Kinetic-spectrophotometric determination of methyl parathion in water and vegetable samples

Article abstract of DOI:10.1016/j.saa.2012.09.084

A new selective and sensitive kinetic method has been developed for spectrophotometric determination of methyl parathion based on its inhibitory effect on the redox reaction between bromate and hydrochloric acid. The decolorization of neutral red by the r

Full text of DOI:10.1016/j.saa.2012.09.084

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 54–58
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Kinetic-spectrophotometric determination of methyl parathion in water
and vegetable samples
⇑
Neetu Tiwari , Anupama Asthana, Kanchan Upadhyay
Department of Chemistry, Govt. V.Y.T. PG Autonomous College, Durg, Chhattisgarh 491 001, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Inhibitory effect of methyl parathion.
Reduction of bromate by hydrochloric acid
"
"
"
Based on a new kinetic approach.
Simple, selective and sensitive.
Applicable in different vegetable and
water samples with satisfactory
results.
1
0 Cl- + 2 BrO3- + 12 H+
5 Cl2 + Br2 + 6 H2O
Oxidation of Methyl Parathion
-O
N+
O
O
Br2 or Cl2
O
P
S
O
Methyl parathion
-O
O
N+
O
O
P
O
O
Oxidised Methyl parathion
Bleaching of neutral-red dye
Neutral-red + Br /Cl Neutral-red (bleached, λmax = 530 nm)
2
2
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 June 2012
Accepted 22 September 2012
Available online 29 September 2012
A new selective and sensitive kinetic method has been developed for spectrophotometric determination
of methyl parathion based on its inhibitory effect on the redox reaction between bromate and hydrochlo-
ric acid. The decolorization of neutral red by the reaction product was used to monitor the reaction spec-
trophotometrically at 530 nm by measuring the change in absorbance at the fixed time of 5 min after the
initiation of the reaction. The variables affecting the rate of the reaction were investigated. Under the
Keywords:
ꢀ1
selected experimental conditions methyl parathion was determined in the range of 0.025–0.3
Sandell’s sensitivity and molar absorptivity for the system were found to be 0.0004
l
g cm
g mL
.
Methyl parathion
Potassium bromate
Neutral red
ꢀ2
l
and
5
ꢀ1
ꢀ1
6.5 ꢁ 10 L mol cm respectively. The proposed method was applied for the determination of methyl
parathion in different vegetable and water samples with satisfactory results. The results were compared
with those obtained by GC–MS, very similar values were found by the two methods.
Ó 2012 Elsevier B.V. All rights reserved.
Spectrophotometry
Introduction
phosphorothioate) is one of the organophosphorus pesticides very
effective against many pests in important crops, such as bulbs,
Organophosphorus pesticides attained the growing importance
in pest control because of their rapid decomposition and less likely
accumulation in environment. They are still of great concern be-
cause of their high solubility in water, excessive usages and acute
toxicity. Methyl parathion (O,O-dimethyl-O-(4-nitrophenyl)
cereals, fruits, vegetables, cotton, peanuts, soybean, potato, sugar
cane, coffee, alfalfa, and pasture [1,2]. It act by contact, ingestion,
and inhalation [3], and is well known inhibitor of acetylcholines-
terase (essential for the operation of the central nervous system
of the insects), therefore, producing serious damage and death
[
4]. It is classified in toxicological class 1 meaning that it is extre-
mely dangerous for mammals. Detection of methyl parathion as
environmental pollutant and their monitoring is important to min-
imize the potential hazards in human health [5,6].
⇑
Corresponding author. Tel.: +91 9755888857.
E-mail address: neetuchem30@gmail.com (N. Tiwari).
1
386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.09.084

N. Tiwari et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 54–58
55
Analysis of methyl parathion in environmental and biological
Table 1
Recoveries from environmental samples.
samples is routinely carried out using analytical techniques, such
as GC–MS [7–11], GC–PFPD [12], HPLC–DAD [13], and electro-
chemical methods [14–16]. Such analysis is generally performed
at centralized laboratories, requiring extensive labor and analytical
resources, and often results in a lengthy turn-around time. Spectro-
photometry is considered the most convenient analytical tech-
nique because of its inherent simplicity, low cost, and wide
availability in most of the laboratories. However, few spectropho-
tometric methods have been reported for its determination which
is based on the estimation of phosphorus present in organophos-
phorus pesticides by reduction of molybdophosphoric acid to
molybdenum blue by using reducing reagents [17–19], or based
on the reduction of the nitro group present in parathion-methyl
with zinc/HCl to form amino-parathion-methyl. The aminoparathi-
on-methyl thus formed is diazotized and subsequently coupled
with a coupling agent [20]. Reported spectrophotometric methods
are indirect, less sensitive, or suffers from interference.
