Differential Pulse Polarography and Voltammetry with an ......DIFFERENTIAL PULSE POLAROGRAPHY AND...

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

Differential Pulse Polarography and Voltammetrywith an Automated Microprocessor-based Polarograph

and a Static Mercury Drop Electrode

W.T. TAN and G.S. TANChemistry Department,

Faculty of Science and Environmental Studies,Universiti Pertanian Malaysia,Serdang, Selangor, Malaysia.

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

ABSTRAK

Polarografi denyut pembeza dengan menggunakan penganalisis polarografik yang berdasar-kan kepada pemerosesan mikro automatik dengan elektrod raksa pegun statik telah dinilai terhadapamplitud denyut, kadar skan, masa menitik (t), dan luas titik. Didapati perubahan punca arus (i )dengan luas titik dan t ~¥z; perubahan punca keupayaan (E ) dengan amplitud denyut bersetujudengan teori secara sederhana. Akan tetapi didapatipengantungan i dan E kepada kadar skan danamplitud denyut yang besar dan terkecil telah didapati menyimpang daripada teori. Keputusanaliran yang serupa juga ditemui untuk voltametri melucut anodik apabila langkah melucut denyutpembeza digunakan. Kepresisan kedua-dua teknik itu adalah baik. Kuprum disadurkan secara indi-vidu semasa voltametri melucut anodik.

ABSTRACT

Differential pulse polarography using an automated, microprocessor-based polarographicanalyzer equipped with a static mercury drop electrode has been evaluated with respect to pulseamplitude, scan rate, drop time (t), and drop area. Variation of peak current, i , with drop area andt ~1/z, with some pulse amplitudes agree reasonably well with theory. However, dependence of i and Eon scan rate and on pulse amplitude at very large and very small amplitudes was found to deviate fromtheory. Similar trends were also found for anodic stripping voltammetry when differential pulsestripping was used. The precision attainable by both techniques was good. Copper peaks demonstrat-ed good precision only when copper was plated out individually during anodic stripping 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 tothe advent of reasonably priced microprocessors(tip), there is an upsurge of interest in the incor-poration of digital microcomputers 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

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 aL,1981). A laboratory built up-controlled polaro-graph which could generate only staircasevoltrammetric and NP mode waveforms wasreported and evaluated by Barrett et aL (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 aL(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 al, 1974; Batley andFlorence, 1974; Valanta et al, 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

theory for DPP and differential pulse anodicstripping voltammetry (DPASV) using thesecond generation ju -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 al., 1981) for a simple, reversibleprocess. A more general and less complicatedexpression for the differential pulse current, Ai,was put forward by Parry and Osteryoung(1965). In their theoretical treatment, maximumA i( A i max) or i is described by the following:

where a = exp time

interval between pulse amplication and currentmeasurement and Eg - E^ = A E, the pulseamplitude (*AE is negative for reduction). Otherparameters have their usual meaning. If AE/2 issmaller than RT/nF, equation 1 is simplified to:

Aimax ' n»F? AC(-AE)4RT

(2)

These expressions have neglected the contri-bution from the dc term, Ai., . A more specific

dc "

Ai expression which takes into account this termwas described recently by Anderson et al. (1981)for a stationary electrode like the SMDE. Thecontribution from both i and Ai . are strictly

p dc '

additive for the reversible electrode process. Theeffect of dc distortion using a SMDE has beenwell documented in the work reported by Ander-son et al (1981). The Aidcterm 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 AE values ( > 5mV) 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 al, 1973; Blutstein and Bond,1976).

According to equation (1), (Ai)max(or i asdetermined experimentally) obtained from areversible reduction process of a simple system, isexpected to be a linear function of the concen-tration of electroactive species, drop area,inverse square root drop time (for constant droparea), 1 - o and pulse amplitudes (A E). Peak

1 + opotential is given as E = E — A E/2 (3)

Even though equation (1) does not take intoaccount the variation of i with scan rate ( v ),certain 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 described by equation (4): i = K X

V\* where K is proportionality constant. Peakcurrent increase with v . 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 position and peak width should alsoconform to the theoretical expectation.

MATERIALS AND METHODS

Apparatus

The polarographic and voltammetric expe-riments described here were performed using anEG & G PARC model 384 second generationmicroprocessor-based polarographic analyzerequiped with an EG & G PARC 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 Ag/AgCl (saturated KC1)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 Cd(II) was prepared by dis-solving CdCl 2 in deionized, distilled water(DDW). DDW was prepared by running labo-ratory prepared distilled HgO through a Milli-pore water purification system and used forsample preparation as required. A more dilutesolution of Cd(II) 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 activated 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 $ supportingelectrolyte.

