THERMAL STUDIES OF ORGANOCHLORINE INSECTICIDE BENZENE HEXACHLORIDE (BHC)

THERMAL STUDIES OF ORGANOCHLORINE INSECTICIDE BENZENE HEXACHLORIDE (BHC)

Arun G. Bhoi (Senior Forensic Expert)

Dy. Director (Former, I/C), Directorate of Forensic Science Labs., State of Maharashtra, (INDIA).

OSD, Consultant (Former), Goa State Forensic Science Laboratory, Verna, Goa (INDIA).

Founder and Head, Arun’s Institute of Forensic Sciences, Research and Education, Pune, Maharashtra (INDIA).

E – mail: arun.bhoi@gmail.com

(e – J. Foren. Crime Inv. 2017, 14 – 22, 1, Art – 1)

Received on: January 2017.

Accepted on: 24^th March 2017.

Published on: 27^th March 2017.

 

ABSTRACT

The thermal decomposition of recrystallized BHC and dusting powder (DP) containing 50 % was studied using thermogravimetry (TG) and differential thermal analysis (DTA) techniques. The behavior of isomeric constituents has also been studied. From thermogravimetric curves, the order (n) and activation energy (Ea) have been elucidated. Additives present in DP have accelerated the rate of the decomposition reaction. However, it did not affect the melting endothermic DTA peaks.

KEYWORDS

Benzene hexachloride, gamma-BHC, Thermogravimetry, DTA, Order, Activation Energy, isomers, Coats and Redfern equation.

INTRODUCTION

BHC (Hexachlorocyclohexane, C₆H₆Cl₆ ) is a synthetic organic chlorine insecticide. It is a stomach poison and contact insecticide with some fumigant action. It has high toxicity towards insects and fleas. Especially, BHC is very much effective against the fleas “Xenopsyllo” which acts as the carrier for plague organisms “Pastewalla postis” from rodents (rats) to human beings. Hence it is extensively used for eradicating diseases like plague and proved to be of great public health importance. It is marketed in the form of emulsion concentrates (EC), wettable dispersible powders (WDP), dust and smoke generators. Some of the countries like UK, Germany, Japan, Poland, Portugal, Spain have restricted its use [1]. The undesirable musty odor of BHC is one of the reasons to restrict its use as agriculture insecticide. However, it is selectively used for protecting seeds and controlling plant pests. Knowingly or unknowingly, many times this organochlorinated insecticide is misused in homicidal and suicidal cases.

BHC consist of eight isomers such as a, b, g, d, e and so on. a- Isomer constitutes major percentage in BHC. However, g- isomer is the prime active ingredient [2]. A number of methods have been reported for the detection and estimation of BHC. For example thin layer chromatography [3-5], spectrometry [6], gas chromatography [7-10] and classical methods [11]. However, least attention has been paid to thermo-analytical methods and kinetics involved therein. Hence, BHC has been chosen for the TG-DTG and DTA studies. The effect of additives (anticaking and deflocculating agents) on the decomposition of BHC has also been studied. Activation energy (Ea) and the order (n) have been deduced by Coats and Redfern equation [12].

EXPERIMENTAL

Thermal properties of BHC are studies using TGA and DTA units installed in our laboratory.

Recrystallised BHC and dusting powder containing 50% BHC passing through 75-micron sieve were used for thermal analysis.

The following specifications were used for TG/DTA studies:

Sample size                                          60-80 mg

Rate of heating                                    5°C/min

Temperature range of study            ambient to 950°C

Atmosphere                                         Static air or ambient

Reference material for DTA               MgO

RESULTS AND DISCUSSION

The thermogravimetric curve for BHC indicates that the weight-loss begins at 90°C and it proceeds through a number of decomposition/weight-loss steps. The first weight – loss step is observed in the temperature range 90°C to 120°C and accounts for 7% weight-loss. The endothermic DTA peak at 80°C is because of melting of g-isomer (Fig.- 1). Hence the loss could be attributed to the volatilization of g-BHC isomer. The presence of other BHC isomers with g-isomer precisely supports the shift in its melting point to the lower temperature.

The observed 8.5% weight –loss in the temperature range 170°C to 230°C corresponds to the simultaneous volatilization of remaining part of gamma-isomer, delta-isomer, and the fraction of the alpha-isomer. The DTA endotherm at 135°C corresponds to the melting of d-BHC and the inflection at 155°C corresponds to the melting of a-BHC. Heating of BHC (isomeric mixture) to 170°C – 180°C isomerizes it to individual isomer. This isomeric change has also been observed at 180°C as DTA endotherm. An endothermic DTA peak at 220°C corresponds to the melting of e-isomer and it could also be contributing partially to the loss occurring in the range described herein (Fig.-1).

All the weight-losses discussed above are clearly depicted on DTG curves (Fig. -1).

The 13% weight –loss is seen in the range 240°-280°C. In this range dehydrochlorination of BHC occurs.

