Analyte stabilization by electrodeposited palladium modifier for electrothermal atomic absorption spectrometry: characterization by scanning electron microscopy and anodic stripping voltammetry

Talanta ELSEVIER

Talanta 44 (1997) 1183 1193

Analyte stabilization by electrodeposited palladium modifier for electrothermal atomic absorption spectrometry: characterization by scanning electron microscopy and anodic stripping voltammetry J.P.

Matousek ~,*, H.K.J. Powell

b

~' Department of Analytical Chemisto,, The University of New South Wales, Sydney 2052, Australia b Department of Chemistry, University o[" Canterbury, Private Bag 4800, Christchurch, New Zealand

Received 24 July 1996; received in revised form 31 October 1996; accepted 4 November 1996

Abstract

In electrothermal atomic absorption spectroscopy (ETAAS) effective stabilization of analytes can be achieved by an initial in situ electrodeposition of 0.2 lag of Pd. This amount of Pd is on average a factor of 50 lower than that typically used for conventional chemical modification. The surface features of this modifier have been characterized by scanning electron microscopy (SEM) measurements on sectioned pyrolytic graphite furnaces and contrasted with those for modifier produced from thermally reduced Pd salts. Electrodeposition produces a uniform array of Pd domains stretching approximately 2 mm from the centrally positioned Pt/Ir anode (which doubles as the autosampler sample delivery tube). In contrast, thermally reduced Pd salts produce a modifier which concentrates in domains near the drying edge of the modifier solution. Anodic stripping voltammetry (ASV) established that Pb electrodeposited onto Pd-modified pyrolytic graphite affixes to the Pd rather than the graphite. ASV measurements using a basal plane pyrolytic graphite working electrode also established that (i) the stripping potentials for monolayer and multilayer Pb are shifted anodically by 0.16 and 0.18 V, respectively by binding to Pd rather than graphite and (ii) deposition of Pb from dilute acidic medium (1% HNO3) leads only to monolayer Pb, in contrast to deposition from acetate buffer (pH 4.0-4.4) which produces predominantly multilayer Pb. © 1997 Elsevier Science B.V. Keywor&': Electrodeposition; Palladium modifier: Stripping potential; Thermal stabilization

1. Introduction

Many analytical protocols in electrothermal atomic absorption spectroscopy (ETAAS) require * Corresponding author. Fax: + 61 2 93856141; e-mail: J. [email protected]

stabilization of the analyte prior to, or within, the atomization phase. Often the objectives are to minimize the premature loss of volatile molecular analyte species, to improve separation of analyte and matrix components during the ashing phase and/or to increase the atomization temperature. The necessary stabilization can be achieved by

0039-9140/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll SO0 39-9140(96)021 55-8

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J.P. Matousek, H.K.J. Powell/ Talanta 44 (1997) 1183 1193

encapsulation of the analyte within a much larger mass of less volatile thermally produced matrix, e.g., MgO, or by alloying the analyte with a large mass of thermally produced metal, such as Ni or Pd [1]. Palladium is also used to preconcentrate hydrides of As, Sb, Se, Bi and Sn as compounds of low volatility prior to atomization from the pyrolytic graphite furnace [2]. A common problem with the use of chemical modifiers is the large mass required relative to the mass of analyte and therefore the associated risk of contamination. For example, the masses of Ni and Pd used are typically in the range I0 50 and 1 20 lag, respectively [1]. Recently we have reported the use of electrodeposited Pd as a modifier [3]. This modifier is significantly more effective than thermally reduced Pd at stabilizing the volatile analytes Pb, Cd, Cu [3] and H2Se [4] to higher temperatures. Typically a mass of only 0.2 gg is required, being produced in situ in the pyrolytic graphite coated furnace. For example, our ETAAS results for stabilization of Pb by thermally generated (3.0 lag) and eletrodeposited (0.25 lag) Pd confirm more efficient stabilization by the latter: based on the ashing loss curves, stabilization of Pb by 500 K using electrodeposited Pd contrasts with only 150 K for thermally produced modifier [3]. The contribution of modifier contaminants with more negative reduction potentials is diminished by programming the autosampler to remove spent electrolyte after modifier deposition. The modifier blank ean be further reduced by an acid wash. The modifier can be used for analysis of analytes by conventional methods, or by their electrodeposition onto the Pd. In the electrodeposition protocol the analyte can be quantitatively separated in 60 s deposition from such troublesome matrices as 0.5 M NaC1; the background absorption by NaCI is reduced by more than 99.5% [3]. In this paper we make a comparison of electrochemically and thermally produced Pd modifiers on the basis of their scanning electron micrographs. Modifiers produced from different electrolytes (HNO3, HC1) are compared. Evidence is presented from differential pulse ASV that electrodeposited analytes affix to the Pd domains rather than to the much larger area of uncoated

