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Electroanalytical study of the pesticide guthion
221
Chem., 244 (1988) 221-233 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
J. Electroanal.
ELECTROANALYTICAL
STUDY OF THE PESTICIDE
GUTHION
J. HERNANDEZ MBNDEZ *, R. CARABIAS MARTiNEZ and E. RODRIGUEZ Department of Analytical Chemistry, Faculty of Chemrstry, 37008 Salamanca (Spain)
GONZALO
University of Salamanca,
(Received 16th September 1987; accepted 7th October 1987)
ABSTRACT An electroanalytical study based on dc and differential pulse polarography (DPP) and coulometry is described for Guthion, S-(3,4-dihydro-4-oxobenzo[d]-(l,2,3)-triazin-3-yl-methyl)-O,O-dimethylphosphorodithioate, in 20% (v/v) MeOH/H,O, B&ton-Robinson buffer, pH 4.3. The results show that the pesticide produces two reduction waves. The first wave is diffusion-controlled and the determination limit (3 s) is 6.0 X lo-’ M for DPP. The second wave is adsorption-controlled and provides a linear response only at low concentrations, in the absence of tensoactive agents. The electrochemical behaviour of Guthion was interpreted according to the polarographic and coulometric data and with the aid of other techniques (IR and NMR spectroscopy). The possible mechanism of the electrode reactions is postulated and a new method for the determination of the pesticide is proposed.
INTRODUCTION
The importance and proliferation of analytical methods for the determination of pesticides and/or their degradation products are of evident interest. Most of the methods employed are spectrophotometric or chromatographic; electroanalytical methods have been used in the determination of numerous pesticides either directly because the pesticide contains electroactive groups [1,2] or indirectly by generating the electroactive group by a previous reaction. Within the latter group of techniques the reactions most frequently used are nitration [3], alkaline hydrolysis [4] and hydrolysis catalyzed by metals [5]. The present paper describes an electroanalytical study of the pesticide Guthion, S-(3,4-dihydro-4-oxobenzo[d]-(l,2,3)-triazin-3-yl-methyl)-O,O-dimethylphosphorodithioate. Its determination was proposed by Bates [6] using cathode-ray polarography for analysis of the pesticide in apples, pears, cucumbers and tomatoes and a * To whom correspondence should be addressed. 0022-0728/88/$03.50
0 1988 Elsevier Sequoia S.A.
222
supporting electrolyte of 0.05 M KC1 + 0.01 M CH,COOH + 6% acetone, pH 3.8. In this medium Guthion produced a wave at -0.8 V (vs. SCE). The limit of detection was quoted to be 0.1 pg/ml. Bates suggested that the wave must be due to the reduction of the carbonyl group. In the present work, a polarographic study was conducted using dc and differential pulse polarography in 20% (v/v) MeOH/H,O medium, B&ton-Robinson buffer. The results showed that the pesticide gives rise to two reduction waves. Its electrochemical behaviour is interpreted according to the polarographic data and with the aid of other techniques (coulometry, spectrophotometry), and a mechanism of electrodic reduction is proposed which does not agree with the scheme proposed by Bates. EXPERIMENTAL
Apparatus
The apparatus consisted of a Metrohm E-505 polarograph, a mercury forced-drop electrode (Metrohm EA-1019-l), a platinum auxiliary electrode (Metrohm EA-285) and a home-made reference calomel electrode. The coulometric study was carried out on an Amel 563 electroanalytical apparatus using a mercury pool cathode; the counter-electrode was placed in a separate compartment, and electrical contact was made through a fritted glass disk in the conventional manner. NMR, IR and UV spectra were recorded on the following instruments: Bruker WP-20054 (200 MHz ‘I-I9 50 MHz 13C); Beckman IR-33 and Varian Techtron 635. Reagents
Solutions of Guthion were prepared by dissolving the solid product (Bayer) in MeGH. Solutions of 2.5 M NaOH, 0.6 M B&ton-Robinson buffer were also prepared. All solutions and reagents (CHCl,, MeOH, EtOH, p-dimethylaminobenzaldehyde) were prepared from analytical grade chemicals. Procedure
The polarographic study of the pesticide Guthion was carried out in B&ton-Robinson buffer, pH 4.3, in 20% (v/v) MeOH/H,O medium. After bubbling N, through the solution for 10 min to deaerate it, the corresponding dc or DP polarograms were recorded; in the latter case, the pulse amplitude was - 50 mV. In both techniques, drop times of 2 and 0.8 s were used. The coulometric study was performed in the same medium except when it was necessary to work with more concentrated solutions (9.4 X low4 M) in which case the percentage of methanol used was 50% (v/v).
223 RESULTS
Polarographic
study
Guthion exhibits one or two processes of electrochemical reduction according to the pH of the medium. In very acidic media (Fig. la), only one very well-defined wave (dc) or peak (DP) can be seen. For pH values above 2.5, two reduction processes are observed (Fig. lb); the second process has a maximum in dc and a minimum in DP polarography. Both disappear in the presence of 0.001% Triton X-100. The intensity of the first wave, Ill, is constant for pH c 9 while the peak intensity, Ipl, decreases for pH > 6.5. The I,, and Ip2 values of the second process remain constant for pH < 9. The half-wave and peak potentials of the first process shift towards negative potential as the pH is increased while in the second process they remain unchanged (Fig. 2). Accordingly, for pH > 9 a single wave or peak is observed; the intensity of the dc wave is the sum of the values of I,, and I,, (Fig. 3a). In alkaline medium (0.10 M NaOH), initially a single wave or peak is observed, whose intensity decreases progressively over time; at the same time, an anodic process appears (E = -0.50 V) accompanied by two new cathodic processes (E = - 0.72 V and E = - 1.60 V) (Fig. 3b). It is known that in this medium Guthion is
b
I
0.2 /JA
I
0.40
0.80
0.40
0.80
1.20
. -E/V
Fig. 1. Polarograms (dc and DP) of Guthion. 2.1X10m4 M Guthion, 0.12 M B&ton-Robinson, MeOH/H,O (v/v) and variable amounts of NAOH. (a) pH 1.9; (b) pH 5.1.