-HCl                                        -HCl                                        -HCl

C₆H₆Cl_{6 —————->}C₆H5Cl5—————–> C₆H₄Cl_{4  ——————>-} C₆H3Cl₃

BHC         Penta – Chlorocyclohexne (CCH)    Tetra- CCH     Trichlorocyclohexane

Hence the observed loss could be attributed partially to the release of HCl. The weight-loss resulted by dehydrochlorination has also been supported by DTG curve. A characteristic odor of pentachloro and tetrachloride products was noticed above 240°C during experimentation. The chloride was tested with the AgNO₃ solution.

The TG curve shows a major weight-loss (73% approx.) in the range 280°-340°C (Fig. 1) leaving sample holder empty. The prominent DTA endotherm at 312°C corresponds to the melting of b-isomer. The b-isomer melts at 312°C and it sublimes after melting [2]. Furthermore, when heated to decomposition BHC emits highly toxic fumes of phosgene.COCl₂ [13]. In the thermoanalytical process above 310°C, the released phosgene gas was detected with p-dimethylaminobenzaldehyde and diphenylamine test paper [2]. Hence the major weight-loss resulted suddenly in the range of 280°-340°C is a composite result of sublimation of b-isomer, rapid release of phosgene and volatilization of residual BHC and products formed therein. This has also been supported by a prominent and sharp peak at 315°C on DTG curve.

From thermogravimetric data, the kinetic parameters activation energy (Ea) and order (n) for the respective decomposition/weight-loss steps have been elucidated using Coats and Redfern equation [12]. The results are presented in Table 1.

The dusting powder contained 50% of BHC and remaining portion consisted anticaking and deflocculating agents like diatomaceous earth, silica, silicates, powdered limestone and sodium tripolyphosphate. The dusting powder was also studied thermogravimetrically. In thermal studies of dusting powder, losses of BHC occurred comparatively at lower temperature. The major loss was observed up to 260°C for dusting powder (Fig. 2). However, the total loss occurred at 340°C in case of recrystallized BHC. This is because in the case of dusting powders the additives could be accelerating the decomposition and thus enhancing the reaction processes. The presence of additives has also increased surface area for the reactions. Therefore, the speedy losses would have occurred by decomposition, volatilization, and sublimation of BHC leaving additives. These steps are also clearly observed on DTG curve. However, this process has not made any remarkable changes on endothermic DTA peaks (Fig. 2). In this range, no interference was observed on TG, DTG and DTA curves by the presence of additives. However, above 340°C, TG-DTG showed many of the decomposition/weight-loss steps corresponding to additives. The exothermic DTA peaks above 340°C could also be attributed to the decomposition and phase changes of additives present therein.

CONCLUSIONS

The present study was undertaken to know the thermal behavior of polychlorinated material isomeric BHC. The TG, DTG and DTA studies throw light on the presence of different isomers in BHC and on the inter-isomerisation and transformations of constituting BHC isomers. It also helped to study decomposition changes and kinetics involving therein. These thermo-analytical methods would also prove helpful to know the presence of different isomers quantitatively. It can also be concluded that additives from dusting powders with 50% BHC only accelerates the changes and remain unchanged till all the BHC is decomposed. Therefore, TG-DTG and DTA methods would prove to be a promising and precise tool for the analysts working in different fields for studying polychlorinated- isomeric compounds.

ACKNOWLEDGEMENTS

Author’s thanks are due to Head, Department of Chemistry, University of Pune, Pune, Maharashtra (INDIA).

REFERENCE

1. K. Gupta and D.K. Salunkhe. Modern Toxicology, Vol. II, The Adverse Effects of Xenobiotics. First Ed., Metropolitan Book Co. Pvt. Ltd., New Delhi, 1985, p.15-17.
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3. Alan Curry. Poison Detection in Human Organs, Third Ed., Charles C. Thomas, Publisher, U.S.A., 1976, p. 225-226.
4. Egon Stahl. Thin Layer Chromatography, A Laboratory Handbook, 2^nd, Springer International Student Edition, Springer-Verlag, New York, 1969, p. 643.
5. A. Khan and J. Paul. Microchem. J., 1979, 24, 333-340.
6. Wood. Analyst, 1960, 85, 21.
7. Pesticides, GC Analysis and Standards, Bulletin 758A, Supelco, Inc., Bellefonte, Pennsylvinia, 1976 p. 1-3.
8. British Standards Institution, BS-5202 Part 10. 1982.
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10. J. de F. Maunder, H. Egan, E.W. Godly, E.W. Hammond, J. Roburn and J. Thomson. Analyst, 1964, 89, 168.
11. S. Sree Ramulu. Methods of Pesticide Analysis, Oxford, and IBH Publishing Co., New Delhi, 1979, p. 52-55.
12. W. Coats and J. P. Redfern. Nature, 1964, 201, 68.
13. Irving Sax. Dangerous Properties of Industrial Materials, Fifth Ed., Van Nostrand Reinhold Company, London, 1979, p. 716.

Table 1: Thermal Data of BHC

Temperature range                       Order                    Activation energy (Ea)

(°C)                                                     (n)                                        (KJ/mole)

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90-120                                                  0.00                                        102.00

170-230                                                0.00                                          24.19

240-280                                                0.00                                        150.50

290-340                                                0.00                                        130.30

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Figures 1 and 2 show the thermal curves for recrystallized BHC and Dusting Powder (50% BHC) as below: