A Semiautomatic Stripping Analysis InstrumentRobert Megargle Citation: Review of Scientific Instruments 43, 43 (1972); doi: 10.1063/1.1685442 View online: http://dx.doi.org/10.1063/1.1685442 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/43/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Analysis of capillary drainage from a flat solid strip Phys. Fluids 26, 062102 (2014); 10.1063/1.4879827 Improving suspicious breast lesion characterization using semi-automatic lesion fractional volume washoutkinetic analysis Med. Phys. 38, 5998 (2011); 10.1118/1.3651635 Systematic and random error suppression in instrumentation for stripping (chrono)potentiometry Rev. Sci. Instrum. 70, 3439 (1999); 10.1063/1.1149934 Analysis of Musical Instrument Tones J. Acoust. Soc. Am. 41, 793 (1967); 10.1121/1.1910409 Semiautomatic UltrasonicVelocity and Absorption Instrumentation for Liquid Media J. Acoust. Soc. Am. 41, 1591 (1967); 10.1121/1.2143616

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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 43, NUMBER I

A Semiautomatic Stripping Analysis Instrument*

ROBERT MEGARGLE

Rnvironmental Trace Substances Center and Surveillance Center, University oj Missouri, Columbia, Missouri 65201

(Received 28 December 1970; and in final form, 23 August 1971)

A stripping analysis instrument was designed and constructed using relays at key points to control the operation. Each relay can be operated manually with switches on the front panel or placed under the control of a digital control circuit. The control circuit takes the instrument sequentially through five modes. In the ready mode the operator fills the cell, sets the controls, and pushes start. The instrument goes into the electrodeposition mode for a preset time where the unknown is deposited on the electrode at constant potential in a stirred cell. This is followed automatically by a wait mode for another preset time, and then by a scan mode where the metals deposited in the electrodeposition mode are stripped off and measured. When the scan mode time is over, the instrument enters the completion mode where the final voltage is held for exhaustive stripping of the electrode. The instrument goes from the completion to the ready mode when the operator intervenes.

JANUARY 1972

Stripping analysis has proved to be a valuable technique for the determination of traces of metal ions in solution. The usual procedure involves three steps. (1) A micro­electrode like a hanging mercury drop is held at a negative potential in a stirred three electrode cell for a few minutes to several hours. During this time, the metal ions of interest are reduced to and concentrated as metals in or on the electrode. (2) The stirring is then stopped for !-2 min to allow the solution to become quiescent. (3) Finally, the potential of the microelectrode is varied linearly in the positive direction and the peak shaped voltammogram is recorded as various metals are stripped from the electrode. Areas under curves or peak heights can be related to original concentrations of constituents in the solution, provided stirring conditions and the time for each step are held constant for standards and unknowns.

The instrumentation described here automatically per­forms all three steps in sequence and requires the operator only to set up and fill the cell, set the controls, and read and interpret the results when the procedure is completed. The instrument is divided into an analog voltage module and an analog cell module, both with digitally operated relay controls, and a digital control module.

FIG. 1. Schematic diagram of the ana­log circuit. Switch S22 positions give scan rates (V /sec) of (A) fast ext, (B) 0.5, (e) 0.2, (D) 0.1, (E) 0.05, (F) 0.02, (G) 0.01, (H) 0.005, (J) 0.002, (K) 0.001, and (L) slow ext.

THE ANALOG CIRCUIT

The voltage module consists of a chopper stabilized Philbrick-Nexus SP656 operational amplifier wired as an integrator to provide a linear voltage scan and a Philbrick­Nexus QFT-2 FET input operational amplifier to add the scan voltage to an adjustable constant voltage. The sum voltage is sent to the cell module to control the cell driver amplifiers.

