Instrumentation for Digital Data Acquisition in Vol ta mmetric Techniques D.C. and A.C. Polarography ERIC R. BROWN, DONALD E. SMITH, and DONALD D. DeFORD Department o f Chemistry, Northwestern University, Evanston, 111.
b An instrument is described which was designed to effect digital readout of current and potential signals on an IBM card in voltammetric techniques involving relatively long experiment times, such as d.c. and a s . polarography. It incorporates a combination of known electrochemical control and signal conditioning procedures with a commercial digital data acquisition system. Results of experiments designed to evaluate instrument performance in measurements involving multichannel digital readout in d.c. and a s . polarography are presented and discussed.
T
SUPERIOR ASPECTS of digital data acquisition over the analog approach have been well documented and proven for some time in measurement systems employed outside the realm of electrochemical experimentation ( 1 1 ) . More recently, the advantages in adaptation of digital techniques to electrochemical measurements have been described (5, 7 , 13-15, 18). I n particular, a general philosophy of application of digital techniques to electrochemical measurements with a detailed description of the attending advantages has been given by Booman ( 6 ) . Of the potential benefits associated with digital readout, those most instrumental in stimulating our efforts along these lines were the direct compatibility of digital readout with high-speed digital computers, the ease of achieving multichannel readout, and the high inherent accuracy. Obtaining voltammetric data in a form which is directly presentable to high-speed digital computers appeared to be the most significant and far-reaching of these advantages. This is suggested by the considerabIe tedium associated with conventional voltammetric data processing, which includes the time-consuming operations of manually digitizing data (reading points from an analog recording), introducing scaling factors, correcting for nonfaradaic influences, plotting data in a variety of potentially informative ways HE
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ANALYTICAL CHEMISTRY
and smoothing noisy data by least squares methods. Through digital readout to IBM card, punched paper tape, or magnetic tape, all of these steps may be performed by accurate, high-speed digital instrumentation, introducing a savings in time of sufficient magnitude (7) to make the acquisition of initially expensive digital equipment an economical approach in the long run. The ready applicability of digital readout procedures to multichannel readout represents a significant consideration in certain voltammetric measurements. By measuring a number of current and/or potential signals of electrochetnical origin in a single experimental run, a considerable improvement in efficiency over conventional data acquisition procedures can be realized. While not always applicable, this form of data acquisition is eminently suited to a number of techniques, a.c. polarographic experiments representing a notable example. A complete ax. polarographic measurement requires readout of three signals; the applied d.c. potential and two components of the faradaic current ( 3 , 8 , 2 3 , 1 5 ) . A significant savings in time can be realized by obtaining these data simultaneously rather than by performing two separate experiments, as is usually done. The possibilities go well beyond this simple example. A variety of current components, all potentially informative, are available for measurement in a.c. polarography. For example, within the framework of the ax. polarographic experiment, simultaneous measurement of the d.c. polarographic current, two components of the fundamental harmonic current-e.g., total and in-phase components-and two components of the second harmonic current would be possible in a single run, provided sufficient capacity in signal conditioning and readout devices were available. By conventional standards, this latter procedure might be looked upon as performing five techniques simultaneously ; d.c. polarography, conventional a.c. polarography (8, 25) phase-selective a x . polarography (1, 8, 10, 12, 25, 28, 19) conventional second harmonic a.c.
