precision polystyrene capacitor, and precision input resistors varying from 99 kohms to 30 megohms ( 1 0 . 1 %except for 10 and 30 megohms which are + 1 all switchable in series to give a wide range of time-constant values. The integrating amplifier is a Philbrick P-25A differential operational amplifier; this unit is followed by a Philbrick P-65A amplifier wired in the inverting mode to give an output voltage of the same polarity as the input voltage to the integrator. The gain of the inverter also is adjustable from 1.00 to 0.10 in four steps (multiplier) to give greater flexibility in voltammetric sweep rates. A low level, current compensation drift-adjustment is included, as well as a (+15)4-(-15) volt d.c. meter wired to the output, which informs the operator of the integrators proximity to its operating limit. A double pole-triple throw Daven switch allows the operator to set the integrator at any of three modes; “reset,” “hold,” or “integrate.” Voltage Dividing Networks (VN1 and VNZ). These are both potentiometer type networks which provide a resistance variable from 100 ohms to 100 kohms ( 1 0 . 1 %) to the input signal, with 100 ohms of this quantity tapped as a recorder input. Thus the input signal can be divided to give a more easily measured quantity for a fixed-range recorder. Power Supply (PS):The Philbrick PR-300R dual power supply provides a precise voltage of i15.000 volts (10.01%) with a stability of 3 ~ 0 . 2 5mv and a current capability of 1 3 0 0 ma (12). Two other power supplies are required to power the Philbrick 6154 high voltage booster (Elcor Model AZ-120-50). Figure 1 shows the complete basic circuit of the instrument set to control the potential of the working electrode. As it is shown, the circuit provides for 11.000 volt from Rb to the integrator input. The integrator, when switched from “reset” to “integrate,” applies a voltage ramp, of the polarity set by S, to the control amplifier which maintains the potential of the working electrode at EIST E,, us. the reference electrode used. The current flowing through the
z),
IN
OUT
IN
Figure 2. Circuit diagrams for the control amplifier, signal generator, and integrator-sweep generator Unless otherwise indicated, resistors are wire-wound, 0.1 and 0.5-watt rated
tolerance,
a. Control amplifier R1, R?,R B IRa, Rj,RB-lOO K; M1-(+15)-0-(-15) volt d.c. meter, (+0.25)-0-( -0.25) ma; Si-DPDT booster selection switch; capacitance (0.000, 0.001 uf, S2-SPDT off-on switch; C-damping 0.01 uf, and 0.1 uf)
*I5
0 -15
OUT
+
b.
D.c. voltage source
R1-12 K variable resistor (Dekastat); R8-15 K ; Rs-l.50 K ; S3double pole 5-position range switch ; Cd-damping capacitance (0.000,0.001 rf, 0.01 pf, and 0.1 pf)
-15
t15 I
1
(12) Bulletin on Models PR-300 and PR-300c Regulated Dual Power Supplies, George A. Philbrick Researches, Boston, Mass., February 1964.
. . . .I . . . 1
I
I
-
OU?
c. Integrator-sweep generator Rio-99 K ; Rii--100 K ; Ri2-300 K ; Ria-500 K ; Ria-1 MEG; Rlj-2 MEG; R165 MEG; Rii-10 MEG, 1.0%; R18-30 MEG, 1.0%; Ria, R?o-50 K, 1.0%; R?i,R?z22 ohms, carbon, l.O%, 1.0 W.; R23-600-ohm potentiometer; Ria-70 K, 1.0%; R2,-l IC, l.O%, 2W; R26-O to 2 kohms wire wound potentiometer, gain adjust; Rli, Rz3-100 K ; R2o-50 K ; &-30 K ; &-lo K ; CI-10.0 pf, polystyrene, +O%, - 1 % (Southern Electronics Corp., Burbank, Calif.); Cd-damping capacitance (0.000, 0.001, 0.01, and 0.1 rf); M~-(+15)4-(-15) volt d.c. meter (+0.25)-0-(-0.25) ma; S4-single pole, 10position time-constant selector; Sj-single pole, 4-position time-constant multiplier switch; S6-dOuble pole, 3-position integrator function switch
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ANALYTICAL CHEMISTRY
I1 c
B
a C
D
0
G
H
__.. _Figure 3. Circuit diagram of potentiostat-amperostat in current control mode
cell may be measured or recorded at VN2. If Sb is switched to the left, current passing through the cell will be integrated and read out at VN2, while the working electrode potential is held at E,, os. the refcrence electrode. The rate of current integration is dictated by the magnitude of current flowing, the value of R,, and the time constant setting of the integrator. To perform cyclic voltammetry with current integration, a function generator such as the Hewlett-Packard Model 202A is connected to an external jack of the control amplifier. It is adjusted to deliver a triangular wave of the correct magnitude, and the d.c. voltage source ( V S ) is set to compensate for any d.c. offset. While the electrode potential is being scanned, Sb is set to intrsgrate the current. If current control is desired, switch Sa is turned to the left to give the circuit shown in Figure 3. In this mode, the operation of the instrumeqt is simpler. Chronopotentiometry is performed by adjustuent of R, and VS to give the desired current level; the control amplifier is switched to “on” and the potential of the working electrode cs. the reference electrode is recorded at VN,. Currents varying linearly with time are obtained by the us(: of a triangular function generator connected to a control amplifier input. The front panel of the instrument with its various controls and jacks is illustrated in Figure 4; additional controls are located on the back of the instrument. The latter are for infrequent adjustments of the damping and of the zero drift for the various components. The amplifiers should be zeroed every six months using a sensitive d.c. vacuum-tube voltmeter. PERFORMANCE OF INSTRUMENT
