Determination of paraquat by flow-injection spectrophotometry

Analytica Chimica Acta, 199 (1987) 203-208 Elsevier Science PublishersB.V., Amsterdam

-

Printed

in The Netherlands

Short Communication

DETERMINATION OF PARAQUAT SPECTROPHOTOMETRY

E. CHICO

GUIJARRO,

P. YANEZ-SEDENO

Department of Analytical Chemistry, Madrid, 28040 Madrid (Spain) (Received

13th

October

BY FLOW-INJECTION

and L. M. POLO

Faculty

of Chemistry,

DIl?Z*

Complutense

University

of

1986)

Summary. The flow-injection determination of Paraquat (l,l’-dimethyl-4,4’-bipyridinium) is based on its reduction with sodium dithionite in alkaline medium and detection at 605 nm. Linear calibration plots are obtained for 0.1-1.0, 1.0-10 and 5.0-30.0 mg 1.’ Paraquat, the lower limit being 40 times less than that of the usual spectrophotometric method. The method is applied to determine Paraquat in spiked potable water and potatoes after preconcentration by column ion-exchange. The determination of Paraquat in different herbicide samples yielded results in good agreement with those obtained by polarographic and manual spectrophotometric methods.

Paraquat (l,l’-dimethyl-4,4’-bipyridinium) is an extensively used herbicide. Several methods for its determination have been described, including techniques such as spectrophotometry [l-6] , differential pulse polarography [7-g], gas chromatography [lo-121 and high-performance liquid chromatography [13-161. The spectrophotometric method most often used is based on the reduction of Paraquat to a blue radical by sodium dithionite in alkaline medium [3]. This method involves measurement of absorbance after 15 min and use of a blank prepared from a 4.0 mg 1-l Paraquat solution, which sets the lower limit of calibration graphs. Moreover, the radical is quite unstable under the experimental conditions because of its fast oxidation by atmospheric oxygen, which produces fading in solutions and causes handling problems, mainly in filling spectrophotometric cells. These inconveniences can be minimized by determining Paraquat in a flow-injection system, thus providing a rapid routine procedure.

Experimental Apparatus.

A Tecator model FIAstar 5020 flow-injection analyzer equipped with an injection valve, two peristaltic pumps with four channels (40 rpm) and a spectrophotometric detector was used in conjunction with a FIAstar 5022 recorder. Polarographic measurements were made by the reference method using a Metrohm E-506 Polarecord equipped with a polarographic stand E-505. Electrodes and electrochemical cells are the same as described previously [ 91. 0003-2670/87/$03.50

o 1987

Elsevier

Science

Publishers

B.V.

204

For the spectrophotometric reference measurements, a double-beam digital Pye-Unicam 8-200 spectrophotometer was used with l-cm glass cells. Reagents. An aqueous 100 mg 1-l Paraquat stock solution was prepared from l,l’-dimethyl-4,4’-bipyridylium dichloride (EGA Chemie). Less concentrated solutions were prepared by suitable dilution. A 1% (w/v) solution of sodium dithionite (Merck) in 0.1 M sodium hydroxide was prepared hourly. The cation-exchange resin Dowex 5OW-X8 was used in the hydrogen form. All other chemicals used were of analytical grade. The analyzed samples were commercial Paraquat and Diquat herbicides from ICI (Gramoxone Extra and Gramoxone Reglone mixtures). Potable water and potatoes spiked with Paraquat were also tested. Calibration graphs. The flow system is shown in Fig. 1. Paraquat solution (120 pl), in the 1.0-10.0 mg 1-l range, was directly injected into the carrier stream (distilled water, or 30% (w/v) NaCl solution when preconcentration was by cation-exchange) and the peak height was measured at 605 nm. Paraquat determination in commercial herbicides. In the absence of Diquat, the sample was diluted to obtain the appropriate concentration level. The sample solution was injected directly into the carrier stream (distilled water) and the above calibration graph was used to determine Paraquat. In the presence of Diquat, samples which contained less than 0.80 mg of Diquat dibromide were added together with 2 ml of 2 M sodium hydroxide to a test tube with a screw cap and diluted to 10 ml with distilled water; then the tube was sealed and left for at least 6 h. The precipitate was removed by centrifugation, and aliquots of the supernatant solution were injected into the carrier stream (distilled water). The above calibration graph for Paraquat determination was used. Paraquat determination in potable water and potatoes. To determine Paraquat in potable water, 250 ml of sample was treated with 0.25 g of disodium-EDTA and adjusted to pH 9 with sodium hydroxide. For potatoes, a representative sample was pulped in a blender and 250 g was weighed and transferred to a l-l round-bottomed flask. Next 75 ml of 9 M sulphuric acid was added and refluxed for 30-60 min. After refluxing, the flask was removed and the condensers washed with water. The contents of the flask were filtered through a sintered glass filter (no. 4) and the filtrate transferred to a l-l beaker. Finally 70 ml of 50% (w/v) sodium hydroxide and 3 g of disodium-EDTA were added and the pH was adjusted to 9 with sodium hydroxide.

