Differential pulse anodic stripping voltammetry at the wall-jet electrode

J. Elecrrounal. Chem., 198 (1986) 21-35 Elsevier Sequota S.A.. Lausanne - Prmted

DIFFERENTIAL PULSE WALL-JET ELECTRODE

H. GUNASINGHAM, ~epurt~ent (Received

K.P. ANG,

27 m The Netherlands

ANODIC

C.C. NGO

STRIPPING

1985: m rewed

AT

THE

and P.C. THIAK

of C~le~zi~t~, ~utiona~ ~n~~~ers~t~of Smgapore, 13th March

VOLTAMMETRY

from 19th August

Kent Rtdge, MI 1 ~S~ng~~or~) 1985)

ABSTRACT

Differential pulse anodtc stnppmg voltammet~ at the wall-jet electrode requires careful consideration of flow conditions in the platmg and stripping steps as well as the electrochemical parameters such as modulatton amphtude and clock period. A systemattc appratsal of the various factors affecting the differenttal pulse strtppmg current ts described. Although the stripping current depends on the strtppmg solution flow rate. the differential pulse mode IS preferred to dc strippmg because of its greater sensttivtty and the abtlity to obviate oxygen interference.

INTRODUCTION

Differential pulse anodic stripping voltammetry (DPASV) is being increasingly used for trace heavy metal analysis in preference to DCASV. primarily because of its inherently greater sensitivity [l]. Unlike Normal Pulse ASV (NPASV), the theory of DPASV has not been rigorously defined. Although Osteryoung and Christie [2] have indicated that, at the limits of the two techniques, the theoretical treatment is comparable, in practice. at the usual operating conditions, NPASV yields a significantly higher response [l]. However, the differential mode yields a superior signalto-noise ratio and Valenta et al. [l] have recommended it for determinations in the ppb and sub-ppb range, a point with which we are in agreement. DPASV has been applied to a wide range of mercury film electrodes with well-defined hydrodynamic characteristics. These include the rotating disk electrode and varied flow-through electrode geometries for continuous flow monitoring applications. Following earlier usage [3,4], we have described DPASV at such electrode systems as hydrodynamic DPASV (HDPASV). Like hydrodynamic DCASV. in practice, HDPASV usually involves a plating step under well-defined convective diffusion conditions and a stripping step where the solution is usually at rest. In a previous paper, however, we showed that in the case of the wall-jet electrode it is, in fact, advantageous to carry out the stripping step in a flowing stream as well [4]. One of the benefits is the ability (especially through the use of microprocessor control) to have subtle control of selectivity by use of matrix exchange. Another benefit is that ~22-0728/86/$03.50

b 1986 Elsevier Sequora

S.A.

28

there is no need to flush out the cell compartment in between analyses. This is an important consideration, because a true wall-jet cell design must have a large volume to prevent interference with the hydrodynamic boundary layer [3-61. Consequently, flushing out of the cell is not a practical proposition where rapid sample injection (for example in FIA applications) is required. Whereas in the case of HDCASV the stripping peak current is insensitive to the stripping flow rate [3]. for HDPASV the stripping flow rate has a significant influence. Nevertheless, with proper optimization of the instrument settings and flow conditions, it is feasible to obtain a reproducible current response which is substantially better than the corresponding HDCASV response without sacrificing true wall-jet behaviour. Also, because of its ability to discriminate against background currents due to capacitance effects and impurities in the carrier stream, more well-resolved voltammograms are feasible. Although the normal pulse theory does not give us equations which can be applied to DPASV, it provides a basis for qualitative explanation. Valenta et al [l] have followed this approach in a study of DPASV at the rotating disk electrode. Little comparable work has been done with regard to flow-through electrode systems and no work has been reported on HDPASV where stripping has been carried out in flowing solutions. EXPERIMENTAL

Flow system Figure 1 is a schematic diagram of the experimental set-up. which differs from that of the previous work [4]. Instead of a stream selector, samples are introduced into the WJ cell through a pneumatically actuated injection valve (Rheodyne model 5020) which employs a 1 ml sample loop. The pneumatic actuator is controlled by two 3-way solenoid valves (Skinner, model 7163). A second, stream selector, valve (Rheodyne model 5010) was used to select the mercury solution or a blank electrolyte solution. The delivery of all solutions was by means of a peristaltic pump (Eyela model MP-3) used in conjunction with a pulse damper. Instrumentation Differential pulse voltammogr~s were obtained with a PAR 174A polarographic analyser (P~nceton Applied Research, NJ) and volt~mograms were recorded on a model REOOX9 X-Y recorder (Houston Instruments). The clock time was set at 0.5 s, which affords a short rest period between pulses. An Apple IIe microcomputer was used to control the sample injection as well as the plate, hold and strip cycle. The computer was patched into the front panel push-button switch control of the PAR 174A through Reed relays. The initial, scan and hold functions would thus be controlled through logic control signals. The control program was written in Applesoft BASIC.