polarography (2,4, 8,19,20,25,27) and phase-selective second harmonic a.c. polarography (26). Third and higher harmonic components could be included in such a multichannel readout procedure. Alternative to or in combination with readout of several different current components originating from a single frequency of applied potential, one could effect simultaneous readout of the ax. polarographic response at several frequencies. This could be achieved by simultaneously applying a number of sinusoidal potentials of varying frequency to the electrochemical cell and measuring the current components corresponding to fundamental, higher, or intermodulation harmonics of the applied frequencies with the aid of frequency selective circuits (26,27). By combining the use of small amplitudes with judicious choice of applied frequencies, one can avoid complications in interpretation which can arise when higher-order current components (second harmonics, intermodulation components) originating from different applied frequencies overlap with each other or with the fundamental harmonic current components. Thus, for example, the measurement of a fundamental harmonic current of a given frequency will be unperturbed by the existence of applied potentials of other frequencies. The act of simultaneously acquiring ax. polarographic information a t a variety of frequencies and/or from a variety of types of current components (fundamental harmonics, second harmonics, etc.) might be entitled “Multiplexing in S.C. Polarography.” The operational amplifier approach to electrochemical instrumentation is well suited without modification t o multichannel data acquisition (26). Simultaneous application of several applied potential signals and/or simultaneous presentation of signals arising from various current components can be achieved readily with operational amplifier equipment described in the literature (6, 17, 22, 24-26). However, to effect readout of a large number of signals, recording equipment of corresponding capacity is required. Analog readout
devices capable of monitoring three signals simultaneously (X-YY’ recorders) are available a t a cost competitive with digital data acquisition systems. However, if more than three signals are to be recorded, the cost of additional analog recording capability is considerable, while the added cost in digital recording equipment is negligible since the capability of recording many (10-20) channels is proved by relatively low cost units. Thus, digital data acquisition appears to be the method of choice if multichannel readout is of interest. The advantages associated with the higher inherent accuracy in digital recording are also significant. Analog recorders of 0.1% accuracy are available, but frequent maintenance is often required to hold this tolerance. The lower accuracy of high speed analog recording devices-e.g., oscilloscopesis well known. At the same time, the least expensive digital recording equipment is capable of an accuracy of about O.l%, and more precise units are readily available. None of these units require significant maintenance. Reported work on adaptation of electrochemical instrumentation to digital readout procedures has appeared only recently. Herman and Bard presented instrumentation for digital readout of transition times in chronopotentiometry (13-15). Breiter (‘7) has described instrumentation for digital readout in low-speed cyclic voltammetric and a x . polarographic measurements with stationary electrodes. The use of a multichannel analyzer for digitization of data in voltammetric methods, particularly potentiostatic measurements, was described by Osteryoung and coworkers (18). Generalized concepts and instrumentation for adaptation of digital techniques to electrochemical measurements were given by Booman (6). The present paper describes instrumentation by which digital readout is effected in voltammetric experiments of relatively long duration (>I minute, approximately) by combining known voltammetric signal conditioning procedures with a commercial digital data acquisition system of standard design. The instrumental system permits the use of digital readout procedures in measurements performed with the dropping mercury electrode, as well as with stationary electrodes. Results obtained in d.c. and ax. polarographic measurements with a dropping mercury electrode are presented. EXPERIMENTAL
Basic Electrochemical Instrumentation. An all solid-state operational amplifier instrument which has been described in detail in a separate communication (9) was employed in this
work. The following main features of this instrumentation are of interest in the present discussion. (a) Positive feedback is employed in the potential control loop to effect compensation for ohmic potential losses. (b) Direct compensation of charging current is achieved by applying identical potential signals to two cells, one containing supporting electrolyte solution and the second containing supporting electrolyte and electroactive component, and subtracting the resulting current signals. (c) A sample-and-hold readout mode is employed in d.c. and a.c. polarographic measurements performed with a dropping mercury electrode. The output signal represents the current component of interest a t the end of each mercury drop life. The hold mode is in effect for all but about 0.1 second late in the life of the mercury drop. (d) Synchronization of mercury drop lives and control of the sample-andhold readout operation is achieved with the aid of a general purpose timing circuit. The output pulses of the timing circuit provide convenient command signals for synchronization of external equipment with events in the polarographic cell. Digital Data Acquisition System. Digital readout to IBM punched card was selected for this work because it provided a readout form immediately interpretable by the operator as well as the high-speed digital computer, a feature not provided by the other choices (typewriter, printed tape, magnetic tape, punched paper tape, etc.). Readability of the data on the part of the operator was deemed important because this permits initial assessment of data quality before presentation to a computer. The choice of IBM card readout sacrificed the somewhat greater speed associated with the tape approaches, but this was not considered important for the applications of interest to us. X Vidar Corporation Model 1025012 10-channel digital data acquisition system designed to record the voltage level a t each channel on IBM punched cards with the aid of an IBM Model 526 card punch was selected for this work. A detailed description of this system is not essential to the present discussion and only the basic features will be given here. -in extensive description may be obtained from the manufacturers (16, SO). While the Vidar system is adequate for our purposes, instruments of similar concept and design which may be equally suitable are available from a variety of other sources. The main components of the Vidar digital data acquisition system consist of a Vidar Model 604 scanner (or “Multiplexer”), a Vidar Model 500 solid-state integrating digital voltmeter (the DVM), a Vidar Model 651-8 coupler and the IBM 526 card punch. The 10-channel scanner employs reed relays to sequentially transfer data from 1 to 10 analog signal sources to the measuring (DVM) and recording (coupler and card punch) equipment.
