Electrochemical reduction of metal ions catalysed by preconcentrated adsorbed ligands: the system Ni(II)-thiabendazole

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ELSEVIER

Journal of ElectroanalyticalChemistry427 (1997) 29-34

Electrochemical reduction of metal 'ons catalysed by preconcentrated adsorbed lJgands: the system Ni(II)-thiabendazole J.A. Garcia Calz6n, A.L Miranda Ordieres, J.L. Mufiiz Alvarez, J.M. L6pez Fonseca * Departamento de Qu{mica Ffsica y Analltica, Universidadde Oviedo, 33071 Oviedo, Asturias, Spain

Received 8 July 1996;revised 28 November 1996

Abstract

The electrochemical reduction of Ni(lI) at mercury electrodes catalysed by the fungicide thiabendazole, has been studied in a sodium acetate solution. The electrode process includes the formation of a catalytic complex between aquo-Ni(H) or acetate-Ni(lI) ions and unprotonated thiabendazole adsorbed on the electrode surface. At low thiabendazole concentrations the surface excess of the catalytic ligand was substantially increased by convective mass transport from solution to the surface of the hanging mercury drop electrode. Thus, adsorptive preconcentration of thiabendazole allows a significant enhancement in sensitivity in the determination of the fungicide based on the electrocatalytic reduction of Ni(lI). © 1997 Elsevier Science S.A. Keywords: Electroeatalysis;Nickel(H);Thiabendazole;Voltammetry

1. Introduction The electrochemical reduction of metal ions on mercury electrodes catalysed by ligands is an important kind of electrode process from both theoretical and practical viewpoints [1-4]. Usually the formation of the intermediate catalytic complex is a surface process, involving the adsorbed ligand on the electrode surface [2-4]. The mechanism of the surface catalytic process includes a set of parallel surface reactions [4,5]. These are described in Scheme 1, where M represents the metal ion, L the catalytic ligand and X a catalytically inactive ligand (for instance, the background electrolyte anion). For the sake of simplicity the charge numbers have been omitted. One of the most important applications of electrocatalysis by ligands is the sensitive determination of polarographically inactive catalytic ligands based on the fact that the limiting catalytic current depends on the ligand concentration [1-4,6]. When this kind of surface process is used for the analytical determination of the catalytic ligand L, its molar concentration c L in the bulk solution (with respect to the analytical concentration of the metal ion c~) is very low and the test solution composition can be selected in such a way that /3txC ~ << 1, where /3ix is the stability col~3tant

of MX and c~ is the analytical concentration of X in solution. In such conditions the inequality ~ILCL<<1 usually holds ( ~lL being the stability constant of ML), the free metal ion concentration in the bulk of solution c M equals the analytical concentration of the metal ion and the mean d.c. polarographic limiting current of the catalytic reduction wave associated to Scheme l (~]i k) is essentially determined by stage I of this scheme. Moreover, F-]lk << I~ ( I ( being the mean d.c. polarographic diffusion current of the uncatalysed metal reduction wave), and so, at the potentials corresponding to the plateau of the catalytic prewave, the free metal ion concentration at the boundary diffusion layer]double layer c ° equals c M and also crY. Taking into account these conditions, ~I~ is given by [4]:

103~k,c~F,~exp

Ei~=0.s86 ~

--~.

(1)

where t~ is the drop time, D is the diffusion coefficient of M, ~ is the mean constant of the Ilkovic equation, FL/mOl cm -2 is the surface excess of L, 01 is the potential in the outer Helrnholtz plane and v is the charge number of M. In the Tur'yan treatment [4], the surface excess FL is given by .,

FL ----KL 10 .3 i + ~3,LC~ exp -- ~ - ~ , * Corresponding author. 0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0022-0728(96)05042-5

(2)

30

J.A. Garcia Calzdn et ai. / Journal of Electroanalytical Chemistry 427 (1997) 29-34

l¢2;Lads ~ ~

MLX IV, stow- +

"1 xiL MX

M

-i-ue- ~ MO + L + Lad s

fast

x

k'l; Lads=

IK slow

M (Lads) +

-Fue" ~ MO + Lads fast

X

kl; Lads~ _ M (Lad s) .I,slow"

If,+ MI..

