edited by
GALEN W. EWlNG ProfesxK Emeritus
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Aaafess P 0 BOX 2573 Las Vegar. NM 87701
CII.
Automated Anodic Stripping
John T. Stock University of Connecticut Storrs, CT 06268 Nowadays instrumental methods are emphasized heavily in the teaching of analytical chemistry. The theoretical and practical aspects are well covered in available texts. However. verv few of these deal with auto-
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John T. Stock received the degrees of I'hD and DSc from the University of London. He joined the Chemistry Department af the University of Connecticut in 1956 and was appointed Professor of Analytical Chemistry in 1959. H e received the 1977 Alumni Award for Excellence in Teaching. John Stock has three broad. research areas,analytical chemistry in its widest sense, the design of instruments and equipment for teaching purposes, and the history of scientific instruments. A few months ago, he was appointed a Fellow of the Science Museum, London for his ongoing studies an historic scientific instruments.
motion, the obvious extension of chemical instrumentation. This situation may he due to the fact that the student rarely has time to make more than one or two runs in a given experiment. Time, both actual and when translated into salary and overhead costs, is a t a premium in actual practice. Repetitive analyses are likely t o be automated. In fact, much of the "analysis" concerning the operation of a large-scale (hence almost certainly continuous) chemical manufactory would appear t o be done "without samples." Here, laboratory automation has been translated into on-line control. The various monitoring devices, situated right in the plant, send signals to a computer which in turn orders such changes in flow rates, pH, temperatures, and the like as are needed to maintain the efficient outflow of a uniform product. The term "chemical" is used in its widest sense, to include such products as fertilizers and even applesauce! It is not suggested that teaching lahoratories should a t once acquire a collection of automated instruments. Far more important are devices and experiments that can be used to demonstrate some of the simpler aspects of automation, as well as some of its problems. An individual organization may not hire a chemistry major to be a mere buttonpusher. More likely, the graduate would be expected to be able toadjust the equipment t o suit a particular analysis. This partieipation is very different from mere buttonpushing! Automatic potentiometric titrators (1-3) have long been used on a lecture demonstration basis in our teaching programs. Some two years ago, they were added to our chemistry major laboratory sequence. The setup consists of three hardwired titratars of different types and one titrator that is controlled by our BASIC programmed Altair microcomputer ( 4 ) . We have now added experiments involving the automation of anodic stripping analysis (ASV). The history, general techniques, and typical applications of ASV have been well covered in the account by Ellis (5).Because of its essential simplicity, high sensitivity, and wide applicability, ASV is an important analytical
Presented a t the 4th National ACS Workshop an Computers in Chemical Education, Eastern Michigan University, Ypsilanti, August 1&17,1979.
technique. A recent paper gives some striking examples of the use of advanced forms of ASV to determine traces of toxic metals in the environment (6). The practical importance of ASV warrants its inclusion in the teaching laboratory program. A bonus is the illustration of a general principle in lowconcentration analysis-first concentrate and then determine. Our ASV setup consists of four units, two manual and two autamated. One manual unit employs a hanging mercury drop electrode and a Beckman Electrosean unit. The other has a rotatine elassv carbon disc electrode IGCDE) thniiaused in conitmetion with n ~L i z &: Narthrup ~lectr&mograph. Both automated units use a GCDE. This