Fig. 1. Flow diagram for the spectrophotometric determination of Paraquat: S, sample injection, 120 ~1; C, reaction coil, 12 cm long, 0.5 mm i.d.; flow rates in ml min-’ .

To preconcentrate the Paraquat by column cation-exchange, a glass-wool plug was placed at the bottom of a 30 X 1.2~cm column and 3 ml of cationexchange resin in water was introduced followed by another glass-wool plug. Saturated sodium chloride solution (25 ml) was passed through the column, followed by 50 ml of water. Next, the Paraquat sample was run through the column (3 ml min-‘) and the resin was washed successively with 25 ml of water, 25 ml of 2 M hydrochloric acid and 25 ml of sodium chloride solution (1: 10 water/saturated NaCl). All these effluents were rejected. Paraquat was then eluted with saturated sodium chloride solution (0.67 ml min-‘), exactly 50 ml of the effluent being collected. It is advisable to keep the resin in contact with the saturated sodium chloride solution for at least one night before eluting. In both cases, the eluted Paraquat solution was injected into the 30% (w/v) NaCl carrier solution. The calibration graph was prepared by using this carrier as indicated above. Results and discussion Optimization’of the flow system.

The sample (120~1) wasinjected directly into the carrier stream, which was distilled water or a 30% (w/v) sodium chloride solution, and reacted with the 1% (w/v) sodium dithionite reagent (Fig. 1). The effects of changed chemical conditions and manifold parameters were studied over the ranges shown in Table 1, in which the optimum values found for each variable are also listed. Analytical characteristics. The calibration graphs for Paraquat were prepared under the optimum conditions listed in Table 1. For aqueous herbicide samples, with distilled water as the carrier stream, linear plots were obtained for three concentration ranges: 5.0-30.0, 1.0-10.0, and 0.1-1.0 mg 1-l Paraquat. The signal obtained from 5 mg 1-l Paraquat was 418 mV, equivalent to 0.167 absorbance. Typical plots are shown in Fig. 2. The effect of Diquat (l,l’-ethylene-2,2’-bipyridinium) on the Paraquat determination was tested. Interference is caused by the formation of a green radical which also absorbs at 605 nm under the experimental conditions used. However, Diquat can be removed previously by precipitation with sodium hydroxide [ 4, 51 as indicated above. The interference of high salt concentrations in solutions taken as eluents TABLE Results

1 of the optimization

Variable

studies

Range studied

Optimum value

0.6-3.0 0.4-2.8 52-112

2.8 0.6 64

Flow rates (ml min.‘) Carrier stream Reagent stream Coil length (cm)

Variable

Sample

volume

(PI) Na,S,O, (%, w/v) NaOH (M)

Range studied

Optimum value

40-220

120

0.1-2.0 0.02-2.0

1.0 0.10

206

Paraquat

(mg I-‘)

Fig. 2. Calibration graphs for Paraquat: eluted samples with 30% NaCl as carrier.

