Metal speciation by coupled in situ graphite furnace electrodeposition and electrothermal atomic absorption spectrometry

Talanta 52 (2000) 1111 – 1122 www.elsevier.com/locate/talanta

Metal speciation by coupled in situ graphite furnace electrodeposition and electrothermal atomic absorption spectrometry Jaroslav P. Matousek a, Stephen D. Money b, Kipton J. Powell b,* a

Department of Analytical Chemistry, The Uni6ersity of New South Wales, Sydney, NSW 2052, Australia b Department of Chemistry, Uni6ersity of Canterbury, Pri6ate Bag 4800, Christchurch, New Zealand Received 25 January 2000; received in revised form 31 May 2000; accepted 1 June 2000

Abstract The technique of coupled in situ electrodeposition – electrothermal atomic absorption spectrometry (ED – ETAAS) is applied to the analytes Bi, Pb, Ni and Cu. Bi, Pb, Ni and Cu are deposited quantitatively from their EDTA complexes at Ecell =1.75, 2.0, 3.0 and 2.5 V, respectively (Ecell = Eanode − Ecathode + iR). By varying the cell potential, selective reduction of free metal ions could be achieved in the presence of the EDTA complexes. For Bi3 + and Pb2 + this utilised the voltage windows Ecell =0.6–1.0 and 1.8 – 2.0 V, respectively. For Ni, deposition at Ecell = 1.7 – 2.0 V achieved substantial, but not complete, differentiation between Ni2 + (ca. 90 – 100% deposition) and Ni(EDTA)2 − (ca. 12–20% deposition). An adequate voltage window was not obtained for Cu. The ability of ED – ETAAS to differentiate between electrochemically labile and inert species was demonstrated by application of both ED – ETAAS and anodic stripping voltammetry to the time-dependent speciation of Pb in freshly mixed Pb2 + – NaCl media. Application to natural water samples is complicated by adsorption of natural organic matter to the graphite cathode. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrodeposition; Palladium modifier; Speciation; Lability; Electrothermal atomic absorption

1. Introduction Each analytical technique has its own attributes and limitations. For example, electrothermal atomic absorption spectrometry (ETAAS) is capable of very high sensitivity and selectivity, but * Corresponding author. Fax: +64-3-3642110. E-mail address: [email protected] (K.J. Powell).

cannot differentiate between the individual species that make up the total element concentration. It is well established that metal speciation is critical to the toxicity of an element in a natural sample. Often it is the simple aqua ion that shows the greatest toxicity [1], although the lability and stability of a metal complex are key parameters affecting its availability within the diffusion layer of a micro-organism’s lipid membrane [2]. Thus to date, ETAAS has had very limited application in

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determining metal toxicity in environmental samples. In contrast, anodic stripping voltammetry (ASV) has high sensitivity and precision but also is able to differentiate between very labile (including ‘free’), moderately labile and (with sample pre-treatment) inert metal species. Another advantage is the ability to determine several elements concurrently. However, it has a much smaller sample throughput than ETAAS and for many samples it is dependent on a sample-bysample in situ calibration, necessitated by the sensitivity of the Hg drop or Hg film to the presence of hydrophobic components in a sample. The technique is limited to metals that are reducible at cathodic potentials more positive than ca. − 1.2 V (vs. SCE) and which can amalgamate to adequate concentration with Hg. Further, the metals must be stripped (oxidised) from the Hg in a reversible reaction and at a potential below that for significant oxidation of Hg (ca. 0.45 V vs. SCE). These provisos discriminate against the application of ASV to elements such as (i) Mn and Fe (Ed B −1.2 V), (ii) Co (limited solubility in Hg), (iii) Ni, Mn, Cr and Co (not stripped reversibly) and (iv) Pt, Pd, Ag, Au (oxidised at E\0.45 V vs. SCE). Recently we have described the combination of quantitative microlitre sample electrodeposition (ED) with ETAAS for analysis of trace elements [3]. The ED is carried out in situ in a pyrolytic graphite-coated atomiser that has been precoated with electrodeposited Pd. From acid solution the analyte is deposited as a monolayer onto Pd domains on the pyrolytic graphite [4]. In some respects there are parallels between this electrodeposition technique and the ASV technique, but there are also marked differences. The ED– ETAAS technique involves quantitative deposition of metal from simple aqua ions (in ca. 40–60 s), compared with B1% deposition in a typical ASV experiment. It does not involve or require amalgamation and a stripping step is not involved. Also the deposition is neither under conditions of controlled stirring and controlled diffusion layer thickness, nor of constant cathode potential (as the cell resistance changes with consumption of H+). Generation of H2 at the

cathode ensures adequate but uneven stirring of the sample. A parallel exists between these two techniques in that only complexes that are labile on the experimental timescale may be reduced at the cathode. In the present work we have explored the possibility of determining metal speciation by ED– ETAAS measurements through control of sample medium and electrolysis potential. We have analysed metal–EDTA systems of varying lability and metal:ligand ratios (M= Bi, Pb, Ni and Cu) and established the cell ‘voltage windows’ in which species differentiation may be achieved. For Bi and Pb, parallel studies by ASV and ED–ETAAS have been performed and compared. This work has also identified the cell voltages at which quantitative deposition of the complexed metals can be achieved. For freshly mixed weakly acidic solutions of Pb (in NaCl or in pH 4.7 acetate media) ASV and ED–ETAAS have been used to monitor the time-dependent changes in lead speciation.