electrode is merely a short length of '/*-in. diameter glassy carbon rod that is either cemented or fused into the end of a 25-emlength of 6-mm ad glass tubing. The carbon is ground flush with the end of the glass and its surface is polished with fine lapping powder. A copper wire that dips into a short column of mercury provides electrical connection to the instrumentation. In use, the GCDE is rotated a t 6M1 rpm. In all cases, potentials are referred to the saturated calomel electrode (SCE). The two-electrode mode is used in introductory experiments; hence, the SCE should be a large area one. When introducing ASV, no attempt is made to exploit its versatility and very high sensitivity. The first example, chosen to stress principles and technique, is the determination of lead in KN03 that has been deliberately contaminated with lead. To do this, a calibration curve is set up a t a mercury plated GCDE. Standard P b ( N 0 s h solution is added progressively to 0.02 M KN03-4 X 10W5M H... e ( N O. l. ) that ~ has been thorouehlv .. , spargrd with nmw,rn After r w h addmon, the purrntial is maintained nt -I).: I \' for 2 min. Thru the strjpping p s t r w n i i rtcc,nlrd after each addition by scanning down to -0.30 V. Having performed ASV manually, parts of the experiment, which is quite a rapid one, ~
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(Continued on page A1261 Volume 57, Number 4. April 1980 1 A125
Chemical Instrumentation are repeated under automation. Automated ASV is by no means new; some very sophisticated devices have been develo~ed(7). %nlieitv. -~~~,~~~ ~ ~with , .a view to the dema&atian c,f principles, was the aim m designing our versions. One of these is hardwired. u h ~ l the e other is controlled by an inexpensive teaching-type microcomputer. Figure 1 is a front view of the hardwired Stripping Analyzer. Its case has a transparent top and back to permit the components t o be viewed. Depression of the START button ~
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F W ~1. Automated sblpplng analyzer.
switches on the magnetic stirrer beneath the electrolysis cell. At the same time, the appropriate light emitting diode (LED) is illuminated. One minute later, the TIMER LED blinks, the deposition potential of -0.70 V is applied to the GCDE, and the applied potential (E.) LED comes on. When the TIMERLED blinks a t the end of the second minute, the stirrer and its LED were turned off. Deposition then continues under the mild agitative conditions caused by the rotation of the GCDE. At the end of the third minute, the TIMER LED goes out and the SCAN LED lights. This light signals the startup of the recorder chart drive and the beginning of the stripping scan. Two minutes are required to reach -0.30 V. Then the GCDE is placed on m e n circuit..~the chart is arrested. and all LED'jerccpt that lahelled 81'OPareextinguishad. Mere pressure un the START button causes the run to he repeated. A STOP button overrides the automatic operation. A block diagram similar to Figure 2 is at-
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",,us " Figure 2. Blockdiagram of analyzer system.
A126 1 Jownal of Chemical Education
Chemical Instrumentation
a 7 ICI
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Figure 3. Circulof automtedswippinganaiyser.Capacitors: C,. C2.47pF: C3, CI. 0.001pF: Cg. Ce. 0.OlpF: Cn IOOpF: Cg,30pF:C~.0.5pK CI0,Cn O.lpF. Diodes: D,, D,, De. D,, IN914 a equivalent. LED'S:D, Timer: Dg Stop: D4. Scan: 4. Stirrer; Ds, Apply E.. Fixedresistors. 'I4W: Rt. R2. Rm. Rl8. 4.7KQ: R3. 470Ka RI. 33Kn: Rs. Re. 330a: R,. 3.3KR; R.. b4,2.2MR R,. 6.8Kfl: R,*. 135Kfk RI3, 1OKfl; Rq5: RZ,. R2@. 1Kn. 1W: R2,. R28, 100R. Baseboard-mounted potentiometers: R8. 50fk R,,, 1Kfl: R,,. 200 0. Transistors: 0,-0,. 2N3415 or equivalent. 1 2 4 relays: L,, DPDT, 50mA: L1, L3. L., SPDT. 12mA. integrated circuit devices: iC1, 7402: iC2.74123: iC3. 7400: iC4.556; iC5.6 and 7,7473: IC8 and 9, 1458. Power connectionsare shown at (a), Push bunon switches: S,, START, S2.STOP.