The described instrument is capable of performing almost any electrochemical stu 3y requiring control of potential or of current. It has been used for measurements based on polarography and volt 2mmetry, cyclic voltammetry, controlled potential electrolysis and coulometry, chronopotentiometry and chronoaniperometry, coulometric titrations, potentiometric titrations and pH measurements. The results of these experiments confirm that the instrument is capable of providing reliable and accurate data. For example, accurate resu ts have been obtained performing cyclic voltammetry of ferricyanide ion at a platinum electrode using a moderate scan rate. Voltage sweep rates from 0.2 mv/ sec to 1 volt/sec are possible by appropriate adjustment of the integrator time constant. Higher scan rates (up to 200 volts/ sec) are possible by connecting a triangular function generator to a control amplifier input. The instrument has been tested further by performing chronopotentiometric measurements on K3Fe(CNh in 0.5F
Figure 4. Front panel of the solid-state potentiostat-amperostat A . Controls for voltage and current boosters; B. On-off switch for instrument; C. Recorder jacks for current voltage divider network; D. Attenuation switch for current sensitivity; E. Dekastat for d.c. voltage source adjustment; F. Control switch for d.c. voltage source polarity and order of magnitude; G. Attenuation switch for EMF sensitivity; H. Recorder jacks for EMF voltage divider network; I. Coaxial connecting jacks for electrode connections; J. Sweep direction for potential scans (&); K. Main function switch for controlled potential or controlled current (Sa); L. Secondary function switch for current integration or voltage scan (&); M . Coaxial connecting jacks for electrode connections, (duplicate set) ; N . Control amplifier output voltage, zero to 1 1 5 volts; 0. Integrator output voltage, zero to f 1 5 volts; P. Independent inverter connectors; Q. Current measuring resistor adjustment; R. Integrator time constant; S. Time constant multiplier; T. Integrator function switch, “reset,” “hold,” and “integrate;” U. D.c. voltage source access jack; V . Additional control amplifier inputs; W . Control amplifier “on-off” (or external-internal) control; X. Control amplifier damping adjustment. Additional controls located inside the housing on the back panel of the chassis; (1) Indeperident inverter damping adjustment. (2) Integrator trim adjustment. (3) Integrator damping adjustment. (4) Integrator drift adjustment. (5) Inverter damping adjustment. (6) Signal generator damping adjustment. (7) Power connector
KCl using a gold electrode; the results are identical to those obtained with vacuum tube-type chronopotentiometric instrumentation. In addition controlled potential coulometric measurements have been compared with the results obtained using a tube-type potential controlling instrument based on the original DeFord design (5). The results are the same for the two instruments. Additional experiments have been performed using an oscilloscope and function generator (Hewlett Packard Model 202A) to test the response of the apparatus. The voltage response at a 500-ohm “dummy” cell for a one-volt step input after turning the control amplifier on is 10 msec with the control amplifier damping switch set at 0.001 pf. With a 200-ohm “dummy” cell and a 100 cps applied square wave, the voltage response is close to 1 msec with the control amplifier damping set at 0.001 pf. A synthetic cell, consisting of a 10-pf capacitor across a 200-ohm resistor, has been used to test the high current booster, with the ends attached to the working and auxiliary electrode connectors and with a 20-ohm resistor placed between the reference and auxiliary electrode connectors. With a square wave of frequency 100 cps applied LO the conVOL. 39, NO. 3, MARCH 1967
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trol amplifier with the high current booster switched in, the working electrode potential was measured, with the control amplifier damping set at 0.01 pf. The electrode potential follows the imposed square wave well, but the same system with the control amplifier damping at 0.00033 pf and an imposed 20 cps square wave yields a wave whose shape is distorted by ringing. Fast scan work is possible, but requires careful adjustment of the damping capacitance. The circuit diagram in Figure 1 is not necessarily the ultimate design, but it illustrates an approach that provides a useful and convenient instrument with considerable versatility. A number of alternatives in terms of construction are possible. For example, the cost of the instrument, which for
parts is about $1800, can be reduced by the use of utility grade operational amplifiers; this will not impair the performance noticeably at ambient temperatures. Also, if the performance of the integrator-sweep generator is not crucial, a Philbrick P-75AU amplifier can be employed in place of the P-25A. On the other hand, if the performance of the integrator-sweep generator holds top priority, a Philbrick P-2A operational amplifier should be used for this component. RECEIVED for review September 12, 1966. Accepted January 5, 1967. Work supported by U. S. Atomic Energy Commission, Contract NO. AT(l1-1 j 3 4 , Project No. 45, and by National Science Foundation, Grant GP-4303.
