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Robert J. Cotter Gettysburg College Gettysburg. Pennsylvania 17325
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Qualitative and Quantitative Analysis Using a Rapid Scanning Polarograph An experiment for undergraduate instrumental analysis
Most texts on instrumental analysis include a brief survey of the recent polarographic methods, including: rapid scanning polarography, a c polarography, cyclic voltammetry, and de. for such rivative polarography ( 1 , 2). ~h~ instrumentation methods is more complex a n d usually more expensive t h a n t h a t used for classical ~ o l a r o g r a ~ havn, d for this reason is generally unavailable in-the u n b e k r a d u a t e laboratory. Thus, few experiments using these techniques have heen published i n undergraduate laboratory texts. A rapid scanning polarography experiment is described below using the Heath Polarograph System. T h i s instrument is already in use in a number of undergraduate laboratories a n d is easily modified t o d o this t y p e of experiment. (3)T h e student is introduced t o t h e basic principles of rapid scanning polarography and, when familiar with t h e technique, can produce excellent qualitative a n d quantitative results.
Theory Rap~dsrnnniny differs fn,m omventional polanlgraphy in that the entire pdnrogmm is taken well within the lifetime of a single mercury drop. As in classical polarography, a rapid rise in current occurs at the point where the voltage sweep (versusthe saturated calomel electrode, SCE) reaches the reduction potential of the metal ion at the dropping merculy electrode (DME). Because the voltage scan begins when the droo size is near maximum and rises verv raoidlv. the diffusion laver a r o k the DME ~-is still mite =~~~~ small & the reh;ction ootentiai is reached. The CUrrQntat this point is much hignrr tnan in clnawcal puluugraphy, due to the increased number oi ions bong redured at the mercury electrode. The diffusion layer does not reach a constant size, as in conventional polarography, but rapidly increases as the ions in the vicinity of the mercury electrode are removed. Diffusion then becomes the limiting process and the current drops, resulting in a current "peak" rather than the familiar diffusion current plateau of conventional oolaroeraohv. .. . Firmre .. 1comoares the essential features of ronventic~naland rapid scan pdarographs. The potentdnt the peak, ur "sumrn~tpotential" is related to the half-wave purential of conventional polarogmphg (41 ~~
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which corresponds to a shift of about 28 mV1n to more negative potentials. The summit potential can he used far qualitative determinations of a metal ion species. Quantitative analysis is possible from measurements of the peak currents, given by the Randles-Sevcik equation (4-7) = kn3/Zm2/3t2/3~L/Zul/Z~ where ia = summit in pA, = 2,72 105, = number of electron equivalents, m = rate ofmercury flow in mgIs, t = drop time ins, D = diffusion coefficient (cm21s),u = speed of the voltage sweep in Vls, and C = concentration of the metal ion in the bulk of the so. lution (mmolell). Comparison with the Ilkovic equation in classical polarography (1.2) reveals that summit currents are much higher than the diffusion currents of conventional polarography and are generally increased with increasing speed of the voltage sweep. The scan speed is limited, however, by the fact that non-faradaie currents, familiar in conductance measurements, occur at higher frequencies. In general, summit currents may be expected to be a factor of 10 or more greater than diffusion currents. resultine,. in hieher .. sensitivitv,for solutions of low cuncentrations ln'addltmn. the use of peaks rather than half-wave menwrements results in greater resolution between species havmg similar reduction potentials. ~
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Equipment Used Heath recording polarograph unit (Model EUA-19-2) Heath operational amplifier unit (Model EUW-19A) Heath dropping mercury electrode apparatus (Model EUA-19-61 Capacitor decade hox Time delay relay (Allied 818R1135,l-15 s "off-delay" relay) Oscilloscope and oscilloscope camera Stock solutions: 1.0 M KN03; 0.01 M CdCL; 0.01 M Co(NOd2. 6Hz0; 0.01 M NiC12.6H20; 0.01 M Pb(NO&; 0.01 M ZnCIz instrumental Modifications and Procedure The 10 pF capacitor in the Heath polarograph sweep unit was removed (31,and a capacitor decade box was connected through external terminals on the polamgraph. The sweep rates shown in Table 1were obtained hv varvine - " the external eaoaeitanee and the "sweev time" settings on the palarograph unit: The output of the time-delay relay was connected to the same terminals as the capacitor decade box. The sweep voltage, present a t the E,rterminal, was connected to the EXT X input of a Telequipment 083 ascillweope,and set to 1.5V/full scale. The 10 V output of the current amplifier was connected to the Y-input of the oscilloscooe. The entire setuv is illustrated in Figure 2. All palarograks were obtained "sing a three electrode system, to eliminate errors in E. due to ir drop across the cell. This is especially important in this technique, because of the much larger currents ohtained in the polarogram. The drop rate for the DME was adjusted to about 5 s. This must be accomplished by a trial and error method with the sweepon, since thedrop rate is significantly affected by the rapid sweep and the field gradients associated with it. The time delay relay was adjusted to 4 s and the voltage scan to 3 Vls, so that the
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Table 1. Sweep Rater Obtained at Varying Capacitance and Sweep Rate Settings capacitance (pF)
"sweep setting" (Vlrninl
sweep rate ( V l r )
E VS. S C E F ~ g w e1 Comparison of conventonal and rapd scan polaro(yams (a) 0 001 MZnClt m 0 1 MKCl a! 0 2 Vim n. lo) rap d scan pc arogram of 0 001 M ZnCI, m 0 1 MKho~at3Vlr C m s n t s muA
Volume 54, Number 7, July 1977 1 457
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110 V A C Figure 2. Connectims to Heath polarography system for rapid scan po&rgraphy experiments.
dE/d t Figwe 4. Summlt currents f w 0.001 MPb(l1) v m u s the square rwt of the speed of voltage sweep. currents are in PA.
