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Investigation into sorption processes in the anodic stripping voltammetric speciation of Cu in natural waters
Analytica Chimica Acta 476 (2003) 33–42
Investigation into sorption processes in the anodic stripping voltammetric speciation of Cu in natural waters Johannes T. van Elteren∗ , Urszula D. Woroniecka Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands Received 1 July 2002; received in revised form 30 September 2002; accepted 16 October 2002
Abstract In the anodic stripping voltammetric speciation of copper significant errors may be introduced as a result of sorption of Cu2+ onto active surfaces of the voltammetric cell assembly. A correction method was developed based on monitoring of the total copper concentration in solution using a 64 Cu-radiotracer; additionally this allowed us to study sorption phenomena in the voltammetric cell assembly. For copper speciation in a fresh water sorption-corrected complexing capacity data (total natural ligand concentration and conditional stability constant of formed complex) showed considerable discrepancies with uncorrected data. From the same sorption data it could be deducted that this was accountable to the presence of two active surface sites in the voltammetric cell assembly. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Speciation; Copper; Anodic stripping voltammetry; Sorption; Correction
1. Introduction Considerable sorption of trace metal ions has been reported on the surfaces of the voltammetric cell assembly during anodic stripping voltammetry (ASV) measurements [1–4]. In the case of speciation analysis, selective disappearance of ionic metal from the cell solution leads to sample re-equilibration and potential inaccurate voltammetric and speciation data, depending on the severity of the sorption (strength and number of surface sorption sites). The surface sorption of trace metal ions may be significantly suppressed by replacing standard components with ∗ Corresponding author. Present address: National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia. Tel.: +386-1-4760-288; fax: +386-1-4760-300. E-mail address: [email protected] (J.T. van Elteren).
components less prone to adsorption [2,5]: the glass cell with a quartz or polystyrene cell, the plastic stirrer with a quartz stirrer and the sinter glass tip of the reference electrode with a quartz one. However, in general pre-equilibration of the voltammetric cell assembly with a representative sample seems to be the norm in speciation analysis to prevent unwanted sorption processes during voltammetric measurement. This procedure is only valid though in case all sorption sites of the cell assembly have been irreversibly saturated so that variation in sample composition does not lead to adsorption onto or desorption from cell wall, counter electrode, etc. It is usually assumed that irreversible saturation indeed occurs notwithstanding the fact that the voltammetric measurement is unable to show it in practical speciation situations and to our knowledge no literature exists to confirm this either.
0003-2670/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 2 ) 0 1 3 5 8 - 2
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In this work we will not focus on overcoming sorption problems during ASV speciation analysis but on gaining insight in sorption phenomena to be able to correct for them. We elaborated on a method to correct for possible sorption artifacts in the ASV speciation of copper in natural waters using a 64 Cu monitor. A 64 Cu2+ -spike was therefore allowed to equilibrate with copper species in the sample prior to addition to an acid-cleaned voltammetric cell assembly where the sorption of Cu2+ and thus re-equilibration process was followed. Mathematical formulae were derived to arrive at corrected ASV speciation data using 64 Cu sorption data.
is the equilibrium ionic Cu concentration after titration step i (mol l−1 ); [Cuinorgcompl,i ] is the equilibrium concentration of inorganic Cu complexes after titration step i (mol l−1 ); [CuLi ] is the equilibrium concentration of Cu bound to natural ligands after titration step i (mol l−1 ); [Lt ] is the total concentration of natural ligands (mol l−1 ); [Li ] is the equilibrium concentration of natural ligands after titration step i (mol l−1 ). Complexing capacity data (KCuL and [Lt ]) may be derived by processing the voltammetric data via the Langmuir [6] or Scatchard [7] linearization: Langmuir
2. Theory
Scatchard It is assumed that in natural waters copper has a 1:1 coordination with discrete ligands and ligand groups, described by the simplified overall equilibrium (charges omitted): (1)
where L and CuL represent the binding sites on discrete ligands/ligand groups and ionic copper bound to these sites, respectively. The conditional stability constant (in l mol−1 ) for the formation of CuL is given by: KCuL
[CuL] = [Cu][L]
(2)
Stepwise titration of L with ionic copper and anodic stripping voltammetric measurement of the remaining reactive copper after equilibration yields copper speciation information. The mass balances after titration step i and subsequent equilibration are as follows (charges omitted): [Cuinit ] + [Cuadd,i ] = [Cui ] + [Cuinorgcompl,i ] + [CuLi ]
(3)
and [Lt ] = [Li ] + [CuLi ]
(5)
Plotting [Cui ]/[CuLi ] versus [Cui ] yields a straight line with a slope 1/[Lt ] and an intercept 1/(KCuL [Lt ]).
2.1. Complexation
Cu + L ↔ CuL
[Cui ] [Cui ] 1 = + [CuLi ] [Lt ] KCuL [Lt ]
(4)
where [Cuinit ] is the initial total Cu concentration before titration (mol l−1 ); [Cuadd,i ] is the ionic Cu concentration added in titration step i (mol l−1 ); [Cui ]
[CuLi ] = −KCuL [CuLi ] + KCuL [Lt ] (6) [Cui ]
Plotting [CuLi ]/[Cui ] versus [CuLi ] yields a straight line with a slope −KCuL and an intercept KCuL [Lt ]. From Scatchard plots it is generally possible to deduct if more than one ligand is present and calculate their complexing capacity data. 2.2. ASV measurement With ASV we measure the equilibrium inorganic copper concentration: [Cuinorg,i ] = [Cui ] + [Cuinorgcompl,i ] = αCu [Cui ] (7) with α Cu the side-reaction coefficient for inorganic copper complexation. In ASV terms this means: [Cuinorg,i ] =
ip,i SASV
(8)
with ip,i the ASV peak current (in nA) in titration step i and SASV the sensitivity (in nA mol−1 l) of the ASV technique. For substitution of [Cui ] and [CuLi ] in Eq. (5) or (6) we use: [Cui ] =
ip,i SASV αCu
(9)
and [CuLi ] = [Cuinit ] + [Cuadd,i ] −
ip,i SASV
(10)
J.T. van Elteren, U.D. Woroniecka / Analytica Chimica Acta 476 (2003) 33–42