20%
224
1.20- EP/V
-0
--o-~-~-~-cl-o-oQo-o
l.OO-
/A’
0.80-
/A AHA
0.60
-
AYA’ /A
.A’
c 2.0
4.0
6.0
6.0
10.0
PH
Fig. 2. Variation of the peak potential with pH. 2.1 X 10m4 M Guthion, 0.12 M Britton-Robinson, 20% MeOH/H20 (v/v) and variable amounts of NaOH. (A) Wave Z,,; (0) wave Zp2; (0) wave Zpl + Zpz.
hydrolysed, giving rise to its major products dithiophosphate, o-aminobenzoic acid [7]: s
and
COOH
II ,OCH,
HS-P,
formaldehyde
+
CHzO
+
OCH,
NH2
The anodic process is attributed to the oxidation of Hg in the presence of dithiophosphate; the cathodic process appearing at -0.72 V corresponds to the reduction of the mercury dithiophosphate formed and the most negative one to the reduction of formaldehyde.
a
b
.
I
0.40
1
0.80
I
1.20
. . _ _. .
I
1.60
0.40
I 0.80
1.20
1.60 - E/V
Fig. 3. Polarograms (dc and DP) of Guthion. (a) pH 9.1; (b) 0.10 M NaOH. For experimental see Fig. 1.
conditions
225
Reversibility and determination of the number of protons involved Owing to the poor definition of wave I,, in the absence of Triton X-100, logarithmic analysis was carried out only for wave I,1 for the 2.0 < pH < 7.0 range. The value of an found was 1.35 & 0.06, which indicates the existence of an irreversible electrode process. The relationship found between El,? and the pH for wave I,, is Q2 = -0.432 - 0.058 pH. Using this relationship and knowing the value of an, it was possible to determine the number of protons involved in the controlling electrode process; this proved to be one. Study of the effect of different variables on the shape of the polarographic waves of Guthion This study was performed at pH 4.3 since this medium is the most suitable for determining the behaviour of waves 1 and 2. The percentage of methanol in the medium MeOH/H,O (v/v) was modified between 10 and 50%. It was found that an increase in the proportion of alcohol led to a slight decrease in 1,i and Ipi. For process 2, a decrease was observed in the maximum and minimum shown by this wave, in dc and DP polarography, respectively, together with a noticeable loss of sensitivity in DPP. Later work was therefore carried out with a MeOH/H,O ratio of 20% (v/v). The influence of temperature was studied between 20 and 45 o C. The I,, wave was seen to have a temperature coefficient of 1.54%. Upon increasing the temperature, the maximum of the I,, wave also rose such that it became necessary to add Triton X-100 at 0.001% to measure it. Under these conditions, the I11 wave had a temperature coefficient of 1.66% and the I, wave 0.98%. Variation of the ionic strength between 0.15 and 0.70 A4 (NaClO,) caused no significant modifications in the values of the intensities and potentials of the two processes, either in the absence or presence of Triton X-100. The relationships found between I,, and Ipl and the drop time (Table 1) show that process 1 is diffusion-controlled. For drop times lower than 1.0 s wave 2 does not exhibit polarographic maxima (for 1.28 X 10e4 A4 Guthion). The decrease in these maxima observed upon decreasing the drop time and temperature seems to indicate that one is dealing with TABLE 1 Relationships between intensities and drop time, (v/v), 7’* = 25 o C, pH 4.3)
i = kt” (1.28 X 10e4 M Guthion, 20% MeOH/H*O
Process
Polarogapbic technique
X,P
Corr. coefficient
(1)
dc DP
0.172 0.668
0.999 0.998
(2)
dc DP
0.254 0.348
0.993 0.996
226
4.0
20
2.0
4.0
2.0
6.0
4:o 104 C/M
6:O
Fig. 4. Variation in the intensity of the II1 wave with the concentration of Guthion. pH 4.3, 20% MeOH/H,O (v/v), Ta = 25O C. (a) t = 2 s; (0) dc; (A) DPP. (b) t = 0.8 s; (@) dc; (A) DPP.
maxima of the first kind, originating from stirring of the layer of solution closest to the electrode [8]. The variation in IP, with the pulse amplitude ( - 10 to - 100 mV) is in keeping with the theoretical predictions for a diffusion process [9]. The intensity of Ipz does not reach a constant value on increasing - AE, which suggests that the second process is not diffusion-controlled Influence of concentration This part of the study was performed using dc and DP polarography for drop times of 2.0 and 0.8 s in the presence and absence of Triton X-100. For wave 1, the
0.40
2.0
4.0
6.0
2.0 104
4.0
6.0
c/M
Fig. 5. Variation in intensity of the I,, wave with the concentration of Guthion. pH 4.3, 20% MeOH/H,O (v/v), Ta = 25OC, 0.001% Triton X-100. (a) f = 2 s; (0) dc; (A) DPP. (b) 2 = 0.8 s; (6)) dc; (A) DPP.
221 TABLE 2 Calibration data (20% MeOH/H,O (v/v), pH 4.3, Ta = 25 o C; A E = - 50 mv). Sn, S,,, and S, are the blank, slope and intercept standard deviation, respectively, i is the intercept, m the slope, and t, the r distribution value (a = 0.005) Process
(1)
Polarographic technique
t/ s
Slope /PA mM_’
Intercept /CA
dc
2.0 2.0 0.8 0.8
2.51 kO.02 10.12* 0.10 2.15 f 0.02 4.65 f 0.08
- 0.002 f 0.015 f - 0.005 f 0.006 f
2.0 2.0 0.8 0.8
1.85+0.03 1.20f0.03 1.78 f 0.10 1.15 *o.os
DPe dc DP (2) *
dc DP
r
106 CL a/M ;;A
b
E
d
0.0004
0.005 0.007 0.005 0.020
0.999 0.999 0.999 0.997
0.0002 0.0006 0.0004
0.48 0.06 0.84 0.26
0.49 0.06 0.85 0.21
6.00 2.08 7.03 12.90
-0.010*0.006 0.018 f 0.006 - 0.035 f 0.020 - 0.020 f 0.020
0.998 0.996 0.993 0.993
0.0006 0.0004 0.0007 0.0005
0.97 1.00 1.18 1.30
1.01 1.06 1.23 1.57
9.78 15.07 33.89 52.32
* CL: concentration limit. b 3$/m. ’ 3Sn/(m - faSm). d 3[Si + SF +(i/m)zS~]‘/2/m. e Concentration lower than 1.6 X 10V4 M. * In the presence of 0.001% Triton X-100; concentration lower than 3.2 x 10m4 M.