2. Experimental

2.1. Reagents Sodium acetate, NaCl, HNO3 and HCl were of Aristar (BDH) quality. Acetic acid and ammonia (BDH Analar) were further purified by isopiestic distillation. EDTA was Koch-Light Analar grade; a 2.4× 10 − 6 M working stock was prepared. Bi, Pb, Ni and Cu working stocks were prepared from 1000 ppm atomic absorption standards (BDH Spectrosol), Cu2 + 100 ppb in 0.125 M HNO3; Bi3 + , Pb2 + and Ni2 + 500 ppb, in 0.125 M HNO3. A stock solution of Pd (100 ppm) was prepared by dissolving PdCl2 (Aldrich, 99.99%) in 1% Aristar HNO3. All dilutions were with Milli-Q water, with addition of 1% HNO3 to metal standards.

2.2. Instrumentation ASV measurements were effected with a PAR 384B polarographic analyser, with a PAR 303A HMDE working electrode and Pt auxiliary elec-

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trode. The Ag/AgCl/Cl− (3 M) reference electrode was separated from the test solution by a Vycor frit. ED–ETAAS measurements were made on a GBC 908 atomic absorption spectrometer coupled with a GBC GF-3000 graphite furnace system. The PAL-3000 autosampler delivery system was modified for analyte electrodeposition, withdrawal of spent electrolyte and rinsing of residual sample from the furnace, these steps being integrated into one concerted procedure [3].

2.3. ED and ASV procedures All ASV and ED – ETAAS measurements were made in a class 100 clean room. For ASV, the instrumental parameters were, stirring 700 rpm, N2 flush 10 min, followed by square wave stripping analysis using a 20 mV pulse height, 2 mV increment and 200 mV s − 1 scan rate. Pb was deposited for 5 min at − 0.6 V and stripped by scanning to 0.0 V. Bi was deposited for 5 min at −0.45 V and stripped by scanning to 0.1 V. The ED– ETAAS procedure involves three steps [3]. Firstly, the pyrolytic graphite-coated tube (ETAAS furnace) is plated with Pd by electrodeposition in situ (from 30 ml of 10 ppm Pd in 0.1% HNO3, by 30 s deposition at Ecell = 2.0 V). Spent electrolyte is withdrawn via the Pt/Ir delivery tube (which also functions as the anode). Secondly, the sample (20 ml) is electrolysed for 40–100 s at a preset cell voltage (0 – 3.0 V). During this time the furnace may be at room temperature, or heated to ca. 45°C to improve convective stirring of the sample. Room temperature is used with high currents (5 – 40 mA) as arise in acidic media, with stirring being effected by evolution of H2 from the Pd cathode. Heating is used with low currents ( B5 mA) as arise with weakly acidified salt media; convective stirring is aided by passing the inert purging gas (N2) over the heated sample. For most analytes quantitative deposition is achieved within 40 – 60 s. Spent electrolyte is removed via the delivery tube. The deposited sample may be (i) rinsed with Milli-Q water (40 ml, 5 s), or (ii) redeposited from 35 ml of dilute acid (3% HNO3) before rinsing. Finally, the sample is atomised using a standard ETAAS protocol. The

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detailed protocols for deposition of modifier and deposition, re-deposition and rinsing of analyte, and the instrument modifications required for electrodeposition and sample withdrawal, have been reported [3].