tached to the side of the case and is useful when explaining the operation of the Analyzer. Details of the circuit, which is based on common, hence inexpensive, IC devices (8) are shown in Figure 3. Two NOR gates in IC1 dehounee the START button signal, which clocks three devices. These are monostable A in the dual IC2 and JK flip-flops A and B in IC5. A fraction of a second later, monostable A starts timer Ain dual IC4. The clockingof flip-flop A drives its output Qn(pin 12) high and locks it into this state. This state deactivates the START button once the run has been started. QB (pin 9) is also driven high, thus providing the signal to start the stirrer. This signal turns on transistor Qa thus causing relay La to pull in. Timer A, set to run for 1min, signals the end of this period by driving its output (pin 5 ) low. This transition, passed by capacitor Ca,starts timer B, which has a w r y brief run time. When output (pin 9) of timer B goes low, monostable B in IC2 is clocked through capacitor C3. This stage causes the reclocking of timer A approximately 0.2 sec after the "one minute" signal has been given. Unless interrupted, the TIMER LED Dl would blink perpetually a t the end of each minute. Each blink of the timer system clocks flip-flopC in IC6. This, withflip-flopsD (the A128 1 Journal of Chemical Education
other half of IC6), E and F (IC7) and one NAND gate in IC3, form alagic system that causes QF (pin 9) t o go high a t the end of the first minute. This signal turns on transistor Q4. Then relay Lnpullsin, thusapplying the depusition potential to the GCDE. At the end of the second minute, flip-flop B is reset, causing $e to go low. This position stops the magnetic stirrer. QE (ICI, pin 12) goes high when the third minute has elapsed. Transistor Q2turns on and DPDT relay L1 pulls in. This action stops the timing system by grounding the reset pins and also starts the stripping scan. Relay L2pulls in a t the same time, thus starting the chart motor. The scan, the rate of which is controlled by potentiometer I%, is generated by operational amplifier OA-A in dual IC8. Normally, integrating capacitor Cg is shorted by the contacts of relay L1 in their "dropped out" position. The output (pin 1)of OA-A is fed to OA-B in the same IC. OA-B acts as an inverting amplifier with a gain of less than unity. Bias, applied to the non-inverting input (pin 5 ) by potentiometer Rll, allows the initial, or deposition potential E , to be set a t -0.70 V. The ootential. which becomes decreasinelv .. . negati\r as the scan PIUCPF~%, is mmiturtd hy Oh-C in 1C9.ThisO.4, which tsn,nnrctrd a* a follower, feeds thc nun-inverting inpur
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Figure 4. Power supplies for strippinganalyzer. (a) iwic system. (b) operational amplifiers. (c) relay system. Capacitar: C1. Ce. CI. 1000pF: CI. O.1pF: 6.. 6. 100pF:6,. C5. C., 220pF. Bridgerectifiers. lA, BR,. BR2, BRI. 1W resistors: R,, RI, RI, 220% R,, 22fl. Transformers:T,. 6.3V: TI. 25.2V CT; TJ 12.6V. 12V 1A Zener diades, 2,.2 . . &. 5V 1A positive regulator. Qr. Pass transistor 0 2 , 2N3403 or equivalent on heat sink. F. 0.5A slow-blow fuse. (Continued on page A1301
Chemical Instrumentation (nin . r 5~) of~OA-D. ~ which ~ - is in. the same IC and is arranged a. a ~ompmstor.P o t r n t i m n ~ t ~ r Hi- hiasrs the mrening input win 6 )of OA-D at -O.JI, V, the chwen "stop" potential. The normally negative output of OA-D therefore swings positive when the scan potential has fallen just below -0.30 V. Transistor Ql then turns on. This on position of QI resets all flip-flops (except B, which is reset earlier in the run hy the logic system), stops integration, cuts off E., and causes STOP LED Da t o light. Sean rate, deposition potential, and stop potential can he altered by changing the settings of Rs, Rn, and Rn. respectively. These changes allow suitable parameters to he chosen for other analyses. If experiments that require different interval timing are contemplated, multi-range switching should he incorporated to permit easy change of the R3-C7 values. As alternatives t o the use of relays for the actual switching of the stirrer and the chert motor, optically coupled triac drivers may be used (9). Suitahly-marked outlets that feed the magnetic stirrer and the chart motor are mounted on the side of the ease. The latter also contains the line-operated power supplies that provide +5 V for the logic, f12 V for the amplifiers and +12 V for the relay systems. The circuits of these supplies are shown in Figure 4. A Heath EUW-20A chart recorder, switched to the 250-mV range, is used to monitor the cell current. This recorder has an internal plug and socket that enables the chart motor to be disconnected from themain switching system and joined to an external lead that plugs into the appropriate outlet on the Analyzer. With a GCDE of average characteristics, the maximum cell current likely t o be encountered is 20 FA which is ~
Figwe 5. Cvrenl-Wvoltage converter. 9V banerles. 0,. Bt: IC, 741. %W resistors, R,. RI. 10Kfk R3. 100KR; Rr. 1Kfl. BALANCE control R4. lOK& OUTPUT conm Re. 500n Switch S,. A run. B balance: Sa.on+H.
