Robert J. Cotter Genysburg College Genysburg. Pennsylvania 17325
An Inexpensive Instrument for Rapid Scanning Polarography
In rapid scanning polarography an entire polarogram is recorded within the lifetime of a single mercury drop. The polarogram is displayed on an oscilloscope as a plot of current versus the applied voltage, where the species reduced a t a dropping mercury electrode appear as current peaks. The positions of the peaks along the voltage axis are used for identification of the ions, while the peak heights are useful for quantitative measurements. Optimum sensitivity and reproducibility are achieved when the potential is scanned near the end of the mercury drop life, when the growth rate of the drop size is a t a minimum. For this reason, a rapid scan instrument must be capable of delaying the voltage scan for some predetermined time after the formation of a new mercury drop. Scan time (speed), voltage range;and offset must also be variable. Also, while it is possible to achieve excellent results by manual triggering, i t is more convenient to provide some means of automatically synchronizing the sweep circuit to the formation of each new mercurv drnn. sive components and uses the oscilloscope to set the sweep times and voltaees. T h e instrument is built around two 555 integrated circuit timers and five741 internally compensated ooerational amolifiers and is shown schematicallv in Fiaure 1.It is composed of three basic sections: a voltage sweep circuit and potentiostat; a current amplifier; and an automatic triggering circuit.
Voltage Sweep Circuit Timer ( A ) is used to delay the voltage sweep after the formation of a new mercury drop. The output of the timer (pin 3) is law until a negative pulse isapplied at pin 2. A 100 K resistor holds pin 2 at the supply voltage so that a negative pulse may be generated by momentarily depressing the pushbutton SI. The output will rise to +12 V and remain in the high state for a time A light pmitting dirrle (LED)is used to indicate when thedelay timer 1s opemring. The p~wntiometerRn is used to vary the delay limr from
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An 0.1 pf rapnrrtor differentiates the square pulse frum the output of tlmw (.41.The negntrve-going pulse at theend ofthedelay pulsr 18 a ~ ~ l l to r d~ l 2n of tlmw ( A )to activate rhe scan timing. in this c i r c k the timine resistor is reolaced hv a constant current source. so that the capacitor Cn is charged linrarly. The height of the linear ramp is 2 ~ tlw 3 supply \,oltagr,or R V.nnd th? time is given hy
where V. is the supply voltage; RE, the p u h p resistor;RI and Rz, bias resistors to set the base of the transistor above 213 the supply voltage; and V B ~the , base to emitter voltage drop for the transistor. The po-
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tentiometer R e is used to vary the scan time, and an LED at pin 3 is used to indicate when the scan timer is operating. Amplifier (1) is a voltage follower, whose output is applied across a voltage divider. The potentiometerR c is then used to set the height of the ramp from 0 to 4 V. The ramp is then applied to the non-inverting input of amplifier (2)where it is summed with a voltage offset applied at the inverting input The potentiometer RD allows variation of the offset. The output of the offset ramp appears at 52. Amplifier (3) is a voltage follower which uses the voltage sensed at the reference electrode in the feedback loop to determine the voltage applied at the working electrode (usually platinum).
Volume 54, Number 7, July 1977 1 455
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RS 1151: 5 0 PIV BRIDGE RECTlFiER Figm 3. Polarogram of a mixture of PbCi,. CdCI2,and NiC12in 0.1 MKCI. Delay time: 4.0 s; scan time: 0.5 s: offset: 0.06 V: andvoltage range: 1.8V. Vertical: 1 Vldivision (10 gAIdivision),horizontal: 0.15 Vldivision.
78L12: 12V. IOOMA POS. VOLTAGE REGULATOR Figure 5. Schematic of f12 V (100 mA) regulated power supply.
turned off.
Power Supplies
Figure 4. Palaragram of 0.004 MCdCI, in 0.1 MKCI. Delay time: 4 s:scan time: 2 S: oftset:0 V; voltage range: 1.5V: and drop time: 5.6 s. Vellical: 0.1 Vldivision (10 uAIdivision). horizontal: 0.15 Vldivision.
