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DETERMINATION OF TRACE DIMETHOATE IN MILK AND RIVER WATER BY KINETIC SPECTROPHOTOMETRY USING MALACHITE GREEN
AND POTASSIUM PERIODATE © 2012 г. S. S. MitiС1, V. V. ZivanoviC2, G. 1. Miletic1, Z. M. Grahovac1, E. T. Pecev1
1Faculty of Natural Sciences and Mathematics, Department of Chemystry, University of Nis Visegradska 33, P.O. Box 224, 18000Nis, Serbia 2The College of Agriculture and Food Technology, Department of Chemistry Cirila i Metodija 1, Prokuplje Received 27.01.2010; in final form 22.06.2011
In the present study, a new sensitive and simple kinetic-spectrophotometric method for the determination of the insecticide dimethoate [O,O-dimethyl-S-(N-methyl-carbomoylmethyl)-phosphoro-dithioate] is developed. The method is based on the inhibition effect of dimethoate on the oxidation of malachite green (MG) by potassium periodate (KIO4) in the presence of Mn(II) ions. Inhibition kinetics of this catalytic reaction was investigated in the presence of dimethoate and the possibility of its analytical application was evaluated. The important variables that affected the reaction rate were investigated and the optimum conditions giving maximum sensitivity were established. Dimethoate was determined with linear calibration graph in the interval from 4.58 to 41.22 p.g/mL. The optimized conditions yielded a theoretical detection limit of 1.24 p.g/mL based on the 3Sb criterion. The RSDs of the method (n = 5) was 1.2—4.9% for the concentration interval of dimethoate from 4.58 to 41.22 p.g/mL. The reaction was monitored spectrophotometrically by measuring the change in absorbance over time at 615 nm. The method was applied to the determination of dimethoate in waters and milk, and was compared with the spectrophotometric method. The quantitive method developed on the basis of inhibition kinetics is practical, fast and economical. For this reason, it is open for new application fields.
Keywords: MG-KIO4, inhibition kinetic, dimethoate, initial rate method and spectrophotometry.
In Fig.1, the chemical structure of dimethoate [O,O-dimethyl-S-(N-methylcarbomoyl-methyl)-phosphoro-dithioate] is presented. Dimethoate is a systemic organophosphorous insecticide, widely applied to crops, trees and ornamental plants. It is also used to control house flies around live stock pens, processing plants and human dwellings as well as grasshoppers on livestock forage. In the enviroment, the pesticide finds its way into the surface water trough agricultural run off and finally into humans through the food grains. Therefore, there is a need for a simple and sensitive method to determine dimethoate and its residue in water to check the hazard for human beings.
Several analytical methods like chromatography [1], polarography [2] and spectrophotometry [3] have been reported for the determination of dimethoate residues. However, those methods suffer from serious drawbacks, like increase in color of blank with increase in concentration of reagent [4], incomplete acid hydrolysis, [5] etc. Since fluorimetry offers high sensitivity in the analysis of organic compounds, several workers [6, 7] have tried to develop a fluorimetric method for the determination of pesticides. However, many pesticides are weak or not at all fluorescent. Ki-netic-spectrophotometric methods are an attractive
alternative for pesticides determination, since they are high sensitive and sufficiency accurate. Besides, sample procedure is simple and less expensive apparatus are used [8]. The most widely used technique to analyze dimethoate and pesticides in general is gas chromatography [9—14]. Many papers reported the determination of dimethoate by HPLC with UV [15—19] or mass spectrometry (MS) detection [20—23]. There are few kinetic-spectrophotometric methods for dimethoate determination [24, 25].
In this paper a sensitive and sample kinetic method for the determination of dimethoate is described. The oxidation of malachite green (MG) with KIO4 in an acetate buffer solution gives a colorless product. The reaction is catalysed by traces of Mn(II) and it is used for its kinetic-catalytic determination [26]. It was also
H S
^y^VCH3
O
Fig. 1. Chemical structure of dimethoate.
observed that small amounts of phenol strongly inhibited the catalysis of this reaction and is was used for their kinetic determination [27]. We was also observed that small amounts of dimethoate strongly inhibited the catalysis of this reaction. The rate of the reaction proportionally decreases with increasing dimethoate concentration. The inhibition effect of dimethoate was used as the basis of the kinetic method for dimethoate determination.
EXPERIMENTAL
Apparatus. The reaction rate was monitored spec-trophotometrically by measuring the rate of change of absorbance at 615 nm. The readings were performed on a Perkin-Elmer Lambda 15 UV/Vis spectrophotometer, connected to a thermo-circulating bath.
A Julabo MP-5A model thermostatic bath ( operating in a temperature range between 20 and 60°C) was used to control the reaction temperature with a ±0.2°C accuracy.