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VOLTAGE MODULE

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r===========================~~~===================== I BQ.\OO I BOOSTER I r---NV'-1-JW_ .... I I I I I I I I I I I I I

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44 ROBERT MEGARGLE

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S32B_-=<n

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In Fig. 1, the 0.01% regulated plus and minus 15 V power supply voltages are applied to voltage dividers formed with resistors R1, R2, and Zener diodes MZ4615 to give + and - 2 V reference signals at the scan direction switch. Resistors R1 and R2 are individually selected so that the variable 100 Q resistor R3 can be adjusted to equalize the two voltages at the scan direction switch. The voltage from the scan direction switch is applied to trimmer resistors R4 and R5. Either the approximately 1 Vat the tap of R4 or the approximately 0.1 V at the tap of R5 is applied to one of the 0.1% precision input resistors R6--Rll of the integrator, provided the mercury contact hold relay is unenergized. The integrating capacitor C1 is a 20% Stabilex capacitor from Industrial Condenser Corporation that is discharged when the reset relay is un­energized. The accuracy of the sweep rate is obtained by adjusting trimmers R4 and R5. If desired, the operator can select other scan rates besides those available with scan rate switch S22 by supplying his own reference voltage to the integrator. Provision is made to use either the largest or smallest input resistor with switch positions L or A for slower or faster scan rates.

r---------------------------------------------~ , 1M R22 10M R32 :

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SM R33

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FIG. 3. Current measuring circuit. Switch S27 positions give ranges in CJ.A/full scale) of (A) 1, (B) 2, (C) 5, (D) 10, (E) 20 (F) 50 (G) 100 (H) 200, (J) 500, and (K) 1000. '"

222K

TO SUMMING POINT OF ADDER AMP

FIG. 2. Switch and resistor network use~ to sele.ct the electrodeposition po­tentIal. ResIstors connected to switches S32-S34 are trimmers adjusted to give the correct ou tpu t voltage.

The output of the integrator is applied to the summing point of the adder amplifier through a 1.3 K resistor. Also applied to this point is either the + 15 or -15 V power supply voltage through a switch and resistor network. Two lO-position switches and one 6-position switch are con­nected with resistors as shown in Fig. 2 so that any voltage between -2.99 and +2.99 V in 0.01 V steps can be ob­tained at the adder amplifier output when the scan ampli­fier output is zero. The resistors used in this network are trimmer potentiometers adjusted to give the right output even though the 1.3 kQ feedback resistor is not high precision. Consequently, the values shown in Fig. 2 are only approximate. A small, continuously variable poten­tial, selected by R49, can be applied to the summing point to be used as a fine trim of the output voltage.

The output of the adder amplifier is a control voltage sent to the noninverting inputs of the cell driver amplifiers in the cell module. Any number of cells can be controlled by this voltage provided that the power capability of the adder amplifier is not exceeded. At present, only two cell units have been constructed. When cell relay A or B is energized, the auxiliary and reference electrodes of the corresponding cell are connected to the output and in­verting input of the QFT-2 cell driver and BQ-100 booster amplifiers, respectively. The driver amplifier thus applies a voltage to the auxiliary electrode so that the potential of the reference electrode with respect to ground equals the control signal from the adder amplifier. Since the working electrode is held at virtual ground, this arrange­ment effectively keeps the voltage between the reference and working electrodes equal to the control voltage.

The current measuring circuit shown in Fig. 3 is a standard operational amplifier configuration.! High pre­cision 0.1% feedback resistors R32-R41 allow different current ranges to be measured. Ordinary 1% precision resistors R22-R31 connect a variable voltage selected by baseline offset control R42 to the summing point of the indicator amplifier. This provides a zero control for the output signal that is nearly independent of the range switch setting. Operational amplifier SN72709N is an inexpensive integrated circuit wired as a follower to reduce loading of the baseline offset divider network. A small

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STRIPPING ANALYSIS 45

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FIG. 4. Digital control logic circuit. Q1 to Q14=2N3415. Mode switch S16 positions: (a) scan, (b) hold, (c) reset, (d) off, and (e) auto.

fraction of the scan amplifier output voltage, adjusted by control R48, can also be applied to the summing point to compensate for the double layer charging current. The output of the operational amplifier is applied to a voltage divider network. Resistors R45-R47 are 0.1% precision resistors that give 100, 10, and 1 m V signals at the corre­sponding output jacks when the indicator amplifier output voltage is 10 V. This precise relationship is obtained by adjusting trimmer resistor R44. Resistor R43 and a 10 J.LF capacitor are provided to damp the output signal.2 Resistor shaft mounted switches S28-S31 allow the various features of the indicator circuit to be disconnected. Input zero switch S30 allows the working electrode to be disconnected from the indicator circuit but still remain in the electro­deposition circuit.

