Electroreduction of the fungicides Folpet, Captan and Captafol on mercury electrodes

Journal of Electroanalytical Chemistry 456 (1998) 193 – 202

Electroreduction of the fungicides Folpet, Captan and Captafol on mercury electrodes R. Carabias-Martı´nez *, E. Rodrı´guez-Gonzalo, M.G. Garcı´a-Jime´nez 1, J. Herna´ndez-Me´ndez Department of Analytical Chemistry, Nutrition and Food Science, Uni6ersity of Salamanca, 37008 Salamanca, Spain Received 18 February 1998; received in revised form 15 May 1998

Abstract An electrochemical study based on direct current (dc) and differential pulse (dp) polarography and coulometry for Folpet, Captan, Captafol and their hydrolyses products was carried out. It was conducted in 10% (v/v) dimethylformamide– water, 0.20 M acetic acid–sodium acetate medium, pH 2.6. The electrochemical behaviour of the fungicides assayed was interpreted according to the polarographic and coulometric data and with the aid of other techniques such as Fourier transform-infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) spectroscopy and gas chromatography-mass spectrometry (GC-MS), that were used to identify the products obtained in the coulometry assays. The possible reduction mechanisms of the electrochemical processes shown by the three fungicides are discussed. Also, different polarographic procedures are proposed for the determination of the fungicides, reporting that it is not possible to perform simultaneous polarographic quantification of the three compounds. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Folpet; Captan; Captafol; Coulometry; Polarography

1. Introduction The fungicides Folpet (N-(trichloromethylthio)phthalimide), Captan (N-trichloromethylthiotetrahydrophthalimide) and Captafol (1,1,2,2-tetrachloroethylthio-tetrahydrophthalimide) are broad spectrum compounds used for seed treatment and for the protection of fruit and vegetables. Most of the methods reported in the literature for the determination of these fungicides are based on gas chromatography [1,2] which has rapidly replaced older colorimetric methods [3,4]. A detailed study of the behaviour of these fungicides in gas chromatography, as a function of the type of stationary phase used, has been described by Gilvydis and Walters [5,6], who

* Corresponding author. Fax: +34 23 224574. 1 Present address: Instituto de Investigaciones Cientı´ficas, Universidad de Guanajuato, Gto, L. Retana no 5, 36000 Mexico.

proposed a multiresidue method for their determination in fruits and vegetables [7]. The fact that Folpet and Captan [8] may decompose during gas chromatographic analysis has led to the development of different liquid chromatographic methods [9–16]. Electroanalytical methods for the determination of these fungicides are few and far between [17,18]. Carabias et al. [19] have previously proposed a chromatographic (HPLC) method with dual glassy carbon electrode detection for the separation and quantification of these fungicides, after a preconcentration step involving cloud point methodology. Owing to the scarcity of references on the electrochemical behaviour of these fungicides, in the present work we have studied their polarographic behaviour and have proposed possible reduction mechanisms for the different electrochemical processes shown by these compounds. Their electrochemical behaviour is interpreted according to the polarographic data obtained

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Fig. 1. Polarograms (dc and dp) of Folpet (a), Captan (b), and Captafol (c), in 10% (v/v) dimethylformamide + water containing 0.20 M acetic acid+ acetate buffer, pH 2.6, T=25°C.

and with the aid of other techniques: coulometry, thinlayer chromatography, Fourier transform-infrared spectroscopy (FT-IR), 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, and gas chromatography-mass spectrometry (GC-MS) applied to the products of the coulometric processes. From the results obtained, it may be concluded that, for the individual determination of each fungicide, in samples containing more than one of them, it is necessary to carry out a prior separation step. In this sense, coupling of electrochemical detection to an HPLC system for the determination of these compounds offers a viable alternative method, as previously described [19]. Furthermore, the studies performed here show that the use of mercury electrodes may well facilitate the detection of these compounds to a considerable extent after their separation in an HPLC system. On a mercury film electrode an applied potential as low as − 0.2 V (vs Ag AgCl) is sufficient to perform the simultaneous determination of all three fungicides via direct reduction. This potential is much less negative than the one that must be applied to carry out direct reduction of these species on glassy carbon electrodes [19], on which all three fungicides give rise to a single reduction wave at potential values between −1.0 and −1.3 V.