2.3.1. Bismuth For ED–ETAAS measurements on Bi as a function of cell voltage applied, two Bi solutions were prepared, (a) 0.96 mM Bi3 + (200 ppb), 0.04 M NaCl, pH 2.0; and (b) a 1:2 Bi:EDTA solution (0.96 mM Bi3 + , 1.9 mM EDTA, 0.04 M NaCl, pH 2.0). These were prepared by dilution of stock solutions, followed by warming of (b) on a water bath for 30 min to achieve complex formation, and allowing to stand overnight. Low pH is necessary to suppress hydrolysis of Bi3 + . Electrodeposition ED was at room temperature with inert gas ‘on’; ETAAS involved pyrolysis at 1100°C and atomisation at 2300°C. For ASV and ED– ETAAS measurements as a function of the Bi:EDTA ratio, solutions were prepared as for (b) above, but using Bi:EDTA stoichiometries in the range 1:0–1:2 (0–1.9 mM EDTA). Speciation in these solutions was calculated using the program SOLGASWATER [5]. 2.3.2. Lead For ED–ETAAS measurements on Pb as a function of cell voltage, two Pb solutions were prepared as required, (a) 2.25 mM Pb2 + in 0.001 M acetate buffer, 0.01 M KNO3, pH 4.5; and (b) 2.25 mM Pb2 + , 3.0 mM EDTA in 0.001 M acetate buffer, 0.01 M KNO3, pH 4.5). ED was for 90 s at room temperature with the inert gas ‘on’ and used an unmodified furnace (no Pd); ETAAS involved pyrolysis at 700°C and atomisation at 2350°C. For ED–ETAAS studies as a function of the Pb:EDTA ratio, solutions were prepared containing 0–0.75 mM EDTA and 0.6 mM Pb2 + in 0.001 M acetate buffer, 0.01 M KNO3, pH 4.5. ED times of 10 and 60 s were used (no Pd); prior to atomisation Pb was re-deposited from 35 ml 0.1% HNO3 (30 s). For ASV studies as a function of the Pb:EDTA ratio, solutions were prepared containing 0–0.3 mM EDTA and 0.3 mM Pb2 + in 0.001 M acetate buffer, 0.01 M KNO3, pH 4.5.

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2.3.3. Nickel and copper For ED– ETAAS measurements on Ni as a function of cell voltage, two solutions were prepared by direct dilution of working stocks, (a) 0.68 mM Ni2 + in 0.14 M NH3/NH+ 4 buffer, pH 7.3; and (b) 0.68 mM Ni2 + , 4.0 mM EDTA in 0.14 M NH3/NH+ 4 buffer, pH 7.3. ED was for 60 s at 45°C with inert gas ‘on’; pyrolysis was at 900°C and atomisation at 2600°C. For analogous studies with Cu as a function of cell voltage two solution stoichiometries were employed, (a) 0.16 mM Cu2 + ; and (b) a 1.00:2.25 Cu:EDTA solution (0.16 mM Cu2 + , 0.36 mM EDTA). The three solution media examined were 0.05 M KNO3, 0.015 M acetate buffer (pH 4.5) and 0.05 M hexamine buffer (pH 4.5). ED was effected for 60 s at room temperature with the inert gas ‘on’; pyrolysis was at 800°C and atomisation at 2500°C. For the Cu–fulvic acid study a solution containing 10 ppb Cu2 + , 2.8 mg l − 1 of soil-derived fulvic acid [6] and 0.1 M acetate buffer (pH 5.5) was prepared and allowed to equilibrate overnight. 2.3.4. Lead in NaCl medium For studies on the lability of Pb2 + in 0.5 M NaCl, solutions were prepared by dilution from stock solutions of NaCl (1.0 M), Pb2 + (500 ppb in 0.125 M HNO3) and HNO3 (conc.) to produce final concentrations of NaCl (0.5 M), Pb (50 ppb) and HNO3 (0.12 M). For parallel studies by ASV and ED–ETAAS the solutions were prepared in the ASV cell and aliquots from the ageing solution were removed for ED – ETAAS measurements. The order of addition of components and water was, solution I (NaCl, Pb2 + , H2O, H+); solution II (Pb2 + , H+, H2O, NaCl). Solution III was the same as solution I except that the 1-ml sample prepared in the ETAAS autosampler vial was microwaved for 10 s (700 W Sharp R-4A11 microwave oven) before measurements. For studies on the lability of Pb in acetate buffer (pH 4.7), the solutions were prepared by mixing in the order, Pb (500 ppb), acetate buffer (0.9 M), H2O. For ED–ETAAS measurements 20-ml samples of chloride or acetate solution were electrolysed for 60 s at Ecell =2.0 V in a Pd-modified furnace, redeposited for 60 s from 40 ml of 1% HNO3 and rinsed for 5 s with 40 ml Milli-Q. ASV involved

120 s deposition at Ecathode = − 0.7 V vs. Ag/ AgCl/Cl− (3 M).

