Pertanika 8(3), 399 - 409 (1985)

Different~al Pulse Polarography and Voltammetrywith an Automated .Microprocessor-based Polarograph

and a Static Mercury Drop Electrode

W.T. TAN and G.S. TANChemz"stry Department,

Faculty of Sdence and Envz"ronmental Studz"es,Unz"versz"tz" Pertanz"an Malaysz"a,Serdang, Selangor, Malaysz"a.

Key words: Analytical evaluation; pulse; stripping; precision.

ABSTRAK

PolarografZ" denyut pembeza dengan menggunakan penganalzszs polarografz"k yang berdasar-kan kepada pemerosesan mz"kro automatz"k dengan elektrod raksa pegun statz"k telah dz"nz"la{terhadapamplz"tud denyut, kadar skan, masa menz"tz"k (t), dan luas tz"tz"k. Dz"dapatz" perubahan punca arus (z";dengan luas tz"tz"k dan t - Y2,. perubahan punca keupayaan (E) dengan amplz"tud denyut bersetuJudengan teorz" secara sederhana. Akan tetapz" dz"dapatz" pengantungan z" dan E kepada kadar skan danamplz"tud denyut yang besar dan terkedl telah dz"dapatz" menyz"mptng daizpada teorz". Keputusanalz"ran yang serupa juga dz"temuz" untuk voltametrz" melucut anodz"k apabz"la langkah melucut denyutpembeza dz"gunakan. Kepreszsan kedua-dua teknz"k z"tu adalah baz"k. Kuprum dzsadurkan secara z"ndz"-vz"du semasa voltametrz" melucut anodz"k.

ABSTRACT

Differentz"al pulse polarography usz"ng an automated, mz"croprocessor- based polarograph~'canalyzer equzpped wz"th a statz"c mercury drop electrode has been evaluated wz"th respect to pulseamplz"tude, scan rate, drop tz"me (t), and drop area. Varz"atz"on ofpeak current, z"p' wz"th drop area andt - Y2, wz"th some pulse amplz"tudes agree reasonably well wz"th theory. However, dependence of z" and Eori scan rate and on pulse amplz"tude at very large and very small amplz"tudes wasfound to devz"dte fronitheory. Sz"mz"lar trends were also found for anodz"c strz"ppz"ng voltammetry when differentz"al pulsestrz"ppz"ng was used. The preczsz"on attaz"nable by both technz"ques was good. Copper peaks demonstrat-ed good precisz"on only when copper was plated out z"ndz"vz"dually durz;n,g anodz"c strzppz"ng voltammetry.

INTRODUCTION

The utility and novelty of computerizedsystems for electrochemical analysis using on-linemini computers have been well. documented(Keller and Osteryoung, 1971; Smith, 1972).However, its high cost and complexity haveprevented its widespread use. Recently, due to

the advent of reasonably priced microprocessors

( Ji p), there i~ an upsurge of interest in the incor-

poration of digital. r:nicrocomputers in electro-

analytical devices (Bond and Grabaric, 1977;

Barrett et al., 1980; Anderson and Bond, 1981,1983). These computerized systems enabledigital methods of data acquisition, dataanalysis, instrument control and hence somedegree of automation;

The first generation up-based polarographconstructed by EG & G Princeton AppliedResearch Corporation (PARC) was the PARCmodel 374. Its performance characteristics fordifferential pulse methods and anodic strippingvoltammetry (ASV) have been described by

W.T. TAN AND G.S. TAN

Mm,x = ~~~2 AC(-lili) (-J 1T~) (2)

interval between pulse amplication and currentmeasurement and E 2 - E 1 = 6. E, the pulseamplitude ( 6 E is negative for reduction). Otherparameters have their usual meaning. If 6E/2 issmaller than RTInF, equation 1 is simplified to:

theory for DPP and differential pulse anodicstripping voltammetry (DPASV) using thesecond generation J1. -p based polarographicanalyzer (PARC model 384) equipped with aSMDE. This study is essential for this fully auto-mated instrument before it can be used forroutine analysis. The analytical evaluation,which is the theme of this paper, is also donewith the future user of this particular instrumentin mind.