pyrolytic graphite. From ASV measurements on a basal plane pyrolytic graphite cathode it is established that the relative amounts of (the more highly stabilized) monolayer and bulk Pb deposited onto Pd (or graphite) depend on sample pH. The additional stabilization of Pb by deposition onto Pd rather than graphite is approximately A G = - 3 1 kJ mol ~

2. Experimental 2.1. Instrumentation

Scanning electron micrographs were taken with a Jeol JXA-840 scanning microanalyzer operating with an accelerating voltage of 30 kV and capable of 12 50000 x magnification. A GBC GF-3000 graphite furnace system, incorporated into a GBC 932 atomic absorption spectrometer was used to clean and condition miniature graphite furnace sections used for depositions. The standard graphite furnace tube (GBC Part No. G 19310121) was modified for this purpose by removing the internal central partitions so as to accommodate the furnace sections for cleaning and for thermal reduction of Pd. ASV measurements were effected with a PAR 384B polarographic analyzer. Solution volumes of 6 ml were held in a perspex cell of 10 ml capacity designed for use with the PAR 303A stirrer and were stirred with a magnetic follower at 700 rpm. The working electrode (3 mm x 4 ram) was fabricated from a thin section of basal plane pyrolytic graphite mounted at the flanged base of a perspex tube (6 mm o.d.) by sealing in Araldite; electrical contact was via a drop of Hg on the reverse side. The auxiliary electrode was a 2 mm dia disk of high density graphite; the Ag,AgC1/C1 (3 M) reference electrode was separated from the test solution by a Vycor tip. 2.2. Reagents

Sodium acetate, HNO 3 and HC1 were Aristar (BDH) quality. Acetic acid (BDH Analar) was further purified by isopiestic distillation. Stock solutions of Pd and Pb (100 mg 1 1) were pre-

J.P. Matousek, H.K.J. Powell Talanta 44 (1997) 1183 1193

1185

pared by dissolving PdC12 (Aldrich 99.99%) and Pb(NO3) 2 (BDH) in dilute Aristar HNO3 or HC1. All dilutions were in Milli-Q water.