Sample
Amt. added (
l
g) Amt. found (l
g)a Recovery (%) RSD (%)
Vegetable sampleb
Cauliflower
5
10
5.13
10.11
15.11
4.94
102.52
101.06
100.75
98.8
0.296
0.205
0.127
0.32
1
5
5
0
5
5
0
5
Tomato
1
1
9.96
14.97
4.92
9.94
14.94
99.56
99.8
98.4
99.4
99.57
0.208
0.106
0.455
0.159
0.139
Spinach
1
1
Waste water samplec
A
B
C
5
10
15
4.89
9.97
14.96
97.92
99.68
99.72
0.47
0.165
0.129
a
b
c
Mean of five replicate analyses.
Amount of sample, 10 g (free from methyl parathion).
Volume of sample, 5 mL (free from methyl parathion).
The proposed new kinetic method is based on the inhibitory ef-
fect of methyl parathion on the decolorization reaction of neutral
red by bromate in acidic medium of hydrochloric acid. The method
is simple, selective, sensitive, and can be easily applied for the
determination of methyl parathion in different vegetable and
waste water samples. The results were compared with those ob-
tained by GC–MS; similar values were found by the two methods.
check the recoveries, various samples free from methyl parathion
were taken and treated with a known amount of the pesticide,
and kept for ꢂ24 h. The samples were then washed with ethanol
and washings were collected. Aliquots of these washings were
used for the determination of methyl parathion by the proposed
method. The recoveries are given in Table 1.
Experimental
Reagent solutions and apparatus
Results and discussion
All chemicals used were of analytical reagent grade and solutions
were prepared from doubly distilled water. Methyl parathion (Bayer,
The reaction of bromate with hydrochloric acid produces chlo-
rine and bromine. The produced bromine and chlorine react with
neutral red and the reaction causes oxidative decolorization. The
reaction was monitored spectrophotometrically by measuring the
decrease in absorbance of the reaction mixture at 530 nm. Methyl
parathion can be oxidized with the product of the reaction (bro-
mine and chlorine) in aqueous medium and by oxidative desulfu-
ration, paraoxon is formed [21–24] as shown in reaction Scheme
1. The induction period of the indicator reaction increases with
increasing methyl parathion concentration. The increase in induc-
tion period affects the change in absorbance with time so the
ꢀ1
India Ltd.) stock solution (1000
lg mL ) was prepared in ethanol.
Potassium bromate (Merck, Mumbai, India) aqueous solution of
ꢀ
3
ꢀ1
6
.0 ꢁ 10 mol L was prepared, and stored in amber colored bottle.
ꢀ3
Neutral red (Merck, Mumbai, India) aqueous solution of 7.0 ꢁ 10
-
ꢀ1
ꢀ1
mol L was prepared. A 2 mol L aqueous solution of HCl (Merck,
Mumbai, India) was prepared. A Varian Cary 50 Bio UV–Visible Spec-
trophotometer was used for spectral measurements, pH measure-
ments were made with Systronics digital pH meter 335.
Recommended procedure
Reduction of bromate by hydrochloric acid
All the solutions and distilled water were kept in a thermostate
water bath at 30 °C for 15 min for equilibration before starting the
experiment. An aliquot of the solution containing 0.025–
-
-
1
0 Cl + 2 BrO3 + 12 H+
5 Cl2 + Br2 + 6 H O
2
Oxidation of Methyl Parathion
-O
ꢀ1
0 lg mL methyl parathion was transferred into a 10 mL volu-
.3
ꢀ
3
ꢀ1
metric flask and 0.3 mL of 6.0 ꢁ 10 mol L potassium bromate,
ꢀ1
N+
0
.6 mL of 2 mol L hydrochloric acid were added and the solution
O
O
Br2 or Cl2
was diluted up to 5 mL and kept for 2 min and then 0.5 mL of
O
P
ꢀ3
ꢀ1
7
.0 ꢁ 10 mol L neutral red solution was added and solution
S
O
was diluted to the mark with distilled water. Time was measured
just after the addition of neutral red. The mixture was mixed and
a portion of the reaction mixture was transferred into spectropho-
tometric cell. The reaction was followed by recording the absor-
bance against water at 530 nm from 0.5 to 5.0 min after the
initiation of the reaction. Then a calibration graph of the difference
between the decrease in absorbance of the blank minus that of the
Methyl parathion
-O
O
N+
O
O
P
O
O
sample (
concentration was constructed.