Based on the Cottrell equation for lineardiffusion to a planar electrode, several theore-tical treatment for DPP have been described(Barker and Gardner, 1958; Parry and Oster-young, 1965; Keller and Osteryoung, 1971 ;Anderson et ai., 1981) for a simple, reversibleprocess. A more general and less complicatedexpression for the differential pulse current, 6. i,was put forward by Parry and Osteryoung(1965). In their theoretical treatment, maximum6. i( 6.i ) or i is described by the following:

max p

(1)

a = exp (E2 - E1 ~ ) l' = time2 RT'

ill =nFAC~ a-Imax -- ---11'1' a+ I

where

Bond and Grabaric (1977). Most of the detailswere concerned with the consequences -of theshort drop times available from a pressurizeddropping mercury electrode (PARC model 302)and the comparison of direct current (DC),normal pulse (NP) and differential pulse polaro-graphy (DPP), also at short drop times « 1 sec),and the DC distortion effect using the mostrecently introduced electrode system, the staticmercury drop electrode (SMDE) (Peterson,1979; Bond and Jones, 1980; Anderson et ai.,1981). A laboratory built up-controlled polaro-graph which could generate only staircasevoltrammetric and NP mode waveforms wasreported and evaluated by Barrett et ai. (1980).Results for the reduction of Cd(II) closely followtheoretical trends. An improved version of a up-controlled polarograph was later constructed byAnderson and Bond (1983) which has the capa-bility to generate square wave, alternatingcurrent (AC), DC, and pulse polarograms.

Analytical evaluation of pulse polarographswas first reported by Parry and Osteryoung(1965) on their laboratory built analog polaro-graph using a DME. The reported instrumentalartifacts in the PARC model 174 (InstructionManual) has been substantiated by Christie et ai.(1973). Anodic stripping voltammetry, whichhas gained much popularity recently and isespecially useful for determining trace elementsat the subppb level, has also been evaluated bymany workers (Copeland et ai., 1974; Batley andFlorence, 1974; Valanta et ai., 1977) using theanalog units, the PARC model 174 and 174A,incorporating a hanging mercury drop electrode(HMDE) or mercury film electrode. Strippingtechniques reported include DC, staircase,square wave and differential pulse.

A recent review in Analytical Chemistry(Ryan and Wilson, 1982) emphasized the need toevaluate theoretically and experimentally thosecomputerized pulse techniques which canimpress a variety of different potential wave-forms.

Thus far there is no report pertaining to amore thorough experimental verification of

These expressions have neglected the contri-bution from the dc term, 6 i de' A more specific6. i expression which ta;kes into account this termwas described recently by Anderson et ai. (1981)for a stationary electrode like the SMDE. Thecontribution from both i and 6 i dare strictl'yadditive for the reversible ~lectrode process. Theeffect of dc distortion using a SMDE has beenwell documented in the work reported by Ander-son et ai. (1981). The 6i determ can be minimiz-

400 PERTANIKA VOL. 8 NO.3, 1985

DIFFERENTIAL PULSE POLAROGRAPHY AND VOLTAMMETRY

ed and hence neglected when reasonably longdrop times (> 0.5 sec) and large L\ E values (> 5m V) are used. For these reasons and also itssimplicity, the theory of DPP described in equa-tion 1 is used in this work. A similar expressionhas been successfully used by other workers totest their instrument (Parry and Osteryoung,1965; Christie et at., 1973; Blutstein and Bond,1976).

According to equation (1), (L\ i) (or i asmax p

aetermined experimentally) obtained from areversible reduction process of a simple system, isexpected to be a linear function of the concen-tration of eh~ctroactive species, drop area,inverse square root drop time (for constant droparea), 1 - a and pulse amplitudes (L\ E). Peak

, 1 + apotential is given as E p = E ~ - L\ E/2 (3)

Even though equation (1) does not take intoaccount the variation of i with scan rate ( v),

. pcertain predictions can be made from the treat-ment of other similar techniques. According toSevcik (1948) and Roundles (1948), the relation-ship between i and v, in linear sweep voltam-metry was degc~ibed by equation (4): i = K X

pv!Y.! where K is proportionality constant. Peak

current increase with VI. No peak shiftingshould occur when scan rate is varied. Thevalidity and hence the usefulness of the instru-ment which performs DPP and DPASV shalllargely depend on the above relationship be-tween i and the various related parameters.Peak pbsition and peak width should alsoconform to the theoretical expectation.

MATERIALS AND METHODS

Apparatus

The polarographic and voltammetric expe-riments described h~re were perforined using anEG & G PARC model 384 second generationmicroprocessor-based polarographic analyzerequiped with an EG & G PARe model 303SMDE. The differential pulse mode was usedthroughout. The 20 ml capacity cell used wasmade of quartz glass. A three electrode systemwas used. It consisted of a working electrode

(DME and HMDE modes in SMDE system wereused throughout for the DPP and DPASVexperiments respectively), a platinum auxilliaryelectrode, and a AgiAgCI (saturated KCI)reference electrode. In DPASV, the stirring wasdone with a teflon covered magnetic stirring barwhich was run by the PARC model 305 magneticstirrer. The polarograms and voltammogramswere recorded by a digital plotter, PARC modelRE0082.