2.3.5. Electrode potentials To estimate the cathode and anode potentials during electrolysis, longitudinally cut half sections of pyrolytic graphite furnace were set up external to the spectrometer. These had been pre-fired at 2400°C then coated with Pd electrodeposited from 20 ml of 10 ppm Pd solution. The position of the Pt/Ir anode in the electrolyte was controlled by its attachment to a micrometer and was set 0.6 mm above the cathode. The 20-ml samples for electrolysis contained 0.16 mM Cu (in 0.01 M HNO3/0.1 M NaCl), or 0.24 mM Pb (in 0.0002 M NaCl/ 0.009 M acetate, pH 4.7), or 0.68 mM Ni (in 0.15 M NH3/NH4Cl buffer, pH 8.0) or 0.96 mM Bi (in 0.15 M acetate, 0.15 M NaCl, pH 4.5). During electrolysis the cathode and anode potentials were measured relative to a Ag/AgCl/Cl− electrode formed by a Ag/AgCl wire dipping into 20 ml of chloride-containing sample solution. A radiometer pHM64 potentiometer was used for potential measurements. 2.4. ETAAS procedures Instrumental parameters for ETAAS procedures following electrodeposition are given in Table 1. The wavelengths, lamp currents and slit widths used for Bi, Pb, Ni and Cu were 306.8 nm, 8 mA and 0.5 nm; 283.3 nm, 4 mA, 0.5 nm; 232.0 nm, 10 mA, 0.2 nm; 324.8, 4 mA, 0.5 nm, respectively. 3. Results and discussion An important feature of the described ED protocol is that it achieves complete deposition of the (reducible) analyte onto the graphite furnace within 40–60 s [3]. Subsequent withdrawal of the spent electrolyte ensures complete separation of the analyte from the sample matrix [3]. Thus, the volatilisation and atomisation processes in the ETAAS step are not influenced by the sample medium. This has obvious advantages when applied to difficult media such as sea-water [3] and biological samples [7].

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In ASV, bound metal may be labilised by pretreatment of the sample with acid or by acid coupled with UV irradiation. One question that arises in ED – ETAAS is whether the deposition of metal is, or can be, quantitative when some or all of the metal is bound in stable complexes; for example, Cu in sea-water or humic waters, or Pb in blood or urine [7]. Two factors may promote electrochemical lability. During ED from an unmodified sample the analyte can be exposed to a higher reduction potential than can be tolerated in ASV (for which H2 evolution must be avoided). The evolution of nascent hydrogen on a (catalytic) Pd surface during ED, and the evolution of nascent oxygen at the Pt/Ir anode, may also affect the speciation, and thus the electrochemical lability of the analyte. Another question is whether selective reduction of the free and labile metal ion can be achieved in unmodified samples by appropriate choice of cell potential, Ecell. That is, can the analysis provide information about metal lability and speciation?

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The present work has addressed these questions by comparing the deposition of Bi, Pb, Ni and Cu from acid or buffer and from NaCl solutions. It has also studied the formation of non-labile Pb species in weakly acidic NaCl solutions.

3.1. Bismuth Fig. 1 shows the ETAAS absorbance versus cell potential following electrodeposition of Bi from 0.96 mM solution (200 ppb, pH 2.0) in the absence and presence of EDTA (1.92 mM). The curves are analogous to polarograms. The Bi(EDTA)− complex is very stable, with a stability constant log K= 25.68 (1.0 M NaClO4, 25°C, [8]). Calculations using the speciation program SOLGASWATER [5] established that at pH 2.0, in micromolar solution, (a) Bi3 + ion is strongly hydrolysed (10% Bi3 + , 80% Bi(OH)2 + and 10% Bi(OH)+ 2 ) and (b) in the presence of EDTA the Bi3 + ion is complexed quantitatively. Although water exchange by the Bi3 + aqua ion is rapid (k\104 M − 1 s − 1

Table 1 Programs for ETAAS measurements following analyte electrodeposition Final temperature (°C)a

Step Bismuth

Lead

Nickel

Copper

a

1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 1 2 3 4 5 6

Calibrated temperature.

85 105 1100 300 2300 2400 75 100 700 300 2350 2400 85 105 900 300 2600 90 110 800 350 2500 2600

Ramp time (s) 1 5 5 1 1 0.1 5 3 5 1 1 0.1 5 1 5 2 1 5 5 5 1 1.1 0.1

Hold time (s) 5 10 5 3 2 1 10 2 5 3 2 1 3 2 10 5 3 5 2 5 5 2 2

Inert gas On On On Off Off On On On On Off Off On On On On Off Off On On On Off Off On

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Fig. 2a and b for ED–ETAAS and ASV experiments, respectively. The ASV results (Fig. 2b) closely match those expected for a non-labile species, i.e. the stripping current decreases linearly as the ratio [EDTA]/[M]

Fig. 1. Atomic absorbance as a function of applied cell potential, Ecell, following 60 s electrodeposition of Bi onto a palladium modified furnace (from 20 ml 0.96 mM Bi3 + , 0.01 M HCl, 0.04 M NaCl) in the absence ( ) and presence ( ) of EDTA (1.92 mM). Bi absorbance measured at the 306.8-nm line.