RESULTS AND DISCUSSION

In this work, peak currents for 6.0 X10 5M and 1.0 X 10 "3M Cd(II) solutions wereobtained for the various scan rates ( v ) available(2, 4, 6, 8 and 10 mV/s) at 5, 10, 25, 50 and 100mV pulse amplitudes using a 1 sec drop time.These data are summarized in Tables 1 and 2.Fig. 1 is a plot of (Ai)maxor i vs scan rate forvarious values of AE. In general, any linear

PERTANIKA VOL. 8 NO. 3, 1985 401


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 ~SM during DPP

Scan rate

AE(mV)

5

10

25

50

100

l - o1 + 0

0.097

0.193

0.453

0.750

0.960

2

i

(A)1.24

2.69

5.93

10.69

13.73

mV/s

E

(mV)

556

555

548

534

508

4

p

(MA)

0.795

1.70

3.68

6.97

14.2

mV/s

p

(mV)

564

560

556

540

536

6

i

(!/A)

0.645

1.40

3.21

6.35

13.3

mV/s

E

(mV)

560

566

554

548

536

10

(MA)

0.447

0.970

2.12

4.80

10.5

mV/s

E

(mV)

540

550

550

540

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 ~5M during DPP

Scan rate

AE(mv)

5

10

25

50

100

2

p

CMA)

0.085

0.190

0.422

0.771

0.954

mV/s

E

(mV)

556

552

546

532

508

4mV/s

p

(MA)

0.065

0.145

0.325

0.635

0.867

E

(mV)

556

552

548

532

508

6mv/s

i

QlA)

0.049

0.099

0.251

0.505

0.840

E

(mV)

554

554

548

530

506

8

p

(MA)

0.039

0.083

0.190

0.396

0.733

mV/s

E

(mV)

548

548

548

532

504

10

p

(MA)

0.032

0.062

•0.139

0.319

0.623

mV/s

Ep

(mV)P

530

548

540

530

510

region seems to be confined to the range of 4 — 8m V / s for various AE. At all the AE 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 and pulse amplitude couldbe attributed to the interplay of the followingeffects: DC distortion associated at low pulseamplitudes 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 — o .

1 + aIn theory, a linear relationship should be observ-ed for all scan rates studied. However, Fig. 3demonstrates clearly that linearity was observedonly at low scan rates ( < 4 mV/s) . The deviationfrom linearity increases with an increase in scanrate. Fig. 4 snows that current increases propor-tionally with tA E up to about 60 mV pulse andincreases slowly for large pulses. This is in agree-



0 .15 .

3 0.08 -

SCAN RATE (mV/s) 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 mV, b - 50 mV, c = 25 mV, d = 10 mV, e = 5 mV

5 JUA A

0.20 V

Fig. 2. (a) Comparison of differential pulse polarograms of 1.0 mM Cd(II) for various pulse amplitudes.V - 2 mV/s, a = 5 mV, b - 50 mV, c - 10 mV

(b) Comparison of differential pulse polarogram of 1.0 mM Cd(I) for various scan rates.A £ 50 mV, drop time = 1 s

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



0.9 .

t 0 . 5 -

0 . 3 -

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

various scan rates.drop time = 1 s, fCd(II)] = 6.0 X 10~'M,

: a = 2 mV/s, b = 4 mV/s,c = 6mV/s, d •e = lOmV/s

20 40

lE(mV)

4. Dependence of peak current on pulseamplitudes for various scan rates.Other parameters: same as in Fig. 3.

ment with equation 2 for small pulse amplitude.Christie et al. (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 /ip-basedpolarograph, the discrepancies observed be-

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

According to equation (3), E should beshifted anodically for a cathodic wave with anincrease in AE. The slope for E vs AE graphshould be - \k for theory to hold. Fig. 5 shows

404 PERTANIKA VOL. 8 NO. 3t 1985


560

560 -*

560 - *

20 mV

• > a, b

40 60

AE (mV)

-rib

Fig. 5. Dependence of peak potential as a functionof pulse amplitudes for various scan rates.Other parameters: same as in Fig. 5.

results which conform reasonably well withtheory for all A E values studied except for A E*<10 mV and v >8mV/s. This is expected aspeak broadening sets in at a high scan rate. Thiseffect leads to error in peak location as explainedabove. DC distortion, evident in Fig. 2a, mayhave had an effect on the cathodic peak shiftingat small pulse amplitude of 5 mV.

According to Baker and Gardner (1958)W,, = 3.52 RT. This gives a W % of 90.4, 45.2

and 30.1 mV for 1, 2 and 3 electron reactionrespectively at 25°C. Table 3 depicts the effect ofA£ on W . For small AE<50mV, W is 50 ±2 mV, agreeing reasonably well with theory for n= 2. Peak broadening which sets in at AE^-50mV is also to be 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 straightline passing through the origin. The i was alsofound proportional to drop area when the dropsize was regulated by the SMDE as it should be(Fig. 7).