A Hanna instruments pH-meter was used to measure the pH values of the solution. Sigma buffers pH 7.00 ± 0.1 and pH 4.00 ± 0.1 were used for the calibration of the pH-meter. In additional, high precision micropipettes of 50, 500 and 1000 |L (LABMATE) were used for handling or pipetting the solutions. The solutions were thermostated at 25.0 ± 0.2°C before the begining of the reaction.
Reagents. Dimethoate standard was obtained from Dr. Ehrenstorfer (Germany) with a certified purity of 99%. The dimethoate solution (1 x 10-3 M) was prepared by dissolving dimethoate in deionised water.
A stock MG solution (1 x 10-3 M) was prepared by dissolving MG in deionised water. The working MG solution (4 x 10-5 M, Merck) was obtained by diluting the stock MG solution with deionised water, and was used within three weeks of preparation.
The periodate solution (1 x 10-3 M, Merck) was prepared by dissolving potassium periodate in deion-ised water. The acetic acid solution (10 M) was prepared from 99.0% reagent. The Mn(II) solution (1 x x 10-3 M, Merck) was prepared by dissolving adequate amount of manganese chloride in deionised water.
Analytical grade chemicals and deionised water (MicroMed high purity water system TKA Wasseraufbereitungssysteme GmbH) were used for the preparation of all solutions. All the glassware used was washed with aqueous HCl (1 : 1) and then thoroughly rinsed with distilled water, and then finally with deionised water.
All stock solutions were stored in polyethylene containers. The working solutions of Mn(II), perio-date and dimethoate were prepared immediately before the usage.
General procedure for dimethoate determination. In order to obtain good mechanical and thermal stability,
the instruments were run for ten minutes before the first measurement. The reaction was carried out in the following way. In a reaction-mixture vessel with four compartments, the solution of MG was placed in one compartment, KIO4 in the second one, Mn(II) in the third one, dimethoate solution was added in fourth compartment and water was added to total volume of 10 mL. The vessel was thermostated at 25.0 ± 0.2°C. The content was mixed well and then immediately transferred to the spectrophotometric cell with a path length of 10 cm. The change in absorbance was recorded at 615 nm as a function of time every 30 sec over a period of 5—8 min.
The rate of the reaction at different concentrations of each of the reactants was obtained by measuring the slope of the linear part of the kinetic curves to the ab-sorbance-time plot (from Beer's law A = etc, dA/dt = = et(dc/dt), slope = dA/dt, rate = dc/dt dc/dt = = (dA/dt)/st). The calibration graph was constructed by plotting the slope of the linear part of the kinetic curve versus concentration of the dimethoate
(cdimethoate, ng/mL) [28].
Dimethoate determination in waters by kinetic method. Spiked water samples for recovery determination were prepared by addition of appropriate amount of standard stock solution (1 mL = 22.90 | g dimethoate in water) in 100 mL of water and were left to stay for 1 day.
The sample was mixed well, then it was filtered and passed through previously prepared and washed cation column, Levatit S-100. After this treatment, 3 mL of samle were used for the recommended procedure.
Dimethoate determination in milk by kinetic method. Spiked milk samples for recovery determination were prepared by addition of appropriate amount of standard stock solution and were left to stay for 1 day. The milk sample was extracted by ethyl acetate. Extract was evaporated to dryness on the rotation vaccum evaporator at the temperature ranging from 35 to 40°C. The dry residue was dissolved in hexane. Extract was evaporated again, and the dry residue was reex-tracted two times with 25 mL of acetonitrile and evaporated to dryness [29, 30].
Dry residue was carefully eroded from the balloon with deionised water and made up to 100 mL (1 mL = = 10 |g dimethoate). After described treatment, 5 mL of the sample were used for the recommended procedure.
RESULTS AND DISCUSSION
Efect of reaction variables. In order to determine the lowest determinable concentration of dimethoate, the working conditions required optimization. Therefore, the dependence of the rate of reactions on the concentration of each of the reactants was determined.
Slope х 102 5
0.5 1.0 1.5 2.0 2.5 3.0
c(CH3COOH), M
Fig. 2. Dependence of catalyzed (1) and inhibited catalyzed (2) reaction on CH3COOH concentration. Initial
concentrations: MG = 1 x 10-6 M; KIO4 = 1 x 10-4 M;
Mn(II) = 2 x 10-6 M; Dimethoate = 5 x 10-4 M; t = = 25.0 ± 0.2°C.
The effect of the acetic acid concentration on the catalyzed and inhibited reaction was studied in the concentration range 0.5—3.0 M, while the dimethoate concentration was kept constant at 5 x 10-4 M. The influence of acetic acid concentra
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