handled hy a simple current-to-voltage converter, the circuit of which is shown in F i i e 5. This circuit is placed with its input in series with the lead ta the GCDE. Potentiometer Fh allows the converter output to he adjusted to suit the recorder input. The tiny power requirements of the converter are satisfactorily met by small internal 9-V batteries.
(Continuedon page A132) A130 / Journal of ChemicalEducation
Chemical Instrumentation
peak height is measured as indicated (10). a calibration curve similar to Figure 7 can be constructed. When a device is required for a specific need, the "hardwired" approach can he both efficient and inexpensive. However, the versatility wiU be limited, because the "program" is built in during the design. With a computer as foundation, the programming of a process t o be automated can be done after the es-
sentials have been assembled. Changes in parameters can be made by a few key strokes. In fact, a maderate-sized computer can control the automation of an entire laborstorv. This kind of approach, invaluable when the acquisition and interpretation of data are the principal needs, has limited teaching value. Further, although a computer may he able to perform all sorts of calculations, it cannot control anything without a suitable interface. Some interfacing may require a considerable amount of hardwiring. The interfacing aspect can he emphasized by repeating a few of the ASV runs under computer control. Because of the limited number of parameters in the determinative stripping of a single metal, only a short program is required. This short program allows control to be imdemented with an inemensive "bread hoard" or training-type m& computer. Devices of this kind are commonly desiened to he roer.. rammed in hexadecimal ~" machine lnnguayr .4lrhough trdims in mutine use, this fwrn of prugrarnrning strecjes what is bring fed into the rornputer. Teaching computers usually allow the contents of memory locations, accumulator, program counter, and the like to be examined a t will. The microcomputer actually used is the Heath ETW-3400, which incorporates a Motorola 6800 central processing unit. Figure 8 i s the circuit of the interface, which is a slight modification of a circuit given in the training manual (111. The additions are (a1 an onboard jumper from LINE to IRQ which supplies a low-voltage 60-Hzsignal for timing. ( b )Connections from IC1, pins 2 and 3,
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Figure 6. Replicate stripping patterns. Lead concentration. 10 pM.
Figure 6 is a composite sketch made from tracings of automated stripping runs. If the
A132 I Journal of Chemical Education
Figure 7. Peak height versus lead concentration response Point X is the response obtained by a typical "unknown."