Figure 3. Rapid scan polarograms for CdClz in 0.1 MKCl at different concentrations. (a) 0.004 MCdCI,: (b)0.002 MCdCI*: (c)0.001 MCdCI2.Current is in
polarogram was taken during the last second of the drop life. (During this period, the increase in drop size is minimal and results in more reproducible currents.) A Telequipment C-5 (Polaroid) camera was used to record the polarogram. The shutter was held open and the time delay relay aetivated manually as the mercury drop fell. The shutter was then released after completion of the voltage sweep.
E VS. S C E Figure 5. Quantitative and qualitative analysis of an unknown mixhjre. Sweep speed was 1.0 Vls. Current is in fiA. Table 2.
Summit Potentials and Currents for
solution (0.001 MI
E. ( V l
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Some Metal Ion$
"A
EX^ ( V I
Experimental Procedure and Results Qualitative Analsis Using the stoek solutions, the following standards were made by pipetting 10 ml of metal ion solution and 10ml of KN03solution into a 100-ml volumetric flask and diluting t o the mark with distilled water
Peak suppressors, such as TRITONX-100,are not used in the rapid scanning technique. The results for summit currents and potentials are summarized in Table 2 and are compared with someliterature values for half-wave potentials.
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Onlntitativo --.....-...- .Andveic ... ,. .
Standard solutions of cadmium ion were made using 10,20, and 40 ml, respectively, of the cadmium stock solution. The polarograms for 0.001 M, 0.002 M, and 0.004 M Cd(l1) are shown in Figure 3.
Effect of Voltage Sweep on Summit Currents Summit currents for 0.001 M Ph(I1) solution were measured for voltage sweeps of 1,2, and 3 Vls. The results are shown graphically 458 1 Journal of Chemical Education
Scan speed: 3 "15. Reference ( I ) . PP. 821-2. for 'Value in 0.01 M KC1 rolution.
Table 3.
0.1 M KC1 rolutionr.
Qualitative and Quantitative Analysis of an Unknown
aSummit currents corrected to 3 Vlr assuming complete reversibility. b Determined from summit currents given in Table 2.
in Figure 4, where the linearity of a plot of summit current versus the square root of the voltage sweep is an indication of the reversihility of the electrode reaction and verifies th'e Randles-Sevcik equation ( I , 5.8).
-. .....- .... .Gnlrltlnn --.-..-.. 1 lnknnwn
Unknown solutions may be prepared using combinations of the stock solutions. The polarogram for a mixture of Cd(II), Co(II), and Zn(11) is shown in Figure 5. The polarogram was scanned a t 1.0 V/s
and the sum mi^ cunentscorrected b t h e l r 3.0 Vls valuestode~ermine r~mrrntrations.The rerultr are summarized in Table 3.
Conclusions Rapid scanning polarography experiments provide a valuable addition t o a study of electrometric methods. Excellent accuracy is obtainable with this method, even with manual triggering. Errors in response timeare minimized by scsnningin &last secondofthedrop life. Other methods, psrticulaiy ae polarography, are perhaps capable of greater resolution and sensitivity and may be the method of choice
for some research problems. The instrumentation in ac polarography is more complex and more expensive, involving the use of a lock-in amplifier and phase sensitive detection (9). The circuitry for rapid scanning, on the other hand, is relatively simple and provides a good introduction to oscillographic methods. In addition, while rapid scanning has obvious advantages in speed, it is perhaps most useful for the study of reaction kinetics for thme inorganic complexreactions which proceed
P ~ (10). Y
tw rapidly to be followed by conventional polarogra-
'l'hii work was supported m pnrt by a grant from the Mellon Foundario~nand Gcttvsl,urr: College. T h e author wishes to acknowledge the assistance of Dorrece L. Bond in preparing solutions for this experient and "student testing" the experiment for use in an instrumental course.
Literature Clted (11 Wfll~rd,HohartH.Merritt,LynneL..andDean. JohnA.."lnstrumentalM~thadsaf Analyois."5th Ed., D. Van Nostrand, New York, 1974,pp. 633670. (2) Skng,Douglas A.. and Wet, Donald M. "Prineiple~ oflnatrumentalA n a l ~ ?Holt, ~," Rioehartand winston.~ew~ork. l971,pp. 5~~78:587-591. (3) Heath (EUW-4011Polarwaphicsynfem ~snusl. (4) Schmidt. Hdmut, and von Stackelberg, Mark, "Modern Polamgraphic Methods," Academic Press, New York. 1963. . see. 44,327-334 (1948). (5) Rsndles, J. E. B., T ~ o n s~orodoy (6) Sevcik,A.. calieetion c~eehosiou.chsm commun., 13.349 (1948). Deiahay,P,J.PhYs. Calioid. 54.630(1950). (81 Cruse,K.,andHoberle,W . Z . Elektrochem.. 57,579i1963). lnstrumentation,"w. B. Saundera. (91 Diefenderfer.A. Jamea;.Princi~k of ~leetronia ~hiiadelphia.1972. pp. 550.557. (10) C w . D.&and W e s t d . J. V.."Pokw~hy of M o f a l C o m p l e r e s , " A u d m i i ~ ~ , New Ymk, 1969. h Y ~ ~dA d d m i i ~ P r ~ , (11) ~ ~ ~ r ~ u t aJ., ~ ~ ~k k ~~ ~i i iVi P ~, I~I ~ ~ P~~ B m g. r a P~ New York. 1966. pp. 497-511 (19661.
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