The number and identity of the channels to be read is controlled by front panel switches. The scanner is designed to receive advance and reset commands from the coupler or by manual operation of front panel pushbutton controls. .4n advance command moves the scanner sequentially to the next input channel, transferring the associated voltage to the DVM. The reset command sets the scanner to the first selected channel, transferring this data to the DVM. From 10 mseconds to 1 second after receipt of a reset or advance command, the scanner produces a command pulse which tells the DVM to begin its measuring operation. The delay time of 10 mseconds to 1 second is adjusted by a front panel control. The digital voltmeter has five ranges from =!=lo0 mv. to *1000 volts full scale. An autoranging operation mode, which was employed throughout the work described here, is included. In the autoranging mode a 100% overranging capability is provided so that the DVM operates on a given range until the signal level exceeds two times the “full scale” value. Data of four significant figures are presented when the instrument is in the overranging configuration while three digits are provided for signal levels less than the full scale value. The DVM measures the voltage presented by the scanner and presents the measurement to the coupler in the form of binary coded decimal (BCD) data, a BCD location signal, and a polarity signal. The DVM effects voltage measurement through a conventional technique which involves three steps: (a) amplification of the input signal, (b) conversion of the amplified input signal to a proportional pulse train with the aid of a voltage controlled oscillator, (c) application of the pulse train to a series of decade counters for ajixed period of time (the integration period). A fixed integration period of 20 mseconds is controlled to better than *0.01% by a crystal oscillator time base. The crystal oscillator output is amplified and converted into short duration pulses which serve as start and stop commands for the counter gate.- A start command cannot be received until a command pulse is provided by the scanner (see above) which activates an auxiliary gate. I t should be noted that the measurement time is insignificant compared to the time required for card punch operation. Upon completion of the measurement, the DVM also produces a print cornl mand which is routed through the control circuitry of the coupler to the card punch. The coupler is the key element of the system. I t translates the BCD data from the DVM into a format compatible with the IBM card punch, supplies this information in a suitably amplified form to the card punch, and coordinates the operation of the other units of the system by providing reset and advance commands for the scanner, print commands for the card punch, etc. As the last character of each data point is recorded by the card punch, an advance command is VOL. 38, NO. 9, AUGUST 1966
1131
1
2
3
4
RUN #1
002493 011844 116043 111263
R U N #2
002503 011864 115993 111143
R U N #3
002493 011814 116033 111183
RUN #4
002503 011584 115983 111123
Figure 1. IForm of digital data readout presented at top of IBM card System: 2.0 X 1 O-*M Pe(lll) in 0.50M K1C204 Applied: 42.0 Hz, 10 mv. peak-to-peak sine wove; d.c. scan rate of 50 mv. per minute Measured: Shows data obtained at - 0 . 2 5 0 volt during four successive runs Column 7 : d.c. potential signal Column 2: d.c. faradaic current signal Column 3: total 42.0 Hz faradaic current signal Column 4: resistive (in-phase) component of 42.0 Hz faradaic current signal All signals measured at end of drop life with compensation for nonfaradaic effects
given to the scanner by the coupler, causing the scanner to sample the next channel. The reset command originates in the card punch each time the card punch prints the last character of information for the last channel. The command is routed through the coupler where it is delayed for 250 mseconds to permit a new card to move into place and then routed to the scanner as the reset command. Provision for control of the readout cycle by external timing circuitry is obtained by routing the reset command through a normallyopen relay. A C. P. Clare Model HGSS1002 single-side-stable, mercurywetted contact relay was used for this purpose. In this way, a reading cycle cannot ensue until