3. Results and discussion

2.3. Procedure

Surface distribution of thermally produced modifiers on pyrolytic graphite coated furnaces has been a subject of several SEM studies [5-7]. There is a general consensus that Pd or Ir modifier is concentrated towards the edges of the deposited solution. This is either ascribed to uneven distribution of the modifier during the drying step [5] or to its migration towards the edges during pyrolysis [6]. However, our experiments at 250 x magnification (not shown) confirm that the uneven distribution of Pd modifier stems from the drying process and is substantially maintained during pyrolysis up to 12000(2. In contrast, electrodeposited modifier is more uniformly and densely distributed. This difference in distribution is consistent with the observation that trapping of H2Se injected through the furnace orifice following external generation is more efficient by Pd electrodeposited from 1% H N O 3 [4]. Modifier (Pd) electrodeposited from HC1 contrasts with that from H N O 3. Evolution of C] 2 at the anode leads to the dissolution of Pd [8] from the cathode area opposite the Pt/Ir probe. Fig. l(a)-(d) shows the scanning electron micrographs (2500 x magnification) and corresponding X-ray dot maps for Pd electrodeposited from 0.5% HC1 and f¥om 1% H N O 3. From HC1 medium a more uniform, higher density of domains was achieved. At 2500 x magnification the Pd domains electrodeposited from HNO3 medium appeared to fluoresce. Fluorescence (cathodoluminescence) is characteristic of non-conducting or semiconductor materials [9]. This suggests that under the conditions of vacuum and electron bombardment in the spectrometer the linkage between the Pd domains and the graphite substrate has a low electrical conductivity. As seen in Fig. l(b), Pd deposits from H N O 3 indeed appear not to adhere as well to the pyrolytic graphite surface as those from HC1 medium. An additional feature associated with Pd deposited from HC1 was a 'glazed' appearance. This feature is consistent with adsorption of C1 on the surface of the Pd domains (despite a 30 min

For scanning electron microscopy (SEM) studies, electrodeposition was onto the concave surface of sections (9 x 3 mm) prepared by grinding pyrolytically coated miniature GBC GF-1000 furnaces (Ringsdorff Werke RWO/PyC) lengthwise to approximately half the original size. The sections were cleaned by heating twice inside a modified GF-3000 tube at 2500°C for 5 s with the internal Ar flow. The electrodepositions, at 5 V uncontrolled potential, were effected from 10 lal aliquots in the half furnaces, using a purpose-built stand in which the position of the Pt/Ir anode was controlled by an attached Agla micrometer. The anode-cathode separation was set at 0.7 mm, similar to that achieved in ETAAS experiments [3]. Following deposition of Pd (60 s, 20 30 mA) from 100 mg 1-1 solution in 1% H N O 3 or 0.5'~, HCI, the furnace section was washed thoroughly in 0.1% Aristar HNO3, then Milli-Q water and dried at 100°C. Lead was deposited over the Pd layer from 100-300 mg 1 ~ Pb in 1% HNO~ or 0.03 M acetate buffer. Conditions were identical to those for Pd except the deposits were washed initially with Milli-Q water under applied voltage and then immersed in Milli-Q water prior to drying. For thermally produced modifier, 10 lal aliquots of 1000 mg 1 1 Pd solution in 5% HC1 were dried in half furnaces, followed by conditioning at 1200°C for 20 s. ASV measurements were performed in a class 100 clean room. The electrode surface was initially cleaned by abrasion on an acid-soaked quantitative filter paper then by anodic polarization at 0.70 V for several minutes in stirred 1% HNO3. Solution concentrations were: Pb, 120 225 gg 1 ~ in 0.03 M acetate buffer, pH 4.4, or in 0.1% HNO3; Pd 5.0 mg 1 1 in 0.05% H N O 3. Instrumental parameters were: N2 flush 10 min, deposition time 60-1500 s, deposition potential - 0.70 V (Pb) or 0.20 V (Pd), final potential 0.60 V, mode DPSV, modulation 25 mV, stripping speed 10 mV s '.

3.1. Scanning electron microscopy

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J.P. Matousek, H.K.J. Powell / Talanta 44 (1997) 1183 1193

(a)

(b)

5629 2BKU

X2,588 18Pm WD39

(c)

(d)

Fig. 1. Scanning electron micrographs at 2500 x magnification of 1 lag Pd electrodeposited from (a) 0.5% HCI and (b) 1% HNO3 with corresponding X-ray dot maps measured at Pd L:~ 2.838 keV line (c) and (d), respectively.