(Lads)

k',; - Lads~ ' ~ ElLslow

4-ue- .~ M o + Lads

thst

(Lads) -l-ue- .~ M O + L + Lad s fast

Scheme 1.

where K L is the c o n s t a n t of the Henry adsorption isotherm and z the charge number of L. Eq. (2) holds if: (i) F L reaches the value corresponding to the adsorption equilibrium of the electrode surface with the bulk of solution, i.e. the concentration of L in the boundary diffusion layerkdectrode double layer c °, equals the concentration of the ligand in the bulk solution; (ii) the adsorption process follows the Henry isotherm. Condition (ii) is fulfilled when the concentration of L in the probe solution is low. However, under such a condition it becomes difficult to attain the adsorption equilibrium within the usual values of drop time of the dropping mercury electrode (DME), especially if L is strongly adsorbed. When the adsorption equilibrium is not attained, the surface excess of L will be given by the equation resulting from the substitution of c L by C0L in Eq. (2). Obviously, the resulting value of F L will be lower, being determined by the rate of transport of L if a high adsorption rate of the ligand is assumed. objective of this paper is to validate the above statement experimentally and, from the viewpoint of the analytical applications, to increase of the sensitivity of the determinations based upon the reduction of metal cations catalysed by adsorbed ligands. The expected improvement in sensitivity would come from an increase in the surface excess of ligand, resulting from the convective transport of L to a stationary mercury electrode. This method is analogous to the corresponding one used i~ conventional adsorptive stripping voltammetry [7,8]. As far as we know, attempts to enhance electrocatalytic effects through ligand accumulation at hanging mercury drop electrodes (HMDEs) have been restricted to thiolic or disulphide compounds [9-12]. The species preconcentrated at the electrode surface is a mercury thiolate produced either electrochemically from the thiolic compound or chemically from the disulphide one. In both cases the catalytic ligands are the adsorbed thiolic species resulting from the electrochemical reduction of the preconcentrated mercury thiolates. For v-penicillamine, an additional pre-

concentration of the ligand by adsorption of the strong chelate complex formed with Ni(II) has been described [12]. With respect to the methods previously described to enhance the electrocatalytic effects [9-12], the direct adsorptive preconcentration of the ligand-catalyst is easier to analyse theoretically, and is also less restrictive regarding the chemical nature of the catalytic species. Another disadvantage of these methods arises from the fact that the reduction of the mercury salt occurs at potentials slightly less negative than those corresponding to the catalysed metal ion reduction. As the cathodic stripping peak current is usually greater than the catalytic one, the evaluation of the catalytic signal may be difficult. The experimental system selected in this paper involves the electrochemical reduction of Ni(II) catalysed by thiabendazole (I). This compound is widely used as a fungicide and its determination in vegetables, milk, wastewater and raw materials has received great attention. H

(1) Techniques applied to the assay of thiabendazole include spectrophotometry [13], gas chromatography with electron capture [14] and flame ionisation [15] detectors, liquid chromatography with ultraviolet and fluorescence detectors [ 16-18], radioimmunoassay [ 19] and enzyme immunoassay [20]. A differential pulse polarographic assay of thiabendazole, based upon the anodic oxidation of mercury associated with the formation of a mercury(ll)-thiabendazole complex, has also been described [21]. 2. Experimental

Direct current 'tast' (DCTP), normal pulse (NPP), reverse pulse (RIP), differential pulse (DPP) and phaseselective alternating current tast (ACTP) polarography as well as differential pulse voltammetry (DPV) were performed using a Metrohm Polarecord E 506 polarograph. Polarographic measurements were carded out using a dropping mercury indicator electrode with a drop time maintained mechanically at 1.0 s, except when studying its effect in DCTP. A Metrohm EA-410 HMDE with a &-op area of 2.2 mm 2 was used as an indicator electrode in voltammetry. In both techniques a Pt-wire was used as the auxiliary electrode and all potentials were measured vs. a saturated calomel reference electrode. The DPP and DPV pulse amplitude was - 5 0 m V and ACP measurements were performed at 75 Hz and a peak-to-peak modulation voltage of 5 mV on the out-of-phase current (~b- 90°).