relationship found between In and the concentration is linear over the entire concentration range studied, both for drop times of 0.8 and 2 s. In DPP, a decrease in the drop time was seen to broaden the range of linearity (Fig. 4). It is recommended that in order to measure wave 2 with precision 0.001% Triton X-100 be added; in its absence, it is only possible to determine concentrations lower than 5.0 X 10d5 and 1.0 X lob4 A4 for drop times of 2.0 and 0.8 s, respectively. In the presence of Triton X-100, the relationships found between intensity and concentration in both dc and DP polarography are not linear and show a shape that indicates an adsorption process. Moreover, a decrease in the drop time does not increase the range of linearity (Fig. 5). The detection limits calculated according to the criteria of Winefordner and Long [lo] are shown in Table 2. The limits are improved in DPP; the best wave for analytical purposes is 1 in view of its better definition and its wider range of linearity. However, the analytical use of one or the other wave would depend on the presence of other interfering species in the solution. Coulometric study This study was conducted using a mercury pool as the working electrode. When electrolysis was carried out at a potential of - 0.75 V, the decrease in the current of wave 2 was parallel to that of wave 1, indicating that the product of wave 1 has undergone a chemical reaction; simultaneously, two anodic waves appear. The species obtained in this coulometry evolve over time (in the presence of oxygen) and a decrease can be observed in the anodic waves and the reappearance of cathodic waves.
228
Fig. 6. Coulometricstudy. Evolution of the polarograms of Guthion as a function of the electrolysis time. pH 4.3, 20% MeOH/H,O (V/V), T* = 25 o C. (a) E = -0.75 V, (b) E = - 1.15 V. The numbers in the plots refer to the electrolysis times in min.
If electrolysis is carried out at a potential of - 1.15 V, the two cathodic waves disappear and at the end of the coulometry only two anodic waves are seen. The reaction products also evolve with time towards other products which exhibit two cathodic waves at potentials close to those of unreduced Guthion. When the electrolysis was interrupted to record the polarographic curves (always in the absence of air) it was observed that at the controlled potential of -0.75 V the electrolysis requires more time than that effected at -1.15 V for total reduction to be reached (Figs. 6a and 6b). If electrolysis at -0.75 V is not interrupted and the absorption spectra of the solution are recorded at different electrolysis times, a decrease is noted in the absorbance when the electrolysis time is increased over the first 80 min. If coulometry is prolonged (bubbling N, through the cell) an increase can be observed in the absorbance (Fig. 7a). This fact indicates that after 80 min of electrolysis a new electroactive species is regenerated which absorbs in the UV zone. This mechanism would imply that the intensity in the coulometry should also undergo an increase unless its efficacy decreases due to deposition on the working electrode [ll]. When electrolysis was conducted at higher concentrations (9.4 X 10e4 M), the formation of a black layer over the mercury pool was observed.
229
b A
1.5(
l.O(
0.5c
2&o
250
360
A/nm
300
hfnm
Fig. 7. Spectra of G&ion and the products obtained in electrolysis. pH 4.3, 20% MeOH/H,O (v/v), Ta = 25 o C. (a) E = -0.75 V; (b) E = - 1.15 V. The numbers in the plots refer to the electrolysis times in min. (- - -) Spectrum after 15 h of contact with air of the products obtained in coulometry at - 1.15 v.
If the same experiment is carried out at a potential of - 1.15 V, a decrease in the absorbance is observed parallel to the increase in electrolysis time; after 80 min of electrolysis, the absorption spectrum remains constant. However, if the solution is placed in contact with air, after a while the spectra resemble those of unreduced Guthion (Fig. 7b). It may be inferred that the electrolysis products at -0.75 V are subject to a chemical reaction (in the absence of oxygen) that generates species which are again reduced at that same potential. However, the products obtained at - 1.15 V are regenerated only in the presence of oxygen. Determination of the number of electrons exchanged The study of the i-t curves at -0.75 V indicates that two (1.97 f 0.25) electrons are exchanged in this process. The In i-t plot is only linear for short periods of time (less than 15 min), which shows that only for times longer than this is it possible to observe the chemical reaction of the products of the electrochemical reaction. If the potential imposed is - 1.15 V, the In i-t plot is not linear, although the values of the intensities of the I,, and I,, waves are similar. It may be concluded
230 TABLE 3 Relationship between the intensities of Phosmet (IF) and Guthion (In) 50% MeOH/H,O (v/v), t = 2 s)
(dc polarography,
10s c/M
Zu/pA
IF/PA
zll /zF
2.0 3.0 4.0 5.0 6.0
0.047 0.073 0.101 0.129 0.155
0.053 0.079 0.106 0.132 0.159
0.89 0.92 0.95 0.98 0.97
that the number of electrons exchanged at a potential of -1.15 corroborate these findings, the values of the 1,, intensity of Guthion with those of the organophosphorus pesticide Phosmet. In 0.10 MeOH/H,O (v/v) medium, Phosmet exhibits a cathodic wave due of the carbonyl group, and hence the number of electrons exchanged relationships found between the Phosmet wave and the 1,i intensity shown in Table 3 for the same medium.