at 25°C [9]), the EDTA complex is known to be of low lability [10]. Despite the high stability and low lability of the Bi(EDTA)− complex, it is possible to achieve deposition on the Pd-coated cathode at Ecell \1.5 V. By inference, deposition of Bi from complex natural matrices should also be possible. The small absorbances measured for Bi(EDTA)− at Ecell B 1.2 V are attributed to adsorption of the complex onto the Pd modifier. Fig. 1 indicates a ‘voltage window’ between Ecell ca. 0.6 and 1.0 V in which Bi(H2O)36 + is reduced but Bi in Bi(EDTA)− is not (Ecell = Eanode − Ecathode + iR). This window was exploited to selectively deposit the non-complexed metal ion. A square wave ASV experiment and a parallel ED– ETAAS experiment were conducted on solutions containing 0.96 mM Bi3 + and varying concentrations of EDTA (0 – 2.0 mM; pH 2.0, 0.04 M NaCl). In the ED – ETAAS experiment cell potentials used were 0.6 and 0.8 V, corresponding to the voltage window cited above. In the ASV experiment the cathode potential was − 0.45 V vs. Ag/AgCl/Cl− (3 M). The results are plotted in

Fig. 2. (a) Atomic absorbance as a function of [EDTA]:[Bi3 + ] ratio following 60 s electrodeposition of Bi from aged solutions (0.96 mM Bi3 + , 0 – 2.0 mM EDTA; pH 2.0, 0.04 M NaCl) at 25°C, 0.8 V ( ), 25°C, 0.6 V (); and 45°C, 0.6 V ( ). Bi measured at the 306.8-nm line. (b) SWASV stripping current as a function of [EDTA]:[Bi3 + ] ratio for aged solutions, as for 2a. ASV conditions: Ep = −0.45 V (vs. Ag/AgCl/3 M KCl), td =5 min.

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larity between the room temperature results simply reflects the inertness of the Bi–EDTA complex.

3.2. Lead

Fig. 3. Atomic absorbance as a function of cell potential, Ecell, following 90 s electrodeposition of Pb (from 20 ml of 2.2 mM Pb, 0.01 M KNO3, 0.001 M acetate buffer, pH 4.7; T =20°C) in the absence ( ) and presence () of EDTA (3.0 mM). Pb absorbance measured at the 283.3-nm line.

increases and approaches zero when [EDTA]/ [M]=1.0. The ED – ETAAS results (Fig. 2a) at room temperature (lower curve) also closely follow this model, although the intercept at [EDTA]/ [Bi3 + ]= 1.15 indicates either a partial (ca. 15%) reduction of the Bi complex, or partial retention of the complex on the furnace by adsorption. Fig. 2a indicates a significantly higher absorbance when electrolysis is effected at 45°C. The enhanced sensitivity at [EDTA]/[Bi3 + ]= 0 indicates that room-temperature deposition is non-quantitative at Ecell =0.8 V. This may be due to inadequate convective stirring of the sample during electrolysis at room temperature, or to slow electrode or dissociation kinetics of hydroxo-bismuth species. In addition to enhanced deposition at 45°C, Fig. 2a shows that the amount of Bi deposited shows little dependence on the EDTA concentration. This suggests that on the experimental timescale the Bi(EDTA)− complex is labile at 45°C. Compared with ASV, the ED process will involve a relatively thick diffusion layer, and a longer residence time of the complex within the larger field gradient. There are fundamental differences between these two techniques; the simi-

Initial investigations failed to locate a cell potential at which the free lead could be selectively reduced in the presence of EDTA-bound lead. This problem was attributed to adsorption of the EDTA–lead complex onto the palladium modifier, giving rise to an ETAAS absorbance irrespective of the cathode potential. The adsorption occurs to a much smaller extent when there is no Pd coating on the pyrolytic graphite, so this approach was used for subsequent experiments. Fig. 3 shows the ETAAS absorbance versus cell potential following electrodeposition of Pb from 50 ppb solution (0.24 mM Pb, 0.09 M acetate buffer, pH 4.7) in the absence and presence of EDTA (0.48 mM). The maximum deposition of ‘free’ lead required Ecell ] 1.8 V. A significant reduction of the Pb–EDTA complex required Ecell \ 2.2 V. The Pb–EDTA complex is less stable than that of Bi3 + (log K=18.52; 0.1 M KNO3, 25°C, [8]), but it is sufficiently stable to ensure quantitative formation at micromolar concentrations and pH 4.7. The reaction chemistry of the Pb(EDTA)2 − complex indicates that it is labile [11] (in apparent contrast to Bi(EDTA)−), although it is reported to be inert on the ASV timescale at a cathode potential of − 0.7 V versus Ag/AgCl/Cl− (3 M) [1]. There is a narrow voltage window, between Ecell ca. 1.8 and 2.0 V, in which Pb2 + is reduced but Pb in Pb(EDTA)2 − is not. Ecell = 1.9 V was used to measure ‘free’ lead in the presence of Pb(EDTA)2 − . Fig. 4a shows the effect of the [EDTA]/[Pb2 + ] ratio on the ETAAS absorbance after ED for 60 s at 1.9 V (0.6 mM Pb2 + and 0–0.75 mM EDTA, 0.01 M KNO3, 0.001 M acetate buffer, pH 4.5). Analogous experiments were carried out by square wave ASV, using solutions containing 0.3 mM Pb2 + and 0–0.3 mM EDTA (0.01 M KNO3, 0.001 M acetate buffer, pH 4.5) and a cathode potential of −0.6 V vs. Ag/AgCl/ Cl− (3 M). The ASV results are expressed as stripping current relative to that for 0.3 mM Pb2 +