The coulometric study was carried out with a mercury pool cathode. The counterelectrode was placed in a separate compartment and electrical contact was made through a fritted glass disk in the conventional manner. FT-IR, NMR, GC-MS spectra and chromatograms were obtained on the following instruments: PerkinElmer M-1730 with a He–Ne laser source; Bruker DRX 400 (400 MHz for 1H and 100 MHz for 13C); Shimadzu 17A-QP5000. Measurements for pH were made with a Crison-2001 fitted with a combined Ingold electrode.

2.2. Reagents Folpet (99.8%), Captan (99.2%), Captafol (99.9%) and phthalimide were supplied by Riedel-de-Hae¨n (Seelze-Hannover, Germany). Solubility in water was 0.8 mg l − 1 (20°C), 3.3 mg l − 1 (25°C) and 1.4 mg l − 1 (20°C) for Folpet, Captan and Captafol, respectively. Fungicide solutions were prepared by dissolving the products in dimethylformamide or in acetonitrile. Table 1 Variation of the half-wave potential (E1/2) with pH Process

Compound

DE1/2mV/pH/DpH

I1

Folpeta Captan Captafol Folpet Captan Captafol Folpet Captan

a

2. Experimental

2.1. Apparatus The apparatus consisted of a Metrohm E-505 polarograph, a mercury forced-drop electrode, Metrohm EA-1019-1, a platinum auxiliary electrode, Metrohm EA-285, and a home-made calomel reference electrode.

I2

I31 I3

r2

a a

−33.5 −38.5 −60.2 −62.8 −49.1

0.932 0.874 0.989 0.990 0.967

a E1/2 independent of pH 10% (v/v) dimethylformamide–water, T = 25°C.

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195

Fig. 2. Polarograms (dc and dp) of Folpet (a), Captan (b), and Captafol (c), at T= 55°C. Other conditions as in Fig. 1.

Acetic acid+ sodium acetate solutions were prepared from acetic acid and sodium hydroxide of analytical reagent grade. All solvents used (dimethylformamide, acetonitrile, dichloromethane, ethyl acetate and ethyl ether) were of analytical reagent grade.

2.3. Procedure Polarographic studies of the fungicides were carried out in 0.20 M acetic acid+sodium acetate, pH 2.6, in 10% (v/v) dimethylformamide – water medium; unless stated otherwise, the fungicide concentration in the working standard solutions was about 10 ppm (mg l − 1). After N2 was bubbled through the solution for 10 min to deaerate it, the corresponding dc and dp polarograms were obtained; in the latter case, the pulse amplitude was −50 mV (pulse duration, 60 ms). In both techniques, a drop time of 1 s was used. The controlled-potential coulometric study was performed in the same medium, with a mercury pool electrode as the cathode and a Pt sheet as the anode, separated from the cathode by a porous glass plate.

The plates for Folpet and phthalimide were observed under a UV lamp (l= 254 nm) and by development with KMnO4. The plates used for Captan and Captafol were developed with KMnO4.

2.4.2. FT-IR and NMR spectroscopy FT-IR spectra were obtained using KBr discs and 1H and 13C NMR spectra with deuterated chloroform. 2.4.3. Gas chromatography-mass spectrometry (GC-MS) The three fungicides, phthalimide and the coulometry products were dissolved in ethyl ether; a 10 mL aliquot was injected into the chromatograph. The column used was a PTE-5 capillary column, 30 m× 0.25 mm ID × 0.25 mm film (Supelco). The temperature program applied was as follows: 80°C for 2 min and then 6 °C min − 1, finishing at 290°C over 20 min. The temperature of the injector was kept at 270°C. Electron impact (EI) mode at 70 eV was used.