J.A. Garcia Calzdn et aL / Journal of Electro~nalytical Chemistry 427 (1997) 29-34

Potential scan rates were 4mV s -~ in the polarographic measurements and 20 mV s-~ in the voltammetric ones. pH was measured using a Crison 2001 micro pH-meter. Thiabendazole was purchased from Sigma, methanol was supplied by Probus and all other chemicals (reagent grade) were supplied by Merck. Water was purified in a Milli-Q (Millipore) system. Stock solutions of 0.01 M thiabendazole in methanol were prepared weekly. Aqueous 5 × 10-3M stock solutions of Ni(ll) were made from the nitrate salt. Sodium acetate + nitric acid buffers were used as supporting electrolytes. ACTP measurements excepted, the concentration of methanol in the probe solutions was never greater than

d.c.-tast polarograms recorded in a solution of 4 x 10 - 4 M Ni(II) + 0.08 M sodium acetate buffer (pH5.5) in the absence and presence of thiabendazole. The Ni(lI) prewave at a half wave potential E~/2 = - 0 . 7 0 V produced by the herbicide is due to the reduction of a Ni(il)-thiabendazole complex with an overpotential lower than that required to reduce the aquo-Ni(II) and acetate-Ni(H) complexes [1-5]. The limiting current of the prewave is dependent upon the stoichiometric concentration of thiabendazole c~ as shown in the inset of Fig. 1: after a sharp initial increase, the current reaches a maximum value much lower than the limiting current of the uncatalysed-Ni(II) wave. As shown also by polarograms in Fig. I(A), O.iabendazole inhibits the uncatalysed reduction of Ni(H) and catalyses the hydrogen ion &scharge. At the highest concentrations assayed, DP polarograms reveal that the global prewave is composed of two steps (Fig. I(B)), demonstrating that the catalytic complex(es) are formed by, at least, two parallel chemical reactions. The influence of acetate buffer pH on the maximum current of the DPP Ni(II) prepeak in a 2 X 10 -s M thiabendazole solution is shown in Fig. 2. At this concentration the two steps observed at higher concentrations merge into a single prepeak. The increase m the maximum current with increasing pH has a shape COlTCSponding to a dissociation curve, indicating that protonation of thiabendazole prevents cataJytic complex(es) formation. By using IR and ligand field spectra, Van Landschoot et al. demonstrate that thiabendazole co-ordinates with Ni(II) and other metal cations via the nitrogen atoms of imidazole and thiazole rings [22]. In the solution of 4 × 1 0 - 4 M Ni(lI)+ 0.08 M sodium acetate buffer (pHS.5) + 2 × 10 -5 M thi'~ndazole, the limiting current of the Ni(ll) prewave in DCTP, (which is 20% lower than the limiting current of the uncatalysed

I% v/v. The probe solution (20m l) was added to the cell and purged with oxygen-free nitrogen for 15 min (and for I min before each new experiment). Except when studying its influence, the temperature was kept at 20 ± 0.20(2 in the polarographic experiments and at 25 ± 0.2°C in the voltammetric ones. In most DP voltammetric measurements the indicator electrode was held at - 0 . 4 V for 10-90s while the solution was stirred at 500revmin-I; the magnetic stirring was stopped and after a 10s rest period the voltammogram was recorded.

3. Results and discussion 3.1. Polarographic studies In a sodium acetate solution, thiabendazole is not electroactive in the potential interval between - 0 . 2 0 and - 1 . 6 0 V . However, when the herbicide is added to a Ni(H) solution, a cathodic wave appears at less negative potentials than the Ni(II) reduction wave. Fig. 1 shows the

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31

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2.5" 10"eA

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I

-0.8

(

I 25-10-eA

-1.2

I

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-0.5 EIV

Fig. 1. Main figure: (A) DC T polarograms of the system 4 X 10-4M Ni(II)+0.08M acetate buffer (pH5.5), in the presence of thiabendazole concentrations of: (1) 0; (2) 2 x 10 -5 M; (3) 8 X 10-5 M. (B) DPP polarogram of the same system in the presence of thiabendazole 8 x 10 -5 M. Inset: effect of the concentration of thiabendazole (0 to 8 x 10 -5 M) on the limiting current of the Ni(ll) prewave recorded by DCTP in the above system.

3.A. Garcia Calzdn et aL / Journal of Eleoroanalytical Chemistry 427 (1997) 29-34

32

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30

0~0

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O

o/Ol

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Fig. 4. Effect of the Ni(II) concentration on the limiting current of the Ni(ID prewave recorded by DCTP. Medium: 0.1M sodium acetate buffer (pH 5.5), containing 2 X 10-5 M thi~bcndazole.

I ......

