Spectrophotometric determination of Isoproturon and Metoxuron using ethylacetoacetate and application to technical and formulation grade samples

Talanta Talanta 43 (1996) 577-581

Spectrophotometric determination of Isoproturon and Metoxuron using ethylacetoacetate and application to technical and formulation grade samples’ K. Ramakrishnam Indian

Raju, S.R.K.M. Institute

of Chemical

Akella, J.V.S. Mm-thy, U.T. Bhalerao* Technology,

Hyderabad

500 007, India

Received12 May 1995; revised25 September1995;accepted5 October 1995

Abstract A simple and rapid spectrophotometric method for the determination of Isoproturon and Metoxuron is described based on alkaline hydrolysis of the compounds to their corresponding primary amines, followed by diazotization and coupling with ethylacetoacetate in alkaline medium. The chromogenic speciesobeys Beer’s law up to 15 and 9 pg ml - ’ for Isoproturon and Metoxuron respectively. The method is successfullyapplied for technical and formulation samples with RSD in the ranges 0.48-0.72, 0.86-1.32 and 0.66-0.74, 0.27-0.69 for technical and formulation grade samples of Isoproturon and Metoxuron respectively. The advantages over the earlier methods are discussed. Keywords:

Spectrophotometry; Isoproturon; Metoxuron

1. Introduction

Among the phenylurea herbicides, N,Ndimethyl-N’-[-( 1-methyl ethyl) phenyl] urea and N’ - (3 - chloro - 4 - methoxy phenyl) - N,N - dimethyl urea popularly

known as Isoproturon

(IPN) and

Metoxuron (MTN) respectively, are widely used for the control of different weed species in cereals and carrots. The need for a simple method for their routine analysis and quality evaluation in technical and formulation grade samples is acknowledged. Titrimetric [l], GC [2] and UV spec* Correspondingauthor. ’ IICT CommunicationNo. 3538.

trophotometric [3] methods are available for this purpose and are unsuitable for routine use for the reasons previously outlined [4,5]. The use of coupling reactions of diazo derivatives of primary amines with phenols, amines and carbanions is known for their analysis. Coupling reactions with carbanions have been by far the least explored, and in particular

been reported. hydrogen

to pesticides has not

with an active

is a simple useful carbanion-

yielding reagent that can be used for coupling with diazonium salts [6-S]. During further investigations on analytical methods, we developed yet another simple and rapid spectrophotometric method based on the coupling reaction of ethyl-

0039-9140~96/$15.00 0 1996ElsevierScienceB.V. All rights reserved SSDI 0039-9140(95)01773-9

atom,

application

Ethylacetoacetate,

578

K. Ramakrishnam

Raju

et al. / Talanta

acetoacetate, a common chemical, in strong alkali medium with the diazonium salt obtained after hydrolysis and subsequent diazotization and the results are presented here.

2. Experimental 2.1. Apparatus

A Parkin-Elmer spectrophotometer was used.

Model Lambda-2 UV-Visible with 1 cm matched quatrz cells

2.2. Reagents and materials

All the chemicals used were of AnalaR grade. The following solutions were prepared: 0.5 N and 0.8 N sulfuric acid (Loba Chemicals, Bombay) in water; 1.0% w/v sodium nitrite (Qualigens, Bombay) in water; 5.0% sulfamic acid (Loba Chemicals, Bombay) in water; 0.5 and 5.0% v/v ethylacetoacetate (Reidel, Germany) in methanol; 50.0% w/v sodium hydroxide (Qualigens, Bombay) in water; 0.5 N sodium hydroxide in methanol. Standard IPN was supplied by M/s Bharat Pulverising Mills, Bombay and used as received. MTN was supplied by the Pesticides Division, IICT, Hyderabad and was used after recrystallization in hot water. 2.3. Preparation

of standard herbicide solutions

Standard herbicide solutions were prepared by dissolving 80 mg herbicide in methanol and diluting to 50 ml in a standard flask from which 3.0 ml was taken and diluted further to 50 ml with methanol and then used as working solutions. 2.4. General procedure

A 1.0 ml portion of working sample solution was transferred into a 10 ml standard flask to which 1.0 ml 0.5 N methanolic sodium hydroxide was added and allowed to stand for 5 min to ensure complete hydrolysis. After evaporating the solvent, 1.25 ml 0.8 N and 0.75 ml 0.5 N sulfuric

43 (1996)

