GALEN W. EWlNG Seton Hall University South Orange. New Jersey 07079
XCVII. Pulse Polarography: A Series of Student Experiments
EQUIPMENT
Wllliam C. Hoyle and Thomas M. Thorpe Miami University, Oxford, Ohio 45056 Since its introduction in 1922, polarography has enjoyed avery remarkable and rapid growth and has been an essential chemical tool for many years. During the mid-sixties, atomic absorption spectrascopy undoubtedly cut into this erowth 11). hut several recent The most important are the development of the pulse palarographie techniques (2) and the availability of moderate cost commercial instrumentation which incorporates the pulsed modes into easily operated instrumental svstems. In addition. the current concern ahmt the trow c o n ~ r i t u r n rin~ air and w s e r rhwld rpark funhrr development of pulse pulamgraphg for the determ~nnrion and speciation of trace metal pollutants and the measurement of eleetroactive organic compounds occurring a t trace levels. As a result. there is an increased need for undereraduite and eraduate lahoratarv,~~ teachine" " experiments w h ~ ~ h I ~ m o n s t mthe r e principlrs, applicatims, and versatility of pulsed voltammetric techniques I t is the purpose of this paper to present a series of experiments illustrating the spplication of pulse or differential . Duke . ~ o l a r o.e raphy to: 1) The study of metal-complex formation 2) End-point deteetion in a titrimetric analysis, and 3) The determination oftrace metals in environmental specimens Each of these experiments is routinely employed in our undergraduate instrumentation courses.
THEORY
I'ulse polaro~mphyran he carried out in two modes; nurmnl pulsr pdamgraphy and differential pulre pnlnroplaphy. Normal pulse polarography involves the imposition of square wave voltage impulses of increasing magnitude upon a constant dc voltage. In the differential Duke mode. fixed maenitude
just before termination of the voltage pulse. For a given pulse, the output is recorded as the difference between the two current flows. For a more complete description of the pulsed techniques see Ref. (3). The advantage of the pulse techniques results from the measurement of the current flow during that portion of the Life of the mercurydrop and pulse when the faradaic current has decayed to the diffusion limited value and the capacitive eurrent is a t a minimal value. This approach yields a n analytical technique with a greatly improved signal-to-noise ratio (sensitivity) and in manv cases greater seleetivitv. Fur~, thrrmore, the current-voltngr outpuu of the pulse tr~ht~iquesareearwr tointrrpret than thosrofdc p u l a m ~ ~ a m s s mtheoscillatims rr due to the growth of the mercury drops are eliminated and far differential pulse polarography the output is peak-shaped rather than the typical step curve of the conventional polarographic methods. ~~
The major piece of equipment required for these experiments is a palarograph capable of operating in the pulse and differential pulse modes. If comparisons of the pulse operation with dc polarography are to he made, then the latter mode must also he available. A Princeton Applied Research Corporation (PARC, Princeton, NJ) Model 174A Polarographic Analyzer and a Hewlett-Packard Model 7040A X-Y recorder were used in these experiments. Other suitable polarographs include the Metrohm Model E506 Polarecord (Brinkman Instruments, Westbury, NY), and the PARC Model 374. Thorough deaeration of the solution being analyzed is required t o avoid interference from reduction of dissolved 0%. Prepurified nitrogen was used for deaeration after scrubbing with a vanadous chloride solution to remove residual Oz and followed by presaturation with water vapor by bubbling through a solution of @upportingelectrolyte (~4 -) , .
The PARC 174A was operated in a three electrode configuration, employing a DME as the working electrode, a Pt-wire auxiliary electrode, and a SCE reference electrode. The SCE was isolated from the test solution with a Vycor-tipped salt bridge.