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Stirrer recorder
F ; Q J 8, ~lnterlacngfor computer control 01 ASV Capacitors. C,. C2. 100pF.C,. 47pF ',W resistors. R,. R2, R5. 1KR. R R, 2.7KI!: R,. Re. IOXL!. Rm, 33r(!!. Basemardmo~medcormolw Re. Re, tK!l Integrated circ~itdevtces Ct. MC6820; .C2.74.S30; IC3.7400: IC4.74LS27: IC5. MC1406; IC6. 301: IC7.741 Power connections to the IC's are shown at (a)
to the STIRRER and CHART relays, respectively. (c) A second OA, IC7 whieh inverts the (positive) output of IC6. The deposition potential, -0.70 V, and end of scan potential, -0.30 V, are set by suitable adjustment of potentiometers Rg and Rs. Computer programming fixes the scan time a t 2 min. The interface is built on a 4%-in. X 4-in. PC board (Radio Shack 276-157 or similar) that fits into a 44-pin connector. The pins that are used carry color-coded 22-gageleads that plug mto the npproprintesorkrrson the cwnputrr lhonrd. output^ rnnrkrd SIIHHEH nnd SCAUdrivr swaratr relm units similar to the Qa-La. Q~-L;arrangements of Figure 3. Interface board, relay units, and line-operated 12-V power supply for the relays are built into a small, well-ventilated box with a transparent top. This box carries outlets for connection to the stirrer and the chart drive motor. Binding posts are provided for connection to the current-to-voltage converter and the ASV cell. Figure 9 is the flow chart upon whieh the program is based. This program, written by Dr. L. M. Doane, is listed as an appendix. The read-only memory (ROM) of the Heath ETW-3400 microcomputer contains suhroutines that allow up to six characters to be displayed in the 7-segment readouts. After entering and checking the program (a quick and easy matter if done by dictation) the "Do" key is pressed and address 0000 is entered. On keying 3F, then FF, the run begins. When "rUnnIn." is displayed, the stirrer starts. Ten seconds later, the deposition po(Continued on page A134)
Figure 9. Programming flow chart Volume 57, Number 4, April 1980 / A133
Chemical Instrumentation
tential is applied. Otherwise, the sequence is the same as with the hardwired Analyzer. When the scan is over, "dOnE." is displayed. The keying of FF initiates a second run. Curve A, Figure 10, was obtained in a preliminary cheek of the computer and interface. Approximately one-sixth of the output from ICI, Figure 8, was picked off by means of a voltage divider and fed to the recorder. Curve B is typical of the recordings obtained during actual runs. Because IC5. Fieure .. 8.. the dieitnl-twanalop. crmwrtrr iDA(',, isonly nsixblr devtce, the steps in rhe scan are clearly viaihle. The use of an eight-bit l)AC wmhl yield much smoother scans. The laboratory handout has a final but optional step. This option in to illustrate the flexibility of programming. Before this step is attempted, the cell system, converter, stirrer, and recorder are disconnected and a voltmeter is attached to the interface binding posts. The interface has indicator LED'Sthat show when the stirrer and the chart motor would be running. As a first step, reprogramming to change the final display from "dOnE." to "FInISH." is suggested.
Literature Clted (1) Staek. J. T.. Annilrt. 87,908 (19621. (2) Stack, J. T and wo1ter. K. D., Anolyat, 101. 786
u -oa -o< -06
-05 Potential, V
Figwe ID. Response M r Canpure, mntrol. C m A, output from interface: B. typical stripping pat-
tern.
(197GI
(3) s&+; T. and Woltpr, K.D.. Sehwl Sei. Re", 87.
(201).728 (1976). (4) Doane.L.M.,Stock, J.T..mdStuart, J. D.. J. CHEW EDUC., 66.415 (1979). (5) Ellis, W D., J. CHEM. EDUC., 50.A131 (19731. (6) Ncrnberg, H. W . , h A d . Diu. Cham. Soc., 15.275 (1978).
(7) Roe, D. K..Anal. Cham., 50.9R (1978). (8) Radio Shack, "Semieonductar Referenee and A d i cation Handbmk," Fort Worth. Tern. 1378. (9) Motomla Semim"due(or Pmd"M. 1nc. "Applicatinu of the MOC3011 Trim Driver" Application Note ~
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(LO) Barendmht.E. in"ElDNavlalytiea1 Chemistry:~V~l. 2, Bard, A. J., (Editor), Marcel Dekkcr. I"