3. Results

2.4. Analysis of coulometry products When electrolysis had been completed, the reduced solutions were neutralised to pH 7.0 by the addition of ammonium hydroxide. They were then extracted with a 1:1 (v/v) mixture of dichloromethane +ether. Na2SO4 was added to the organic phase and, after filtration, the solvent was evaporated off. The solid residue thus obtained was analysed by the techniques described below.

2.4.1. Thin-layer chromatography Silica-gel plates were used and the analytes were eluted with a 1:1 (v/v) mixture of ethyl acetate+ ether.

3.1. Electroreduction of the fungicides Folpet, Captan and Captafol on mercury electrodes In dc polarography, Folpet displays three reduction waves: process I1, which appears at a potential close to 0.0 V; process I2,which is not very well defined; and process I3, which under certain conditions may split into two, designated I31 and I32. In differential pulse polarography four peaks corresponding to these reduction processes appear, process I31 being the one with the best definition and highest intensity (Fig. 1). Captan also shows three reduction waves (I1, I2 and I3) but, unlike Folpet, the I3 process of Captan does not

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Fig. 3. Evolution of the polarograms of Folpet (20.6 ppm) as a function of the hydrolysis time: (a) initial, (b) 5 days, (c) 25 days. Experimental conditions as in Fig. 1.

split. The reduction potentials of these processes are similar to those of Folpet. For this fungicide, the I1 process is the most prominent (Fig. 1). The fungicide Captafol shows only two well defined reduction waves in dc polarography, the I1 and I2 processes. In differential pulse polarography, the I1 process produces several poorly resolved reduction peaks. For Captafol, the wave from the I2 process shows greater intensity and a better defined shape than those obtained from the other two fungicides (Fig. 1). The I3 process shown by Folpet and Captan does not appear in the reduction of Captafol. The disturbance observed in the baseline, in the region of potentials corresponding to the I3 process, also appears in the absence of the fungicide and must be due to some impurity present in the supporting electrolyte. Since these fungicides are susceptible to hydrolysis in aqueous medium, a study was carried out in which the pH value and the percentage of dimethylformamide were varied. From the results obtained it may be concluded that on increasing the percentage of dimethylformamide between 1.0 and 10% (v/v), the size of the signals decreases but the stability of the fungicides increases. Thus, at pH 2.6 and room temperature the peak intensity of the I31 process of Folpet decreased by 18, 4 and 0%, over a period of 54 min when the working medium contained 1.0, 5.0 and 10% (v/v) dimethylformamide – water, respectively. Additionally, in 10% (v/v) dimethylformamide – water media, and for more acidic pH values, the polarograms of these fungicides are better defined; thus, the most suitable medium for performing later studies proved to be one containing 10% (v/v) dimethylformamide – water at pH 2.6. The variations in half-wave potential with pH for the different processes shown by the three fungicides are shown in Table 1. For all three compounds the I1

process is independent of pH; neither its intensity nor its potential are modified when the pH is altered. However, it may be deduced that in processes I2 and I3 a protonation reaction occurs prior to electron transfer. This acid-base equilibrium is established rapidly since no dependence of limiting current on the pH of the medium is observed. The magnitudes of the slopes of the variation in E1/2 with pH (DE1/2/DpH in Table 1) together with the values of ac (vide infra) permit the determination of the number of protons exchanged in the I2 and I3 processes since process I1 remains independent of pH. The experimental data obtained for these I2 and I3 processes are close to the theoretical values expected when a proton exchange occurs. The differences between the experimental and theoretical values are due to the imprecision in the measurement of E1/2 owing to the shape of the waves. The influence of temperature was studied between 15 and 55°C. For all three fungicides, the intensities of the I1 and I2 processes show a non-linear variation with temperature. This kind of behaviour suggests that temperature exerts two effects on the analytical signal. On the one hand, it would modify the electrode process and, on the other, it would induce hydrolysis of the fungicides, with a consequent decrease in the intensities of processes I1 and I2. Process I1 of Captafol, which at 25°C shows more than one peak, gives rise to a single signal at high temperatures (Fig. 2). The I2 processes of the fungicides have a temperature coefficient close to 3% for T5 30°C. This coefficient decreases at higher temperatures. This type of behaviour seems to rule out the diffusive nature of the processes; instead, it suggests the simultaneous participation of some kinetic process together with hydrolysis phenomena.