EXPERIMENTAL
Am~erometricdetection methods e m he applied to a wide vanety of litrations mvulving oxidafion.redurtion, complex f ~ l r matim, ond prroipitatim rcartiuni. In fact amperometry can he used to determine any substance-molecular or ionic, organic or inorganic-if only one of the reactants or products in the titration can he oxidized or ;educed a t the working electrode. Neither the (Continued on page A230) ~
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William C. Hoyle received B.S. and M S . degrees in Chemistry from Northern Illinois University in 1966 and 1969. He earned his Ph.D. under the supervision of H. Diehl a t Iowa State University in 1973. After spending one year with the Illinois Environmental Protection Agency, he became an assistant professor of chemistry a t Miami University in Oxford, Ohio. Dr. Hoyle is now an advisory scientist with the Continental Group Inc., of Chicago, Illinois. His research interests include electroanalytieal, co-ordination and environmental chemistry as well as various aspects of health and safety related chemistrv. Thomas M. Thorpe received a B.A. degree in Chemistry from Lafayette College in 1971. He earned M.S. and Ph.D. degrees a t the University of Illinois in 1973 and 1975 under the direction of D. F. S. Natusch. He has been an assistant professor of chemistry a t Miami University in Oxford, Ohio, since August, 1975. Dr. Thorpe's research interests include the application of gas-liquid chromatography, atomic absorption spectroscopy, electrochemical techniques and evolved gas analysis ta the speciation of inorganic and organometallic substances in environmental samples.
Volume 55, Number 5, May 1978 1 A229
Chemical Instrumentation titrant nor the titrated species need undergo a reversible electrode reaction, nor is a welldefined limiting current plateau a requirement. Consequently the scopeof possible titrations by amperometry is very large. In an amperometric titration, the polaro..eraohie diffusion current of an ion is measurcd and plotted against thc volume of an added reagrnt. In this rxperimcnt zinc will i r titrnted with EDTA.
the student an application of polarography as an end point detection device, the effects un rhr current caused b).chan:rs in mrenrmtion, pH, and t'mmatiun o i a shghtly disamatcd cvrnplr~ran Ire pointed our. The,? rffrrtr are more thon9ughlv dcsrrhrd in the follwing expermenti.
M+"
+ ne-
s M"(Hg)
(1)
is described by the equation
Where E is the applied potential; El/z, the half wave potential for reaction (I); i, the current a t the value of the applied potential; and id, the diffusion limited current. If, upon addition of a ligand X to the solution containing the metal ion M+n the reaction
.
Procedure Place 50.00 ml of standard 0.100 M zinc solution and 100 ml of 1.0 M sodium acetate-acetic acid (pH 5) buffer in a 250-ml volumetric flask and dilute t o volume. Pipet 50.00 ml of this 0.0200 M Zn solution into the polarographic cell. The electrode tip should be immersed a t least 1cm; if necessary add just enough water tocover theelectrode tip. I t is essential to know the total volume of solution in the cell before the titration is started so that a correction far dilution can be made later. Add 1drop of 1% Triton-X 100 and deaerate. Measure the diffusion current for zinc in the solution, then add a small amount of EDTA; the resulting diffusion current a t -1.20 V versus SCE will he reduced by an amount equivalent t o the zinc complexed (see Fig. 1). When sufficient
working electrode to the metal dissolved in mercury:
~ r c u r sthr , hnlf-wave potrnt~nlis obierved to change according to the equntim
-
where K n isthedisst,cistiuncunstnnt fm the \IS,,series = K I ; p , the mmherof ligands nssoc:arPd with eavh metal ion: 1x1.the molar concentration of the ligand; GI;;,the shift in half-wave potential. Equations (2) and (4) together with the polarographic data obtained from the following procedure allow students to determine n, p and KdI(Kr) for the lead-oxalate complex. Prepare the following solutions in 100 ml volumetric flasks:
'
Figure 2: Ampemmetric titration curve of 0.0200 M Zn2+ with 0.10 MEDTA
Table 1. Typical Polarographlc Condltlons.
Condition
Lead-Oxalate Complex Formation
Amperometric Tination
Tap Water
Anallyses
Made
Pulse ffi
Drop Time,
1
Nahlral 1
1
-0.75
-0.20
-0.20
+0.10
0.75
0.75
0.75
1.5
1
1
1
2
Neg.