Reagents and Procedures

Reagent grade chemicals were used. A 0.10M stock solution of ed(n) was prepared by dis-solving CdCl

2in deionized, distilled water

(DDW). DDW was prepared by running labo-ratory prepared distilled H p through a Milli-pore water purification system and used forsample preparation as required. A more dilutesolution of Cd(n) was prepared by 1 : 10 dilutionof the stock solution. 0.10 M KNO 3was used as asupporting electrolyte. The nitrogen used todeoxygenate the solution was purified by passagethrough activat~d molecular sieve and deioniz-ed, distilled water.

The blank was initially run on a 10.0 ml0.10 M KNO 3solution followed by stepwise addi-tion of 10 to 100 ul of Cd(II) stock solution asrequired. Blank subtraction was done instru-mentally for all experimental runs. Tangent fit,an iterative numerical routine which fits the besttangent to the baseline of each peak was usedthroughout to measure peak heights. For sensi-tivity and reproducibility studies, nine experi-mental runs were. performed using preparedCd(II) standard and 0.10 M KNO 3supportingelectrolyte.

RESULTS AND DISCUSSION

In this work, peak currents for 6.0 X10 -5M and 1.0 X 10 -3M Cd(n) solutions wereobtained for the various scan rates (v ) available(2,4, 6, 8 and 10 m VIs) at 5, 10, 25, 50 and 100m V pulse amplitudes using a 1 sec drop time.These data are summarized in Tables 1 and 2.Fig. 1 is a plot of (L\li) or i vs scan rate for

max p

various values of L\ IE. In general, any linear

PERTANIKA VOL. 8 NO.3, 1985 401

W.T. TAN AND G.S. TAN

TABLE 1Dependence of peak current and peak potential on scan rate and pulse amplitude at constant

drop time of 1 sec. and Cd(II) concentration of 1.0 X 10 -3M during DP}>

Scan rate 2 mV/s 4mV/s 6 mV/s 10 mVIs

i E i E i E i Ep p p p p p p p

~E "I - iJ (JlA) (mV) Q}iA) (mV) (IJl,A) (mV) qJlA) (mV)(mV) 1 + a 1

5 0.097 1.24 556 0.795 564 0.645 560 0.447 540

10 0.193 2.69 555 1.70 560 1.40 566 0.970 550

25 0.453 5.93 548 3.68 556 3.21 554 2.12 550

50 0.750 10.69 534 6.97 540 6.35 548 4.80 540

100 0.960 13.73 508 14.2 536 13.3 536 10.5 520

TABLE 2Dependence of peak current and peak potential on scan rate and pulse amplitude at constant

drop time of 1 sec. and "Cd(II) concentration of 6 X 10 -5 M during DPP

Scan rate 2 mV/s 4mV/s 6mv/s 8 mV/s 10 mVIs

~E(mv) i E i E i E i E i Ep p p p p p p p p p

(IJl A) (mV) (IJl A) (mV) (JlA) (mV) ~,uJA) (mV) CfJ;A) (mV)

5 0.085 556 0.065 556 0.049 554 0.039 548 0.032 530

10 0.190 552 0.145 552 0.099 554 0.083 548 0.062 548

25 0.422 546 0.325 548 0.251 548 0.190 548 ·0.139 540

50 0.7.71 532 0.635 532 0.505 530 0.396 532 0.319 530

100 0.954 508 0.867 508 0.840 506 0.733 504 0.623 510

region seems to be confined to the range of 4 ..:.. 8m V/ s for various ~ E. At all the I~ E valuesstudied, i decreases with scan rates contrary towhat is expected from theory (refer equation (4».The increase in scan rate is also associated withpeak shifting in a rather irreproducible mannerdeviating somewhat from theory~ The non-constancy of peak potential with respect to thechange in scan rate arid pulse amplitude couldbe attributed to the interplay of the followingeffects: DC distortion associated at low pulseamplitude's with peak broadening at higher scanrates; peak broadening at large pulse amplitudesand high scan rates (Fig. 2a, b). These effects

cause some difficulty in locating accurately thepeak potential.