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Fig. 4. Coulometric reduction at − 0.2 V. Evolution of the polarograms of Folpet (155 ppm) as a function of the electrolysis time: (a) initial, (b) 5 h, (c) 19 h. Experimental conditions as in Fig. 1.

Process I3 of Folpet, which at low temperatures is split into two waves (I31 and I32), shows a single reduction wave at high temperature (I31). This behaviour suggests that the I31 process corresponds to an adsorption pseudo-prewave (Fig. 2). The effect of drop time on the polarographic currents does not permit clear assignment of a kinetic, adsorptive or diffusional character to the reduction processes of the three fungicides studied since the variation in limiting current with drop time does not follow the dependence expected for any of the cases. Modification of the working medium may offer new data about the electrochemical behaviour of a species since, in general, the kinetic and adsorptive components of polarographic processes are strongly affected by the composition of the medium. Accordingly, it was decided to modify the working medium and to study the behaviour of the compounds in acetonitrile –water medium. The results obtained on studying the effect of pH, temperature and drop time in this new medium confirm those obtained previously in dimethylformamide –water Table 2 Thin-layer chromatographic analysis of phthalimide, Folpet, Captan, Captafol and the products of their coulometric reduction at −0.2 V Compound

Rf

Phthalimide

0.48

Folpet Folpeta

0.23

Captan Captana

0.25

Captafol Captafola

0.26

a

Reduced product.

Rf

0.50 0.48 0.55

0.56 0.54

medium. Thus, regarding pH, process I1 remains independent of the pH of the medium for all three fungicides whereas the half-wave potentials of the I2 processes, common to all three compounds, as well as the I3 Folpet and Captan processes, shift to more negative values when the pH is increased.

3.1.1. Determination of electron transfer coefficient ac To elucidate the reaction mechanism, electron transfer coefficients were determined by logarithmic analysis of the I-V curves obtained by dc polarography. The transfer coefficient for a cathodic process (ac) is related to parameters [20,21] which are determined by the reaction mechanism and serves as a useful criterion g for determination of the mechanism: ac = c +rb 6 In this expression, valid under steady-state conditions, gc is the number of electrons transferred before the rate-determining step (rds); n is the stoichiometric number; i.e. the number of times the rate determining step must take place for the overall reaction to occur once; r is a factor whose value is 1 when the rds is a charge transfer step and has a value of 0 when the rds is a chemical step; and b is the symmetry factor, whose value can be assumed to be 0.5. For the I1 processes of the three fungicides, an ac value of 1.0 was obtained; it may thus be proposed that the electrochemical process could occur under the control of a chemical step (r=0). This chemical step would be the slowest one and would be preceded by an electrochemical reaction (gc = 1), the controlling chemical step taking place only once (6 =1). For the I2 and I3 processes, the values of ac are close to 1.5; with this value, the slowest step would be the electrochemical one (r= 1), which would be preceded by another electrochemical reaction (gc = 1), the controlling electrochemical step taking place only once (6 =1).

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Fig. 5. RMN

13

C spectra of Captan and Captafol standard. The symbol (*) means ‘reduced product’.