Jin and Gogan [8] calculated a value of 6.25 for α Cu in fresh water using average Cl− , SO4 2− and CO3 2− concentrations, a pH of 7.6 and an ionic strength of 0.1 mol l−1 . For our particular 1:10 diluted fresh water sample, recalculation with our conditions (see Section 3) resulted in an α Cu of 5.85 which value was used in this work to derive complexing capacity data. [Cuinit ] is usually measured with the ASV technique as well but after digestion of the sample. 2.3. Sorption artifacts In the determination of the copper complexing capacity of natural waters, re-equilibration of the copper equilibrium (Eq. (1)) may take place due to sorption of ionic copper onto the voltammetric cell assembly. Natural ligands are supposed to compete with surface sites S for Cu binding and may be crucial to keep Cu in solution. Assuming that copper has a 1:1 coordination with surface sites the following equilibrium describes the sorption onto the voltammetric cell assembly (charges omitted): Cu + S ↔ CuS
(11)
The conditional stability constant for the formation of CuS is given by: KCuS =
[CuS] [Cu][S]
(12)
with [CuS] and [S] the concentration of occupied and unoccupied surface sites (in moles per liter voltammetric cell volume), respectively. In this work an ASV-based copper speciation method is presented which corrects for possible sorption artifacts by monitoring the total soluble copper concentration in a standard voltammetric assembly using 64 Cu as radiotracer. To this end subsamples in inert polystyrene vials are both titrated (i steps) with copper ([Cuadd,i ]) and spiked with radioactive copper ([64 Cuadd ]). The total activity concentration [64 Cuadd ] (in Bq l−1 ) is the same for each subsample. Equilibration may take a considerable time (up to 1 day) and therefore titrations are in general performed in inert vials and not in the voltammetric cell. After transfer of (aliquots of) these equilibrated subsamples to the acid-cleaned voltammetric cell assembly re-equilibration may take place due to Cu2+ surface
35
sorption. When fully equilibrated the specific activities (in Bq mol−1 ) for all copper species involved should be equal, i.e.: [64 Cuinorgcompl,i ] [64 Cui ] [64 CuLi ] = = [Cui ] [Cuinorgcompl,i ] [CuLi ] =
[64 CuSi ] [CuSi ]
(13)
It is essential that the specific activity of the initial radioactive copper spike is sufficiently high so that the amount of Cu added with the spike is negligible compared to the total amount of Cu already present. To this end high-specific 64 Cu, produced via the 64 Zn(n,p)64 Cu reaction, was used [9]. 2.4. Sorption correction Sorption of ionic copper onto the voltammetric cell assembly in the titration process leads to a decrease of [Cui ] and [64 Cui ] and thus of the total soluble copper concentration [Cusol,i ] and the total soluble activity concentration [64 Cusol,i ]: [64 Cusol,i ] [Cusol,i ] = ([Cuinit ] + [Cuadd,i ]) 64 (14) [ Cuadd ] where [64 Cusol,i ] follows from direct counting of an aliquot of the voltammetric cell solution and [64 Cuadd ] is the known added radiotracer spike. For calculation of the complexing capacity data we use Eq. (5) or (6) and substitute Eq. (9) for [Cui ] and a modified form of Eq. (10) for [CuLi ]: [CuLi ] = [Cusol,i ] −
ip,i SASV
(15)
2.5. Sorption characteristics In analogy with the binding of Cu to L, the sorption characteristics of Cu onto the voltammetric cell assembly may also be derived using the Langmuir or the Scatchard linearization. When the system is assumed to have a fixed concentration of sorption sites [St ] (in moles per liter voltammetric cell volume) and a conditional stability constant for the formation of CuS of KCuS , linearizations may be achieved similar
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to Eqs. (5) and (6): Langmuir
3.2. Preparation of high-specific activity
[Cui ] [Cui ] 1 = + [CuSi ] [St ] KCuS [St ]
(16)
Plotting [Cui ]/[CuSi ] versus [Cui ] yields a straight line with a slope 1/[St ] and an intercept 1/(KCuS [St ]). Scatchard
[CuSi ] =−KCuS [CuSi ] + KCuS [St ] (17) [Cui ]
Plotting [CuSi ]/[Cui ] versus [CuSi ] yields a straight line with a slope −KCuS and an intercept KCuS [St ]. For calculation of the complexing capacity data we use Eq. (16) or (17) and substitute Eq. (9) for [Cui ] and the following equation for [CuSi ]: [CuSi ] = [Cuinit ] + [Cuadd,i ] − [Cusol,i ]
(t1/2 = 12.7 h; Eγ = 0.511 MeV) was obtained by irradiation of 150 mg zinc oxide (99.9995%, Johnston Matthey, Materials Technology, UK) in the Delft nuclear reactor in a fast neutron flux of 1.9 × 1016 m−2 s−1 for 12 h. After 3 h of cooling the zinc oxide target was dissolved in acid and the copper was separated from the zinc on a Dowex 2 × 8 ion exchange resin as described in literature [9] yielding an activity concentration of 54 GBq l−1 and a specific activity of 1.9 × 1017 Bq mol−1 , 8 h after the irradiation. 64 Cu2+
3.3. Chemicals and sample preparation
(18)
3. Experimental 3.1. Sorption simulation A Pentium® II processor based IBM-compatible PC was used to simulate sorption processes during the ASV determination of the Cu complexing capacity of natural waters in an electrochemical cell with carefully chosen input data (see Table 1). Calculations were performed with Mathcad software (©1986–1993, MathSoft, Inc., Version 4.0) based on equilibria (1) and (11) with their respective mass balances, generating concentrations of all copper species involved ([Cui ], [Cuinorgcompl,i ], [CuLi ] and [CuSi ] as a function of [Cuadd,i ]). The output data were linearized according to a Langmuir or a Scatchard transformation, with and without sorption correction, using Excel software (Microsoft® Excel 97 SR-2). Table 1 Input data for simulation of copper sorption in a voltammetric cell assembly; all parameters are fixed except the log KCuS values [Cuinit ] (nmol l−1 ) [Cuadd,i ] (nmol l−1 ) [Lt ] (nmol l−1 ) [St ] (nmol l−1 ) V (l) α Cu log KCuL log KCus
64 Cu2+
5 5–1500 (23 points) 500 250 0.02 10 9 8, 9, 10
All chemicals were at least of analytical reagent grade. Milli-Q-Plus (MQ) water (Millipore-waters, Milford, MA, USA) with a resistivity of 18 M cm was used for all solution preparations. A 1-l freshwater sample from a canal in Delft (The Netherlands) was collected in an acid-cleaned (soaking for at least 24 h in 1 mol l−1 HCl) high density polyethylene (HDPE) bottle and immediately filtered through a 0.4-m polycarbonate filter (Nuclepore) using Swinnex filterholders (25 mm diameter; Millipore). Aliquots of 2 ml were added to 30-ml polystyrene containers (Sterilin) and titrated with Cu2+ (10−4 mol l−1 , 0–300 l) and spiked with high-specific activity 64 Cu2+ (54 GBq l−1 , 10 l); further 0.2 ml 1 mol l−1 HEPES (mono-sodium N-hydroxyethylpiperazine-N -2-ethanesulfonate) and 2 ml 0.5 mol l−1 NaClO4 were added and the whole sample was adjusted to a pH of 7.7 and made up to a volume of 20 ml with MQ-water giving final added Cu2+ concentrations up to 1500 nmol l−1 and a final added 64 Cu2+ activity concentration of 27 MBq l−1 . It can be calculated that only 0.14 nmol l−1 Cu2+ was added with the 64 Cu2+ -spike. Cu2+ and 64 Cu2+ were allowed to equilibrate overnight with the natural ligands. The next day each equilibrated sample was transferred to an acid-cleaned standard voltammetric cell and allowed to re-equilibrate for 30 min prior to ASV measurement and collection of a 0.1 ml aliquot for counting. The polystyrene equilibration containers were inert towards copper sorption as established by counting of sample aliquots against a reference (described later).
