Investigation of the reactivity of propanil using cyclic voltammetric methods

*Co resp nding uthor Abstract: The stability and hence the reactivity of the herbicide, propanil [N-(3,4-dichlorophenyl) propionamide], strongly depends on pH of the medium and freshness of the solution. According to cyclic voltammetric investigation, propanil is fairly stable in mixed water/ethanol medium between pH = 4 and pH = 7 for a period of ten weeks. However, in strongly acidic medium (pH = 1), its degradation which results in 3,4dichloroaniline (3,4-DCA) follows first order kinetics with an apparent rate constant of 5.33×10-8 s-1, while in strongly basic medium (pH =13), it undergoes faster degradation. 3,4-DCA is electroactive and its fresh solutions mimic the behaviour of propanil after degradation. Adsorption characteristics of propanil are also pH dependent, and its main degraded product, 3,4-DCA, is irreversibly adsorbed onto glassy carbon surfaces in an extremely basic medium.

Many analytical methods have already been developed for the detection of propanil and its residues.Gas chromatography with nitrogen-phosphorus 6 , electron capture 7 and mass spectrometric detection 8 , liquid chromatography with diode array detection 9 , various immunoassay techniques 10,11 and amperometry based on catalytic reduction in the presence of metalloporphyrins are a few modern methods that have been recently reported for the detection of propanil in different types of sample matrices.Solid-phase microextraction steps have also been coupled with either chromatographic detection or enzyme-linked immunoassay 14 to improve sensitivity of the method 12 .Furthermore, electroanalytical methods have been developed to detect the primary residue of propanil, 3,4-DCA, using modified electrodes 15 .
Although detection methods for propanil and its residues have been significantly advanced in the recent past, not much attention has been focused on the investigation of kinetics of degradation and adsorption aspects.Such studies would be essential to understand the fate of propanil in the environment.Among reported studies, photodegradation of propanil in water and soil under normal environmental conditions, as well as in aqueous solutions under simulated solar irradiation with a photocatalyst 16,17 , adsorption-desorption characteristics in soil-fly ash mixtures through isotherm analysis 18 , and electrochemical oxidation of propanil and related N-substituted amides through voltammetric methods have been attractive.Additionally, electro-catalytic action of organo halides on metallic surfaces and mechanistic investigation of the reductive cleavage of the carbonhalogen bond have been recently reported 20,21 .However, determination of the fate of propanil requires continuous monitoring of the levels of propanil in the environment to reach a firm conclusion.
The objective of this research was to employ cyclic voltammetric methods for the detection of propanil, in order to investigate the stability and reactivity of propanil together with its adsorption characteristics over a wide pH range of 1.0 to 13.0.The conclusions of such studies would be important, not only for the development of kinetics and adsorption models to be extended for the prediction of the fate of propanil in the environment, but also for updating the tolerance limits and risk assessment of this heavily used anilide pesticide.

Sample preparation:
All aqueous electrolyte solutions were prepared in 20 % ethanol using freshly distilled water.All electrochemical experiments were carried out under N 2 atmosphere.The pH was measured with an Orion Model 420A meter, equipped with an Orion pH electrode (Thermo Orion, USA).
Instrumentation: Cyclic voltammetry was performed with a CV-1B cyclic voltammograph and recorded on a Model 240 R X-Y recorder (both from Bioanalytical Systems, USA).All potentials were reported with respect to the saturated calomel reference electrode (SCE).Glassy carbon (GC) electrode and the platinum wire electrode served as the working and the counter electrodes, respectively.