D
A
blank
ꢀ
DAsample) at a fixed time vs. methyl parathion
Oxidised Methyl parathion
Determination of methyl parathion in water and vegetable samples
Bleaching of neutral-red dye
Neutral-red + Br /Cl Neutral-red (bleached, λmax = 530 nm)
2
2
The proposed method was applied satisfactorily to the determi-
nation of pesticides in waste water and vegetable samples. To
Scheme 1. Inhibitory effect of methyl parathion.

5
6
N. Tiwari et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 54–58
Concentration of HCl, × 10-3 mol L-1
Fig. 1. Effect of HCl concentration on the sensitivity. Condition – methyl parathion:
Fig. 3. Effect of neutral red concentration on the sensitivity. Condition – methyl
ꢀ
1
ꢀ3
ꢀ1
ꢀ3
ꢀ1
0
.2
lg mL ; neutral red: 7 ꢁ 10 mol L ; potassium bromate: 6 ꢁ 10 mol L
;
ꢀ1
ꢀ1
parathion: 0.2
l
g mL
;
hydrochloric acid: 2 mol L
; potassium bromate:
temperature: 30 °C.
ꢀ3
ꢀ1
6
ꢁ 10 mol L ; temperature: 30 °C.
with methyl parathion and therefore decreases the induction per-
iod. An increase in the production rate of Cl and Br also cause an
increase in the rate of reaction with neutral red and therefore the
slope of the absorbance changes after the initiation of the reaction.
calibration graph was plotted between the decrease in absorbance
of the blank minus that of the sample ( sample) at a fixed
time vs. methyl parathion concentration.
2
2
D
A
blank
ꢀ
DA
A
good sensitivity was observed at final concentration of
ꢀ3
ꢀ1
6
.0 ꢁ 10 mol L (Fig. 2).
Effect of acid concentration
The effect of hydrochloric acid concentration on obtaining max-
Effect of neutral red concentration
ꢀ
3
ꢀ1
imum sensitivity was investigated with 6 ꢁ 10 mol L potas-
ꢀ
3
ꢀ1
sium bromate, and 7 ꢁ 10 mol L neutral red at 30 °C (Fig. 1)
The influence of neutral red concentration on the sensitivity of
ꢀ1
ꢀ3
ꢀ3
over the range of 0.5–4 mol L . In order to find the optimum con-
centration of hydrochloric acid, the absorbance changes for the
blank reaction (the reaction in the absence of methyl parathion)
at a fixed time of 5 min were measured as a function of HCl con-
centration. The results have been shown in Fig. 1. The difference
between the change in absorbance for the blank and sample reac-
the reaction was studied in the range of 2 ꢁ 10 to 10 ꢁ 10
-
ꢀ
1
ꢀ3
ꢀ1
mol L
with 6.0 ꢁ 10 mol L
potassium bromate and
ꢀ1
2.0 mol L hydrochloric acid at 30 °C. The results show that by
ꢀ
3
ꢀ1
increasing the neutral red concentration up to 7.0 ꢁ 10 mol L
the sensitivity increases, whereas a greater amount of reagent de-
ꢀ3
ꢀ1
creases sensitivity, thus 7.0 ꢁ 10 mol L
neutral red was se-
ꢀ
1
tion shows a maximum at 2.0 mol L HCl, therefore, a final con-
centration of 2.0 mol L acid was selected as optimum.
lected for the study (Fig. 3).
ꢀ
1
Effect of temperature and time
Effect of potassium bromate concentration
The effect of temperature on the inhibitory reaction was studied
in the range of 10–70 °C with the optimum of the reagent concen-
trations. The results showed that as the temperature increases up
to 30 °C the sensitivity increases, whereas higher temperature va-
The effect of potassium bromate concentration on the reaction
ꢀ
1
rate was studied with 2.0 mol L
hydrochloric acid and
.0 ꢁ 10ꢀ mol L neutral red at 30 °C. Potassium bromate con-
3
ꢀ1
7
ꢀ3
ꢀ3
ꢀ1
lue decreases the sensitivity (
was selected for further study and 5.0 min time was suitable for
D
Ablank
ꢀ
DAsample). Therefore, 30 °C
centration in the range of 2 ꢁ 10 to 10 ꢁ 10 mol L was inves-
tigated. An increase in bromate concentration causes a decrease in
the induction period. It was also observed that the calibration
range differed according to the concentration of bromate hence
the concentration of bromate was selected on the basis of the rate
the study of inhibitory reaction (Fig. 4).