3.2. Hydrolysis of the fungicides Folpet, Captan and Captafol Another issue addressed to determine the mechanism of polarographic reduction was the polarographic behaviour of the hydrolysis products of these compounds. The evolution of the polarograms, at different hydrolysis times is shown for Folpet in Fig. 3; when the fungicide is completely hydrolysed, only two cathodic waves appear, one at −0.30 V and the other at − 0.85 V. The first is due to the reduction of sulphur and the second one coincides with the reduction potential of phthalimide in this medium. Identical results were obtained in a study of the hydrolysis of Captan and Captafol, the only difference being the absence of the reduction wave at − 0.85 V since tetrahydrophthalimide is not electroactive. The hydrolysis products of the three fungicides do not show the I1 and I2 processes. Since phthalimide and dichloromethylthiol are generated in the hydrolysis of Folpet, and dichloromethylthiol degenerates to yield Cl − , CO2, and S, the disappearance of the I1

and I2 processes in the three fungicides indicates that these processes are linked to the reduction of electroactive groups present in the aliphatic part of the fungicides.

3.3. Coulometric study of Folpet, Captan and Captafol The coulometric study was performed at constant potential, with a mercury pool cathode. The potential applied was − 0.2 V, corresponding to the limitingcurrent portion of the reduction wave of process I1. As the electrolysis of Folpet and Captan progressed, a brown precipitate formed, whereas in the coulometry of Captafol the precipitate formed was grey. Fig. 4 shows the evolution of the polarograms for Folpet as a function of time of electrolysis. After the coulometry, only a cathodic wave at − 0.9 V appears; this corresponds to the reduction process shown by phthalimide. For Captan and Captafol, after application of a constant potential of − 0.2 V for sufficient time, their polarograms show no reduction processes.

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Table 3 GC-MS analysis of the fungicides and the products obtained in electrolysis at −0.2 V Compound

Retention time/min−1

Phthalimide Folpet Folpeta Captan Captana Captafol Captafola

16.3 28.5 16.3 28.3 16.7 29.8 16.7 29.9

a

mz−1 (% relative abundance) 147 (80) 297 (7.3) 263 (7.3) 313 (2.4) 313 (1.2)

104 (70) 104 (95) 104 (63)

260 (83)

167 (2.4) 165 (4.9)

147 156 151 150 151 150

(71) (9.8) (37) (4.9) (31) (4.9)

123 (11) 123 (9.8)

76 (100) 76 (100) 107 (39) 94 (4.9) 107 (7.3) 94 (4.9) 107 (6.9)

50 50 50 79 79 79 79 79

(98) (96) (94) (100) (100) (100) (89) (32)

Reduced product.

3.3.1. Analysis of products generated in coulometry at − 0.2 V In order to identify the species generated by coulometric reduction, these products were analysed by means of the different techniques detailed below. 3.3.2. Thin-layer chromatography The results obtained in thin-layer chromatographic analysis are shown in Table 2. The reduction products of Folpet display Rf values of 0.23 and 0.48. The latter value coincides with that of standard phthalimide. The reduction products of Captan gave rise to only one spot, with an Rf of 0.25 whereas the reduction products of Captafol give two spots, one of them with an Rf value close to that of unreduced Captafol and the other with an Rf close to the value observed for the coulometry products of Folpet and Captan. 3.3.3. FT-Infrared spectroscopy In infrared spectroscopic analysis, the band corresponding to the carbonyl group together with another absorption band between 3200 and 3400 cm − 1 appeared as characteristic signals common to all three pesticides. The latter band is due to the stretching mode of the-N-H group. The presence of this group was confirmed in the 1 H NMR spectra, which showed the signal corresponding to an N–H amine proton. 3.3.4. Nuclear magnetic resonance spectroscopy The 13C NMR spectra of Captan and Captafol, together with those of the coulometry products are shown in Fig. 5. In the spectrum of the coulometry products of Captan, the signal corresponding to the

carbonyl groups (177.6 ppm) is seen, but with a difference in the chemical shift of 2 ppm with respect to standard Captan. This indicates that the reduced products are not identical to the parent compound. Moreover, in the reduction products of Captan, the signal corresponding to the aliphatic carbon atom disappeared. The spectrum of the electrolysis products of Captafol did not reveal any significant difference with respect to the standard product. A possible explanation for this could be that tetrahydrophthalimide and a dimer could be generated in the reduction of this fungicide. The presence of the dimer would yield signals identical to those of standard Captafol in the NMR spectra since in the dimer, owing to its symmetry, the signals are magnetically equivalent to those of standard Captafol.