8.0

pH Fig. 2. Effect of the pH on the maximum current of the prepeak of Ni(II) induced by thiabendazole (DPP). System: 4×10-4M Ni(II)+0.1M sodium acetate buffer + 2 x l0 -5 M thiabendazole.

Ni(ll) reduction wave) is proportional to t °'6. This shows that this current is essentially controlled by the rate of the chemical reaction(s) leading to the formation of the catalytic complex(es). The kinetic nature of this prewave is also supported by the temperature coefficient of the corresponding limiting current: a value of 5.0% °C- ~ was measured over the interval 10 to 4(r-C. The reverse pulse polamgraphic curve recorded using a base potential corresponding to the plateau of the Ni(II) prewave, exhibits only a cathodic current, as expected for a totally irreversible electrode reaction [23]. The limiting current of the Ni(II) prewave reaches a maximum value which is always significantly lower than the limiting current of the uncatalysed Ni(II) reduction wave (see Fig. 1). This fact suggests that the formation of the catalytic complex(es) is a 'surface' process (i.e. it starts from the ligand specifically adsorbed on the electrode surface), and not a 'bulk' process, occurring in the reaction layer. The adsorption of thiabendazole on mercury electrodes was studied in an acetate medium in order to confirm the surface nature of the electrode process. Capacity-potential curves were recorded, using out-of-phase alternating current polamgraphy, for the background elec-

trolyte and solutions containing increasing amounts of thiabendazole (Fig. 3). The fungicide adsorbs on the mercury drop electrode within a potential interval limited on the negative side by an adsorption-desorption peak starting at a potential depending upon the thiabendazole concentration. At slightly less negative potentials, corresponding to the limiting current of the Ni(II) prewave, the adsorbed thiabendazole reaches a maximum surface excess concentration for a bulk concentration of 8 × 10-5 M. This is almost the same concentration at which the prewave's limiting current reaches its maximum value (Fig. l). Further evidence of the 'surface' nature of the catalytic complex(es) formation follows from the stationary mercury electrode experiments (see below). The influence of Ni(II) concentration (at a fixed thiabendazole concentration) on the DCTP limiting current of the Ni(H) prewave is displayed in Fig. 4. The observed curvature towards the X-axis indicates that increasing concentrations of Ni(II) produces an increase in bulk Ni(II)-thiabendazole complex concentration. This provokes a decrease in the free thiabendazole concentration, shifting the adsorption equilibrium and decreasing the surface excess of the adsorbed ligand. The polarographic studies described above allow us to conclude that at the stoichiometric thiabendazole concentrations used, c~ (which are smaller than the analytical concentrations of Ni(II), cT), the mechanism of the pre-

I 60' t0"eA

3 7

6 54

4

-0.2

-0,5

-0.8

-11

-t3

E/V Fig, 3. Out.of-phase ( ~ = 9 0 °) AC T polarograms of a 0.08M sodium acetate medium (pH8.2) in the presence of the following thiahendazole concentrations: (]) 0; (2) ! x 10-5 M; (3) 4 x 10-5 M; (4) 8 x 10-5 M; (5) 1.2 X 10-4 M; (6) 1.4 x 10-4 M; (7) 1.6 x 10-4 M.

ZA. Gmcia Calzdn et aL / Journal of Electroanalytical Chemistry 427 (1997) 29-34

wave's electrode reaction may be described by means of stages I and [ ] of Scheme 1, where L represents the unprotonated thiabendazole and X the acetate anion. The simultaneous formation of the catalytic complex by reactions I and IIl is supported by the changes of the catalytic current observed when the acetate concentration is modified in solutions of constant pH and ionic strength. For the assayed solutions, the stoichiometric concentration of acetate c~ is much higher than c~4 and, according to this mechanism, the instantaneous limiting current E I k (at q) of the Ni(II) prewave in DCP (i.e. the current registered in DCTP) is given by

33

o~O--9--° o

_

h

/o/

o~

/o I

I

i

I

I

30

I 60

I

I

t s Is

10- lO-ek

h exp(, - - -2~Fb ' ) F..llk = 1.16(--~)I/2103KCMFL

[

X k, + k', flzxC~ exp " ~ 0 .