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(Fig. 4b). For a non-labile complex, the intercept on the absorbance or current axis should be at [EDTA]/[Pb2 + ]= 1.0. The ASV result approximates to this but under ED conditions the Pb(EDTA)2 + complex is partially labile.

Fig. 5. Atomic absorbance as a function of cell potential, Ecell, following 60 s electrodeposition of Ni (from 0.68 mM Ni2 + , 0.14 M ammonia/ammonium chloride buffer, pH 7.3; T= 45°C) in the absence ( ) and presence ( ) of EDTA (4.0 mM). Ni absorbance measured at the 232.0-nm line.

3.3. Nickel

Fig. 4. (a) Atomic absorbance as a function of [EDTA]:[Pb2 + ] ratio following 60 s electrodeposition of Pb onto pyrolytic graphite (from 0.6 mM Pb2 + in 0.001 M acetate buffer, 0.01 M KNO3, pH 4.5). Pb measured at the 283.3-nm line. (b) SWASV stripping current for 0.3 mM Pb as a function of [EDTA]:[Pb2 + ] ratio (0.01 M KNO3, 0.09 M acetate buffer, pH 4.7). ASV conditions, Ep = − 0.6 V vs. Ag/AgCl/Cl− (3 M), td = 60 s. Stripping current expressed as a percentage of that for [EDTA]=0.0.M

The optimum solution conditions for quantitative reduction of Ni are an ammonia/ammonium buffer at pH 7.5–8.5 [12]. SOLGASWATER calculations indicate that formation of Ni(EDTA)2 − (log K= 18.52; 0.1 M KNO3, 25°C, [8]) is quantitative in the pH range 2.6–10 for micromolar solutions with 1:1 stoichiometry and in the presence of 0.14 M ammonia/ammonium buffer. Fig. 5 shows the effect of Ecell on measured ETAAS absorbance for nickel deposition in the presence and absence of excess EDTA. The curve for Ni(EDTA)2 − has a much smaller limiting slope than that observed for the bismuth, lead, or copper–EDTA complexes. This points to irreversibility of the Ni(EDTA)2 − reduction process. At Ecell ca. 3.0 V, reduction of both Ni2 + and Ni(EDTA)2 − becomes quantitative (confirmed by comparison with conventional ETAAS measurements). There is no voltage window in which Ni2 + is selectively reduced while Ni in Ni(EDTA)2 − is not. The best resolution between these two Ni

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species is achieved at Ecell ca. 1.7 – 2.0 V, at which Ni2 + reduction is almost quantitative and ca. 12–20% of Ni in Ni(EDTA)2 − is reduced. Fig. 6

Fig. 6. Atomic absorbance as a function of [EDTA]:[Ni2 + ] ratio for deposition of Ni at 45°C. Solutions as for Fig. 5; Ecell = 2.0 V; tdep = 60 s. Ni absorbance measured at the 232.0-nm line.

Fig. 7. Atomic absorbance as a function of cell potential, Ecell, following 60 s electrodeposition of Cu onto Pd (from 0.16 mM Cu; 0.05 M hexamine buffer, pH 4.5; T= 25°C) in the absence ( ) and presence ( ) of EDTA (0.36 mM).

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shows the effect of [EDTA] on the ETAAS absorbance for 20 ml of 0.68 mM Ni, after electrodeposition from 0.14 M ammonia buffer (pH 7.3) at Ecell = 2.0 V. The decrease in absorbance from A= 0.55 for [EDTA]/[Ni2 + ]= 0.0 to A= 0.10 for [EDTA]/[Ni2 + ]= 5.0 is consistent with the formation of a Ni(EDTA)2 − complex that has measurable lability on the ED timescale and thus is partially reduced at this voltage. Margerum et al. [13] established that the rate-determining step for release of Ni2 + from Ni(EDTA)2 − involves the half-dissociation of the ligand (one iminodiacetate moiety) from the metal ion. The rate constant varies with ionic strength and pH. At pH 5.3 (the highest studied) and I= 0.10 M (this work, 0.14 M) the first order rate constant is 2.7 × 10 − 4 s − 1. For a 0.68 mM Ni(EDTA)2 − solution at 25°C the calculated percent dissociation at 60 s is 1.6%. The results in Fig. 6 indicate that the higher temperature (45°C) and the forcing conditions of the ED experiment somewhat enhance this dissociation.