577-581

acid were added for IPN and MTN respectively and the flasks were heated in a water bath for about 30 min (IPN) and 5 min (MTN). The flasks were then removed from the water bath and cooled to room temperature after which 1.0 ml and 0.75 ml of 1.0% sodium nitrite solution were added for IPN and MTN respectively and the flasks allowed to stand for 5 min. Then 0.5 ml (IPN) and 0.25 ml (MTN) 5.0% sulfamic acid solution were added, the flasks were stood for 5 min, 1.0 ml 0.5% ethyl acetoacetate solution and 2.0 ml 50.0% sodium hydroxide solution were added to each flask and distilled water was added to 10 ml. The absorbances of the solutions were measured at 398 nm (IPN) and 400 nm (MTN) against distilled water blank.

2.5. Technical grade samples

About 80 mg of technical grade sample was weighted accurately and dissolved in 50 ml methylene chloride. The solution was treated with 1 N hydrochloric acid and the organic layer was evaporated. The residue was dissolved in methanol and diluted to 50 ml in a standard flask. A 3.0 ml portion of solution was further diluted to 50 ml with methanol and then 1.0 ml was extracted and subjected to analysis using the general procedure.

2.6. Formulation

samples

About 160 mg and 110 mg of formulations of IPN and MTN respectively were weighed accurately, 50 ml of methylene chloride was added and the samples were centrifuged. The organic layer was evaporated after treatment with 1 N hydrochloric acid. The residue was dissolved in methanol, diluted to 50 ml, and 3.0 ml portions of the solutions were diluted to 50 ml with methanol and 1.0 ml was used for subsequent analysis using the general procedure. Synthetic formulations (80%) as wettable powders for MTN were prepared [9] and analysed owing to the non-availability of commercial formulations.

K. Ramkrishnam

Raju

et al. 1 Talanta

43 (1996)

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3. Results and discussion

Table 1 Beer’s law, precision and accuracy

3.1. Optimisation

Parameter

IPN

MTN

A,,, (nm) Beer’s law limit (a ml-‘) Molar absorptivity (E) (1 mol-’ cm-‘) Slope Intercept Correlation coefficient Regression equation

398 I5

400 9

1.37 x I04

2.33 x IO“

0.0608 0.0351 0.9995 y = 0.0608x f0.0351

0.0963 0.0192 0.9958 y = 0.0963x +0.0192

of reaction variables

Typical absorption spectra of IPN and MTN are shown in Fig. 1. The reaction variables have been optimised to obtain maximum colour development by varying one variable at a time while keeping the others constant. The optimum concentrations of the various reagents for IPN and MTN respectively are found to be 1.0 ml 0.5 N methanolic sodium hydroxide, 1.25 ml 0.8 N and 0.75 ml 0.5 N sulfuric acid, 30 min and 5 min heating time, 1.O ml and 0.75 ml 1% sodium nitrite, 0.5 ml and 0.25 ml 5% sulphamic acid and 1.0 ml 0.5% ethylacetoacetate followed by 2.0 ml 50% sodium hydroxide. Although methanolic sodium hydroxide brings about rapid hydrolysis [lO,l l] of the present herbicides, methanol affects the colour development

and therefore its evaporation is recommended for further procedure after hydrolysis. Variations in absorption measurements are observed when made against reagent blank. Upon further investigation it is found that the reagent blank is not stable. It is found that the variation in the absorbance of the blank is due to insufficient quantity of sulfamic acid being used to destroy the excess nitrite [12]. However, with the use of further excess of sulfamic acid, although the blank absorption is found to be constant, the absorbance of the sample decreases (vide optimum conditions). However, the absorption measurements for samples are stable and reproducible when made against distilled water blank and hence the present work is carried out accordingly. 3.2. Beer’s law, precision and accuracy

The absorption maxima (&,,,), the molar absorptivities, Beer’s law limits and correlation coefficients are given in Table 1. Although the reaction is fast, 3 min standing time is necessary to obtain reproducible results and the colour is stable for 30 min in both cases. 3.3. Reaction mechanism

0 Wovclcngthlnm)

Fig. 1. Absorption spectra of IPN and MTN: (-) IPN; (-----) MTN. (1.0 ml 0.025% IPN/MTN+ 1.0 ml 0.5 N methanolic NaOH + 1.0 ml 0.8 N H,SO, + 1.0 ml 1.0% NaNO,+ 0.5 ml 5.0% sulfamic acid+ 1.0 ml 0.5% ethylacetoacetate + 2.0 ml 50% NaOH against distilled water blank.)