0.10 M HCl,
V is four. To were compared M HCl + 50% to the reduction is two [12]. The of Guthion are
Studies carried out with the products obtained in coulometry at - 1.15 V The solution obtained by coulometry at -1.15 V gives a positive reaction of amines (primary and secondary) with p-dimethyl aminobenzaldehyde. In the presence of oxygen, it evolves progressively, as shown in the polarograms obtained at different times: the appearance of two cathodic waves can be seen at the same time as the gradual disappearance of the anodic waves. For times longer than 150 h, a
b
0
0.20
0.60
1.00
0.20
0.60
1.00
0.20
0.60
Fig. 8. Polarograms of Guthion. pH 4.3, 20% MeOH/H,O (v/v), T’ = 25OC. (a) Unreduced; and (c) 9 days, respectively, after terminating coulometry at - 1.15 V.
1.00
-E/V
(b) 14 h
231
decrease can be observed in the more cathodic wave, and from 220 h onwards the polarograms do not show any modifications in their shapes (Fig. 8). In one solution, at 216 h after coulometry at - 1.15 V the solvent was removed and the dry residue was extracted with CHCl,. The IR spectra of the chloroform extract show a band at 1700 cm-’ characteristic of the C=O group. In the NMR spectra, in deuterated chloroform, splitting of the aromatic hydrogens can be observed; the most characteristic feature of these spectra is the presence of methoxyl and methylene groups. When coulometry is performed in ethanolic medium instead of in methanol, the NMR spectrum reveals the presence of the ethoxyl group.
DISCUSSION
In view of the results obtained, it may be concluded that the electrochemical reduction of Guthion takes place in two steps, each involving two electrons. The electrochemical process taking place at the potential of the first wave (- 0.75 V) can be attributed to the reduction of the -N=N-group according to the following scheme: 5
0
II
N-CH2
,OC’-‘,
- S-pP'OCH3 + H+ + e-
i
(B)
-
(1)
(A)
(5)
+ e- +
(2)
l-l+ ___f
0
(C)
+
H,O
-
+
CH,O
+
HS-!<;;;:
(3)
(D)
The products generated in the coulometry performed at -0.75 V are unstable and, like the hydrazo compounds [13], undergo dismutation reactions to generate azo and diamino groups.
(4)
232
Moreover, the amino compound (F) is oxidized progressively in the presence of air until it forms an a20 group again: 0
0
02
(5)
NH2
(F)
This mechanism would explain why the reaction at -0.75 V requires more time than at -1.15 V, since partial regeneration of the oxidized form occurs. It is similarly possible to account for the experimental observation that cathodic waves reappear after the solution enters into contact with atmospheric oxygen. The formation of a black layer over the mercury pool is due to the chemical reaction between the dithiophosphate generated in reaction (3) and the electrode. In the reduction at a potential of -1.15 V, one can propose breakage of the -NH-NH of the hydrazo compound formed in the first electrochemical process: 0
0
+
CD)
2e-
+
2H+-
(6) (G)
Moreover, the species generated in the coulometry at - 1.15 V gives a positive reaction of primary and secondary amines, although like other hydrazines it is not stable and in the presence of oxygen undergoes oxidation reactions, reordering and condensations [14,15]. Experimentally, it was observed that freshly prepared solutions of phenylhydrazine exhibit an anodic wave that decreases with time; at the same time, two cathodic waves appeared at -0.72 and -0.92 V; however, after 48 h neither cathodic nor anodic waves were observed. This kind of behaviour is in agreement with the mechanism proposed by Ring and Bard [14], according to whom phenylhydrazine undergoes oxidation by oxygen to form an azo group which in acid solutions decomposes slowly. In the lifetime of the azo compound, reduction waves appear but later both the oxidation and the reduction waves disappear. Therefore, although the products of coulometry at - 1.15 V show an electrochemical behaviour identical to that of the hydrazines during the first 20 min after obtaining them (Fig. 8), the oxidation of compound G alone would not justify the electrochemical and spectroscopic behaviour of the products generated after coulometry. The band appearing in the IR spectrum at 1700 cm-’ suggests the presence of carbonyl in a five-membered ring. Thus, it may be proposed that at least the following reactions take place:
(7)
(8)
233
The presence of the azo compound generated in reaction (8) would explain why after coulometry at -1.15 V the products in contact with atmospheric oxygen exhibit, after a certain time, a reduction wave instead of the two waves shown by unreduced Guthion. Even so, reactions (7) and (8) do not justify the NMR spectroscopic behaviour of these products for times longer than 216 h after coulometry at - 1.15 V. This reaction is now under investigation. ACKNOWLEDGEMENT
This research was supported by the CAICYT (Project No. 2999/83). REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
M.R. Smyth and J.G. Osteryoung, Anal. Chim. Acta, 96 (1978) 335. P. Nangniot, Rech. Agron. Gembloux, 3 (1968) 182. R. Engst, W. Schnack and H. Woggon, Z. Anal. Chem., 207 (1965) 30. K. Dulak, J. Kovk and M. Michalek, Z. Anal. Chem., 195 (1963) 350. J. Hem&ndez Mendez, R. Carabias Martinez and J. Sanchez Martin, Anal. Chem., 58 (1986) 1969. J.A. Bates, Analyst, 87 (1962) 786. E. Morifusa, Organophosphorus Pesticides: Organic and Biological Chemistry, C.R.C. Press, Cleveland, OH, 1974, pp. 66-67. S. Wolf, Angew. Chem., 72 (1960) 449. A.M. Bond, Modem Polarographic Methods in Analytical Chemistry, Marcel Dekker, New York, 1980, p. 249. J.D. Winefordner and G.L. Long, Anal. Chem., 55 (1983) 712A. M.D. Morris, Anal. Chem., 39 (1967) 476. R. Carabias Martinez and M.E. Gonz&lez Lopez, personal communication, 1985. T.M. Florence, D.A. Johnson and G.E. Batley, J. Electroanal. Chem., 50 (1974) 113. D.M. King and A.J. Bard, J. Am. Chem. Sot., 87 (1965) 419. I.M. Kolthoff and P.J. Elving, Treatise on Analytical Chemistry, Part 2, Vol. 15, Wiley, New York, 1976, p. 268.