rESuLtS And dIScuSSIon
Cyclic voltammetry of 1.0×10 -3 mol dm -3 propanil, freshly prepared in pH = 7 phosphate buffer (0.1 mol dm -3 ) in ethanol/water (1:4) solvent system, indicates that propanil is electrochemically active, showing a strong oxidation peak centred at +1.45 V (Figure 2a).Due to the low solubility of propanil in distilled water, a mixed solvent system (ethanol/water 1:4) was used in this study.It is also important to state that the medium is still 80 % water, and hence pH corrections to account for solvent effects are not necessary.Careful voltammetric characterization experiments conducted at regular time intervals for a period of ten weeks showed that the major oxidation peak changed neither its position nor its magnitude significantly at pH = 7 (Figure 2b).Similar results were also observed in solutions of pH between 4 and 7 suggesting that propanil is fairly stable within these pH values, which is in agreement with our previous studies 15 .
However, drastic changes in the voltammetric behavior were observed beyond this pH range.Fresh solutions of propanil at 1.0×10 -3 mol dm -3 concentration level did not show any electrochemical activity within the working potential range at pH = 1 (Figure 3a), indicating the electroinactivity of propanil at this pH.This is probably due to the interference of the oxidation peak of propanil with the solvent breakdown process.However, one irreversible oxidation peak, and one oxidation/ reduction couple appeared after one week, indicating the change in reactivity of propanil with exposure to very low pH.Monitoring the activity of propanil for longer periods at pH = 1 indicated that the intensity of the peaks, which appeared after one week, significantly increased with time (Figure 3b).
Further electrochemical investigation reveals that the intensities of these peaks were decreased during continuous scanning due to the decrease in the concentration gradient under unstirred condition, as expected (Figure 4).However, no chemisorption effects were encountered at this pH because thorough rinsing of the electrode with water and efficient stirring of the electrolyte solution were able to regenerate the electrochemical activity as observed in the first scan.
Interestingly, electrochemical behaviour of propanil after 10 weeks and that of a fresh solution of 3,4-DCA are equivalent at pH = 1 (Figures 3b and 5).It is therefore suggested that propanil undergoes slow degradation yielding 3,4-DCA under extremely acidic conditions.Consequently electrochemical characteristics of propanil observed under such condition are attributed to the electroactivity of 3,4-DCA.Detailed investigation of cyclic voltammetric features in Figure 3b indicated that peak 3 did not appear in the first scan which was started from the cathodic end although peak 1 appeared, followed by peak 2. Thus the product of the oxidation reaction associated with peak 1 is the reactant for the process associated with peak 2 followed by subsequent oxidation giving rise to peak 3. It is suggested that headto-tail oxidative coupling of 3,4-DCA, the degraded product of propanil as shown below, is responsible for      time.Furthermore, the electrochemical behaviour of a fresh solution of 3,4-DCA at pH = 13 is very much similar to the electrochemistry of a fresh solution of propanil at the same pH (Figure 7b).Therefore, it is suggested that propanil undergo base hydrolysis due to the nucleophilic attack of OH -at the carbonyl position of propanil forming 3,4-DCA.According to electrochemical observations, base hydrolysis is faster, and the electroactivity of propanil observed for a fresh solution is due to the presence of 3,4-DCA.Further, the presence of different anionic constituents, namely KCl/NaOH mixture (both at 0.1 mol dm -3 ), does not alter the electrochemistry of propanil solutions at pH =13.
Continuous cyclic voltammetric scans recorded at pH = 13 indicate that the intensity of all the oxidation peaks is drastically decreased even at the second scan (Figure 7a).However, in contrast to the observations made at pH = 1, rinsing the electrode and stirring the solution did not reproduce the initial voltammetric behaviour, indicating that the electrochemically degraded product, 3,4-DCA, which is formed rapidly, would have strongly adsorbed on to the glassy carbon surface during the first few potential scans.An attempt to make electrochemical cleaning of the surface through application of the starting potential for a period of few minutes prior to the potential scan also failed to reproduce the voltammetric behaviour, confirming irrerversible adsorption of 3,4-DCA, or electrochemical degradation of 3,4-DCA forming electroinactive compounds.Such observations lead to the suggestion that irreversible adsorption of the degraded products of propanil take place at slower rates under real environmental conditions even at moderate pH values.
Adsorption characteristics of propanil were further investigated by analyzing the variation of peak current (i p ) of the oxidation peak at +1.50 V (pH = 13) with the potential scan rate (ν).The plot of log (i p ) vs. log (ν) results in a slope of 0.7 at low scan rates and of 0.4 at high scan rates (Figure 8).This clearly suggests that the oxidation process at low scan rates is a combination of adsorption and diffusion.However, increase in the scan rate would change the rate-limiting step of the oxidation process as diffusion-controlled, although strong adsorption may appear as a follow-up step.

concLuSIon
[N- (3,4-dichlorophenyl)propionamide], propanil, is fairly stable between pH = 4 and pH = 7, and its electroactivity within this pH range remains unchanged for a period of ten weeks.However, it undergoes degradation producing 3,4-dichloroaniline (3,4-DCA) in strongly acidic or basic media, and the rate of degradation strongly depends on the the oxidation peak at +1.0 V (peak 1).The product of the above oxidation reaction would undergo the following reversible process giving rise to peaks 2 and 3. A similar mechanism has already bean proposed for p-chloroaniline at pH < 4 using an electrochemical method 22 .The kinetics of the degradation process of propanil at pH = 1 can be followed by monitoring the rate of increase in the current of the major oxidation peak at +1.0 V, which is due to the presence of 3,4-DCA.This reaction follows first order kinetics according to the linear plot between the logarithmic peak current and the time of measurement (Figure 6).The slope of the plot, the apparent rate constant associated with the degradation process, is determined to be 5.33×10 -8 s -1 .This demonstrates that the reactivity of propanil can be indirectly monitored by observing the electrochemical activity of the degraded product, 3,4-DCA.
The behaviour of propanil in basic medium is much different from that in acidic medium in many respects.Fresh solutions of propanil at pH = 13 result in three irrerversible oxidation peaks at +0.47 V, + 0.80 V and +1.50 V (Figure 7a).However, unlike at pH = 1, electrochemical activity is not significantly altered with pH of the medium.It is slow at pH = 1 with an apparent rate constant of 5.33 × 10 -8 s -1 , while the degradation is faster at pH = 13 due to the nucleophilic attack of OH - at the carbonyl position.Electrochemical features of a fresh solution of 3,4-DCA and a solution of propanil ten weeks after preparation are equivalent at pH = 1.Nevertheless, electrochemical activity of fresh solutions of 3,4-DCA at pH = 13, which is markedly different from that at pH =1, is similar to the activity of a fresh solution propanil at the same pH.It is therefore concluded that the electroactivty of propanil at extreme pH values is due to the presence of 3,4-DCA, and not due to propanil.Similar studies would lead to the understanding of the fate of pesticides in the environment through design of models for kinetics, adsorption/desorption, and pesticide degradation pathways.

Figure 1 :
Figure 1: The structure of propanil

Figure 4 :
Figure 4: Continuous scanning responses of 1.0×10 -3 mol dm -3 propanil in pH = 1 KCl/HCl (0.2 mol dm -3 ) under N 2 saturated at a scan rate of 50 mV s -1 recorded one week after solution preparation.Arrows indicate the direction of the shift during scanning.

Figure 6 :
Figure 6: The variation of the logarithmic peak current for the oxidation of propanil at pH = 1 with time (The regression coefficient, R 2 = 0.95812).

Figure 8 :
Figure 8: The plot of log (i p /µA) vs. (log (ν/mV s -1 ),where i p is the peak current and ν is the potential scan rate, for the oxidation of propanil at pH = 13.