wash in 0.1% HNO3). An intense C1 fluorescence, its source physically coincident with Pd fluorescence, was observed in the SEM. Although the mass of Pd used was well in excess of that required for a monolayer coverage (but of the same order as the amount required for the production of electrodeposited modifier), the Pd deposited from 0.5% HC1 (Fig. l(a) and (c)) is concentrated in a series of domains representing less than 20% of the surface area. Fig. 2 documents that, when viewed at the same magnification, thermally produced modifier provides a more uniform coverage (but less dense) relative to the same quantity of Pd electrodeposited from HC1. The scattered domains which arise from thermal evaporation and reduction of Pd salts (as documented previously [7]) indicate that sites with

discrete activity are a feature of the adsorption and nucleation processes. The formation of highly concentrated domains of Pd by electrodeposition on graphite surfaces, as observed for metal deposition on a glassy carbon electrode, indicates the presence of electroactive sites [10]. It may be expected that Pd electrodeposition is directed preferentially towards the most active sites. However, recent observations for thermally generated Ir modifier have confirmed that its distribution is non-reproducible even when carried out under the same conditions and that thermal pretreament results in modifier redistribution dictated by the surface features (rather than active sites) of the graphite tube [5]. Our observations confirm that surface morphology of the pyrolytic graphite coating plays a

J.P. Matousek, H.K.J. Powell Talanta 44 (1997) 1183 1193

significant role in determining the deposition pattern. The grade of graphite used displays a range of raised features and it is on these that preferential deposition takes place. In ETAAS the amount of analyte determined is typically much smaller than the quantity of Pd modifier used. However, the low sensitivity of the XRF technique necessitates the use of unrealistically high amounts of analyte to obtain usable results. Despite using three orders of magnitude higher amount of Pb for SEM investigations (relative to the ASV work), detection still proved difficult. In order to highlight X-ray dot maps, these are presented in Fig. 3 and Fig. 4 as negative images. It appears from overlaps of X-ray dot maps for Pd and Pb that, for both HNO3 and

(a)

(b) Fig. 2. Scanning electron micrographs at 2500 x magnification of 10 p.g Pd in 5% HCI after drying and conditioning at 1200°C (a) and corresponding X-ray dot map measured at Pd L:q 2.838 keV line (b).

1187

acetate buffer, these elements favour the same regions of the pyrolytic graphite surface. It should be noted that the dot maps are not exactly coincident, however, coincidence of the deposits is indicated by the displacement of the Pb ashing loss curve in the presence of electrodeposited Pd [3], and from the ASV results presented below. It was observed that the intensity of fluorescent radiation for Pd was noticeably weaker when Pb was deposited over the Pd layer (Fig. 3(b) and Fig. 4(b) compared with Fig. l(c)).

3.2. Anodic stripping voltammetJ 3, 3.2.1. Deposition of Pb on graphite Fig. 5 shows the differential pulse stripping voltammograms as a function of deposition time for Pb deposited at - 0 , 8 0 V from a 120 pg 1solution in 0.03 M acetate buffer, pH 4.0. Broad stripping peaks are observed at E v = - 0 . 4 5 , 0.06 and 0.21 V versus Ag,AgC1/CI (3 M). The more cathodic peak at potential E v = - 0 . 4 5 V, is ascribed to the stripping of Pb from multilayer (bulk) deposits. For reference, the E ° for Pb 2 +/Pb is - 0 . 3 2 V and E1/2 for Pb 2+/(Pb/Hg) is - 0 . 3 1 V, both relative to Ag,AgC1/CI (3 M) [11]. The most anodic peak is ascribed to the stripping of monolayer Pb (designated Type I Pb). This indicates an underpotential of 0.66 V. The peak at - 0 . 0 6 V may arise from Pb deposits at surface sites different from those at which the more stable monolayer is formed [12]. Szabo [13] has noted the energetic heterogeneity of substrate surfaces and thus the existence of multiple energy states for monolayers. Alternatively, the peak at - 0 . 0 6 V may arise from stripping of the Pb layer next to that in direct contact with the graphite substrate. The atomic environment in this layer will be unique and the atoms will be partially stabilized by interaction with the electrode material. Fig. 6 shows a plot of peak height versus deposition time for the three stripping peaks. The peak height for monolayer stripping increases systematically with deposition time, but much less rapidly than that for multilayer Pb. The peak at E r = - 0.06 V is almost invariant with deposition time. This is consistent with it representing monolayer deposition at sites for which the activation

I 188

J.P. Matousek, H.K.J. Powell /Talanta 44 (1997) 1183 1193

...... • .