Effect of foreign species
of the reaction of bromate with HCl. As the rate of production of Cl
and Br increases by increasing HCl and bromate concentration,
this causes an increase in the rate of the reaction of Cl and Br
2 2
2
To evaluate the analytical applicability of the proposed method,
the method was applied to determination of methyl parathion in
2
Concentration of Bromate, × 10-3 mol L-1
Fig. 2. Effect of bromate concentration on the sensitivity. Condition – methyl
Fig. 4. Effect of temperature on the sensitivity. Condition – methyl parathion:
ꢀ
1
ꢀ1
ꢀ3
ꢀ1
ꢀ3
ꢀ1
ꢀ1
parathion: 0.2
lg mL
;
hydrochloric acid: 2 mol L
;
neutral red: 7 ꢁ 10
-
0.2
lg mL ; neutral red: 7 ꢁ 10 mol L ; hydrochloric acid: 2 mol L ; potassium
ꢀ
1
ꢀ3 ꢀ1
mol L ; temperature: 30 °C.
bromate: 6 ꢁ 10 mol L
.

N. Tiwari et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 54–58
57
Table 2
Table 3
Effect of diverse ions (concentration of methyl parathion 0.2
l
g mLꢀ1).
Spectral characteristics, precision and accuracy of the proposed method.
Foreign species
Tolerance limit
(lg mL )
Parameters
max (nm)
Range of Beer’s law (
Molar absorptivity (L mol cm
Sandell’s sensitivity (
Results
ꢀ
1 a
k
530
0.02–0.3
Pyrethroid pesticides, atrazine
Carbamate pesticides
Phorate
400
250
200
100
l
g mLꢀ1)
ꢀ
1
ꢀ1
5
)
6.5 ꢁ 10
g cm 2
ꢀ
)
0.0004
l
PO 34 ꢀ, CH
Cr(VI)
COO
ꢀ
3
Relative standard deviation (%)
Intra-day
Inter-day
50
20
0.54–2.65
0.39–4.89
0.009
0.028
0.047
K , Na , Ba2+, Ca2+, NH , Mg2+, Mn2+, Cu2+, Cr3+, Al3+
+
+
þ
4
,
Zn2+, Co2+, Cd+2, Fe3+, Pb2+, As(V), NO , SO
3ꢀ
2ꢀ
ꢀ1
Limit of detection (lg mL
)
4
Limit of quantification (
SD of slope
SD of intercept
l
g mLꢀ1)
Phenol
10
5
4
ꢀ
NO
2
SO 23 ꢀ, Fe2+, Mo(VI)
0.004
Chlorpyrifos, endosulfan malathion, dimethoate
1.5
Regression equation (Y = bx + a)
Slope (b)
Intercept (a)
a
2.94
0.005
0.999
a
Tolerance limit causing ±2% variation in absorbance value.
Correlation coefficient
a
ꢀ1
Concentration in
lg mL
.
0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
0
absorbance at this wavelength. Beer’s law was obeyed over the
ꢀ1
concentration range of 0.025–0.3
lg mL
(Fig. 6). The molar
absorptivity and Sandell’s sensitivity are given in Table 3. The
slope, intercept, and the correlation coefficient were calculated
by least squares regression analysis (Table 3). The detection limits
(DL = 3.3
r/S) and quantitation limits (QL = 10
r/S) (where ‘S’ is the
slope of the calibration curve and
r
is the standard deviation of
4
40
460
480
500
520
540
560
580
600
620
blank), SD of slope and intercept values calculated are given in
Table 3.
Wavelength, nm
To check the precision of the method three different concentra-
tions of methyl parathion (within the working limits) were ana-
lyzed in seven replicates during the same day (intra-day
precision) and seven consecutive days (inter-day precision). The
RSD (%) values range of intra-day and inter-day studies showed
that the precision was good for the method (Table 3).