an external pulse effects relay closure, transmitting the reset command to the scanner. Once the reset command is provided, readout of from 1 to 10 channels of information is achieved on a single IBM card at a rate of 100 channels per minute (0.6 second per channel) after which a new card is moved into place and the system ceases operation until an externally induced relay closure again provides a start command. The form of the data provided is illustrated in Figure 1. Results of four measurements (four IBM cards) performed under essentially identical conditions are shown. Each measurement involved readout of four channels of information. For the present discussion it is essential to note only the form of the data. Further details regarding the significance of the data are discussed below. As shown in Figure I , data are given in the form of six characters per word. The first character (left hand character) indicates polarity. The next four characters represent the data with the most significant to least significant digits ordered from left to right. The last character (right side) indicates the range. In the polarity character, a zero indicates positive polarity input, a one indicates negative polarity, a two 1132
0
ANALYTICAL CHEMISTRY
indicates positive overload, and a three indicates negative overload. Range identification is effected through indication of the appropriate negative exponent of ten in the last character. Thus, the word 017653 represents f1.765 volts, and word 143204 indicates -0.4320 volt, etc. Coupling Basic Electrochemical Instrumentation with Digital Data Acquisition System. It should be apparent from the foregoing discussion that the electrochemical instrumentation and the digital data acquisition system (the DDAS) are ideally compatible for a.c. and d.c. polarographic studies without further modification of either unit. The key elements in achieving this compatibility are the sample-and-hold readout capability of the electrochemical system and the provision for external control of the reading cycle of the DDAS. Coupling of the electrochemical instrument and the DDAS simply required the following steps. First, each channel of analog information from the electrochemical instrument was applied to the scanner inputs of the DDAS via sampling circuits (9). Second, the source of a voltage pulse from the timing circuit of the electrochemical instrument which signals the end of each mercury drop life [the output of Trigger 2 of Figure 3, Ref. (9)]was routed to the coil of the relay whose closure initiates a reading cycle of the DDAS. An example of one type of system resulting from such a combination of DDAS and electrochemical instrument is illustrated in a schematic form in Figure 2. Figure 2 depicts an instrument which is designed to compensate for nonfaradaic effects (9) and simultaneously to read out the d.c. polarographic current and two components of both the fundamental and second harmonic currents (total a x . signal and a vectorial component). The rationale for the various analog signal paths has been discussed elsewhere (26-27). With a system such as illustrated in Figure 2,
electrochemical data obtained with a dropping mercury electrode can be read out digitally to an IBM card, with each IBM card containing data points corresponding to instantaneous signal levels a t the end of drop life (9) for all channels of information. The sequence of operations performed by this instrumentation in the course of effecting digital readout is as follows. (a) Late in the life of a mercury drop, a pulse from the timing circuit simultaneously initiates sampling of all analog output signals of interest. (b) About 0.1 second later, a pulse from the timing circuit terminates the sampling operation, sending the sampling circuits into the hold mode. The same pulse activates the dropdislodger circuitry, ending the mercury drop life, and signals the DDAS to begin a reading cycle. (c) During the first few seconds of the life of the new mercury drop, the DDAS goes through the operation of printing on a single IBM card the voltage outputs of the sampling circuits from each channel of information. (d) The IBM card on which the data has been punched is moved to the storage rack, a new card is moved into place and the DDAS ceases operation until stages (a) and (b) recur a few seconds