3.3.5. Gas chromatography-mass spectrometry The chromatogram corresponding to the reduction products of Folpet revealed a single chromatographic peak at 16.3 min, whose mass spectrum was identical to that of phthalimide (Table 3). The reduction products of Captan give rise to a chromatographic signal whose mass spectrum corresponds to that of tetrahydrophthalimide. The reduction products of Captafol afford two signals at retention times of 16.7 and 29.9 min; the mass spectrum of the first peak is identical to that afforded by reduced Captan, and the mass spectrum of the peak appearing at 29.9 min is identical to that of unreduced Captafol. The results obtained in the analysis of the coulometry products indicate that one of the reduction products of Folpet is phthalimide; that, in the reduction of Captan, tetrahydrophthalimide is generated; and that in the

Table 4 Calibration data for the differential pulse polargraphic determination of Folpet, Captan and Captafol Fungicide

Process

Slope/mA mol−1

Intercept/mA

r2

[LOD]a/mol l−1

S br /%

Folpet Captan Captafol

I31 I1 I2

(3.359 0.08) 103 (4.7390.08) 103 (1.5090.10) 103

(−0.8197.4) 10−3 (−4.459 3.5) 10−3 (0.57 9 5.2) 10−3

0.999 0.999 0.998

1.1 10−6 7.9 10−7 2.5 10−6

1.5 2.1 1.7

a

LOD, limit of detection (3Sb m−1), Sb is the blank standard deviation and m the slope of the calibration graph. Sr, relative standard deviation (n=6). 10% (v/v) dimethylformamide+water; 0.20 M acetic acid+acetate; pH 2.6. T =25°C, DE=−50 mV. b

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reduction of Captafol, two species (tetrahydrophthalimide and a dimer of the Captafol) can be identified.

4. Discussion

4.1. Process I1 From the results of the studies, it may be proposed that the electrochemical process occurring for wave I1 can be attributed to reduction with rupture of the \ N –S group, or to the loss of a halide. The reduction of \ N–S would not explain why, in the coulometric reduction of Captafol, a dimer displaying the signals of the carbon atoms in the aliphatic part is generated. Accordingly, it is proposed that the electrochemical process occurring at the potential of wave I1 may be due to the loss of a halide via the following mechanism:

The mercury radical [RHg’], formed in reaction [II], may well be adsorbed onto the electrode and may undergo different reactions (here, pathways (1), (2) and (3)) that may take place to different extents:

This mechanism is consistent with the ac values found. The controlling step would be the chemical reaction between the radical and the mercury (reaction [II]), which is preceded by an electrochemical step. Appearance of the R2 dimer (reaction [V]) is also possible from simple coupling of R’ radicals (reaction [I]).The experimental fact that the I1 process does not appear when a glassy carbon electrode is used suggests the participation of mercury in this reduction process (reaction [II]). The proposed mechanism is in agreement with that described for most organic halides [22–28]. These are complex reactions and the electrochemical reaction can progress via any of the three pathways or a combination of them. This mechanism would imply that the half-wave potentials of I1 processes do not depend on pH since the entry of protons occurs after the electrochemical step (pathway 3). The products generated in coulometry (reaction [VII]) are not stable and, like the fungicides themselves, undergo a hydrolysis reaction, thus accounting for the presence of phthalimide in the results of the NMR, IR and mass spectral analyses shown below.