(0)

aqueous

samples

with

water

as carrier;

(0)

for the cation-exchange procedure was also tested. Saturated ammonium chloride solution, which is the eluent most often recommended in the literature [ 171, provides good recoveries in spectrophotometric determinations of the herbicide at wavelengths near 396 nm. However, this eluent is unsuitable for the flow-injection procedure because it produces acid conditions quite different from the optimum necessary for the reduction reaction, thus decreasing the signal. This effect was avoided by using sodium chloride as eluent, but the recorded signals were increased by the refraction effects of the sample when water was used as carrier. To avoid this, the refractive indexes of the carrier and sample must be as close as possible. Good results were obtained by using a 30% (w/v) sodium chloride solution as the carrier when eluted samples were tested. The interference of calcium(I1) and magnesium(H) ions, which are usually present in water and potato samples, was also tested. Slight increments in the signals were observed for concentration levels between 25 and 400 mg 1-l in both species, probably because of precipitation reactions with the alkaline dithionite reagent. This interference can be avoided by treating the sample with EDTA at pH 9, before introduction to the column. In contrast, interferences from the different materials usually present in aqueous herbicide formulations, such as common surfactants, were found to be negligible after the sample solutions had been diluted to give suitable Paraquat concentration. Determination of Paraquat in commercial herbicides, potable water and potatoes. In the case of commercial herbicides, the accuracy of the method was established by determining Paraquat in commercial herbicides containing Paraquat and Diquat in the usual ratios. Results obtained were statistically compared with those from polarographic [9] and manual spectrophotometric [3] methods using the Student t-test (Table 2). The F values

207 TABLE

2

Determination

of Paraquat

in commercial Paraquat

herbicides

Paraquat present?

Diquat presenta

(% w/v)

(% w/v)

Flow injection

Polarography

texp

Manual spectrophotometry

t exP

20 20 20 8 10 12

I. 2 10 8

22.9 22.0 23.2 8.1 10.3 11.8

22.6 21.2 22.8 7.9 9.8 11.9

+ + t + + +

1.58 2.07 1.66 1.58 2.25 0.63

22.8 22.3 23.4 8.3 9.7 12.1

0.34 1.63 1.26 1.26 1.75 1.90

+ * + f f i

foundb

0.3 0.7 0.2 0.2 0.3 0.3

(% w/v)

aNominal values. bMean of 5 determinations Student t-test: t.,,,.,, = 2.31.

0.3 0.5 0.5 0.2 0.4 0.2

with standard

+ + t+ iI

deviation

0.6 0.3 0.3 0.3 0.7 0.2 and texp from

the

showed no significant differences at the 0.05 significance level and the t values also showed no significant differences between means at the 95% confidence level, testifying to t,he reliability of the proposed method. In the case of potable water and potato, recovery experiments were done by analyzing spiked potable water and potato samples to which the cationexchange method was applied. The results obtained (Table 3) were common for this kind of study and showed that major losses of Paraquat occur at the extraction step, particularly for potato samples. The mean recoveries of Paraquat from these samples are lower than expected when saturated ammonium chloride is used as eluent. However, reproducibility is satisfactory and it is not necessary to correct the absorbance of the sample for background absorption by means of equations [l]. It can be concluded that the flow-injection technique allows Paraquat to be determined at concentrations up to forty times less than the lower concentration limit possible with the usual spectrophotometric method. The flow-injection method also provides economy of reagent and sample and higher sample throughput (80 h-‘). The financial support gratefully acknowledged. TABLE

Spanish

C.A.I.C.y.T.,

project

2251/83,

is

3

Recoveries

of Paraquat

Sample

Standard solution Potable water POhtOeS

of the

by applying

the ion-exchange

method

Size of sample

Paraquat added (mg 1-I)

Recovery

R.s.d.

(70)

(70)

250 ml 250 ml 250 g

0.2-1.0 0.5 0.2-0.4

64-70 65-69 54-57

2.1 2.8 3.1

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