Use of lsosbestic Point as a Basie Line in Differential Spectrophotometry Norman A.
Shane
Bristol-Myers Products Division Hillside, N . J . Joseph
I. Routh
Department of Biochemistry, University of Iowa,Iowa City, Iowa
METHODS FOR THE QUANTITATION of organic compounds often employ measurement of the absorbance or absorptivity of solutions in the ultraviolet. When the absorption spectra of organic acids are determined at different pH values that include molecular and ionic forms of the acid, isosbestic points are frequently observed. Absorbance values at the wavelength of the isosbestic point do not assist in the determination of the ratio of the molecular to the ionic form of the acid or the absolute amounts of each form. Quantitative measurement, however, may be possible when differential absorption spectra of organic acids are employed. To illustrate the principle of the use of the isosbestic point as a base line in an analytical method the determination of acetylsalicylic acid in the presence of salicylic acid will be considered. Salicylic acid solutions when examined at pH 9 (monosodium salicylate) and at pH 13.5 (disodium salicylate) exhibit a shift in their spectral absorbance curves ( I ) . When a series of concentrations of salicylic acid are used to prepare differential absorption spectra (monosodium salicylate in the reference cell rx. disodium salicylate in the sample cell) two maxima at 246 and 319 mp, two minima at 233 and 283 mp, and two isosbestic points at 268 and 300 mp are observed. If differential absorption spectra of acetylsalicylic acid solutions are prepared by the same procedure (monosodium acetylsalicylate in the reference cell 1;s. disodium salicylate equivalent to the monosodium acetylsalicylate of the reference cell, in the sample cell) a maximum at 300 mp, a minimum at 268 mp, and an isosbestic point at 272 mp are observed. Figure 1 illustrates the differential absorption spectra of three concentrations of salicylic acid ( A , B, and C) compared to the spectra of three concentrations of acetylsalicylic acid ( A ’ , B’, and C’) in the region from 260 to 340 mp. Each curve or differential absorption spectra in Figure 1 is constructed from the two spectra of the same concentra( 1 ) L. A. Williams, R. A. Linn, and B. Zak, J . Lab. Clin. Med., 53, 156 (1959).
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ANALYTICAL CHEMISTRY
+0f -1 260
270
280 290 300 310 320 330 340 WAVELENGTH (my1
Figure 1. Differential absorption spectra of salicylic acid ( A , B, C)and acetylsalicylic acid (A’, B’, C’).
tion of either salicylic acid or acetylsalicylic acid obtained at pH 9 and at pH 13.5 by subtracting one spectrum from the other spectrum. The isosbestic point in differential spectrophotometry would be the wavelength at which each difference curve crosses the zero line. The correlation of the isosbestic point at 300 mp for salicylic acid and the maximum at the same wavelength for acetylsalicylic acid permits the use of the isosbestic point as the zero or base line for the quantitative determination of acetylsalicylic acid in the presence of salicylic acid. This principle may be applied to the quantitation of other pairs of organic compounds of similar structure, such as the barbiturates, if the isosbestic point of the differential absorption spectra of one compound corresponds to a maximum in the differential spectra of the other compound, RECEIVED November 30,1966. Accepted January 9,1967.