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For the I1 process of Captan and Captafol, a mechanism similar to that of Folpet (reactions [I] to [VII]) is proposed. The reduction of Captafol would be accompanied by the formation of an organomercury compound (pathway (1)), a dimer (pathway (2)) or the product with halide loss (pathway (3)). The hydrolysis of this latter to yield tetrahydrophthalimide as well as the presence of the dimer would explain the results obtained in the 1H, 13 C NMR, FT-IR and GC-MS analyses.

This mechanism implies that the limiting step is the entry of the second electron (reaction [XI]), preceded by an electrochemical reaction (reaction [X]) and a protonation reaction (reaction [IX]), the latter being responsible for the variation in the potential of this process with pH. Similar mechanisms have been proposed by other authors for the polarographic reduction of phthalimide [29,30].

4.2. Process I2

5. Quantitative determination of Folpet, Captan and Captafol

In the case of process I2, which is common to all three fungicides, it cannot be categorically stated that the observed electrochemical process is due to the reduction of the organomercury compound (product of reaction [VI]) or of the dimer (reaction [V]). The experimental observations to the effect that this process appears better defined in the polarographic reduction of Captafol, and that the presence of the dimer was detected in the case of this fungicide, suggest that the I2 process corresponds to the reduction of the dimer obtained in the I1 process. This compound is not soluble in the medium in which the coulometry is carried out and hence the I2 process disappears when the coulometry is performed at the I1 wave potential.

4.3. Process I3 For process I3, shown by Folpet and Captan, it is proposed that the electrochemical reaction taking place is the reduction of the carbonyl group. In view of the ac values found and the dependence of the half-wave potential on pH, the following polarographic reduction mechanism is proposed:

Quantitative determination of these fungicides was performed by means of dp polarography. To do so, we used the polarographic processes that, owing to their greater sensitivity and better definition, are of greatest analytical interest and are the most appropriate for quantifying each of the analytes. Table 4 shows the analytical characteristics of the polarographic determination in 10% (v/v) dimethylformamide –water containing 0.2 M acetic acid+ sodium acetate buffer of pH 2.6. The minimum amount detectable in the case of Folpet was 0.31 ppm, using measurement of the I3 process; for Captan, this value was 0.25 ppm (process I1), and for Captafol, 0.87 ppm (process I2). The relative standard deviations of these values were close to 2%, which is within the usual range for polarographic determinations. We also investigated the possibility of performing the polarographic determination of Folpet in the presence of Captan and Captafol, based on measurement of the process designated I31 since this process is discriminatory for Folpet. The polarographic processes labeled I1 and I2 are common to all three fungicides and occur at very similar reduction potentials such that their use for analytical purposes would permit the joint determination of the three fungicides but would not allow their individual discrimination. Studies performed with solutions of mixtures of the three fungicides revealed that the polarographic behaviour of these compounds is modified by the simultaneous presence of more than one of them with respect to the type of behaviour observed when they are studied individually. The height of the I31 peak of Folpet is abnormally low and does not correspond to the concentration of the fungicide when Captan and/or Captafol are also present in the solution. This could be due to competition for the electrode surface owing to the participation of adsorption phenomena in most of the reduction processes of these analytes. It may therefore be concluded that in mixtures of these analytes it is not possible to perform individual polarographic determination of any of the compounds without previously carrying out a separation step.

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However, the polarographic studies described here do offer a new possibility for the simultaneous determination of the three fungicides as long as chromatographic separation is performed. The reduction processes designated I1 and I2, which are common to all three fungicides, appear only when a mercury electrode is used, but not when glassy carbon electrodes are employed. The use of mercury electrodes, coupled to a separation system such as HPLC, would allow the detection of these species at potentials close to −0.2 V, a potential value corresponding to the upper plateau of process I1. This potential is much less extreme than that required when glassy carbon electrodes are used. The use of mercury film electrodes as a detection system, after an HPLC separation, offers one possibility for the simultaneous detection of the three fungicides which is currently under study.

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