Setting the Scan Timers and Voltages Sweep timing and voltages are set on the oscilloscope. The horizontal axis is set a t 0.5 s/divisian and is triggered externally from the negative pulse a t JJ. The vertical axis of the scope is set a t 0.5 V/ division. The output of 52 is connected t o the vertical input (DC). When Sl is depressed, a waveform similar to that illustrated in Figure 2 will appear on the scope. Delay time, scan time, sweep range, and voltaee -. re" offset can then be set bv adiustine " Ra. Ra. Rr. and Rn. spectively. In the exampleshow, the scan circuit will delay for 3 sand scan the voltage range from 0.5 to 2.5 V in 1.5 s.
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Current Amplifier Circuit
Recording a Polarogram T o record a polarogram the voltage sweep a t 52 is connected to the EXTERNAL-X input of the oscilloscope, while the output of the current amplifier a t J 3 is connected to thevertical input (DC). SJ may be triggered manually when the mercury drop falls. Figure 3 shows a polaragram of a mixture of Pb(II1, Cd(ll1, and Ni(I1) in 0.1 M KCI.
Triggering Circuit It is possible to automatically trigger the sweep circuit to coincide with the beginning of each new mercury drop. The drop time is adjusted so that the falling drop appears as a fast negative-going pulse near the end of the oscilloscope trace. Figure 4 shows the appearance of the drop pulse near the end of s palarogram of 0.004 M CdW) solution. The hieb freauencv oulse is oositive a t theoutout of amolifier ( 4 ) . It oassesfhrou~ha 0.61 uf caoacitor and fed inio smolifier 15) where it is invertedand amplified and coupled t o the input of the delay timer, when switch S3 is closed. The polarograph will "free run," repeating a polarogram on the oscilloscope for every mercury drop.
456 1 Journal of Chemical Education
A power supply with positive and negative outputs is necessary t o run the circuit. Supply voltages o f f 9 V and i- 12 V have been used. The positive side of the circuit draws 25 mA (to run the op-amps, timers, and LEDs), while the negative side draws around 5 mA. A suitable power supply is shown in Figure 5. This instrument has been used for experiments in the undergraduate instrumental analysis course a t Gettysburg College. Some basic principles of electronics are covered in this course, and the design and use of integrated circuitry far chemical analysis is well illustrated with this instnnnent. . .~ I ~ l ~ cpol.in,graph i-.alw suml,le n n r r w m h inatru~rnrnr.7'hr rrlatwr speed the m e t h d hwomri irnponnnt when n lnrgr numbrr of cn\ inmmnltal s:~rnl,lri;wct,, I,P ~ t t ~ l ~The e d .present? of "penks" rather than diffusion plateaus increases the resolution in caseswhere many soecies are oresent in a eiven solution. Also. raoid scan methods havkbden aoo~iedto kinetic &dies of some metal ckrnolex reactions which prorzerl togl mp~dl\.fcr ca.nvrntim:il m e t h d r ur du rhun ruitaldc a l r i q n ~ t mprnkc to he tctlluurd hv rwviiil,le ipcrtruphw tometric methods.' The principles of rapid scan polarography are covered in most undergraduate texts in instrumental a n a l y s i ~ ,although ~,~ experiments for this purpose have not been included in laboratory texts. The author has recently submitted an experiment in Rapid Scan polaragraphy to this Journal," which uses the Heath Polarographic System. This present article offen an alternative instrument in the hope that others will find this method a valuable addition to a course in instrumental analysis. The author would like to gratefully acknowledge the assistance of several individuals in answering his many electronics questions. In particulgr, I am indebted to Stanley Orrell (Analytical Systems, Inc.), Theodore C. Daniels (Gettysburg College), and Louis C. Frees (The Johns Hopkins University). This work was supported by the Mellon Foundation and Gettysburg College.
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'Crow, D. R., and Westwood, J. V., "Polarography of Metal Complexes," Academic Press, New York, 1969. 2Willard, Hobart H., Merritt, Lynne L., and Dean, John A., "Instrumental Methods of Analysis," 5th Ed., D. Van Nostrand, New York, 1974, pp. 633-670. %koog, Douglas A., and West, Donald M., "Principles of Instrumental analysis," Holt, Rinehart and Winston, New York, 1971, pp. 553-578; 587-591. 'Cotter, Robert J., "Qualitative and Quantitative Analysis Using a Rapid Scanning Polarograpby," d. CHEM. EDUC., 54, 251 (1977).