3.4. Copper The differences between the results for copper and those for the other cations may be attributed to the much greater lability of copper complexes. The effect of Ecell on the ETAAS absorbance following Cu deposition in the presence and absence of excess EDTA was medium dependent. Fig. 7 shows the ED–ETAAS results for reduction of Cu2 + and Cu(EDTA)2 − in 0.05 M hexamine buffer (pH 4.5). In this case, distinctive curves were obtained, giving a useful potential window in which Cu2 + may be selectively reduced in the presence of Cu(EDTA)2 − . A cell potential of 1.0 V will give maximum deposition of free copper with minimum Cu(EDTA)2 − deposition. In contrast, the plots of Ecell versus absorbance were almost coincident for Cu2 + and Cu(EDTA)2 − in 0.05 M KNO3 and in 0.015 M acetate buffer media (use of a 10- or 100-fold excess of EDTA over Cu in the acetate medium did not shift the Cu(EDTA)2 − curve significantly). This indicates that in the latter media the Cu(EDTA)2 − complex is labile on the ED timescale. In contrast, on the ASV timescale,

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Fig. 8. Effect of solution age on the measurement of labile Pb by SWASV () and Pb atomic absorbance by ED – ETAAS ( ) for a Pb2 + /NaCl solution (solution I, 50 ppb Pb, 0.5 M NaCl, 0.12 M HNO3). ASV conditions, Ep = − 0.7 V vs. Ag/AgCl/Cl− (3 M), td = 120 s. ED–ETAAS conditions, tdep =60 s, T = 20°C, Ecell = 2.0 V, Pd-modified furnace. Pb measured at the 283.3-nm line. For each technique values are expressed as a percentage of that for t= 300 min.

Cu(EDTA)2 − in acetate medium is inert [14]. It is possible that the activation energy for deposition of Cu onto Pd/C is medium dependent.

3.5. Lead speciation in NaCl solution The ability of ED – ETAAS to effect speciation analyses was demonstrated in studies on the timedependent speciation of Pb2 + in freshly prepared, weakly acidic Pb2 + /NaCl solutions. In contrast to conventional ETAAS, it was observed that the order of mixing and dilution of analyte, acid and electrolyte (NaCl) had a significant effect on the ED–ETAAS result and, by inference, on the speciation of Pb. In particular, 10-fold dilution of a 500 ppb Pb (0.125 M HNO3) solution into 0.5 M NaCl, followed by acidification with HNO3 (solution I, Section 2.3) gave a low absorbance which increased with time. Fig. 8 shows the absorbance, expressed as a percentage of the limiting value,

increasing from ca. 83% (t= 0) to 100% at 4 h. A similar increase in ASV response is recorded on the same graph, although a higher percentage of the pseudo-labile or non-labile Pb species formed is reduced under the more forcing ED conditions in the Pd-coated graphite furnace (Ecell = 2.0 V). When the Pb standard was acidified before dilution and addition of NaCl (solution II, Section 2.3) the initial absorbance was increased to 92% of the limiting value. Similar results were obtained for solutions acidified with HCl, for which the initial (t= 0) absorbances for solution I and solution II were 72 and 91%, respectively, of the limiting values at 24 h. This problem of time-dependent speciation in standard Pb solutions was resolved by a 10 s microwave treatment in a 1.5-ml autosampler cuvette. ED–ETAAS and ASV measurements then confirmed complete labilisation of Pb. Similar effects (not shown) were observed when Pb standards were prepared in a range of acetate buffers at pH 4.7 (0.045– 0.45 M acetate).

3.6. Copper ful6ate solutions The copper(II)-fulvate complex is non-labile on the ASV timescale [15]. An attempted application of the ED–ETAAS technique to the speciation of copper(II) in fulvate solutions highlighted one of the persistent properties of the graphite cathode, viz. adsorption of hydrophobic substances. Fig. 9 shows results from ED–ETAAS analyses as a function of Ecell for Cu (10 ppb) in 0.1 M acetate (pH 5.5) in the absence and presence of fulvic acid (2.8 mg l − 1). These indicate that at Ecell \ 2.0 V both free and fulvate-bound Cu is measured quantitatively, but that at lower voltages some 80–100% of the fulvate-bound Cu is also measured. It is unlikely that the fulvate bound Cu is being reduced at a lower voltage than is Cu2 + in acetate medium. It follows that adsorption of the hydrophobic fulvate complex to the graphite leads to the retention of Cu in the furnace and its subsequent detection by ETAAS. It is clear that applicability of the technique to effect metal speciation requires media with very low organic content (e.g. sea-water [3] and potable waters).