later. The total time required to perform the preceding sequence of operations depends upon the number of channels to be monitored and places an obvious limit on the minimum drop time that can be used. This has been no problem in our applications to date, although one can envision situations where this restriction would be a disadvantage. The operation of the instrument has been discussed only within the context of experiments employing the dropping mercury electrode because the foregoing approach t o digital data acquisition is best suited to such applications where the experiment time (time required to run one polarogram) can be made relatively long without influencing the nature of the data. However, straightforward applications in experiments with stationary electrodes also exist, provided the time of the experiment is sufficient. With this restriction, adaptation of the instrument just described to measurements with stationary electrodes would require simply the removal of the dropdislodger circuitry and possibly the sampling circuits, although the latter step is not recommended. The sample-and-hold operation makes possible the recording of data points corresponding to the same instant in time for all information channels. This is desirable regardless of the nature of the electrode because of the associated ease of comparing data from various channels. It should be recognized that the combination of more-or-less standard electroanalytical instrumentation with the digital data acquisition system described here amounts to a special case of the generalized system configuration outlined by Booman ( 5 ) . Supporting Equipment. An Electro Instruments Model 480 X-YY' re-
corder with Model 468 (X-axis) and Model 420 (Y-axes) plug-in modules was employed to record polarograms in the usual analog fashion. Analog recordings were obtained simultaneously with the digital readout t o assess the consistency between the two forms of data acquisition, to determine whether the DDAS was influencing data quality, etc. A Hewlett-Packard Model 5243L electronic counter with a Model 52658 plug-in served as a second digital voltmeter which permitted on-line checking of the data produced by the Vidar DDAS. A California Computer Products, Inc., Model 565 digital incremental plotter made available at the Northwestern University Computing Center was employed to obtain analog plots of raw digital data produced by the DDAS. The plotter was operated in the mode in which straight lines are drawn between adjacent data points. By comparing on-line analog recordings with those produced from the digital data with the aid of the digital incremental plotter, an assessment of the noise and other sources of error introduced in analog-digital-analog conversion was possible. This was considered important because analog forms are expected to remain a most informative and efficient approach to data presentation. Thus, while data acquisition and processing may be performed by digital techniques, data presentation in both analog and digital forms is likely to be preferred. The remaining supporting electronic equipment, thermostats, electrodes, cells, etc., were described in a separate paper (9) as were their applications. The ferric-ferrous oxalate system in 0.5M potassium oxalate (21, 27, 29) was employed as a model in the instrument evaluation experiments. Solution preparation was performed in the usual manner (9).
SAMPLE-AND -HOLD CIRCUIT
SAMPLE-AND -HOLD CIRCUIT
SAMPLE-AND -HOLD CIRCUIT
SAMPLE-AND -HOLD CIRCUIT
SAMPLE-AND -HOLD
TIMING CIRCUITRY
'q
-
I
RESULTS AND DISCUSSION
An assessment of the performance of the electrochemical instrumentation and the DDAS as a unit was the primary objective of the experimental work presented here. Excellent individual performance of these two basic components has been established (9, SO). However, successful implementation of the combined unit to produce accurate digital data demands reliable, precise, coordinated performance of a number of critical units (potentiostats, signal amplifiers, timing circuitry, sampling circuits, drop-dislodger circuitry, scanner, digital voltmeter, coupler, and card punch). A secondary objective was to assess the feasibility of the multichannel readout procedures discussed above. h variety of experiments showed that the accuracy and reproducibility of digital data obtained with this system was comparable with the results obtained in analog readout with the same basic electrochemical instrument (9). For example, Figure 1 illustrates the