(3)

where K is the constant for the maximum instantaneous diffusion current in the llkovic equation. (K = Id/c T, with c r expressed in moles per cubic decimetre), fllX the stability constant of the Ni(lI)-acetate complex, c u the bulk concentration of the Ni(lI)-aquo complex given by the expression CM = 1 + flzx cT

(4)

and with the remaining symbols as described previously. Assuming that in the adsorption process the surface excess of thiabendazole reaches its equilibrium value with the bulk of the solution, for the lower concentrations assayed (with adsorption following Henry's law), the surface excess of the ligand is given by FL = K L I 0 -3

(5) 1+

+

+

where ~IL is the stability c o n s t a n t of the Ni(II)-L complex, flm~ the stability constant of the Ni(II)-LX complex, c H÷ the hydrogen ion concentration in the bulk of solution and K a the dissociation constant of the protonated thiabendazole, LH +. Otherwise, for the higher thiabendazole concentrations assayed

F, = l',..m

(6)

where FL,m is the maximum surface excess corresponding to the complete coverage of the electrode surface. Eqs. (3)-(6) explain the complete set of experimental results obtained from the polarographic studies.

3.2. Voltammetric studies These studies were performed to confirm that the surface excess of thiabendazole at the DME surface does not reach the equilibrium value with the bulk solution at concentrations lower than those examined in the polarographic study. Fig. 5 shows the differential pulse voitam-

-0.4

-0.8

-0.6

E~ V Fig. 5. Main figure: DP voltammograms at an HMDE of the system 4 × 10 -4 M Ni(lI)+0.1M sodium acetate buffer (pH4.6)+ 1 x 10 -7 M thiabendazole after a stirring period of: (1) 0s; (2) 30s; (3) 60s; (4) 90s. Inset: effect of the stirring period of the above solution on the maximum current of the Ni(H) prepeak induced by thiabendazole (DPV).

mograms corresponding to the system 4 × 10 -4 M Ni(II) + 0.1M acetate buffer (pH4.6)+ 1 X l 0 -7 M thiabendazole. Curve 1 was recorded starting immediately after the stationary mercury drop was formed. Curves 2, 3 and 4 were recorded after the mercury drop was poised at an applied potential of - 0 . 4 V for periods of 30 s, 60 s and 90 s respectively in a stirred solution, followed by a rest period of 10 s. The effect of this time period on the/max of the Ni(H) prepeak (see Fig. 5) reveals that the surface excess of iigand corresponding to the equilibrium with the bulk of solution is reached after stirring for 60 s. For lower stirring times, the surface excess of thiabendazole is determined by the rate of convective mass transport to the electrode surface. The experiments described above confmn the surface nature of the electrode process and also reveal that convective transport of thiabendazole significantly increases the sensitivity of analytical determinations based on the Ni(II) prewave produced by the herbicide. In fact, Fig. 5 shows that thiabendazole cannot be determined at the I × ! 0-7 M level neither by DPV at the HMDE without a convective preconcentration nor by DPP. However, a well-defined signal can be detected after only !0 s of preconcentration at the HMDE in a stirred solution. For routine analytical application of DPV with convective accumulation, the influence of the potential applied to the working electrode during the pteconceatradon step was examined. It was found that Ima~ for the Ni(II) prepeak remains almost unchanged when the applied potential was modified between - 0 . 2 and - 0 . 5 V. This agrees with ACTP data (Fig. 3), which demonstrates a strong thiabendazole adsorption over this potential interval. The dependence of the maximum current of the DPV prepeak on

J.A. Garcia Calzdn et aL /Journal of Electroanalytical Chemistry 427 (1997) 29-34

34

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h t-

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~ L ,o2" -... I~81 I I I I I. t,

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o/°//o/o / ,,"o / -" " / d "

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4. Conclusions The sensitivity of analytical determinations based upon the electro-reduction of metal cations catalysed by ligands adsorbed onto the electrode surface can be significantly enhanced when the surface excess of the ligand increases as a result of convective transport accumulation at the HMDE. For the reduction of Ni(ll) catalysed by thia['endazole this approach, using DPV after ligand accumulation periods of 10-30 s, allows the determination of the herbicide to be carried out at a concentration level 100 times lower than the minimum concentration which can be determined by DPP. The detection limit obtained in this paper for thiabendazole is also 100 times lower than the corresponding one for the differential pulse polarographic determination based upon the anodic wave associated with the formation of mercury-thiabendazole complex(es) [21].