J.T. van Elteren, U.D. Woroniecka / Analytica Chimica Acta 476 (2003) 33–42
3.4. ASV measurement An Autolab potentiostat (Eco Chemie bv, The Netherlands) interfaced with a Metrohm model 663 model hanging mercury drop electrode (HMDE) was used for the inorganic copper determination; reference and counter electrodes were Ag/AgCl, KCl (3 mol l−1 ) and glassy carbon, respectively. The 50-ml cell was made out of borosilicate glass and the stirrer and purge tubing out of teflon. For measurements the computer program General Purpose Electrochemical System 3.1 (Eco Chemie bv) was used. An anodic stripping voltammetric procedure as described in the literature [5] was slightly adapted. After transfer of the overnight-equilibrated samples (20 ml) to the voltammetric cell deaeration of the solution took place by purging with water-saturated nitrogen for 30 min after which the ASV cycle was started. A new mercury drop was extruded (drop size setting, 3) and copper was deposited at a deposition potential of −0.9 V for 1 min under continuous stirring (stirrer setting, 3) followed by a period of quiescence of 5 s. Copper was stripped from the mercury drop using a differential pulse scan (pulse height 50 mV) at a scan rate of 20 mV s−1 . Under these conditions a peak for inorganic copper appeared at −0.05 V. It should be noted that prior to each sample transfer the electrochemical cell was acid-cleaned. For measurement of the total copper concentration in the filtered sample an acid-digestion was applied followed by the ASV method described earlier using the method of standard additions. Therefore 2 ml of an acidified sample in the presence of 0.2 ml 30% H2 O2 and 2 ml 65% HNO3 was heated for 15 h at 140 ◦ C in a Parr bomb with teflon liner, followed by addition of 0.2 ml 1 mol l−1 HEPES and 2 ml 0.5 mol l−1 NaClO4 , and made up to 20 ml with MQ-water. The total concentration of copper in the undiluted sample was 158 nmol l−1 . 3.5.
64 Cu
counting
The 0.1-ml counting aliquot from the voltammetric cell was counted for 10 min in an automatic ␥-counter (NaI detector; Wallac 1480 Wizard); the counting error was <5%. Corrections were made for background radioactivity and 64 Cu decay. As a reference 0.1 ml of an acidic solution (pH = 1) was used containing the
37
same 64 Cu activity concentration (27 MBq l−1 ) as the initial samples prior to sorption. 4. Results and discussion 4.1. Computer simulation In Fig. 1A–D a simulation is given of copper species expected in a voltammetric cell assembly during titration of excess ligand with ionic copper (Cuadd,i ) under various sorption regimes (log KCuS ). Realistic input data (Table 1) were used with an initial total copper concentration [Cuinit ] of 5 nmol l−1 , a total ligand concentration [Lt ] of 500 nmol l−1 with a log KCuL of 9 and a total surface sorption site concentration [St ] of 250 nmol l−1 with varying strength (log KCuS from 8 to 10). As can be seen from Fig. 1A mostly CuL is formed during titration of L with ionic copper up to the equivalence concentration of 500 nmol l−1 where [Cuinit ] + [Cuadd,i ] = [Lt ]. At twice the equivalence concentration the CuL concentration is 500 nmol l−1 , or 50% of the total copper present in the system. Competition of surface sorption sites with ligand for ionic copper binding results in formation of CuS, the amount of which depends on log KCuS . With increasing log KCuS the contribution of CuS gets considerable as illustrated in Fig. 1B–D looking at the shaded area under the CuS curves. Due to the presence of 250 nmol l−1 surface sorption sites the equivalence concentration is increased to 750 nmol l−1 . At twice this equivalence concentration the CuL and CuS concentration are 500 and 250 nmol l−1 , respectively, or 33.3 and 16.7% of the total copper present in the system, independent of log KCuS . The CuL/CuS ratio is 2 in this case; however, in the trajectory to 1500 nmol l−1 ([Cuinit ] + [Cuadd,i ]) the CuLi /CuSi ratio strongly depends on log KCuS as shown in Fig. 2. When log KCuS = log KCuL , the CuLi /CuSi ratio is 2 over the whole trajectory; when log KCuS < log KCuL the CuLi /CuSi ratio ≥2 and when log KCuS > log KCuL the CuLi /CuSi ratio ≤2. Using the distribution data of Fig. 1A–D a simulated voltammetric output (=[Cuinorg,i ]) may be calculated as a function of titration with ionic copper, not knowing (=not correcting) for sorption on surface sites (Fig. 3A). The input concentration of 500 nmol l−1 for total ligand may be estimated from extrapolation of the linear part of the graph to the abscissa. From the “no
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Fig. 1. Simulation of the copper distribution in a voltammetric cell assembly using the input data from Table 1: (A) no sorption, (B) log KCuS = 8, (C) log KCuS = 9, and (D) log KCuS = 10.
Fig. 2. CuL/CuS ratio using the copper distribution data from Fig. 1B–D taking into account a fixed log KCuL of 9 and varying log KCuS values as indicated.
sorption” graph in Fig. 3A it follows that [Lt ] is indeed 500 nmol l−1 whereas for increasing log KCuS values the intercept with the abscissa shifts to higher values and thus more inaccurate total ligand concentrations. When we linearize the graphs in Fig. 3A according to Langmuir or Scatchard (Eq. (5) or (6)), log KCuL and [Lt ] data as given in Table 2A are derived. As expected, the “no sorption” data yield the exact input data whereas an increasing surface site “strength” (log KCuS from 8 to 10) gives serious deviation of the input data. In particular, all [Lt ] values are much too high whereas log KCuL values may be equal to the input data or too high/low. When simulating the voltammetric output (=[Cuinorg,i ]) as a function of titration with ionic copper from the distribution data of Fig. 1A–D, knowing (=correcting) for sorption on surface sites, all input conditions yield the same graph (Fig. 3B); this graph is identical to the “no sorption” graph in
J.T. van Elteren, U.D. Woroniecka / Analytica Chimica Acta 476 (2003) 33–42
39
Fig. 3A. Understandably this yields the log KCuL and [Lt ] input data (Table 2B). Using Eqs. (16) and (17) also the log KCuS and [St ] data can be derived via linearization. For all three surface site cases the exact KCuS and [St ] input data were retrieved (Table 2B). 4.2. Titration in practice
Fig. 3. Simulated titration graph using the copper distribution data from Fig. 1A–D: (A) not corrected for sorption and (B) corrected for sorption.