A deviation from theoretical behaviour wasalso observed when i was plotted against 1 - '0 .

p -1 + '0,

In theory, a linear relations~ipshould be observ-ed for all scan rates studied. However, Fig. 3demonstrates clearly that linearity was observedonly at low scan rates (I~ 4 mVIs). The deviationfrom linearity increases with an increase in scanrate. Fig. 4 snows that current increases propor-tionally with i~ E up to about 60 m V pulse andincreases slowly for large pulses. This is in agree-

402 PERTANIKA VOL. 8 NO.3, 1985

DIFFERENTIAL PULSE POLAROGRAPHY AND VOLTAMMETRY

·1

'.~.

8 ~•7 ,.~.

~..

6

~, a

5

.~-

•

~4

.~-

.~ •3 '" b

2.~

.~.~

C

1o.

0.9

o.

o.

o.

o.

~ o....Po

O.

O.

10

SCAN RATE (mV/s)

Fig. 1. Peak current as a function of scan rate for various pulse amplitudes.[Cd(II)} = 6.0 x 10 - 5M, drop time = 1.0 s,a = 100 m V, b = 50 m V, c = 25 m V, d = 10m V, e = 5 m V

\\ 0.20 V

L--

Fig. 2. (a) Comparison of differential pulse polarograms of 1.0 mM Cd(II) for various pulse amplitudes.V = 2 m VIs, a = 5 m V, b = 50 m V, c :; 10m V

(b) Comparison of differentz'al pulse polarogram of 1.0 mM Cdr!) for various scan rates./).:£ 50 mV, drop time = 1 s

a = 2 mVIs, b = 4 mV/s, c = 6 mVIs, d = 8 mV/s, e = 10 mV/s

PERTANlKA VOL. 8 NO.3, 1985 403

W.T. TAN AND C.S. TAN

1008060

.4 E( mV)

0.2

0.3 ~:If:0.1 ~~.~..~

0 0.2 0.4 0.6 0.8 1.0

1 - 6i'"+7

•.-0.9

0.8

0.7

/•0.6

~ 0.5 i0. .....P-.....

().4 '.

Fig. 3. Dependence of peak currents on 1 - a for1 + a

various scan rates.drop time = 1 s, [Cd(II)} = 6.0x 10- 5M,

: a = 2 mV/s, b 4 mV/s,c - 6 mV/s, d = 8 mV/s,e 10 mV/s

Fz'g. 4. Dependence of peak current on pulseamphtudes for various scan rates.Other parameters: same as in Fig. 3.

ment with equation 2 for small pulse amplitude.Christie et at. (1973) and Blutstein and Bond(1976) also observed a divergence from theoryfor large pulse amplitude and large scan rates intheir analog polarograph. They attributed thediscrepancies to an instrumental artifact whicharises from the small ratio of the sampling inter-val to the rather long time- constant of thememory circuit. The long time constant wasused to improve signal to noise ratio in verydilute solutions. However, in this ,up-basedpolarograph, the discrepancies observed be-

tween theory and practical results could be dueto the instrumental artifact mentioned above orto the combination of the ,up and the instru-mental artifact inherent in the polarograph or toa faculty mathematical model fot the behaviourof the electrode processes. No attempt is made toidentify the real cause of these discrepancies.

According to equation (3), E should bep

shifted anodically for a cathodic wave with anincrease in li E. The slope for E vs 6. E graphshould be - ~ for theory to hold. Fig. 5 shows

404 PERTANIKA VOL. 8 NO.3, 1985

DIFFERENTIAL PULSE POLAROGRAPHY AND VOLTAMMETRY

560

560

s-:;1110. 560 -I

o 20 40AE (mV)

60 80 ido

and 30.1 m V for 1, 2 and 3 electron reactionrespectively at 25°C. Table 3 depicts the effect ofI::iEon W~. For small AE~50mV,W ~is50 ±2 m V, agreeing reasonably well with theory for n= 2. Peak broadening which sets in at AE ,).50m V is also to oe expected from the theoreticaltreatment of Parry and Osteryoung.

For a stationary electrode system, faradaiccurrent decays inversely as t ~where t is the droptime or experimentally the interval betweenpulse steps. Fig. 6 demonstrates this relationshipwhich holds true for this automated polarographas i vs t ~ (at constant V ) produces a straight

P .line passing through the origin. The i was alsofound proportional to drop area whenPthe dropsize was regulated by the SMDE as it should be(Fig. 7).

Fig. 5. Dependence of peak potentz'al as a functionof pulse ampHtudes for various scan rates.Other parameters: same as in Fig. 3.

results which conform reasonably well withtheory for all I::i E values studied except for I::i E

-

W.T. TAN AND G.S. TAN

observed i should be directly proportional to theelectrolysi: time. However, this is not often thecase in practice.