c/10"7M Fig. 6. Main figure: effect of the thiabendazole concentration (0 to 10X 10 -l M) on the maximum current of the Ni(H) prepeak ind:,ced by thiabendazole (DPV). System: 4 x 10-4M Ni(II)+0.1M sodium acetate buffer (pH4.6). Stirring period: (O) 10s; (A) 20s; (12]) 30s. Inset: effect of the thiabcndazole concentration ((0-22)x 10-s M) on the maximum current of the Ni(H) prcpeak induced by thiabendazole (DPV) after a stirring period of the above system of 20s.

thiabendazole concentration is plotted in Fig. 6 for three different solution stirring periods. At low concentrations the Imax vs. c [ plots are linear, whereas at higher concentrations negative deviations from linearity indicate that *,he surface excess of thiabendazole approaches the adsorption equilibrium value. The inset in Fig. 6, corresponding to an accumulation period of 20 s, shows a linearly increasing lma, between concentrations of 2.0× 10 -s and 2.2 × 10 -7 M. Analogous plots were obtained for stirring periods of 10 and 30 s. The voltarnmetric curves corresponding to Fig. 6 were recorded using a pH4.6 sodium acetate buffer as a background electrolyte. In this medium the catalytic effect of thiabendazole on the electrochemical reduction of Ni(II) is not at a maximum (see Fig. 2) due to the partial protonation of the ligand. This pH was selected because blank voltammograms recorded at higher pH show that a compound reducible at potentials close to the Ni(ll) catalytic prepeak is accumulated at the HMDE. The cathodic current associated with this species, for a fixed stirring period, increases with increasing pH and also with increasing Ni(lI) concentration, suggesting that it must be an hydroxylated Ni(II) complex. Moreover, when thiabendazole is added to these solutions it was found that convective accumulation of the hydroxylated species at the HMDE inhibits the reduction of Ni(II) catalysed by thiabendazole adsorbed onto the electrode surface. At pH4.6 this inhibitory effect is almost eliminated, while the adsorbed thiabendazole maintains a significant catalytic activity.

Acknowledgements This work was supported by the Research Fund of the Universidad de Oviedo. Authors gratefully acknowledge Dr. J.R. Barreira for technical support.

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H.B. Mark, Rev. Polarogr., 15 (1968) 2. Ya.l. Tur'yan, Russ. Chem. Rev., 42 (1973) _'227. H.B. Mark, Analyst, 115 (1990) 667. Ya.I. Tur'yan, J. Electroanal. Chem., 338 (1992) 1. Ya.I. Tur'yan, J. Gen. Chem., 60 (1990) 1453. Ya.I. Tur'yan, J. Electroanal. Chem., 365 (1994) 185. J. Wang, Am. Lab., (1985) 41. R. Kalvoda and M. Kopanica, Pure Appl. Chem., 61 (1989) 97. F.G. Banica, J.C. Moreira and A.G. Fogg, Analyst, 119 (1994) 309. F.G. Banica, A.G. Fogg and J.C. Moreira, Analyst, 119 (1994) 2343. EG. Banica, A.G. Fogg and J.C. Moreira, Talanta, 42 (1995) 227. A. Ion, F.G. Banica, A.G. Fogg and H. Kozlowski, Electroanalysis, 8 (1996) 40. [13] M.G. Bardiey, M.Y. Mohamed and M.S. Tawakkol, Anal. Lea., 23 (1990) 1385. [14] N. Nose, S. Kobayashi, A. Tanaka, A. Hirose and A. Watanabe, J. Chromatogr., 130 (1977) 410. [15] A. Tanaka and Y. Fujimoto, J. Chromatogr., 117 (1976) 149. [16] D.M. Victor, R.E. Hall, J.D. Shamis and S. Whitlock, J. Chromatogr., 283 (1984) 383. [17] S.S.C. Tai, M. Cargile and C.J. Barnes, J. Assoc. Off. Anal. Chem., 73 (1990) 368. [18] D.M. Gilvydis and S.M. Waiters, J. Assoc. Off. Anal. Chem., 73 (1990) 753. [19] W.H. Newsone and LB. Shields, J. Agric. Food Chem., 29 (1981) 220. [20] W.H. Newsone and P.G. Collins. J. Assoc. Off. Anal. Chem., 70 (1987) 1025. [21] V. Smola and G. Sontag, Mibochim. Acta, (1986) 239. [22] R.C. Van Landschoot, J.A.M. Van Hest and J. Reedijk, J. Inorg. Nucl. Chem., 38 (1976) 185. [23] K.B. Oldham and E.P. Parry, Anal. Chem., 42 (1970) 229.