Table 2 Copper complexing capacity data derived from linearization of titration data obtained from simulated copper distributions in a voltammetric cell assembly under different sorption regimes without (A) and with (B) correction of sorption artifacts Input log KCuL
[Lt ] (nM)
No sorption log KCuS = 8 log KCuS = 9 log KCuS = 10
9.00 8.80 9.00 9.47
500 724 750 745
No sorption log KCuS = 8 log KCuS = 9 log KCuS = 10
– 9.00 9.00 9.00
– 500 500 500
log KCuS
[St ] (nM)
A – – – –
– – – –
B – 8.00 9.00 10.00
Using a 1:10 diluted fresh water sample with a large amount of organic matter (brownish coloring) the sorption correction technique as proposed in this paper is illustrated experimentally. In Fig. 4A a typical titration graph as produced by ASV is given. Note that each pre-equilibrated sample was allowed to re-equilibrated in the voltammetric cell for 30 min prior to ASV measurement. This time was sufficient for all copper species to reach new equilibrium values as shown in Fig. 5. In contrast to common practice the electrochemical cell assembly was acid-cleaned for each individual sample. After re-equilibration a sample aliquot was taken from the electrochemical cell for ␥-counting of 64 Cu to establish sorption (Fig. 4B). Up to ca. 30% of all the copper present ([Cuinit ] + [Cuadd,i ]) undergoes sorption and therefore delivers inaccurate complexing capacity data. After sorption correction of Fig. 4A with Fig. 4B, Fig. 4C results. Linearization of the data in Fig. 4C according to Langmuir for CuL and Scatchard for CuS is visualized in Fig. 6A and B, respectively. Interestingly the Scatchard plot shows two different Cu surface sorption sites. In Table 3 the data retrieved from these linearizations are summarized. There are roughly four times as many “weak” than “strong” surface sorption sites which may be explained by the presence of several surface active materials in the electrochemical cell assembly (described later). However, in view of the low log KCuS2 of the “weak” sites
– 250 250 250
Table 3 Corrected complexing capacity data for a 1:10 diluted fresh water sample with additionally calculated surface sorption phenomena from 64 Cu monitoring log KCuL [Lt ] (nM) log KCuS1 [S1t ] (nM) log KCuS2 [S2t ] (nM)
8.70 449 9.45 153 7.28 667
± ± ± ± ± ±
0.23 13 0.02 5 0.11 78
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Fig. 5. Surface sorption of Cu2+ in the electrochemical cell assembly as a function of equilibration time using the 1:10 diluted fresh water sample with 64 Cu monitor.
Fig. 4. Titration results for the 1:10 diluted fresh water sample: (A) ASV measurement, (B) radiochemical sorption monitoring, and (C) sorption-corrected ASV measurement.
their sorption influence is probably negligible. In case we would use linearization of the (uncorrected) data points in Fig. 4A we obtain the following complexing capacity data for natural ligands, viz. 8.71 ± 0.28 for
Fig. 6. Linearized data for the 1:10 diluted fresh water sample: (A) Langmuir transformation for copper bound to natural ligands and (B) Scatchard transformation for copper bound to active surface sites.
J.T. van Elteren, U.D. Woroniecka / Analytica Chimica Acta 476 (2003) 33–42
log KCuL and 601 ± 15 nmol l−1 for [Lt ]. Compared to the corrected data in Table 3 it follows that log KCuL is similar whereas the uncorrected [Lt ] is much too high. 4.3. Sorption details Since in Fig. 4B such serious sorption problems were encountered, in a separate experiment potential sorption sites were investigated. To this end the standard electrochemical cell assembly was equilibrated for 30 min with 20 ml of a solution containing 1 mol l−1 Cu2+ , 27 MBq l−1 64 Cu2+ , 50 mmol l−1 NaClO4 and 10 mmol l−1 H3 BO3 (pH 8.2). After equilibration all the parts of the electrochemical cell assembly were scrutinized for Cu sorption by rinsing with acid solutions followed by ␥-counting of the 64 Cu in the acid-rinses. Fig. 7 gives an idea which parts of the electrochemical cell assembly are most notorious for copper sorption under the conditions applied, viz. the glass wall and the teflon stirrer rod. One way to overcome sorption problems in the voltammetric complexing capacity measurement of natural waters
41
is using the alternative cathodic stripping voltammetry (CSV) method. In contrast to ASV this method measures indirectly, i.e. after competitive ligand equilibration (CLE) with a known ligand which forms an electrochemically-active complex [10]. The presence of an additional “strong” ligand keeps copper in solution and prevents sorption of Cu2+ onto surface sites. This was proven experimentally by adding 50 mol l−1 salicylaldoxime, a known well-defined competitive ligand [11], to the electrochemical cell solution above yielding negligible sorption. This suggests that CLE-CSV methods may be more suited to the measurement of the complexing capacity of natural waters, although no a priori adverse decision against the use of ASV should be made as multifarious ASV methods with minimal sorption problems exist, depending on metal/sample matrix combination. However, we have to realize that the detection window of CLE-CSV is such that much “stronger” ligands are measured than with the ASV technique which is susceptible to ASV labile copper (ionic and labile-bound copper) only. For ASV measurements the proposed
Fig. 7. Sorption characteristics of copper onto electrochemical cell assembly parts from a standard solution using a
64 Cu
monitor.
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correction method by monitoring the sorption using a radiotracer of the metal under study is a way to deal with usually unknown sorption phenomena.
Acknowledgements The authors wish to thank Mr. K.J. Kroon for preparation of the 64 Cu-radiotracer. References [1] J.M. D´ıaz-Cruz, M. Esteban, M.A.G.T. van den Hoop, H.P. van Leeuwen, Anal. Chem. 64 (1992) 1769.