J.P. Matousek et al. / Talanta 52 (2000) 1111–1122

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3.7. Cathode and anode potentials

Fig. 9. Atomic absorbance as a function of Ecell, following 60 s electrodeposition of Cu onto Pd (from 0.16 mM Cu, 0.1 M acetate buffer, pH 5.5; T=25°C) in the absence ( ) and presence () of fulvic acid (2.8 mg l − 1). Cu absorbance measured at the 324.8-nm line.

The electrodeposition experiments were carried out at cell potentials of 0.0–3.0 V. Cathode and anode potentials at these Ecell values were measured relative to a Ag/AgCl electrode which was dipped into the analyte/chloride-electrolyte solution. These experiments used pre-fired, Pd-coated half furnaces, mounted below a micrometer-positioned Pt/Ir capillary in a set up external to the ETAAS instrument. Similar results were obtained for each medium; those for Bi3 + are shown in Fig. 10, with voltages expressed relative to the Ag/AgCl/Cl− (3 M) electrode. Processes occurring at the anode include evolution of Cl2 and of O2. The processes at the cathode include evolution of H2 from the electrolysis of H2O, a reaction that generates OH−. At typical working potentials for quantitative deposition of complexed analytes (Ecell ca. 2.0–2.5 V) the cathode potential is ca. − 0.6 to −0.8 V, an adequate potential to reduce the cited metals, given that metal reduction onto Pd has a significant underpotential [4]. These measurements provided only approximate values for the electrode potentials. During electrolysis the current decreases with time, due to the consumption of H+. The decrease in iR causes Eanode − Ecathode to increase with time (Ecell constant). The potentials shown in Fig. 10 were measured within 10 s of commencement of electrolysis. Thus, the measurements provide only approximate values for the initial electrode potentials during electrolysis.

4. Conclusions

Fig. 10. Plot of applied potential, Ecell, vs. Ecathode (Pd-coated pyrolytic graphite electrode, ) and Eanode (Pt/Ir capillary, ) for electrolysis of 0.96 mM Bi3 + in 0.04 M NaCl, 0.01% HNO3.

The ED–ETAAS technique is able to differentiate between free metal ions and stable complexes when the latter are non-labile on the ED timescale. This was demonstrated for EDTA complexes of Bi, Pb and Ni, but not for Cu. For Bi, Pb and Ni there was a voltage window in which the free metal ion was reduced but metal in the EDTA complex was not. Plots of ETAAS absorbance versus Ecell are analogous to pseudopolarograms for stripping voltammetry except that the amount of deposited metal is determined by

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J.P. Matousek et al. / Talanta 52 (2000) 1111–1122

ETAAS. The plots indicate that, despite the stability and inertness of the EDTA complexes, these metals can be deposited at Ecell \ca. 2.5 V. The technique is affected by adsorption of ionic or hydrophobic complexes to the (Pd modified) pyrolytic graphite surface. This was illustrated for non-labile Cu – fulvic acid complexes. Adsorbed material produces an ETAAS signal and so contributes to the ‘reducible metal fraction’. This is a distinct limitation in the application of the ED– ETAAS method to speciation in real systems. Other important factors that may affect the species distribution in a sample are the need for buffering (to counter the effect of OH− generated at the cathode) and for a pH conducive to efficient electrodeposition.

References

[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

[1] T.M. Florence, Electrochemical techniques for trace ele-

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ment speciation in natural waters, in: G.E. Batley (Ed.), Trace Metal Speciation: Analytical Methods and Problems, CRC Press, Boca Raton, FL, 1989. T.M. Florence, Analyst 111 (1986) 489. J.P. Matousek, H.K.J. Powell, Spectrochim. Acta Part B 50 (1995) 857. J.P. Matousek, H.K.J. Powell, Talanta 44 (1996) 1183. G. Eriksson, Anal. Chim. Acta 112 (1979) 375. J.E. Gregor, H.K.J. Powell, J. Soil Sci. 37 (1986) 577. J.P. Matousek, K.J. Powell, Spectrochim. Acta Part B 54 (1999) 2105. L.D. Pettit, K.J. Powell, SC-database, Stability Constants Database, IUPAC, Oxford, Academic Software, 1997. J. Burgess, Metal Ions in Solution, Ellis Horwood, Chichester, 1978. T.M. Florence, J. Electroanal. Chem. 26 (1970) 293. G. Schwarzenbach, H. Flaschka, Complexometric Titrations, Methuen, London, 1969. J.P. Matousek, K.J. Powell, unpublished results. D.W. Margerum, D.L. Janes, H.M. Rosen, J. Am. Chem. Soc. 87 (1965) 4463. J.R. Tuschall, P.L. Brezonik, Anal. Chem. 53 (1981) 1986. J.E. Gregor, H.K.J. Powell, Anal. Chim. Acta 211 (1988) 141.