(a)

,

t

:

(b)

w

m

N (c)

.'

mlt

(d)

Fig. 3. Scanning electron micrographs at 2500 x magnification of 3 p~g Pb electrodeposited from 1% H N O 3 over 1 ~g Pd electrodeposited from 0.5% HC1 (a) and corresponding X-ray dot maps measured at Pd L ~ 2.838 keV line (b) and Pb M ~ 2.346 keV line (c). X-ray dot map (d) is a composite of (b) and (c). Negative images are used for (b), (c) and (d).

energy for deposition is low and the number of sites is unaffected by the impact which cumulative deposition or gas evolution has on the surface properties. (This is designated Type II Pb.) It is not consistent with a unique second layer of Pb deposited over the expanding monolayer coverage. Both of the monolayer peaks are of significant magnitude after zero deposition time (70-90 s scan from - 0 . 7 0 V in unstirred solution). The unique stability of the monolayer Pb deposits is illustrated by the effect of a 20 s rinse of the electrode in 0.5% HNO3 following a 360 s deposition at - 0 . 8 0 V. Fig. 7, curve A indicates that all of the multilayer Pb (from 360 s deposition) is dissolved by the acid treatment. From

the relative peak heights at 0.21 V and - 0 . 0 6 V it is inferred that, compared with Fig. 5, a partial dissolution of the less stable Type II monolayer also occurs. Fig. 7, curves B and C show the stripping voltammograms following 60 s and 500 s depositions of Pb from a 0.1% HNO3 solution, respectively. Only the more stable monolayer, Type I, accumulates; the monolayer Pb is more resistant to oxidative dissolution in the presence of 02 [14], consistent with its more positive stripping potential, The Ep value for Type I Pb indicates a stabilization of A G ° = - 1 2 7 kJ mol J ( = - z F A E ) relative to multilayer Pb. The resistance of the graphite-Pb monolayer to acid dissolution is consistent with the negative value of E for Pb(C) + 2H + = Pb 2 + + C + H2.

1189

J.P. Matousek, H.K.J. Powell Talanta 44 (1997) 1183 1193

n

m

(a)

(b)

. . . .

•

.

• .-;;.,.z

•, "_.,, ) L

N (c)

.?

IIl°mF-1[l~ O

(d)

Fig. 4. Scanning electron micrographs at 2500 x magnification of 1 btg Pb electrodeposited from 0.03 M acetate buffer over 1 p.g Pd electrodeposited from 0.5% HC1 (a) and corresponding X-ray dot maps measured at Pd L:q 2.838 keV line (b) and Pb Mcq 2.346 keV line (c). X-ray dot map (d) is a composite of (b) and (c). Negative images are used for (b), (c) and (d).

3.2.2. Deposition o f P d on graphite Deposition o f Pd onto pyrolytic graphite was effected at 0.20 V from a solution containing 5.0 mg 1-1 Pd in 0.05% (0.08 M) H N O 3. The stripping v o l t a m m o g r a m taken from - 0.70 V, immediately following 100 s deposition showed a double peak with maxima at - 0 . 1 8 V and approximately 0.00 V (Fig. 8, curve A). Because E~,a2 + ,.pd = 0.95 V (versus N H E ) these peaks must involve a species other than (or in addition to) Pd~s). Possible assignments include the oxidation processes:

Pd2 H --- 2Pd + H + + e and

1)

Pd(H2) = Pd + 2H + + 2e -

(2)