reproducibility obtained in an experiment in which the d.c. potential, the d.c. polarographic current and two components of the fundamental harmonic a x . polarographic current were recorded simultaneously. Each set of four data points was obtained in the course of a normal polarographic run a t a particular d.c. potential (-0.250 volt) along the polarographic wave. The time elapsed between readout of the first and the last set of data was approximately 1 hour, indicating that drift in the various instrument components is negligible. Table I gives a
more extensive example of typical precision observed in digital data readout in experiments of the type illustrated in Figure 1. The degradation of precision in the d.c. polarographic signal at the foot of the wave (data a t more positive potentials in Table I) is partly due to a drift in d.c. offset in some of the amplifiers, a factor which is of no consequence in the a.c. measurements. This problem is readily soluble by incorporating into the instrument amplifiers characterized by less d.c. drift. It should be mentioned that d.c. polarograms obtained under these circumstances are esVOL. 38, NO. 9, AUGUST 1966
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Table 1. Reproducibility in Simultaneous Digital Readout of D.C. Polarographic Current, Total Fundamental Harmonic A.C. Polarographic Current and In-Phase (Resistive) Component of Fundamental Harmonic A.C. Polarographic Current"
D.C. potential
Run 1
0.230 0.235 0.239 0.245 0.250 0.255 0.259 0.264 0.269 0.274 0.279 0.284 0.288 0.293 0.300 0.305
0.0790 0.0885 0.0983 0.1084 0.1184 0.1282 0.1376 0.1466 0.1549 0.1627 0.1699 0.1765 0.1823 0.1874 0.1937 0.1974
D.C. polarographic current Total fundamental harmonic In-phase fundamental harmonic Run3 R.A.D.b Run 1 Run2 R u n 3 R.A.D.* Run2 Run3 R.A.D.b Run 1 Run2 0.0800
0.0892 0.0988 0.1087 0.1186 0.1285 0.1379 0.1470 0.1555 0.1633 0.1705 0.1771 0.1830 0.1881 0.1946 0.1983
0.0766 0.0862 0.0959 0.1059 0.1158 0.1258 0.1358 0.1450 0.1535 0.1614 0.1687 0.1751 0.1809 0.1859 0.1925 0.1960
1.6
1.3 1.2
1.1
1.0 0.9 0.6 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.4
1.517 i.575 1.611 1.621 1.604 1.563 1.497 1.416 1.318 1.209 1.095 0.980 0.868 0.762 0.616 0.530
1.510
i.569
1.605 1.617 1.599 1.558 1.493 1.410 1.311 1.199 1.087 0,973 0.859 0,754 0.609 0.524
1.503
i.565
1.602 1.612 1.598 1.556 1.493 1.410 1.313 1.205 1.092 0,977 0.866 0.757 0.613 0.527
0.3 0.2 0.2 0.2 0.1 0.2 0.1 0.2 0.2 0.3 0.3 0.2 0.4 0.4 0.4 0.4
1.069 - ... 1.118 1.135 1,140 1.126 1.093 1.045 0.982 0.916 0.836 0.749 0.667 0.590 0,514 0.413 0.355
1.057 i.099 1.123 1.113 1.114 1.089 1.038 0.974 0.903 0.823 0.742 0.654 0.576 0.503 0.406 0.348
1.056 1.098 1.123 1.126 1.112 1.080 1.032 0.971 0.899 0.824 0.742 0.663 0.585 0.506 0.407 0.353
0.5 0.8
0.5 0.8
0.5 0.5 0.4 0.4 0.7 0.7 0.4 0.8 0.9 0.9 0.7 0.8
Signal levels given are those actually applied to digital data acquisition system in units of volts; data obtained with the iron system in oxalate media; applied alternating potential of 10 mv. peak-to-peak and 40 Hz. b R.A.D. = relative average deviation from the mean in percentage units.
sentially identical with d.c. polarograms recorded in absence of the a.c. signal. That is, the d.c. faradaic rectification component is negligible under the experimental conditions associated with the data in Figure 1 and Table I. This was verified by both experiment and theoretical calculation which showed that the faradaic rectification component did not exceed 1-2y0 of the normal d.c. polarographic current and was usually much less. This contribution could be made even smaller by use of smaller amplitudes of applied alternating potential (8,25). Illustrated in Table I1 is the reproducibility of data obtained in experiments in which several sinusoidal potentials of differing frequency were applied to the polarographic cell and the fundamental harmonic current components a t each frequency were recorded simultaneously. As shown, the reproducibility was found to be quite satisfactory. Tests also showed that the application of several simultaneous alternating potentials of different frequencies caused insignificant (