A-Reagent blank
B-Concentration of methyl parathion: 0.15 g mL
C-Concentration of methyl parathion: 0.25 g mL
-
-
1
1
Fig. 5. Absorption spectra of the dye.
various samples. The influence of various ions, several organic and
inorganic compounds and pesticides was examined under the opti-
mum conditions with the developed method. The effect of various
Determination of kinetic parameters
The reaction rate of bromate with hydrochloric acid in the pres-
ence of methyl parathion was determined at four different temper-
ꢀ1
interferences on the determination of 0.2 lg mL of methyl para-
thion concentration was studied. A variation of ±2% in the absor-
bance value was considered tolerable. The results are given in
Table 2, it was found that sensitivity of the reaction was not af-
fected by most of interferences.
atures. A plot of ln K versus 1/T gives a straight line, from which the
–
energy of activation
equation:
D
E
was calculated according to the Arrhenius
K ¼ A ꢃ eꢀ
DE–=RT
where K is the reaction rate constant and R is the gas constant. The
Spectral characteristics and method validation
other thermodynamic parameters were calculated at 30 °C. The en-
–
thalpy of activation (
D
H ) was calculated using the relation:
The absorption spectra of colored product showed a maximum
absorbance at 530 nm (Fig. 5). The reagent blank had negligible
–
–
D
E
¼
D
H þ RT
–
The entropy of activation (
D
S ) was obtained from the
1
equation:
0
0
0
0
0
0
0
0
0
.9
.8
.7
.6
.5
.4
.3
.2
.1
0
y = 2.936x + 0.004
R² = 0.999
A ¼ KT=h ꢃ eðRþ
D
S–=RÞ
where K and h are rate constant and Planck’s constant, respectively.
–
The free energy of activation (
equation:
DG ) was calculated according to
Table 4
Kinetic parameters for the decolorization of neutral-red at 30 °C.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
A (s 1
1
ꢀ
)
D
E
–
(kJ/mol)
38.10
D
H
–
(kJ/mol)
35.59
D
S
–
(J/mol K)
ꢀ628.05
DG
–
(kJ/mol)
Concentration, µg mL-1
5
.15 ꢁ 10
225.88
Fig. 6. Calibration data for the determination of methyl parathion.

5
8
N. Tiwari et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 54–58
Table 5
Application of the method for determination of methyl parathion in real samples.
Sample
Methyl parathion found (
Proposed method
l
g)a
Reported method [17]
GC–MS method [25]
Agricultural waste waterb
A
B
4.14 (0.029)
3.58 (0.064)
3.95 (0.039)
3.19 (0.041)
4.57 (0.011)
3.81 (0.009)
Vegetable samplesc
Cauliflower
Tomato
3.09 (0.019)
4.21 (0.039)
3.98 (0.026)
2.96 (0.032)
3.99 (0.061)
3.88 (0.061)
3.33 (0.016)
4.71 (0.018)
4.06 (0.021)
Spinach
a
b
c
Mean ± standard deviation (n = 5).
Volume of sample, 5 mL (from field where methyl parathion has been sprayed).
Amount of sample, 10 g (from field where methyl parathion has been sprayed).
–
–
–
D
G
¼
D
H ꢀ T
DS
compared with reported spectrophotometric methods. The easy
availability of the reagent and freedom from a large group of inter-
fering species are some advantages of the method. As the method
is based on the kinetic study it can be further applied for simulta-
neous determination of different organophosphorus pesticides by
partial least square method.
The results are given in Table 4.
Applications
Determination of methyl parathion in waste water samples
Acknowledgment
Water samples were collected from nearby runoff agricultural
field where methyl parathion was sprayed as a pesticide. Water
was filtered through a Whatman no. 40 filter paper and the aliquot
of the filtrate was analyzed as described above. Results were com-
pared with reported spectrophotometric [17] and GC–MS method
The authors are thankful to Government V.Y.T. PG Autonomous
College, Drug for laboratory facilities.
[
25] (Table 5).
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327–332.
(
Table 5).
Conclusion
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etc. are available but the factors such as the low cost of the instru-
ment, ease of handling, and almost no maintenance have caused
spectrophotometry to remain a popular and inevitable technique,
particularly in the laboratories of developing countries. Most of
the spectrophotometric methods reported for the determination
of methyl parathion suffer from drawbacks including reagent cost,
instability of color, interference, low sensitivity and selectivity. The
proposed spectrophotometric method is based on a new kinetic
approach which is more sensitive, simple and selective as
[19] J. Raju, V.K. Gupta, Microchem. J. 39 (1989) 166–171.
[
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