Chem., 244 (1988) 221-233 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
J. Electroanal.
ELECTROANALYTICAL
STUDY OF THE PESTICIDE
GUTHION
J. HERNANDEZ MBNDEZ *, R. CARABIAS MARTiNEZ and E. RODRIGUEZ Department of Analytical Chemistry, Faculty of Chemrstry, 37008 Salamanca (Spain)
GONZALO
University of Salamanca,
(Received 16th September 1987; accepted 7th October 1987)
ABSTRACT An electroanalytical study based on dc and differential pulse polarography (DPP) and coulometry is described for Guthion, S-(3,4-dihydro-4-oxobenzo[d]-(l,2,3)-triazin-3-yl-methyl)-O,O-dimethylphosphorodithioate, in 20% (v/v) MeOH/H,O, B&ton-Robinson buffer, pH 4.3. The results show that the pesticide produces two reduction waves. The first wave is diffusion-controlled and the determination limit (3 s) is 6.0 X lo-’ M for DPP. The second wave is adsorption-controlled and provides a linear response only at low concentrations, in the absence of tensoactive agents. The electrochemical behaviour of Guthion was interpreted according to the polarographic and coulometric data and with the aid of other techniques (IR and NMR spectroscopy). The possible mechanism of the electrode reactions is postulated and a new method for the determination of the pesticide is proposed.
INTRODUCTION
The importance and proliferation of analytical methods for the determination of pesticides and/or their degradation products are of evident interest. Most of the methods employed are spectrophotometric or chromatographic; electroanalytical methods have been used in the determination of numerous pesticides either directly because the pesticide contains electroactive groups [1,2] or indirectly by generating the electroactive group by a previous reaction. Within the latter group of techniques the reactions most frequently used are nitration [3], alkaline hydrolysis [4] and hydrolysis catalyzed by metals [5]. The present paper describes an electroanalytical study of the pesticide Guthion, S-(3,4-dihydro-4-oxobenzo[d]-(l,2,3)-triazin-3-yl-methyl)-O,O-dimethylphosphorodithioate. Its determination was proposed by Bates [6] using cathode-ray polarography for analysis of the pesticide in apples, pears, cucumbers and tomatoes and a * To whom correspondence should be addressed. 0022-0728/88/$03.50
0 1988 Elsevier Sequoia S.A.
222
supporting electrolyte of 0.05 M KC1 + 0.01 M CH,COOH + 6% acetone, pH 3.8. In this medium Guthion produced a wave at -0.8 V (vs. SCE). The limit of detection was quoted to be 0.1 pg/ml. Bates suggested that the wave must be due to the reduction of the carbonyl group. In the present work, a polarographic study was conducted using dc and differential pulse polarography in 20% (v/v) MeOH/H,O medium, B&ton-Robinson buffer. The results showed that the pesticide gives rise to two reduction waves. Its electrochemical behaviour is interpreted according to the polarographic data and with the aid of other techniques (coulometry, spectrophotometry), and a mechanism of electrodic reduction is proposed which does not agree with the scheme proposed by Bates. EXPERIMENTAL
Apparatus
The apparatus consisted of a Metrohm E-505 polarograph, a mercury forced-drop electrode (Metrohm EA-1019-l), a platinum auxiliary electrode (Metrohm EA-285) and a home-made reference calomel electrode. The coulometric study was carried out on an Amel 563 electroanalytical apparatus using a mercury pool cathode; the counter-electrode was placed in a separate compartment, and electrical contact was made through a fritted glass disk in the conventional manner. NMR, IR and UV spectra were recorded on the following instruments: Bruker WP-20054 (200 MHz ‘I-I9 50 MHz 13C); Beckman IR-33 and Varian Techtron 635. Reagents
Solutions of Guthion were prepared by dissolving the solid product (Bayer) in MeGH. Solutions of 2.5 M NaOH, 0.6 M B&ton-Robinson buffer were also prepared. All solutions and reagents (CHCl,, MeOH, EtOH, p-dimethylaminobenzaldehyde) were prepared from analytical grade chemicals. Procedure
The polarographic study of the pesticide Guthion was carried out in B&ton-Robinson buffer, pH 4.3, in 20% (v/v) MeOH/H,O medium. After bubbling N, through the solution for 10 min to deaerate it, the corresponding dc or DP polarograms were recorded; in the latter case, the pulse amplitude was - 50 mV. In both techniques, drop times of 2 and 0.8 s were used. The coulometric study was performed in the same medium except when it was necessary to work with more concentrated solutions (9.4 X low4 M) in which case the percentage of methanol used was 50% (v/v).
223 RESULTS
Polarographic
study
Guthion exhibits one or two processes of electrochemical reduction according to the pH of the medium. In very acidic media (Fig. la), only one very well-defined wave (dc) or peak (DP) can be seen. For pH values above 2.5, two reduction processes are observed (Fig. lb); the second process has a maximum in dc and a minimum in DP polarography. Both disappear in the presence of 0.001% Triton X-100. The intensity of the first wave, Ill, is constant for pH c 9 while the peak intensity, Ipl, decreases for pH > 6.5. The I,, and Ip2 values of the second process remain constant for pH < 9. The half-wave and peak potentials of the first process shift towards negative potential as the pH is increased while in the second process they remain unchanged (Fig. 2). Accordingly, for pH > 9 a single wave or peak is observed; the intensity of the dc wave is the sum of the values of I,, and I,, (Fig. 3a). In alkaline medium (0.10 M NaOH), initially a single wave or peak is observed, whose intensity decreases progressively over time; at the same time, an anodic process appears (E = -0.50 V) accompanied by two new cathodic processes (E = - 0.72 V and E = - 1.60 V) (Fig. 3b). It is known that in this medium Guthion is
b
I
0.2 /JA
I
0.40
0.80
0.40
0.80
1.20
. -E/V
Fig. 1. Polarograms (dc and DP) of Guthion. 2.1X10m4 M Guthion, 0.12 M B&ton-Robinson, MeOH/H,O (v/v) and variable amounts of NAOH. (a) pH 1.9; (b) pH 5.1.