[2] D. Omanovi´c, Ž. Peharec, I. Pižeta, G. Brug, M. Branica, Anal. Chim. Acta 339 (1997) 147. [3] W. Davison, S.J. de Mora, R.M. Harrison, S. Wilson, Sci. Total Environ. 60 (1987) 35. [4] V. Cuculi´c, M. Branica, Analyst 121 (1996) 1127. [5] S. D´ıaz-Cruz, J.M. D´ıaz-Cruz, M. Esteban, Anal. Chem. 66 (1994) 1548. [6] R.F.C. Mantoura, J.P. Riley, Anal. Chim. Acta 78 (1975) 193. [7] I. Ruži´c, Anal. Chim. Acta 140 (1982) 99. [8] L. Jin, N.J. Gogan, Anal. Chim. Acta 412 (2000) 77. [9] J.T. van Elteren, K.J. Kroon, U.D. Woroniecka, J.J.M. de Goeij, Appl. Radiat. Isot. 51 (1999) 15. [10] J. Barek, A.G. Fogg, A. Muck, J. Zima, Crit. Rev. Anal. Chem. 31 (2001) 291. [11] M.L.A.M. Campos, C.M.G. van den Berg, Anal. Chim. Acta 284 (1994) 481.
Investigation into sorption processes in the anodic stripping voltammetric speciation of Cu in natural waters Johannes T. van Elteren∗ , Urszula D. Woroniecka Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands Received 1 July 2002; received in revised form 30 September 2002; accepted 16 October 2002
Abstract In the anodic stripping voltammetric speciation of copper significant errors may be introduced as a result of sorption of Cu2+ onto active surfaces of the voltammetric cell assembly. A correction method was developed based on monitoring of the total copper concentration in solution using a 64 Cu-radiotracer; additionally this allowed us to study sorption phenomena in the voltammetric cell assembly. For copper speciation in a fresh water sorption-corrected complexing capacity data (total natural ligand concentration and conditional stability constant of formed complex) showed considerable discrepancies with uncorrected data. From the same sorption data it could be deducted that this was accountable to the presence of two active surface sites in the voltammetric cell assembly. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Speciation; Copper; Anodic stripping voltammetry; Sorption; Correction
1. Introduction Considerable sorption of trace metal ions has been reported on the surfaces of the voltammetric cell assembly during anodic stripping voltammetry (ASV) measurements [1–4]. In the case of speciation analysis, selective disappearance of ionic metal from the cell solution leads to sample re-equilibration and potential inaccurate voltammetric and speciation data, depending on the severity of the sorption (strength and number of surface sorption sites). The surface sorption of trace metal ions may be significantly suppressed by replacing standard components with ∗ Corresponding author. Present address: National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia. Tel.: +386-1-4760-288; fax: +386-1-4760-300. E-mail address: [email protected] (J.T. van Elteren).
components less prone to adsorption [2,5]: the glass cell with a quartz or polystyrene cell, the plastic stirrer with a quartz stirrer and the sinter glass tip of the reference electrode with a quartz one. However, in general pre-equilibration of the voltammetric cell assembly with a representative sample seems to be the norm in speciation analysis to prevent unwanted sorption processes during voltammetric measurement. This procedure is only valid though in case all sorption sites of the cell assembly have been irreversibly saturated so that variation in sample composition does not lead to adsorption onto or desorption from cell wall, counter electrode, etc. It is usually assumed that irreversible saturation indeed occurs notwithstanding the fact that the voltammetric measurement is unable to show it in practical speciation situations and to our knowledge no literature exists to confirm this either.
0003-2670/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 2 ) 0 1 3 5 8 - 2
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In this work we will not focus on overcoming sorption problems during ASV speciation analysis but on gaining insight in sorption phenomena to be able to correct for them. We elaborated on a method to correct for possible sorption artifacts in the ASV speciation of copper in natural waters using a 64 Cu monitor. A 64 Cu2+ -spike was therefore allowed to equilibrate with copper species in the sample prior to addition to an acid-cleaned voltammetric cell assembly where the sorption of Cu2+ and thus re-equilibration process was followed. Mathematical formulae were derived to arrive at corrected ASV speciation data using 64 Cu sorption data.
is the equilibrium ionic Cu concentration after titration step i (mol l−1 ); [Cuinorgcompl,i ] is the equilibrium concentration of inorganic Cu complexes after titration step i (mol l−1 ); [CuLi ] is the equilibrium concentration of Cu bound to natural ligands after titration step i (mol l−1 ); [Lt ] is the total concentration of natural ligands (mol l−1 ); [Li ] is the equilibrium concentration of natural ligands after titration step i (mol l−1 ). Complexing capacity data (KCuL and [Lt ]) may be derived by processing the voltammetric data via the Langmuir [6] or Scatchard [7] linearization: Langmuir
2. Theory
Scatchard It is assumed that in natural waters copper has a 1:1 coordination with discrete ligands and ligand groups, described by the simplified overall equilibrium (charges omitted): (1)
where L and CuL represent the binding sites on discrete ligands/ligand groups and ionic copper bound to these sites, respectively. The conditional stability constant (in l mol−1 ) for the formation of CuL is given by: KCuL
[CuL] = [Cu][L]
(2)
Stepwise titration of L with ionic copper and anodic stripping voltammetric measurement of the remaining reactive copper after equilibration yields copper speciation information. The mass balances after titration step i and subsequent equilibration are as follows (charges omitted): [Cuinit ] + [Cuadd,i ] = [Cui ] + [Cuinorgcompl,i ] + [CuLi ]
(3)
and [Lt ] = [Li ] + [CuLi ]
(5)
Plotting [Cui ]/[CuLi ] versus [Cui ] yields a straight line with a slope 1/[Lt ] and an intercept 1/(KCuL [Lt ]).