In contrast, stripping following a 60 s electrolysis at - 0.70 V in acetate buffer gave a single peak at - 0 . 1 6 V (Fig. 8, curve B). F o r the reduction process Eq. (1), the standard reduction potential E ° = 0 . 0 4 8 - 0 . 0 5 9 p H versus N H E [15], giving E = - 0 . 2 1 V versus Ag,AgC1/C1 (3 M) at [H +] = 0 . 0 8 M. F o r the reduction process Eq. (2), E ° = 0 . 0 0 V versus N H E , giving E = - 0 . 2 6 V versus Ag,AgC1/C1- (3 M) at [H +] = 0 . 0 8 M. These values are for bulk Pd; different values could be expected for small Pd domains on a graphite substrate [16]. As indicated below, the position o f the peak(s) is very sensitive to the deposition o f analyte (Pb) o n t o Pd.

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J.P. Matousek, H.K.J. Powell /Talanta 44 (1997) 1183 1193

3.2.3. Deposition of Pb on Pd The electrode was precoated with Pd by 600 s deposition from 5.0 mg 1-1 solution at 0.20 V. Deposition of Pb from 0.03 M acetate buffer (pH 4.4) onto electrodeposited Pd on pyrolytic graphite gave a stripping voltammogram with peaks at - 0 . 2 7 , - 0 . 1 7 to 0.09 and 0.37 V (Fig. 9, curves A E) for 0 1500 s depositions. The former was assigned to multilayer Pb and the latter to monolayer Pb on Pd. The shift in stripping potential for multilayer Pb (relative to Ep on graphite) was unexpected; it may be a consequence of the low mass of analyte involved, hence the proximity of all Pb atoms to the electrode (Pd) surface. The difference between Ep for monolayer Pb deposited on graphite and for monolayer deposition on Pd, ZXEp= 0.16 V, corresponds to a stabilization (on Pd) of AG ° = - 3 1 kJ t o o l ( = - zFAEp). This is the energy difference for the processes Pb(C) = Pb(aq~ 2 + + 2e

2.00

1.60

=L

C i.

(J

0.21V

1

0.40

-O.06V

(3)

and Pb(Pd) = Pb(~q) 2 ÷ + 2e

(4)

~)'

1

300

I

t

600

900

I

1200

t dep (s)

Fig. 6. Plot of differential pulse stripping peak height versus deposition time for bulk Pb (Ep = - 0 . 4 5 V) and monolayer Pb bound to graphite, type I ( E p - 0 . 2 1 V) and type II (Ep = - 0 . 0 6 V). Deposition conditions as for Fig. 3.

2.G

@

1,C

0

-0~.7

'

-0~.3 ' 011 ' Voltage (vs Ag,AgCl/CI'(3M))

0'.5

Fig. 5. Differential pulse stripping voltammograms for Pb deposited on pyrolytic graphite from acetate buffer (0.03 M, pH 4.0). Conditions: 120 gg 1 ~ Pb, conditioning potential 0.70 V, deposition potential - 0.80 V. Voltammograms shown for (A) 60, (B) 180 and (C) 600 s deposition from stirred solutions.

Yan and Ni [17] determined the activation energies for release (first order kinetics) of Pb from graphite and palladium surfaces (301 and 418518 kJ mol 1, respectively). They inferred that these energy terms are a measure of the analytesurface interaction energy. If this is so, then the difference in stability of the two surface states of the analyte is c. -(100-200) kJ t o o l J. However these terms are kinetic parameters (activation energies) rather than thermodynamic parameters. Further, there is no guarantee that the process leading to the observed activation energy involves monolayer, as distinct from polylayer atomic species. For comparison, the stabilization of monolayer Pb on Pd relative to bulk Pb on Pd is calculated as A G ° = - 124 kJ mol i from the shift in stripping potentials.