20%
224
1.20- EP/V
-0
--o-~-~-~-cl-o-oQo-o
l.OO-
/A’
0.80-
/A AHA
0.60
-
AYA’ /A
.A’
c 2.0
4.0
6.0
6.0
10.0
PH
Fig. 2. Variation of the peak potential with pH. 2.1 X 10m4 M Guthion, 0.12 M Britton-Robinson, 20% MeOH/H20 (v/v) and variable amounts of NaOH. (A) Wave Z,,; (0) wave Zp2; (0) wave Zpl + Zpz.
hydrolysed, giving rise to its major products dithiophosphate, o-aminobenzoic acid [7]: s
and
COOH
II ,OCH,
HS-P,
formaldehyde
+
CHzO
+
OCH,
NH2
The anodic process is attributed to the oxidation of Hg in the presence of dithiophosphate; the cathodic process appearing at -0.72 V corresponds to the reduction of the mercury dithiophosphate formed and the most negative one to the reduction of formaldehyde.
a
b
.
I
0.40
1
0.80
I
1.20
. . _ _. .
I
1.60
0.40
I 0.80
1.20
1.60 - E/V
Fig. 3. Polarograms (dc and DP) of Guthion. (a) pH 9.1; (b) 0.10 M NaOH. For experimental see Fig. 1.
conditions
225
Reversibility and determination of the number of protons involved Owing to the poor definition of wave I,, in the absence of Triton X-100, logarithmic analysis was carried out only for wave I,1 for the 2.0 < pH < 7.0 range. The value of an found was 1.35 & 0.06, which indicates the existence of an irreversible electrode process. The relationship found between El,? and the pH for wave I,, is Q2 = -0.432 - 0.058 pH. Using this relationship and knowing the value of an, it was possible to determine the number of protons involved in the controlling electrode process; this proved to be one. Study of the effect of different variables on the shape of the polarographic waves of Guthion This study was performed at pH 4.3 since this medium is the most suitable for determining the behaviour of waves 1 and 2. The percentage of methanol in the medium MeOH/H,O (v/v) was modified between 10 and 50%. It was found that an increase in the proportion of alcohol led to a slight decrease in 1,i and Ipi. For process 2, a decrease was observed in the maximum and minimum shown by this wave, in dc and DP polarography, respectively, together with a noticeable loss of sensitivity in DPP. Later work was therefore carried out with a MeOH/H,O ratio of 20% (v/v). The influence of temperature was studied between 20 and 45 o C. The I,, wave was seen to have a temperature coefficient of 1.54%. Upon increasing the temperature, the maximum of the I,, wave also rose such that it became necessary to add Triton X-100 at 0.001% to measure it. Under these conditions, the I11 wave had a temperature coefficient of 1.66% and the I, wave 0.98%. Variation of the ionic strength between 0.15 and 0.70 A4 (NaClO,) caused no significant modifications in the values of the intensities and potentials of the two processes, either in the absence or presence of Triton X-100. The relationships found between I,, and Ipl and the drop time (Table 1) show that process 1 is diffusion-controlled. For drop times lower than 1.0 s wave 2 does not exhibit polarographic maxima (for 1.28 X 10e4 A4 Guthion). The decrease in these maxima observed upon decreasing the drop time and temperature seems to indicate that one is dealing with TABLE 1 Relationships between intensities and drop time, (v/v), 7’* = 25 o C, pH 4.3)
i = kt” (1.28 X 10e4 M Guthion, 20% MeOH/H*O
Process
Polarogapbic technique
X,P
Corr. coefficient
(1)
dc DP
0.172 0.668
0.999 0.998
(2)
dc DP
0.254 0.348
0.993 0.996
226
4.0
20
2.0
4.0
2.0
6.0
4:o 104 C/M
6:O
Fig. 4. Variation in the intensity of the II1 wave with the concentration of Guthion. pH 4.3, 20% MeOH/H,O (v/v), Ta = 25O C. (a) t = 2 s; (0) dc; (A) DPP. (b) t = 0.8 s; (@) dc; (A) DPP.
maxima of the first kind, originating from stirring of the layer of solution closest to the electrode [8]. The variation in IP, with the pulse amplitude ( - 10 to - 100 mV) is in keeping with the theoretical predictions for a diffusion process [9]. The intensity of Ipz does not reach a constant value on increasing - AE, which suggests that the second process is not diffusion-controlled Influence of concentration This part of the study was performed using dc and DP polarography for drop times of 2.0 and 0.8 s in the presence and absence of Triton X-100. For wave 1, the
0.40
2.0
4.0
6.0
2.0 104
4.0
6.0
c/M
Fig. 5. Variation in intensity of the I,, wave with the concentration of Guthion. pH 4.3, 20% MeOH/H,O (v/v), Ta = 25OC, 0.001% Triton X-100. (a) f = 2 s; (0) dc; (A) DPP. (b) 2 = 0.8 s; (6)) dc; (A) DPP.
221 TABLE 2 Calibration data (20% MeOH/H,O (v/v), pH 4.3, Ta = 25 o C; A E = - 50 mv). Sn, S,,, and S, are the blank, slope and intercept standard deviation, respectively, i is the intercept, m the slope, and t, the r distribution value (a = 0.005) Process
(1)
Polarographic technique
t/ s
Slope /PA mM_’
Intercept /CA
dc
2.0 2.0 0.8 0.8
2.51 kO.02 10.12* 0.10 2.15 f 0.02 4.65 f 0.08
- 0.002 f 0.015 f - 0.005 f 0.006 f
2.0 2.0 0.8 0.8
1.85+0.03 1.20f0.03 1.78 f 0.10 1.15 *o.os
DPe dc DP (2) *
dc DP
r
106 CL a/M ;;A
b
E
d
0.0004
0.005 0.007 0.005 0.020
0.999 0.999 0.999 0.997
0.0002 0.0006 0.0004
0.48 0.06 0.84 0.26
0.49 0.06 0.85 0.21
6.00 2.08 7.03 12.90
-0.010*0.006 0.018 f 0.006 - 0.035 f 0.020 - 0.020 f 0.020
0.998 0.996 0.993 0.993
0.0006 0.0004 0.0007 0.0005
0.97 1.00 1.18 1.30
1.01 1.06 1.23 1.57
9.78 15.07 33.89 52.32
* CL: concentration limit. b 3$/m. ’ 3Sn/(m - faSm). d 3[Si + SF +(i/m)zS~]‘/2/m. e Concentration lower than 1.6 X 10V4 M. * In the presence of 0.001% Triton X-100; concentration lower than 3.2 x 10m4 M.