29

Electrochemc

Fg.

1. Schematic

representation

of expermental

set up.

Reagents All chemicals were of Analar Grade. Water was first passed through a mixed bed ion-exchange resin and then distilled twice from potassium permanganate. Stock solutions of mercury (2 X lo-’ M) were prepared by treating triply distilled mercury with concentrated nitric acid. Lead solutions were prepared from lead nitrate. The blank carrier electrolyte consisted of 0.1 M potassium nitrate and 0.05 M nitric acid. All solutions were deaerated with oxygen-free nitrogen before use and a nitrogen atmosphere was maintained over sample reservoirs during analyses. Wall-jet

cell

The wall-jet cell was of identical design to the one used in the earlier work [4]. The inlet-electrode separation was usually 4 mm. All potentials are quoted with respect to a saturated Ag/AgCl reference electrode. Experimental

procedure

All HDPASV scans were carried out at a preplated mercury film. The preplated film was formed by switching the stream selector valve to the mercury solution (2 X lop4 M, flow rate = 2.73 ml/mm, deposition potential = - 1.0 V and deposition time = 10 min). The film could be used for several hours without any apparent deterioration. However. checks for consistency were made from time to time by replating a fresh mercury film,

30

Once a suitable mercury film had been plated, the carrier stream was switched to the blank electrolyte solution. The analysis of the lead sample solution begins by switching the injection valve to divert the carrier stream into the sample loop. thus injecting the sample into the WJC. Stripping was carried out in the flowing blank carrier solution with the sample injector valve in the load position. The entire sequence of operations was under the control of the computer and the operator only has to type in the required durations for the various operations. The pre-electrolysis potential for lead was - 1.0 V. RESULTS

AND DISCUSSION

Effect of flows rute during the deposltlon step In the theoretical treatment of NPASV by Osteryoung major conclusions was that the normal pulse stripping quantity of metal deposited in the deposition step. It reasoning to HDPASV at the MFWJE. Given that the under steady-state conditions is dictated by the limiting bly assume that 1 =

and Christie [2], one of the current is determined by the is reasonable to extend this quantity of metal deposited current [2], we can reasona-

kca~‘,‘~v5/“~R3/4V3,‘4td

P

(1)

where k is a constant, c = bulk concentration of metal ions, CI= inlet diameter, v = kinematic viscosity, R = electrode radius, V = volume flow rate and t, = deposition time. The effect of solution flow rate during deposition was studied for a range of lead solution concentrations (between 4 X lop6 and 2.4 X 10e5 M). while the stripping flow rate was held constant. The average slope for plots of log peak current vs. log flow rate obtained for a range of stripping flow rates was found to be 0.75 f 0.3. This is in good agreement with eqn. 1. Effect of stripping flow rate As already mentioned, for HDCASV scans at the MFWJE, the stripping peak current is relatively insensitive to stripping flow rate. The same is not true. however, for the differential pulse mode. This is, without doubt, caused by the increased rate of removal of oxidized metal amalgam under conditions of convective diffusion, which diminishes the replating effect. Figure 2 gives plots of stripping peak current against stripping flow rate for a blank stripping solution and a stripping solution containing sample ions. With regard to the latter, in order to obviate the practical problem of metal deposition during the anodic scan itself. the scan is carried out initially in a flowing blank solution until the potential just reaches the point before stripping commences. (Because of the unique flow characteristics of the WJE. metal ion from the bulk solution is prevented from reaching the electrode surface and. thus, no additional plating is possible [3,4].) The solution was switched to the sample

31 Peak current/pA

i

80

AE=‘;OmV

60

I

Stnppmg Flow Rate /ml rnm-‘l

Ftg. 2. Companson between strIpping in sample solulron and blank electrolyte for different fhw rates. (1). (4) HDPASV scan m sample solution. (2). (5) HDPASV scan m blank electralyte. (3) HDCASV scan m blank electrolyte. Flow rate during deposItson = 2.58 ml/min, deposItion ttme = 15 s. scan rate ;= 10 mV/s.

stream and the actual stepping was thea done in the flowing sample solution. The potential for switching from flowin,0 blank to flowing sample was determined experimentally, Figure 3 describes the sequence of operations more clearly. From Fig. 2, we can see that the effect of stripping flow rate is most significant for the flowing blank solution where the differential pulse mode is employed with large modulation amplitude. Interestingly, for the case where stripping is done in the sample solution, the flow rate does not appear to have as great an influence. The difference presumably occurs because, with the flowing sample solution, the concentration gradient of metal ion in the diffusion layer adjacent to t!se MFE is diminished by the fact that sample ions are continuousIy being brought to the surface by Forced convection, even as they are being removed from the surface. For the fbving blank solution, however, the concentration gradient increases signifi-

32

E (electrode

potentloll

T

Introduce blank electrolyte

Inject 5Mlple

IT

q

depose tlon

Introduce sample

In!roduce blank electrolyte

time

WA =woltlng time at electrode potential sample solution 15 being introduced

(Sl) while

HT = hold time Ep = peak

potentlo.

