2.3.1. Selection of the Optimum DMG and Catechol Concentrations
The concentrations of DMG and Catechol were varied in order to obtain the optimal conditions for the simultaneous determination of Cu, Ni and V. Aware of the slow kinetics of complex formation between Ni and DMG (Pihlar et al., 1981; Colombo and van den Berg, 1997), a preliminary study was undertaken in order to obtain the time needed to attain equilibrium. The Ni peak height was recorded as a function of time after the addition of 0.3 mM of DMG to the UVSW containing 0.01 M HEPES (pH ~ 7.4) and 0.8 mM catechol. The peak height of Ni became constant after 3 min; a further addition of 14 nM of Ni also required 3 min to obtain a constant signal.
The dependence of Ni sensitivity with DMG was checked varying the ligand concentration up to 0.85 mM (Figure II.1) in the presence of 0.2 mM catechol; the Ni cathodic peak currents increased with sucesive additions of DMG in this concentration range checked. Cu and V sensitivity was affected by the presence of DMG.
The cathodic peaks of Cu and V sharply increased (Figure II.1) with the addition of catechol to a sample containing 0.5 mM DMG, reaching the highest sensitivity at ligand concentrations of ~0.6 mM (V) and ~1.0 mM (Cu). Further additions of catechol provoked a decrease in the peak height, probably due to saturation of the mercury drop. On the other hand, the addition of catechol to the sample also caused a slight increase of Ni sensitivity.
From the results obtained, an optimum ligands concentrations of 0.5 mM DMG and 0.7 mM catechol was found for the direct simultaneous determination of Cu, Ni and V.
2.3.2. Selection of the Optimum pH
The optimum pH ranges for the metals in a single ligand solution are 6-9 for the Cu-catechol complexes, 7-10 for Ni-DMG and 6.6-7.2 for V-Catechol (van den Berg, 1991). Therefore, the optimum pH for the simultaneous determination of Cu, Ni and V was studied varying the pH from 6.2 to 8.4 in the UVSW containing 0.01 M HEPES, 0.3 mM DMG and 0.8 mM Catechol, at a deposition potential of - 0.4 V. As observed in Figure II.2a, highest Cu sensitivity was found around pH 6.7- 7.3, from 7.2 to 8.0 for Ni and from 6.5 to 7.5 for V. Therefore, an optimum pH of 7.3-7.4 was chosen. The peak potential became more negative with increasing pH, indicating that metal-ligand complexes were stronger and less easily reduced, which is indicative of the lower competition of protons for DMG and catechol at higher pH (Donat and Bruland, 1988).
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Figure II.1. Optimisation of the ligand – catechol and DMG –
concentrations for the simultaneous determination of Cu, V and Ni. (a) Optimization of DMG concentration in UVSW at pH 7.4 containing 0.01M HEPES and 0.2mM catechol; (b) Optimization of catechol concentration in UVSW at pH 7.4 containing 0.01 M HEPES and 0.5mM DMG.
2.3.3. Selection of the Optimum Adsorption Potential
The adsorption of Cu, Ni and V on the HMDE was studied varying the deposition potential in the 0.0 to -1.0 V range (Figure II.2b); UVSW contained 0.01 M HEPES (pH ~ 7.4), 0.2 mM DMG and 0.2 mM Catechol. After deposition, the solution was allowed to rest for 20 s at a potential of -0.05 V, from where the cathodic scan was initiated and terminated at -1.2 V. The peak height of Ni and Cu showed a sigmoidal shape with deposition potential, markedly increasing at potentials more negative than -0.2 V (Ni) and -0.3 V (Cu), from where a plateau was observed. For V, however, the peak height begins to increase at potentials more positive than -0.6 V, reaching the maximum values at around -0.3 V.
DMG (mM) 0.0 0.2 0.4 0.6 0.8 1.0 Ip (nA ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Cu V Ni Catecol (mM) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Ip (nA ) 0 1 2 3 4 5 Cu V Ni
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Therefore, a deposition potential of -0.35 V was found to be optimal for the simultaneous determination of these three metals.
2.3.4. Effect of the Adsorption Time, Working Range and Mutual Interferences
The sensitivity of Cu, Ni and V peaks was studied as a function of adsorption time using UVSW containing 0.01 M HEPES (pH ~ 7.4), 0.3 mM DMG and 0.8 mM Catechol and with metal concentrations 7.7 nM (Cu), 4.1 nM (Ni) and 22.7 nM (V) (Figure II.2c). Under these conditions, the cathodic peak heights varied linearly with adsorption time from 0 to 3 min for Cu and Ni, and from 0 to 2 min for V. From these results, a deposition time of 60 sec was chosen as the most suitable for the determination of these three metals at typical concentrations of near-shore coastal waters. For other samples with substantially lower or higher concentrations, the appropriate sensitivity can be easily modified using a higher or lower deposition time. The working range was examined using the abovementioned conditions. The cathodic stripping peak was linear with metal concentrations up to 80 nM for Cu (R2=0.998), up to 100 nM for V (R2=0.998) and >150 nM for Ni (R2>0.999).This linear ranges allow the determination of the metals in all types of oceanic, coastal and estuarine waters. Determination in highly contaminated near-shore waters can be also easily achieved by reducing the deposition time below 60 sec. On the other hand, experiments were carried out in order to check the mutual interferences, finding that metal peaks were not affected by the addition of the other metals at low concentrations.
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Figure II.2. Optimisation of the pH (a), adsorption potential (b) and
deposition time (c) for the simultaneous determination of Cu, V and Ni in seawater.Samples contained 0.3 mM DMG and 0.8 mM catechol in UVSW. pH 6 7 8 9 Ip (nA ) 0 1 2 3 4 5 6 Cu V Ni Udep (-V) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Ip (nA ) 0 1 2 3 4 5 6 Cu V Ni
Tiempo de deposición (min)
0 2 4 6 8 10 Ip C u( nA ) 0 2 4 6 8 10 12 14 Ip N i, V (n A) 0 1 2 3 4 Cu V Ni
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