1191

J.P. Matousek, H.K.J. Powell / Talanta 44 (1997) 1183-1193

~

4.0

5.0

3.O

4.0

~ E

v

A

= 2.0

<= 3.0

D

e .'-

C 2.0

1.0

B 1,0

-oi,

-o13

olt

ols

A

Voltage (vs Ag,AgCI/CI-(3M))

Fig. 7. Differential pulse stripping voltammograms for Pb deposited on graphite in acid (HNO3) solution. Curve A, 200 s conditioning at 0.70 V, 360 s deposition from acetate buffer (0.03 M, pH 4.0) at 0.80 V, followed by 20 s in 0.5% HNO 3 before stripping. Curves B and C for deposition (60 and 500 s, respectively) and stripping in 0.1% HNO3 (curve C at 1/3 sensitivity). T h e p o s i t i o n o f the central stripping p e a k was very sensitive to p l a t i n g time (mass o f P b deposited), Fig. 9, curves A - E . It shifted from - 0 . 1 7 V for 0 s d e p o s i t i o n to 0.09 V for deposition times greater t h a n 1000 s, an effect which parallels that o f Pb ' a d a t o m s ' on Pd catalysts; M a l l a t et al. [16] r e p o r t the o x i d a t i o n o f H 2 f r o m 2.0

-0.7

-0 3

0.1

0.5

Voltage (vs Ag,AgCI/CI-(3M))

Fig. 9. Differential pulse stripping voltammograms for depositions from 225 p.g 1-i Pb in 0.03 M acetate buffer (pH 4.4) onto pyrolytic graphite precoated with electrodeposited Pd. Curve A, blank (Pd only), B 60 s. C 240 s, D 480 s, E 1500 s deposition at - 0.70 V. Curve F for 480 s deposition followed by 20 s in 1% HNO 3 before stripping. f l - P d shifts f r o m 0.15 V (versus N H E ) for clean Pd to 0,30 V after Pb p o i s o n i n g . In contrast, the stripping c u r r e n t was i n d e p e n d e n t o f p l a t i n g time b u t was larger for scans in 0.5% H N O 3 t h a n in acetate buffer. T h e s t r i p p i n g c u r r e n t increased with mass o f d e p o s i t e d Pd. A 15 s wash with 1% HNO3 following a 480 s d e p o s i t i o n from p H 4.4 acetate buffer shifted this p e a k f r o m 0.02 V b a c k to - 0.19 V (curve F). This w a s h also caused the loss o f the - 0 . 2 7 V peak. T h e r e was also an a p p a r e n t three-fold increase in the m o n o l a y e r p e a k at 0.37 V, p r e s u m a b l y arising from the reactions:

1.O

2Pb +

A

~

B

0 2

-}- 4 H + = 2Pb 2 + + H20

and Pb ~ + + Pd(H2) = P b / P d + 2H +

_017

_013

011

O'.S

Voltage (vs Ag,AgCI/CI'(3M))

Fig. 8. Differential pulse stripping voltammograms for Pd deposited on pyrolytic graphite. Curve A: 100 s deposition from 5.0 mgl L Pd (0.05% HNO3) at 0.20 V. Curve B: 60 s electrolysis at 0.70 V of Pd-coated pyrolytic graphite electrode (curve A) in 0.03 M acetate buffer (pH 4.4).

(5) (6)

Thus, a n a l o g o u s to d e p o s i t i o n on graphite, only the Pb m o n o l a y e r resists dissolution in dilute acid. D e p o s i t i o n o f Pb o n t o Pd f r o m 1% (or 3%) H N O 3 gave m o n o l a y e r Pb only (Fig. 10, curve A). The stripping c u r r e n t was p r o p o r t i o n a l to (tdep) n, where n is less t h a n 1.0. T h e s t r i p p i n g v o l t a m m o g r a m also s h o w e d the Pd(H2) o x i d a t i o n p e a k