Fig. 3. Sequence

of

steps

for strIppIng in sample solution

cantly with the flow rate, because the replenishing of sample ions is diminished. The peak current should reach a limiting value at high flow rates. The relative insensitivity of the HDCASV peak current to stripping flow rate arises because dc stripping is a straightforward thin-layer process involving fast electron transfer. Figure 2 also shows that, at low modulation amplitude, the effect of stripping flow rate is less apparent. This is because the replating effect is less significant. Dependenqv

on bulk concentration

A linear dependence tion range (10e6- lop5

on concentration M ) for different

was found even for a high lead concentrastripping flow rates.

Effect of scan rate During the DPASV scan, the amalgam is progressively oxidized by the application of successive pulses. The deleterious effect of this progressive depletion on the peak current may be diminished by using a fast scan rate. However. there are limiting factors to this, such as the necessity to have sufficient points to define the peak profile and capacitance effects. Figure 4 shows the effect of scan rate on the peak current for different stripping

33

(:-::;

‘;IA 10

20

Statfonary

0.49 ml/mfn

30

40

50

scan rate / mV <’

Fig 4 Effect of scan rate on peak current (for different strlp~lng flow rates) mV. depositlon flow rate = 2.26 ml/mm. deposhon time = 15 s

~~odulatl~~

amphtude

= 50

flow rates. A maximum is found around 10 mV/s, which is in accord with Osteryoung and co-workers [7], although, in that work, stripping was carried out in a stationary solution. Buckground effeects-comparison

of dc and DP modes

One of the differences between the DP and dc modes we have found is that the former affords poorer precision when a mercury film prepared in situ is used. Presumably this is because during a dc scan complete exhaustion of the mercury amalgam occurs, whereas for DP a significant amount remains at the end of a scan because of the replating effect [l]. However, a preplated mercury film gives good results. Figure 5 shows stripping voltammograms obtained for dc and DP stripping modes in a flowing carrier stream for a preplated mercury film. The dc scan slopes severely at more negative potentials due to trace oxygen. This is a distinct disad-

Cd

I Differentml pulse ----Undeoxygenated --

Deoxrjenated

Pb

electrolyte electrolyte

11Dtrect current 1.2 -Undeox~enated electrolyte 3,L5,6-Deoxygenoted electrolyte (2.6.10,20 rmn respectlvely)

-06

1

Pb

- OL

’

- 02

’

0 E/V vs.Ag/AgC :I

’

J

Fig. 5. HDPASV and HDCASV scans for undeaerated and deaerated stnppmg solutions. Scan rate = 5 mV/s, modulation amplitude = 50 mV. DeposItIon and strlppmg flow rates = 1.31 ml/mm. Deposition and strippmg solutions = 0.005 M HNO,, 0.1 M KNO,. DeposItIon time = 45 s. Concentration of Pb+. Cd+. Cu+. and Zn*+ = 6.0 x 10m6 M.

vantage, because it is difficult in practice to purge oxygen completely from the carrier stream in continuous flow monitoring. As can be seen, the zinc peak is obscured in the dc scan, In contrast, the DP plot has a flat baseline and the voltammogram is better resolved at more negative potentials even when the solution is not deaerated. In fact, although there is a slight shift in the peak potentials, the presence of oxygen does not affect the peak currents. Also, background subtraction is easily achieved for the DP mode whereas it is not feasible for the dc stripping mode.

35 CONCLUSION

Because the DP stripping mode is more prone to variations in the MFE conditions, compared to the dc mode, there is a need to use a preplated mercury film rather than one prepared in situ. With this precaution. however. HDPASV is a better technique practically than dc stripping. If stripping is carried out at low carrier flow rates (about 0.5 ml/min), the peak current is more than 90% of the value if stripping is done in a static solution. Apart from the fact that the sensitivity is much greater, voltammograms are better resolved while being reproducible. Most important, oxygen interference is obviated. REFERENCES I 2 3 4 5 6 7

P Valenta, L. Mart and H. Rutzei, J. Electroanal. Chem., 82 (1977) 327 R.A. Osteryoung and J.H. Chnstle. Anal Chem.. 46 (1974) 351. H. Gunasmgham, K.P. Ang and C.C. Ngo. Anal. Chem.. 57 (1985) 505 H. Gunasmgham, K.P. Ang, C C. Ngo, P C. Thlak and B. Fleet, J Electroanal. Chem.. 186 (1985) 51 H. Gunasmgham and B. Fleet. Anal. Chem., 55 (1983) 1409. W.J Aibery and C.M.A. Brett. J. Eiectroanal. Chem.. 148 (1983) 201, 211 T.R Copeland, J.H. Christie. R.A. Osteryoung and R.K. Skogerboe. Anal. Chem.. 45 (1973) 2171