Compounds containing active methylene groups such as ethylacetoacetate can be used as reagents for the determination of primary amines as it is known that diazo derivatives undergo coupling reactions by means of electrophilic sub-

580

K. Ramakrishnam

CHJ . CO. CH2 COOC2H)

-

01 koli

CH3 X0.

Raju

CH

et al. / Talanta

CC4X2H5

R.&N -

R.N=N.?i.COOC2H5

43 (1996)

Table 2 Assay of IPN and MTN” Sample

co CH3 Colourcd

Where

R i

*(I’

Comparison method

Present method

Recovery RSD (%) (Present method)

99.10 98.90 99.10 98.30 50.00 49.60 49.30

99.50 100.50 99.80 99.60 99.60 100.80 99.20

0.48 0.58 0.37 0.72 0.86 1.03 1.32

93.18 99.13 99.37 80.30 79.60 79.70

99.83 100.17 99.67 99.60 100.20 100.60

0.66 0.73 0.74 0.69 0.27 0.52

(Ref. Ill)

product

or

Qocn3

IPN Technical grade 99.80

3

(IPN)

577-581

,MTk

Scheme 1.

stitution with phenols, amines, and carbanions with active methylene groups [7]. Thus, the authors are also of the opinion that diazo derivatives of the amines formed after alkaline hydrolysis of IPN and MTN [lO,l l] are likely to couple with the carbanions of ethylacetoacetate, as reported earlier [8], yielding the yellow chromogenic species shown in Scheme 1.

Formulation (labelled 50%)

98.40 99.30 98.70 50.20

49.40 49.70

MTN Technical grade 93.30 Formulation

98.90 99.72 80.60 79.40 79.20

a Averages of seven determinations.

3.4. Application

A series (7) of standard samples of IPN and MTN has been analysed and the average percent relative error is found to be 0.74 [1.04, 0.441 for IPN and 0.68 [0.99, 0.371 for MTN at the 95% confidence level. A number of technical and formulation grade samples of IPN and MTN are analysed using the present method and the data are shown in Table 2. The RSDs were in the ranges 0.48-0.72, 0.86-1.32 and 0.66-0.74, 0.270.69 for technical and formulation grade samples of IPN and MTN respectively. The recoveries with respect to a comparison method [l] are also given in Table 2 to indicate the applicability of the method. 3.5. Adoan tages

The present method does not involve time-consuming distillation, thermal and photo-decomposition as reported earlier [l-3]. Furthermore, the present method is superior to our earlier methods [4,5] in terms of the simplicity of the reagent and is much more sensitive than the latter method and comparable with the former. Therefore, the

present method is simple, sensitive, rapid and useful for routine analysis.

Acknowledgement The authors acknowledge M/s Bharat Pulverising Mills, Bombay for the gift of IPN sample.

References [I] 2. Giinthar, Analytical Methods for Pesticides and Plant Growth Regulators, Vol. 8, Academic Press, New York, 1976, p. 417. [2] H. Buser and K. Grolimund, J. Assoc. Off. Anal. Chem., 57 (1974) [3]

[4] [5] [6] [7]

1294.

Indian Standards Institution, Delhi, IS No. 12004, 1987. K. Ramakrishnam Raju, T.N. Parthasarathy and S.R.K.M. Akella, Analyst, 115 (1990) 455. K. Ramakrishnam Raju, T.N. Parthasarathy, S.R.K.M. Akella and U.T. Bhalerao, Talanta, 39 (1992) 1387. S. Bela], E.A. El Neanaey and S. Soliman, Talanta, 25 (1978) 290. A.K. Connors, Reaction Mechanisms in Organic Chemistry, Wiley Interscience, New York, 1973, p. 244.

K. Ramakrishnam

Raju

[8] S.M. Hassan, F. Belal, M. Sharaf El-Din and M. Sultan, Analyst, 113 (1988) 1087. [9] W.V. Valkenburg, Pesticide Formulations, Marcel Dekker, New York, 1973, p. 178. [lo] D. Hartley and H. Kidd, The Agrochemicals Handbook, 2nd edn., Royal Society of Chemistry, London, 1987.

et al. / Talanta

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[I l] C.S.P. Sastry, D. Vijaya and K. Ekambareswara Rao, Food Chem., 20 (1986) 157. [12] M.I. Walash, M. Rizk and A. El-Brashy, Talanta, 35 (1988) 895.