J.P. Matousek, H.K.J. Powell / Talanta 44 (1997) 1183-1193

1192

at - 0.01 V. An interesting feature of this voltammogram is a ten-fold enhancement of the monolayer peak relative to that produced from acetate buffer for the same deposition time (Fig. 10, curve B). From this it may be inferred that in acetate buffer the rate (activation energy) for deposition of Pb onto Pb is greater (smaller) than that for deposition onto Pd. In the application of the electrodeposition--ETAAS technique it was found advantageous to effect 1% HNO3 redeposition of Pb initially deposited from more weakly acidic acetate buffer solutions. This minimized losses during the subsequent rinsing stage (used to dilute matrix components such as NaC1). This can now be understood in terms of the dissolution of bulk Pb and its redeposition in more stable monolayer forms which resist dissolution during rinsing with air-saturated Milli-Q water. A potential problem with quantitative deposition of analytes from dilute solutions is the competitive reaction from dissolved 02, or other oxidants generated at the anode [14,18,19]. An advantage arising from monolayer deposition onto a Pd modifier is the substantial increase in stability of the Pb(s) phase. This will contribute to the success of the electrodeposition--ETAAS method.

e

A

u 2

4.3

o'.5

Voltage (vs Ag,AgCI/CI'(3M))

Fig. 10. Differential pulse stripping voltammograms for 225 p.g 1 ~ Pb deposited on pyrolytic graphite coated with electrodeposited Pd. Curve A, 480 s deposition from and stripping into 1% HNO3. Curve B, 480 s deposition from acetate buffer (pH 4.4).

3.2.4. Effect of analyte stabilization on appearance temperatures Using a simplified approach it is possible to estimate the effect on the ETAAS appearance temperature of Pb (Tapp) arising from its stabilization as a monolayer on Pd. From the relationship A(AG °) ~ -zF(AEp) applied to the redox reactions Eq. (7) and Eq. (8) the relative stability (Gibbs free energy) of the monolayers Pb(C) and Pb(Pd) can be estimated. Pb 2 + + 2e + C~s~= Pb(C)

(7)

Pb e + + 2e + Pd(~) = Pb(Pd)

(8)

Using the relationship A(AG °) = - R A ( T In Kp) applied to reactions Eq. (9) and Eq. (10) Pb(C) = Pb(g) + C(s) Pb(Pd) = Pb(g~ + Pd(s)

(9) (10)

A(AG °) can be used to estimate: (i) the ratio of partial pressures of Pb above the two monolayers at a given temperature, or (ii) the shift in appearance temperature (Tavv) corresponding to a specific vapour pressure of Pb, A(AG °) = - RA (T, pp) in Papp However, some approximations are necessary. It is assumed that A(AG °) is not affected significantly by a change in temperature. If it is assumed that Tapp corresponds to a mass of analyte similar to the characteristic mass (8 pg for Pb [3]) and that the effective volume occupied by the gaseous analyte at T,p v is ca. 100 gl then at (say) 1000°C the ideal gas equation gives Papp = 4 x 10 8 atm. An ideal gas assumption is reasonable considering the high temperature involved. From the observed shift in stripping potentials (0.16 V) the calculated value for /~(Tapp) is approximately 220 K, a value in reasonable agreement with the displacement of ashing loss curves by 250 K reported previously by Matousek and Powell [3]. Such calculations do not take cognisance of the kinetic factors involved in the atom release processes, nor the fact that the system may not be at equilibrium around the appearance temperature. However, they serve to indicate the insight to be gained about surface stabilization of analytes from measurement of their stripping potentials.

J.P. Matousek, H.K.J. Powell /Talanta 44 (1997) 1183 1193

Acknowledgements One of the authors (J.P.M.) gratefully acknowledges financial support from The University of New South Wales Special Studies Program and from Deutscher Akademischer Austauschdienst. The authors are thankful to Miss V. Piegerova, School of Materials Science and Engineering, The University of New South Wales for assistance with measurement and interpretation of scanning electron micrographs.

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