relationship found between In and the concentration is linear over the entire concentration range studied, both for drop times of 0.8 and 2 s. In DPP, a decrease in the drop time was seen to broaden the range of linearity (Fig. 4). It is recommended that in order to measure wave 2 with precision 0.001% Triton X-100 be added; in its absence, it is only possible to determine concentrations lower than 5.0 X 10d5 and 1.0 X lob4 A4 for drop times of 2.0 and 0.8 s, respectively. In the presence of Triton X-100, the relationships found between intensity and concentration in both dc and DP polarography are not linear and show a shape that indicates an adsorption process. Moreover, a decrease in the drop time does not increase the range of linearity (Fig. 5). The detection limits calculated according to the criteria of Winefordner and Long [lo] are shown in Table 2. The limits are improved in DPP; the best wave for analytical purposes is 1 in view of its better definition and its wider range of linearity. However, the analytical use of one or the other wave would depend on the presence of other interfering species in the solution. Coulometric study This study was conducted using a mercury pool as the working electrode. When electrolysis was carried out at a potential of - 0.75 V, the decrease in the current of wave 2 was parallel to that of wave 1, indicating that the product of wave 1 has undergone a chemical reaction; simultaneously, two anodic waves appear. The species obtained in this coulometry evolve over time (in the presence of oxygen) and a decrease can be observed in the anodic waves and the reappearance of cathodic waves.
228
Fig. 6. Coulometricstudy. Evolution of the polarograms of Guthion as a function of the electrolysis time. pH 4.3, 20% MeOH/H,O (V/V), T* = 25 o C. (a) E = -0.75 V, (b) E = - 1.15 V. The numbers in the plots refer to the electrolysis times in min.
If electrolysis is carried out at a potential of - 1.15 V, the two cathodic waves disappear and at the end of the coulometry only two anodic waves are seen. The reaction products also evolve with time towards other products which exhibit two cathodic waves at potentials close to those of unreduced Guthion. When the electrolysis was interrupted to record the polarographic curves (always in the absence of air) it was observed that at the controlled potential of -0.75 V the electrolysis requires more time than that effected at -1.15 V for total reduction to be reached (Figs. 6a and 6b). If electrolysis at -0.75 V is not interrupted and the absorption spectra of the solution are recorded at different electrolysis times, a decrease is noted in the absorbance when the electrolysis time is increased over the first 80 min. If coulometry is prolonged (bubbling N, through the cell) an increase can be observed in the absorbance (Fig. 7a). This fact indicates that after 80 min of electrolysis a new electroactive species is regenerated which absorbs in the UV zone. This mechanism would imply that the intensity in the coulometry should also undergo an increase unless its efficacy decreases due to deposition on the working electrode [ll]. When electrolysis was conducted at higher concentrations (9.4 X 10e4 M), the formation of a black layer over the mercury pool was observed.
229
b A
1.5(
l.O(
0.5c
2&o
250
360
A/nm
300
hfnm
Fig. 7. Spectra of G&ion and the products obtained in electrolysis. pH 4.3, 20% MeOH/H,O (v/v), Ta = 25 o C. (a) E = -0.75 V; (b) E = - 1.15 V. The numbers in the plots refer to the electrolysis times in min. (- - -) Spectrum after 15 h of contact with air of the products obtained in coulometry at - 1.15 v.
If the same experiment is carried out at a potential of - 1.15 V, a decrease in the absorbance is observed parallel to the increase in electrolysis time; after 80 min of electrolysis, the absorption spectrum remains constant. However, if the solution is placed in contact with air, after a while the spectra resemble those of unreduced Guthion (Fig. 7b). It may be inferred that the electrolysis products at -0.75 V are subject to a chemical reaction (in the absence of oxygen) that generates species which are again reduced at that same potential. However, the products obtained at - 1.15 V are regenerated only in the presence of oxygen. Determination of the number of electrons exchanged The study of the i-t curves at -0.75 V indicates that two (1.97 f 0.25) electrons are exchanged in this process. The In i-t plot is only linear for short periods of time (less than 15 min), which shows that only for times longer than this is it possible to observe the chemical reaction of the products of the electrochemical reaction. If the potential imposed is - 1.15 V, the In i-t plot is not linear, although the values of the intensities of the I,, and I,, waves are similar. It may be concluded
230 TABLE 3 Relationship between the intensities of Phosmet (IF) and Guthion (In) 50% MeOH/H,O (v/v), t = 2 s)
(dc polarography,
10s c/M
Zu/pA
IF/PA
zll /zF
2.0 3.0 4.0 5.0 6.0
0.047 0.073 0.101 0.129 0.155
0.053 0.079 0.106 0.132 0.159
0.89 0.92 0.95 0.98 0.97
that the number of electrons exchanged at a potential of -1.15 corroborate these findings, the values of the 1,, intensity of Guthion with those of the organophosphorus pesticide Phosmet. In 0.10 MeOH/H,O (v/v) medium, Phosmet exhibits a cathodic wave due of the carbonyl group, and hence the number of electrons exchanged relationships found between the Phosmet wave and the 1,i intensity shown in Table 3 for the same medium.