Differential Pulse Anodic StrippingVoltammetry (DPASV)

The effects of various parameters common-ly associated with DPP on i , E and W inDPASV were found to follow similar trends tothose discussed above for DPP (Fig. 1-7).Besides these parameters, electrolysis time whichincludes deposition time (tde ) and equilibriumperiod (t ) also determines the performance ofDPASV. Deposition time exerts a direct controlon the amount of metal plated onto the electrodewhile t controls the homogeneity of amalgamformation in the electrode. Theoretically the

TABLE 3Effect of scan rate and pulse amplitude on half-peak width (W )

(a) AE(mV)

5

10

25

50

80

100

DPP

50

50

50

63

—

105

l (±2mV)

DPASV

48

48

48

56

80

96

(b)U,(mV/s)

2

4

6

8

10

W]

DPP

63

71

94

113

150

\i (±2mV)

DPASV

56

72

96

120

232

0 U = 2 mV/ji) A E = 50 mV

[Cd(II)] = 1.0 X 10 SM for DPP8.9 X 10 ~5M for DPASV



o.i

0.6

t „,«.

0.2

I - " T

-

—i—

i

1 1

0.2 0 .4 0.6 0 . 8 1.0

Fig. 6. Plot of peak current as a function oft~^(proportional to electrode area) at constantscan rate.

[Cd(II)] = 1.0 X 10 ~*M, AE = 50 mV,V = 2 mV/s

-2 2Drop Area (x 10 ) cm

Fig. 7. Plot of peak current against electrode arearegulated by SMDE.Other parameters; Same as in Fig. 6.

observed i should be directly proportional to theelectrolysis time. However, this is not often thecasein practice.

Fig. 8, 9 depict the dependence of i ontde

and t . Direct proportionality between i andthe electrolysis time was maintained up to depo-sition time of 120 sec and equilibrium period of300 sec. The increase in electrolysis time was alsoassociated with an anodic shift in E .It appearsthat an equilibrium surface concentration isreached when longer deposition times are used(Fig. 8). Also sensitivity can be increased by in-creasing tde and te . This approach would havelittle merit since it would require a rather longanalysis time. More over it would lead to elec-

deposition tine (x 10 s)

Fig. 8. Plot of peak current as a function ofdeposition time during DPASV.[Cd(II) = 8.9 x 10~3M, V= 2mV/s,AE = 50 mV, t = 10 s.

eq



equilibrium time (x 10 s)

Fig. 9. Plot of peak current as a function ofequilibrium time during DPASV.[Cd(II) = 8.9 x 10~>M, 0= 2 mV/s,AE = 50 mVt t . = 60s.

dep

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

p dep 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.

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. 10a, b shows

6 . 0 Cu(ll)

-0.9 -0.7 -0.5E(V vs Ag/ApCl)

2.0

- 0 . 2 -0.U -0.6E(V v$_ Ag/AgCl)

Fig. 10. Comparison of (a) differential pulse anodicstripping voltammograins and (b)differential pulse polarograms of Cd(II),Pb(H) and Cu(II) in 0,10 M KNO ?

background solution.V= 2 mV/s, AE = 50 mV,

drop area = 2.61 x 102cm2',t, = 60 s, t = 30 s.

dep eq

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 1A<M Cd(II) and 11 JLXM Cd(II)Prespectively. InDPASV, when nine duplicate runs, each with



one analysis on one mercury drop, was consider-ed RSD of i at a level of 10 ~8M Cd(II) andPb(II) were &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. AE remains reasonably constant i.e.± 2 mV for <5d(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- O.$58 V ( ± 28 mV). 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) in 0.1 M KNO 3 was plated out individual-ly, its RSD for i and E improved to 6.1% and4.4% respectively. From a standard additionstudy, the amount of Cu(II) present in the 0.10M KNO 5 supporting electrolyte was estimated tobe 1.8 X 1 0 ~ 8 M o r l . l p p b .

CONCLUSION

The dependences of i on 1 — o at high1 + a

and of i and E on scan rates were found todeviate from theory. 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 size, and of i with drop area agreewell with theory indicating the validity of usingexpression (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 inextrapolating distorted based lines commonly

associated with dc distortion and high matrixeffects. The time saving capability and thecommendable precision attainable would un-doubtedly enhance the status of differentialpulse techniques especially DPASV for traceanalysis at ppb or subppb level.

ACKNOWLEDGEMENTS

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

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(Received 23 January, 1985)


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