2.1. Complexation
Cu + L ↔ CuL
[Cui ] [Cui ] 1 = + [CuLi ] [Lt ] KCuL [Lt ]
(4)
where [Cuinit ] is the initial total Cu concentration before titration (mol l−1 ); [Cuadd,i ] is the ionic Cu concentration added in titration step i (mol l−1 ); [Cui ]
[CuLi ] = −KCuL [CuLi ] + KCuL [Lt ] (6) [Cui ]
Plotting [CuLi ]/[Cui ] versus [CuLi ] yields a straight line with a slope −KCuL and an intercept KCuL [Lt ]. From Scatchard plots it is generally possible to deduct if more than one ligand is present and calculate their complexing capacity data. 2.2. ASV measurement With ASV we measure the equilibrium inorganic copper concentration: [Cuinorg,i ] = [Cui ] + [Cuinorgcompl,i ] = αCu [Cui ] (7) with α Cu the side-reaction coefficient for inorganic copper complexation. In ASV terms this means: [Cuinorg,i ] =
ip,i SASV
(8)
with ip,i the ASV peak current (in nA) in titration step i and SASV the sensitivity (in nA mol−1 l) of the ASV technique. For substitution of [Cui ] and [CuLi ] in Eq. (5) or (6) we use: [Cui ] =
ip,i SASV αCu
(9)
and [CuLi ] = [Cuinit ] + [Cuadd,i ] −
ip,i SASV
(10)
J.T. van Elteren, U.D. Woroniecka / Analytica Chimica Acta 476 (2003) 33–42
Jin and Gogan [8] calculated a value of 6.25 for α Cu in fresh water using average Cl− , SO4 2− and CO3 2− concentrations, a pH of 7.6 and an ionic strength of 0.1 mol l−1 . For our particular 1:10 diluted fresh water sample, recalculation with our conditions (see Section 3) resulted in an α Cu of 5.85 which value was used in this work to derive complexing capacity data. [Cuinit ] is usually measured with the ASV technique as well but after digestion of the sample. 2.3. Sorption artifacts In the determination of the copper complexing capacity of natural waters, re-equilibration of the copper equilibrium (Eq. (1)) may take place due to sorption of ionic copper onto the voltammetric cell assembly. Natural ligands are supposed to compete with surface sites S for Cu binding and may be crucial to keep Cu in solution. Assuming that copper has a 1:1 coordination with surface sites the following equilibrium describes the sorption onto the voltammetric cell assembly (charges omitted): Cu + S ↔ CuS
(11)
The conditional stability constant for the formation of CuS is given by: KCuS =
[CuS] [Cu][S]
(12)
with [CuS] and [S] the concentration of occupied and unoccupied surface sites (in moles per liter voltammetric cell volume), respectively. In this work an ASV-based copper speciation method is presented which corrects for possible sorption artifacts by monitoring the total soluble copper concentration in a standard voltammetric assembly using 64 Cu as radiotracer. To this end subsamples in inert polystyrene vials are both titrated (i steps) with copper ([Cuadd,i ]) and spiked with radioactive copper ([64 Cuadd ]). The total activity concentration [64 Cuadd ] (in Bq l−1 ) is the same for each subsample. Equilibration may take a considerable time (up to 1 day) and therefore titrations are in general performed in inert vials and not in the voltammetric cell. After transfer of (aliquots of) these equilibrated subsamples to the acid-cleaned voltammetric cell assembly re-equilibration may take place due to Cu2+ surface
35
sorption. When fully equilibrated the specific activities (in Bq mol−1 ) for all copper species involved should be equal, i.e.: [64 Cuinorgcompl,i ] [64 Cui ] [64 CuLi ] = = [Cui ] [Cuinorgcompl,i ] [CuLi ] =
[64 CuSi ] [CuSi ]
(13)
It is essential that the specific activity of the initial radioactive copper spike is sufficiently high so that the amount of Cu added with the spike is negligible compared to the total amount of Cu already present. To this end high-specific 64 Cu, produced via the 64 Zn(n,p)64 Cu reaction, was used [9]. 2.4. Sorption correction Sorption of ionic copper onto the voltammetric cell assembly in the titration process leads to a decrease of [Cui ] and [64 Cui ] and thus of the total soluble copper concentration [Cusol,i ] and the total soluble activity concentration [64 Cusol,i ]: [64 Cusol,i ] [Cusol,i ] = ([Cuinit ] + [Cuadd,i ]) 64 (14) [ Cuadd ] where [64 Cusol,i ] follows from direct counting of an aliquot of the voltammetric cell solution and [64 Cuadd ] is the known added radiotracer spike. For calculation of the complexing capacity data we use Eq. (5) or (6) and substitute Eq. (9) for [Cui ] and a modified form of Eq. (10) for [CuLi ]: [CuLi ] = [Cusol,i ] −
ip,i SASV
(15)
2.5. Sorption characteristics In analogy with the binding of Cu to L, the sorption characteristics of Cu onto the voltammetric cell assembly may also be derived using the Langmuir or the Scatchard linearization. When the system is assumed to have a fixed concentration of sorption sites [St ] (in moles per liter voltammetric cell volume) and a conditional stability constant for the formation of CuS of KCuS , linearizations may be achieved similar
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to Eqs. (5) and (6): Langmuir
3.2. Preparation of high-specific activity
[Cui ] [Cui ] 1 = + [CuSi ] [St ] KCuS [St ]
(16)
Plotting [Cui ]/[CuSi ] versus [Cui ] yields a straight line with a slope 1/[St ] and an intercept 1/(KCuS [St ]). Scatchard
[CuSi ] =−KCuS [CuSi ] + KCuS [St ] (17) [Cui ]
Plotting [CuSi ]/[Cui ] versus [CuSi ] yields a straight line with a slope −KCuS and an intercept KCuS [St ]. For calculation of the complexing capacity data we use Eq. (16) or (17) and substitute Eq. (9) for [Cui ] and the following equation for [CuSi ]: [CuSi ] = [Cuinit ] + [Cuadd,i ] − [Cusol,i ]
(t1/2 = 12.7 h; Eγ = 0.511 MeV) was obtained by irradiation of 150 mg zinc oxide (99.9995%, Johnston Matthey, Materials Technology, UK) in the Delft nuclear reactor in a fast neutron flux of 1.9 × 1016 m−2 s−1 for 12 h. After 3 h of cooling the zinc oxide target was dissolved in acid and the copper was separated from the zinc on a Dowex 2 × 8 ion exchange resin as described in literature [9] yielding an activity concentration of 54 GBq l−1 and a specific activity of 1.9 × 1017 Bq mol−1 , 8 h after the irradiation. 64 Cu2+
3.3. Chemicals and sample preparation
(18)
3. Experimental 3.1. Sorption simulation A Pentium® II processor based IBM-compatible PC was used to simulate sorption processes during the ASV determination of the Cu complexing capacity of natural waters in an electrochemical cell with carefully chosen input data (see Table 1). Calculations were performed with Mathcad software (©1986–1993, MathSoft, Inc., Version 4.0) based on equilibria (1) and (11) with their respective mass balances, generating concentrations of all copper species involved ([Cui ], [Cuinorgcompl,i ], [CuLi ] and [CuSi ] as a function of [Cuadd,i ]). The output data were linearized according to a Langmuir or a Scatchard transformation, with and without sorption correction, using Excel software (Microsoft® Excel 97 SR-2). Table 1 Input data for simulation of copper sorption in a voltammetric cell assembly; all parameters are fixed except the log KCuS values [Cuinit ] (nmol l−1 ) [Cuadd,i ] (nmol l−1 ) [Lt ] (nmol l−1 ) [St ] (nmol l−1 ) V (l) α Cu log KCuL log KCus
64 Cu2+
5 5–1500 (23 points) 500 250 0.02 10 9 8, 9, 10
All chemicals were at least of analytical reagent grade. Milli-Q-Plus (MQ) water (Millipore-waters, Milford, MA, USA) with a resistivity of 18 M cm was used for all solution preparations. A 1-l freshwater sample from a canal in Delft (The Netherlands) was collected in an acid-cleaned (soaking for at least 24 h in 1 mol l−1 HCl) high density polyethylene (HDPE) bottle and immediately filtered through a 0.4-m polycarbonate filter (Nuclepore) using Swinnex filterholders (25 mm diameter; Millipore). Aliquots of 2 ml were added to 30-ml polystyrene containers (Sterilin) and titrated with Cu2+ (10−4 mol l−1 , 0–300 l) and spiked with high-specific activity 64 Cu2+ (54 GBq l−1 , 10 l); further 0.2 ml 1 mol l−1 HEPES (mono-sodium N-hydroxyethylpiperazine-N -2-ethanesulfonate) and 2 ml 0.5 mol l−1 NaClO4 were added and the whole sample was adjusted to a pH of 7.7 and made up to a volume of 20 ml with MQ-water giving final added Cu2+ concentrations up to 1500 nmol l−1 and a final added 64 Cu2+ activity concentration of 27 MBq l−1 . It can be calculated that only 0.14 nmol l−1 Cu2+ was added with the 64 Cu2+ -spike. Cu2+ and 64 Cu2+ were allowed to equilibrate overnight with the natural ligands. The next day each equilibrated sample was transferred to an acid-cleaned standard voltammetric cell and allowed to re-equilibrate for 30 min prior to ASV measurement and collection of a 0.1 ml aliquot for counting. The polystyrene equilibration containers were inert towards copper sorption as established by counting of sample aliquots against a reference (described later).