0.8

0.6

'

DIFFERENTIAL PULSE POLAROGRAPHY AND VOLTAMMETRY

a

35

)0

Ii .0

'<coI 4.0S

7.0

CdOl)

CuO!)

25 •

/•

20 ./

/•,.4 ~ 8 10 12

equilibrium time (x 102

s)

0.0

'< 4.0ctlI

S

".r<

"0-

2.0

-0.9

b

-0.7 -0.5

E(V ~ Ag/Ap.Cl)

-0.3 -0.1

Fz'g. 10. Comparison of (a) differential pulse anodicstripping voltammograms and (b)differential pulse polarograms of Cd(II),Pb(II) and Cu(II) in 0.10 M KNO)backgrounq, solution.

V = 2 mV/s, 6E = 50 mV,drop area = 2.61 x 10 2 cm 2 ,t dep 60 s, t eq 30 s.

that DPASV, but not DPP, can easily pick upthree distinct peaks of Cd(II), Pb(II) and Cu(II)even at a concentration of "'" 10 -8M. Fromreproducibility studies involving nine duplicateruns, DPP shows a precision of 3 and 1% relativestandard deviation (RSD) for i at a level of 1

J1 M Cd(II) and 11 J.L M Cd(II)Prespectively. InDPASV, when nine duplicate runs, each with

Fig. 9. Plot of peak current as a function ofequilibrium time during DPASV.[Cd(II) = 8.9 x 10 -5M, V = 2 mV/s,6,E = 50 mV, t = 60 s.

dep

tronic instabilities which may explain the in-crease in anodic shift of E with t d and t . With

p ep eq

electrodes formed by a mercury drop hanging ona mercury column as used in this work, back dif-fusion also plays a part.' This phenomenoncauses peak broadening (as observed in thiswork) and partial loss of the metal pre-elec-trolysed, thus leading to an apparent decrease ini and peak shifting.

p

From sensitivity and reproducibility studies,it was found that DPASV is still by far a moresensitive technique than DPP due to the precon-centration step of DPASV. Fig. lOa, b shows

-0.2 -0.4 -0.6

E(V ~ Ag/AgCl)

-O.B

PERTANIKA VOL. 8 NO.3, 1985 407

W.T. TAN AND G.S. TAN

one analysis on one mercury drop, was consider-ed RSD of i at a level of i 0 - 8M Cd(II) andPb(II) were 9.3 and 12.7% respectively. How-ever, when nine duplicate runs, each with threeanalyses on different mercury drops and itsmeans considered, the RSD for determiningCd(II) and Pb(II) improves to 8.0 and 6.8% res-pectively. b. E remains reasonably constant i.e.± 2 mV for Cd(II) and Pb(II) in both techni-ques. However, when Cu(II) is plated out toge-ther with Cd and Pb, the RSD is as high as 34%for i and E fluctuates greatly from - 0.086 to- 0.058 V (± 28 m V). A possible reason for thisanomally is the interference between metals.Many studies have shown that Cd and Cu mayform an intermetallic compound when platedout simultaneously (Whitfield, 1975). WhenCu(II) inO.l M KNO:swas plated out individual-ly, its RSD for i . and E improved to 6.1 % and4.4% respectivefy. Fro~ a standard additionstudy, the amount of Cu(II) present in the 0.10M KNO :ssupporting electrolyte was estimated tobe 1.8 X 10 -8M or 1.1 ppb.

CONCLUSION

The dependences of i p on.l -. a. at high1+0

and of i and E on scan rates were found todeviate from th~ory. These would inevitablyprevent the use of this polarograph in the investi-gation of electrode mechanisms. However,variation of i with drop time at constant scanrate and drop ~ize, and of i with drop area agreewell with theory indicating the validity of usingexpressio~ (1) for the SMDE using the PARC384. Peak potential was also found to vary withmost pulse amplitudes employed in accordancewith theory. Peak half-width is acceptable onlyfor pulse amplitudes smaller than about 50 mV.Peak broadening occurs at large pulse ampli-tudes and scan rates.

In general, the PARC 384 and PARC 303SMDE make a good combination as an essential-ly fully automated system. There is also theadded features of background subtraction andtangent fitting which help to remove difficulty in .extrapoiating distorted based lines commonly

associate

ACKNOWLEDGEMENTS

This work was supported by a UniversitiPertanian Malaysia Research Grant. Theauthors wish to thaI}k Noraini bte Abu Bakar fortyping the manuscript.

REFERENCES

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(Received 23January, 1985)

PERTANIKA VOL. 8 NO.3, 1985 409