0.10 M HCl,
V is four. To were compared M HCl + 50% to the reduction is two [12]. The of Guthion are
Studies carried out with the products obtained in coulometry at - 1.15 V The solution obtained by coulometry at -1.15 V gives a positive reaction of amines (primary and secondary) with p-dimethyl aminobenzaldehyde. In the presence of oxygen, it evolves progressively, as shown in the polarograms obtained at different times: the appearance of two cathodic waves can be seen at the same time as the gradual disappearance of the anodic waves. For times longer than 150 h, a
b
0
0.20
0.60
1.00
0.20
0.60
1.00
0.20
0.60
Fig. 8. Polarograms of Guthion. pH 4.3, 20% MeOH/H,O (v/v), T’ = 25OC. (a) Unreduced; and (c) 9 days, respectively, after terminating coulometry at - 1.15 V.
1.00
-E/V
(b) 14 h
231
decrease can be observed in the more cathodic wave, and from 220 h onwards the polarograms do not show any modifications in their shapes (Fig. 8). In one solution, at 216 h after coulometry at - 1.15 V the solvent was removed and the dry residue was extracted with CHCl,. The IR spectra of the chloroform extract show a band at 1700 cm-’ characteristic of the C=O group. In the NMR spectra, in deuterated chloroform, splitting of the aromatic hydrogens can be observed; the most characteristic feature of these spectra is the presence of methoxyl and methylene groups. When coulometry is performed in ethanolic medium instead of in methanol, the NMR spectrum reveals the presence of the ethoxyl group.
DISCUSSION
In view of the results obtained, it may be concluded that the electrochemical reduction of Guthion takes place in two steps, each involving two electrons. The electrochemical process taking place at the potential of the first wave (- 0.75 V) can be attributed to the reduction of the -N=N-group according to the following scheme: 5
0
II
N-CH2
,OC’-‘,
- S-pP'OCH3 + H+ + e-
i
(B)
-
(1)
(A)
(5)
+ e- +
(2)
l-l+ ___f
0
(C)
+
H,O
-
+
CH,O
+
HS-!<;;;:
(3)
(D)
The products generated in the coulometry performed at -0.75 V are unstable and, like the hydrazo compounds [13], undergo dismutation reactions to generate azo and diamino groups.
(4)
232
Moreover, the amino compound (F) is oxidized progressively in the presence of air until it forms an a20 group again: 0
0
02
(5)
NH2
(F)
This mechanism would explain why the reaction at -0.75 V requires more time than at -1.15 V, since partial regeneration of the oxidized form occurs. It is similarly possible to account for the experimental observation that cathodic waves reappear after the solution enters into contact with atmospheric oxygen. The formation of a black layer over the mercury pool is due to the chemical reaction between the dithiophosphate generated in reaction (3) and the electrode. In the reduction at a potential of -1.15 V, one can propose breakage of the -NH-NH of the hydrazo compound formed in the first electrochemical process: 0
0
+
CD)
2e-
+
2H+-
(6) (G)
Moreover, the species generated in the coulometry at - 1.15 V gives a positive reaction of primary and secondary amines, although like other hydrazines it is not stable and in the presence of oxygen undergoes oxidation reactions, reordering and condensations [14,15]. Experimentally, it was observed that freshly prepared solutions of phenylhydrazine exhibit an anodic wave that decreases with time; at the same time, two cathodic waves appeared at -0.72 and -0.92 V; however, after 48 h neither cathodic nor anodic waves were observed. This kind of behaviour is in agreement with the mechanism proposed by Ring and Bard [14], according to whom phenylhydrazine undergoes oxidation by oxygen to form an azo group which in acid solutions decomposes slowly. In the lifetime of the azo compound, reduction waves appear but later both the oxidation and the reduction waves disappear. Therefore, although the products of coulometry at - 1.15 V show an electrochemical behaviour identical to that of the hydrazines during the first 20 min after obtaining them (Fig. 8), the oxidation of compound G alone would not justify the electrochemical and spectroscopic behaviour of the products generated after coulometry. The band appearing in the IR spectrum at 1700 cm-’ suggests the presence of carbonyl in a five-membered ring. Thus, it may be proposed that at least the following reactions take place:
(7)
(8)
233
The presence of the azo compound generated in reaction (8) would explain why after coulometry at -1.15 V the products in contact with atmospheric oxygen exhibit, after a certain time, a reduction wave instead of the two waves shown by unreduced Guthion. Even so, reactions (7) and (8) do not justify the NMR spectroscopic behaviour of these products for times longer than 216 h after coulometry at - 1.15 V. This reaction is now under investigation. ACKNOWLEDGEMENT
This research was supported by the CAICYT (Project No. 2999/83). REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
M.R. Smyth and J.G. Osteryoung, Anal. Chim. Acta, 96 (1978) 335. P. Nangniot, Rech. Agron. Gembloux, 3 (1968) 182. R. Engst, W. Schnack and H. Woggon, Z. Anal. Chem., 207 (1965) 30. K. Dulak, J. Kovk and M. Michalek, Z. Anal. Chem., 195 (1963) 350. J. Hem&ndez Mendez, R. Carabias Martinez and J. Sanchez Martin, Anal. Chem., 58 (1986) 1969. J.A. Bates, Analyst, 87 (1962) 786. E. Morifusa, Organophosphorus Pesticides: Organic and Biological Chemistry, C.R.C. Press, Cleveland, OH, 1974, pp. 66-67. S. Wolf, Angew. Chem., 72 (1960) 449. A.M. Bond, Modem Polarographic Methods in Analytical Chemistry, Marcel Dekker, New York, 1980, p. 249. J.D. Winefordner and G.L. Long, Anal. Chem., 55 (1983) 712A. M.D. Morris, Anal. Chem., 39 (1967) 476. R. Carabias Martinez and M.E. Gonz&lez Lopez, personal communication, 1985. T.M. Florence, D.A. Johnson and G.E. Batley, J. Electroanal. Chem., 50 (1974) 113. D.M. King and A.J. Bard, J. Am. Chem. Sot., 87 (1965) 419. I.M. Kolthoff and P.J. Elving, Treatise on Analytical Chemistry, Part 2, Vol. 15, Wiley, New York, 1976, p. 268.