J.T. van Elteren, U.D. Woroniecka / Analytica Chimica Acta 476 (2003) 33–42
3.4. ASV measurement An Autolab potentiostat (Eco Chemie bv, The Netherlands) interfaced with a Metrohm model 663 model hanging mercury drop electrode (HMDE) was used for the inorganic copper determination; reference and counter electrodes were Ag/AgCl, KCl (3 mol l−1 ) and glassy carbon, respectively. The 50-ml cell was made out of borosilicate glass and the stirrer and purge tubing out of teflon. For measurements the computer program General Purpose Electrochemical System 3.1 (Eco Chemie bv) was used. An anodic stripping voltammetric procedure as described in the literature [5] was slightly adapted. After transfer of the overnight-equilibrated samples (20 ml) to the voltammetric cell deaeration of the solution took place by purging with water-saturated nitrogen for 30 min after which the ASV cycle was started. A new mercury drop was extruded (drop size setting, 3) and copper was deposited at a deposition potential of −0.9 V for 1 min under continuous stirring (stirrer setting, 3) followed by a period of quiescence of 5 s. Copper was stripped from the mercury drop using a differential pulse scan (pulse height 50 mV) at a scan rate of 20 mV s−1 . Under these conditions a peak for inorganic copper appeared at −0.05 V. It should be noted that prior to each sample transfer the electrochemical cell was acid-cleaned. For measurement of the total copper concentration in the filtered sample an acid-digestion was applied followed by the ASV method described earlier using the method of standard additions. Therefore 2 ml of an acidified sample in the presence of 0.2 ml 30% H2 O2 and 2 ml 65% HNO3 was heated for 15 h at 140 ◦ C in a Parr bomb with teflon liner, followed by addition of 0.2 ml 1 mol l−1 HEPES and 2 ml 0.5 mol l−1 NaClO4 , and made up to 20 ml with MQ-water. The total concentration of copper in the undiluted sample was 158 nmol l−1 . 3.5.
64 Cu
counting
The 0.1-ml counting aliquot from the voltammetric cell was counted for 10 min in an automatic ␥-counter (NaI detector; Wallac 1480 Wizard); the counting error was <5%. Corrections were made for background radioactivity and 64 Cu decay. As a reference 0.1 ml of an acidic solution (pH = 1) was used containing the
37
same 64 Cu activity concentration (27 MBq l−1 ) as the initial samples prior to sorption. 4. Results and discussion 4.1. Computer simulation In Fig. 1A–D a simulation is given of copper species expected in a voltammetric cell assembly during titration of excess ligand with ionic copper (Cuadd,i ) under various sorption regimes (log KCuS ). Realistic input data (Table 1) were used with an initial total copper concentration [Cuinit ] of 5 nmol l−1 , a total ligand concentration [Lt ] of 500 nmol l−1 with a log KCuL of 9 and a total surface sorption site concentration [St ] of 250 nmol l−1 with varying strength (log KCuS from 8 to 10). As can be seen from Fig. 1A mostly CuL is formed during titration of L with ionic copper up to the equivalence concentration of 500 nmol l−1 where [Cuinit ] + [Cuadd,i ] = [Lt ]. At twice the equivalence concentration the CuL concentration is 500 nmol l−1 , or 50% of the total copper present in the system. Competition of surface sorption sites with ligand for ionic copper binding results in formation of CuS, the amount of which depends on log KCuS . With increasing log KCuS the contribution of CuS gets considerable as illustrated in Fig. 1B–D looking at the shaded area under the CuS curves. Due to the presence of 250 nmol l−1 surface sorption sites the equivalence concentration is increased to 750 nmol l−1 . At twice this equivalence concentration the CuL and CuS concentration are 500 and 250 nmol l−1 , respectively, or 33.3 and 16.7% of the total copper present in the system, independent of log KCuS . The CuL/CuS ratio is 2 in this case; however, in the trajectory to 1500 nmol l−1 ([Cuinit ] + [Cuadd,i ]) the CuLi /CuSi ratio strongly depends on log KCuS as shown in Fig. 2. When log KCuS = log KCuL , the CuLi /CuSi ratio is 2 over the whole trajectory; when log KCuS < log KCuL the CuLi /CuSi ratio ≥2 and when log KCuS > log KCuL the CuLi /CuSi ratio ≤2. Using the distribution data of Fig. 1A–D a simulated voltammetric output (=[Cuinorg,i ]) may be calculated as a function of titration with ionic copper, not knowing (=not correcting) for sorption on surface sites (Fig. 3A). The input concentration of 500 nmol l−1 for total ligand may be estimated from extrapolation of the linear part of the graph to the abscissa. From the “no
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Fig. 1. Simulation of the copper distribution in a voltammetric cell assembly using the input data from Table 1: (A) no sorption, (B) log KCuS = 8, (C) log KCuS = 9, and (D) log KCuS = 10.