Neg.
Neg.
Neg.
Pulse Diff. Pulse
.t ~s.cCI ~...
e for the amperomelric Figure 1. P ~ l r polarograms t Ration of 0.0200 MZn7* wlh 0 10 MEDTA
EDTA has been added to just complex all the zinc, the wave will completely disappear and the addition of excess EDTA will have no effect. A plot of diffusion current versus volume of EDTA added will appear as shown in Figure 2. The end-point is the intersection of the two straight lines. Note that dissolved oxygen gives an mtrrfrrmg u m c in this rpgion and musr lw remored. After the add~tion ufcach inrrpment of EDTA bubble nltnrcen throueh the solution for about one minut; to u stir the wlutim and tu flush our the O X ~ Y C I I that was mtnduced with thetirmnt.Typical polnrographic nmditions arc shown in Table 1.
This pracedure can be used to standardize the EDTA solution and may he followed by an analysis of an unknown solution for the Zn content. Since there is no electrode calibraof the analvsis is tion reouired. the accurnev ,~ ~* ~~not~limitedby the ac&&y with which the current is measured; the limitation is in the sample preparation. In addition to showing
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A230 1 Journal of Chemical Education
Initial Potential (V vs. SCE) Potential Range (V)
Scan Rate (mvlsec) Scan Direction Modulation Amplitude (mv)
...
...
...
50
Polarographic Study of Metal Complex Formation In recent years voltammetry has become a prominent tool in the investigation of slightly dissociated metal complexes ( 5 ) .When oxalate ions are added to asolutian of lead(II), a white precipitate is immediately formed; however, with the addition of excessoxalate the precipitate dissolves indicating the formation of a sliahtlv . . dissociated lead oxalate ion. In this exoeriment the formation constant and stoirhiomerr) d t h e lend-uxalate 5)srrm will be in\,estigntecl ( 6 1 .The polaragram for the rrductlun uf a metal ion, \I"., at rhe
To each solution add 2 drops of 1%Triton-X 100 and dilute to volume with deionized water. Transfer solution 1 t o the polarographic cell, deaerate for 10 min and record a polarogram a t applied potentials from -0.20 to -0.95 V versus SCE. Set the initial voltage carefully since accurate potential readings are required. In a similar fashion run polarograms of solutions 2 through 5. For each of the five polarograms plot log ((id - i)/i) versus the applied potential, E. Determine from the plots the El/z for each of the solutions and the number of electrons, n, involved in the reduction. Then plot the E,r, for each solution versus the log [CzOa2-1 and determine from the slope the number of oxalate ions,p, associated with each lead ion in the slightly-dissociated complex. From the plot of ElIz versus log [ C z O F ] and from the Elis of the lead ion in 1.0 M KNOa calculate the formation constant for the lead oxalate complex. The data for this experiment can he acquired in either the de or pulse modes of operation. Typical polarographic operating conditions for both modes are given in Table 1. Some results we have obtained for each mode are given in Table 2. Interesting take-home exercises for the student, using the above data, could include proposing a structure for the lead oaalate complex and determining the coordination number for lead in this complex. The student should also be capable of discussing qualita(Continued on page A2321
Chemical Instrumentation tively the effect of pH on this experiment given pKl = 1.2 and pK2 = 4.3 for oxalic acid.
Determination of Copper and Zinc in Tap Water Differential pulse polarography (DPP) has been utilized for the determination of trace metals in a variety of natural samples, for example drinking and waste waters, and biological fluids (7,s). The high sensitivity and low detection limits of DPP (1-5 X 10W M ) are essential for its application to measure-
ments of As, Cd, Cr, Cu, Ph, or Zn a t the pgA (pph) to mgA (ppm) level8 of these metals commonly found in natural specimens. For the determination of very low concentrations of metals to he successful, extreme care must he exercised in the handline..and oreoaration of samples and standnnls. Tram metal-rontmning contaminants rhould be a w i d d , and high purity reagents shwld he wed at all stages 01 the analyiia. 1)I'I'may bewbjert t