In addition to recorder outputs, the signals from the two indicator amplifiers can be sent to the difference

amplifier through switches S25 and S26 and attenuators R20 and R21. Resistors R12-R19 are 0.1% high precision resistors so the output of the difference amplifier is given precisely by (a V A +b V B) where V A and VB are the outputs of the A and B indicator amplifiers and a and b are the adjustments of R20 and R21, respectively. The output arrangement of the difference amplifier is similar to the output of the indicator amplifier except that the damping has been eliminated. Switch S24 causes the output of the difference amplifier to be zero to aid the adjustment of the device used to record the difference signal. The difference amplifier is useful for differential determinations to reduce the effects of residual currents.

Included in the instrument, but not shown in the diagrams, is a four place autopolarity digital panel meter that covers the two ranges 0-0.9999 or 0-9.999 V. A rotary switch allows the operator to display various

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46 ROBERT MEGARGLE

TABLE 1. Relay positions and timing during the cycle of the stripping analysis instrument.

Relay positions

Hold Reset Mode

Ready Hold Reset Electrodeposition Hold Reset Wait Hold Operate Scan Operate Operate Completion Hold Operate

voltages of interest from different points within the instrument. Also not shown are toggle switches that allow each amplifier to be placed in a simple voltage-gain-of-lO circuit with grounded input to allow balancing, and the networks suggested by the manufacturer for balancing each amplifier. Test points are provided at convenient points in the circuit.

THE DIGITAL CONTROL CIRCUIT

The relays used to control the operation of the analog circuit can be operated manually from the front panel or automatically by the digital control module. Switch S16 in the voltage module controls the hold and reset relays as shown in Fig. 4. The scanning circuit will scan when the hold relay is un energized and the reset relay is energized. In the cell module, all relays are controlled individually with an on-off-on toggle switch and a gate. In addition to those previously described, for each cell there is a chart relay to provide an SPDT switch during the scan step to activate a recorder, and a stirrer relay to energize a standard 110 V socket during the electrodeposition step. Jacks are provided for auxiliary control of all relays, if desired.

The digital control circuit consists of a clock circuit, four decade counters, and appropriate decoding and sequencing circuitry. The clock circuit produces 60 Hz pulses that pass through gate v to a frequency divider to give an out­put of pulses with a period of 1 sec. These are counted by decade counters DC1-DC4, whose outputs are decoded so that each value of the count produces a logical 1 at one of 10 logic outputs. The count is also displayed on decade indicator tubes so that the time gate v is open is shown in seconds. The logic outputs of some of the DC units are connected through rotary switches Sl-S9 to gates n, q, and p. These switches select the time the instrument will remain in the electrodeposition, wait, and scan modes.

At the start, the operator clears flipflop FF4 with reset switch S 17 to put the instrument into the ready mode. Clock pulses pass through gate a to trigger the 1 msec monostable. Since the 100 msec monostable is not trig­gered at this time, FFl cannot set, and a maximum of three pulses through gate a is required to clear the shift register made up of FF1-FF3. Gate e then disables gate v to prevent the timer from running, and the counters are

Cell

Off On On On On

Timing

Chart Stir Max time Resolution (sec) (sec)

Off Off Determined by operator Off On 9990 10 Off Off 990 10 On Off 9999 1 Off Off Determined by operator

reset by the 10 f.Lsec monostable. The seemingly unneces­sary number of gating steps in relay control was done so that the voltage and cell modules could be operated with­out the control module, and to reduce the number of lines needed to connect the modules together.

To begin an automatic analysis, the operator sets FF4 to trigger the 100 msec monostable. During its 100 msec pulse, at least one 1/60 sec pulse will trigger the 1 msec monostable to set FFl to begin the electrodeposition mode. Unless switches Sl-S3 are all set to zero, gate b disables gate a when FFl sets. No pulses come through to clock the shift register or reset the counters. Also, gate e opens gate v allowing the timer to run.

When the counters have reached a count equal to the value set by switches Sl-S3, gate n disables gate b to open gate a, allowing the 1 msec monostable to clock the shift register once to put the instrument into the wait mode, and the 10 f.Lsec monostable to reset the counters. Now gate d closes gate a. In the same way, the instrument goes into the scan mode after a time set by switches S4 and SS, and into the completion mode after a time determined by S6-S9. In the completion mode, gate a is open, but counter reset and shift register clock pulses have no effect after the first one. This mode is useful for exhaustively stripping the working electrode at the end of a run, and it is ended when the operator clears FF4. The operations of the different modes are summarized in Table 1.