Fig. 2. CuL/CuS ratio using the copper distribution data from Fig. 1B–D taking into account a fixed log KCuL of 9 and varying log KCuS values as indicated.
sorption” graph in Fig. 3A it follows that [Lt ] is indeed 500 nmol l−1 whereas for increasing log KCuS values the intercept with the abscissa shifts to higher values and thus more inaccurate total ligand concentrations. When we linearize the graphs in Fig. 3A according to Langmuir or Scatchard (Eq. (5) or (6)), log KCuL and [Lt ] data as given in Table 2A are derived. As expected, the “no sorption” data yield the exact input data whereas an increasing surface site “strength” (log KCuS from 8 to 10) gives serious deviation of the input data. In particular, all [Lt ] values are much too high whereas log KCuL values may be equal to the input data or too high/low. When simulating the voltammetric output (=[Cuinorg,i ]) as a function of titration with ionic copper from the distribution data of Fig. 1A–D, knowing (=correcting) for sorption on surface sites, all input conditions yield the same graph (Fig. 3B); this graph is identical to the “no sorption” graph in
J.T. van Elteren, U.D. Woroniecka / Analytica Chimica Acta 476 (2003) 33–42
39
Fig. 3A. Understandably this yields the log KCuL and [Lt ] input data (Table 2B). Using Eqs. (16) and (17) also the log KCuS and [St ] data can be derived via linearization. For all three surface site cases the exact KCuS and [St ] input data were retrieved (Table 2B). 4.2. Titration in practice
Fig. 3. Simulated titration graph using the copper distribution data from Fig. 1A–D: (A) not corrected for sorption and (B) corrected for sorption.
Table 2 Copper complexing capacity data derived from linearization of titration data obtained from simulated copper distributions in a voltammetric cell assembly under different sorption regimes without (A) and with (B) correction of sorption artifacts Input log KCuL
[Lt ] (nM)
No sorption log KCuS = 8 log KCuS = 9 log KCuS = 10
9.00 8.80 9.00 9.47
500 724 750 745
No sorption log KCuS = 8 log KCuS = 9 log KCuS = 10
– 9.00 9.00 9.00
– 500 500 500
log KCuS
[St ] (nM)
A – – – –
– – – –
B – 8.00 9.00 10.00
Using a 1:10 diluted fresh water sample with a large amount of organic matter (brownish coloring) the sorption correction technique as proposed in this paper is illustrated experimentally. In Fig. 4A a typical titration graph as produced by ASV is given. Note that each pre-equilibrated sample was allowed to re-equilibrated in the voltammetric cell for 30 min prior to ASV measurement. This time was sufficient for all copper species to reach new equilibrium values as shown in Fig. 5. In contrast to common practice the electrochemical cell assembly was acid-cleaned for each individual sample. After re-equilibration a sample aliquot was taken from the electrochemical cell for ␥-counting of 64 Cu to establish sorption (Fig. 4B). Up to ca. 30% of all the copper present ([Cuinit ] + [Cuadd,i ]) undergoes sorption and therefore delivers inaccurate complexing capacity data. After sorption correction of Fig. 4A with Fig. 4B, Fig. 4C results. Linearization of the data in Fig. 4C according to Langmuir for CuL and Scatchard for CuS is visualized in Fig. 6A and B, respectively. Interestingly the Scatchard plot shows two different Cu surface sorption sites. In Table 3 the data retrieved from these linearizations are summarized. There are roughly four times as many “weak” than “strong” surface sorption sites which may be explained by the presence of several surface active materials in the electrochemical cell assembly (described later). However, in view of the low log KCuS2 of the “weak” sites
– 250 250 250
Table 3 Corrected complexing capacity data for a 1:10 diluted fresh water sample with additionally calculated surface sorption phenomena from 64 Cu monitoring log KCuL [Lt ] (nM) log KCuS1 [S1t ] (nM) log KCuS2 [S2t ] (nM)
8.70 449 9.45 153 7.28 667
± ± ± ± ± ±
0.23 13 0.02 5 0.11 78
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Fig. 5. Surface sorption of Cu2+ in the electrochemical cell assembly as a function of equilibration time using the 1:10 diluted fresh water sample with 64 Cu monitor.
Fig. 4. Titration results for the 1:10 diluted fresh water sample: (A) ASV measurement, (B) radiochemical sorption monitoring, and (C) sorption-corrected ASV measurement.
their sorption influence is probably negligible. In case we would use linearization of the (uncorrected) data points in Fig. 4A we obtain the following complexing capacity data for natural ligands, viz. 8.71 ± 0.28 for
Fig. 6. Linearized data for the 1:10 diluted fresh water sample: (A) Langmuir transformation for copper bound to natural ligands and (B) Scatchard transformation for copper bound to active surface sites.
J.T. van Elteren, U.D. Woroniecka / Analytica Chimica Acta 476 (2003) 33–42
log KCuL and 601 ± 15 nmol l−1 for [Lt ]. Compared to the corrected data in Table 3 it follows that log KCuL is similar whereas the uncorrected [Lt ] is much too high. 4.3. Sorption details Since in Fig. 4B such serious sorption problems were encountered, in a separate experiment potential sorption sites were investigated. To this end the standard electrochemical cell assembly was equilibrated for 30 min with 20 ml of a solution containing 1 mol l−1 Cu2+ , 27 MBq l−1 64 Cu2+ , 50 mmol l−1 NaClO4 and 10 mmol l−1 H3 BO3 (pH 8.2). After equilibration all the parts of the electrochemical cell assembly were scrutinized for Cu sorption by rinsing with acid solutions followed by ␥-counting of the 64 Cu in the acid-rinses. Fig. 7 gives an idea which parts of the electrochemical cell assembly are most notorious for copper sorption under the conditions applied, viz. the glass wall and the teflon stirrer rod. One way to overcome sorption problems in the voltammetric complexing capacity measurement of natural waters
41
is using the alternative cathodic stripping voltammetry (CSV) method. In contrast to ASV this method measures indirectly, i.e. after competitive ligand equilibration (CLE) with a known ligand which forms an electrochemically-active complex [10]. The presence of an additional “strong” ligand keeps copper in solution and prevents sorption of Cu2+ onto surface sites. This was proven experimentally by adding 50 mol l−1 salicylaldoxime, a known well-defined competitive ligand [11], to the electrochemical cell solution above yielding negligible sorption. This suggests that CLE-CSV methods may be more suited to the measurement of the complexing capacity of natural waters, although no a priori adverse decision against the use of ASV should be made as multifarious ASV methods with minimal sorption problems exist, depending on metal/sample matrix combination. However, we have to realize that the detection window of CLE-CSV is such that much “stronger” ligands are measured than with the ASV technique which is susceptible to ASV labile copper (ionic and labile-bound copper) only. For ASV measurements the proposed
Fig. 7. Sorption characteristics of copper onto electrochemical cell assembly parts from a standard solution using a
64 Cu
monitor.
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correction method by monitoring the sorption using a radiotracer of the metal under study is a way to deal with usually unknown sorption phenomena.
Acknowledgements The authors wish to thank Mr. K.J. Kroon for preparation of the 64 Cu-radiotracer. References [1] J.M. D´ıaz-Cruz, M. Esteban, M.A.G.T. van den Hoop, H.P. van Leeuwen, Anal. Chem. 64 (1992) 1769.
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