The electrodeposition, wait, and scan modes can be terminated immediately with switches S19, S20, and S21, respectively. The ready mode can be obtained immediately by clearing FF4. Lamps LlO, L11, L12, and L13 indicate the instrument is in the electrodeposition, wait, scan, or ready modes, respectively. Lamps L7, L8, and L9 indicate the scan circuit is in the scan, reset, or hold operations.

The automatic timing and sequencing of the instrument are important because it eliminates the need for constant attention by the operator, reduces the chances of switch throwing or timing errors, and gives better precision and accuracy by electronically controlling the time parameters that effect peak height.

RESULTS

During testing, it was found that stirring conditions, dissolved oxygen, stripping scan rate, Hg drop size, and

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STRIPPING ANALYSIS 47

electrodeposition time have the most influence on peak height. The first four were held constant and the last was varied along with indicator amplifier gain and recorder attenuators to change the sensitivity of the instrument.

A Sargent hanging drop electrolysis cell was used during testing. The cell had an SCE and an Hg pool as reference and auxiliary electrodes, respectively. A glass propeller stirrer passing through the Teflon cell top was turned at 600 rpm with a Sargent synchronous motor. All parts of the cell top and stirring motor were rigidly clamped to insure reproducible stirring. Only the cell body was re­moved for cleaning and refilling. Mercury drop size was controlled by carefully adjusting the height of Hg above a DME inserted in the cell. After a number of drops fell in a quiescent solution, a drop was caught in a Teflon scoop and attached to the hanging Hg electrode. Solutions were purged for 15 min with N 2 to eliminate O2•

The first tests were made by analyzing lead samples in 0.0068M sodium tartrate supporting electrolyte. The lead was electrodeposited at -0.70 V vs SCE for times selected for optimum results for the concentration range. Solutions as dilute as 1 ppm could be analyzed using 60 sec electro­deposition times at the most sensitive current range. More concentrated solutions were determined using 60 sec electrodeposition and attenuated current scales. For the dilute solutions shown in Fig. 5, 1000 sec electrodeposition was used. A wait time of 30 sec was used for all work. A stripping scan rate of 0.05 V /sec was chosen as the fastest rate (most sensitive) consistent with recorder abilities.

One solution placed in the cell can be electrodeposited and stripped a number of times provided care is taken to insure the working electrode is exhaustively stripped each time. A new drop was used for each cycle. Successive stripping waves were reproducible with standard devia­tions around 8% except at the lowest levels where the standard deviation was as large as 12% in some cases. Variations in stirring or Hg drop size may account for the variations. The data shown in Fig. 5 represent the average and ± standard deviation for three or four determinations for each solution.

Results were recorded with a Honeywell 530 X - Y recorder. The X signal was obtained from the indicator

FIG. 5. Lead strip­ping wave peak height vs added PbH .

12

10

8

::'6 .; :z: .... .. .. II. 4

2 0 0.005 0.01 Added Pb (ppm)

amplifier and the Y signal from the adder amplifier. With gain and attenuator switches in the most sensitive posi­tions, pen deflections correspond to working electrode currents of 5.0XlO-3 /-LA/cm. Recordings at this setting (e.g., those of Fig. 5) were free of noticeable noise, indi­cating additional amplification is possible. At this sensi­tivity, charging current compensation is necessary.

At present, a more important limitation on sensitivity is purity of reagents. Figure 5 shows a plot of results for the lowest concentrations studied to date. The y intercept and the blank are not at zero peak height, indicating contamination of the water used, the supporting electro­lyte, and/or the mercury. Contamination of the cell from previous use has been eliminated as a possible source of the lead peaks in the blanks because careful acid cleaning and rinsing and leaching with purified water did not affect the height of the residual lead peak. Attempts are now being made to purify reagents by electrolytic stripping and it is hoped to be able to extend the sensitivity another order of magnitude.

* The author wishes to acknowledge the contribution of the En­vironmental Trace Substances Center and Surveillance Center at the University of Missouri. This work was supported in part by USPHS Grant 5P01-ES00082.

1 H. V. Malmstadt, C. G. Enke, and E. C. Toren, ElectronicsforSci­entists (Benjamin, New York, 1963), p. 353.

2 C. G. Enke and R. A. Baxter, J. Chern. Educ. 41, 202 (1964).

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