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Journal of Scientific & Industrial Research Vol. 58, January 1999, pp 25-30

Silica Gel Detector Tubes for Toxic Chemicals and Their Evaluation

Beer Singh*, P P Bhise, M V S Suryanarayana, S S Yadav, V K Rao, B G Polke, D Pandey, K Ganesan and N B S N Rao

Defence Research & Development Establishment, Jhansi Road, Gwalior 470 002, India

Received: 19 March 1 998; accepted: 5 August 1 998

Silica gel of surface area, 250-400 m2/g was impregnated with different chemicals and used in detector tubes to detect different toxic gases like ammonia, hydrogen cyanide, cyanogen chloride, phosgene and sulphur mustard with the detection limits achieved being 0.00497 mg/l , 0.00500 mg/l , 0.00522 mg/l , 0.00500 mg/I and 00.00045 mg/l respectively. These detector tubes offered very low air way resistance when compared with those of foreign origin. However, detection limits were found to be a little lower when tested with the generated gas mixtures. Shelf life of indigenous detector tubes was found to be 2 y generally with the exception of hydrogen cyanide detector tubes, the latter being 16 months.

Introduction Analysis of air is not only essential to determine its

contamination but also a requirement to meet industrial hygiene. An industrial hygienist requires readily usable and rapid methods for analysis of air contaminants so that industrial workers can work in a chemical environment below their TLV values, as needed, in safe levels of con­taminations. The service personnel also require such rapid methods for detection of war gases to take necessary steps for physical protection. Detector tubes offer an easy, rapid and specific detection useful both in industries and warfare.

The quantification and detection of the toxicants depend mainly on the adsorbents, impregnants, air volume, dura­tion, area etc. Silica gel was found to be a suitable adsorbent due to its porous structure and high adsorptive potential. It offers a good support to hold the active chemical reagents which on reaction with the toxic gases give rise to the chromophoric reaction products enabling the identification of specific gases based on the generated colour. Silica gel with suitable characteristics, therefore, was used world­wide for the detection of the industrial gases and the gases of interest after impregnation with suitable chemical im­pregnants. Various impregnants were used for the detection of hydro¥en cyanide ! , cyanogen chloride ! , ammonia ! , phosgene and sulphur mustard ! -4.The literature is full of detection methods for these gases but did not provide much information about the resistance of the detector tubes, impregnation methods of silica gel and its surface charac­teristics. This study was undertaken with a view to inves­tigate the utility of these detector tubes not only to war * Author for correspondence

gases but for industrial gases as well with modification in impregnants.

Experimental Procedure Silica gel of 250-400 m2/g surface area and 20 x 40 BSS

mesh size was obtained from MIS Chempure Ltd, Calcutta. All the impregnants used were of analytical grade.The methods of impregnation of silica gel for use in detector tubes, specific to each gas, are described.

1 Ammonia Various silica gel(surface area-400 m2/g) samples were

prepared by impregnating with an ethanolic solution of bromophenol blue (0.025-2.5 weight %) in incipient quan­tity and mixing. The impregnated silica gel was dried in fume free atmosphere at room temperature.

2 Hydrogen cyanide Si lica gel of 250 m2/g surface area was used for impreg­

nation with mercuric chloride and thymol blue (0.025-0.377 and 2.25 x 1 0-3 to 0.025 weight percent of silica gel respectively). Mercuric chloride was dissolved in metha­nol, and thymol blue was added to it shaking well . The solution thus made was added to s ilica gel in incipient quantity and mixed gently. Impregnated gel was dried in fumes-free atmosphere at room temperature.

3 Cyanogen chloride Silica gel of 400 m2/g surface area was used to impreg­

nate for cyanogen chloride detector tubes. Primary impreg­nants were 4- benzylpyridine in acetone and saturated aqueous barbituric acid solution. Sil ica gel was first im­pregnated with 4-benzylpyridine by adding and mixing its solution in incipient quantity to the gel. Sil ica gel thus impregnated was dried in fumes free atmosphere at room

26 J SCI IND RES VOL 58 JANUARY 1 999

temperature. The dried gel was then impregnated with satu­rated aqueous solution of barbituric acid in a similar man­ner. Weight percentages of 4-benzylpyridine and barbituric acid were 0.01 4-7.00 and 9.5- 1 .9 respectively to silica gel.

4 Phosgene Silica gel of 250 m2/g surface area was used in the

phosgene detector tubes. Impregnants were 4-( 4-nitroben­zyl) pyridine, N- phenylbenzylamine and sodium carbon­ate. Solutions of the first two impregnants were made in benzene and the third was in distilled water. Si lica gel was first impregnated with 4-(4- nitrobenzyl) pyridine by add­ing and mixing its solution, in incipient quantity with respect to silica gel. The impregnated gel was dried in fumes- free atmosphere at room temperature. It was then impregnated with N-phenylbenzylamine and sodium car­bonate in two separate steps, by similar method. Impreg­nants 4-(4- nitrobenzyl) pyridine, N-phenylbenzylamine and sodium carbonate in the impregnated silica gel were 2.3 x 10-3 to 4.6, 4.6 x 1 0-3 to 0.92 and 2.25 to 9.0 weight percent of silica gel.

5. Mustard gas Silica gel of250 m2/g surface area was impregnated with

4-(4- nitrobenzyl) pyridine and mercuric cyanide in 2.02 to 4.04 and 0. 1 1 25 to 1 .8 weight percent. 4-(4-nitrobenzyl) pyridine was dissolved in benzene and mercuric cyanide in methanol. Si lica gel was first impregnated with 4-(4-ni­trobenzyl) pyridine by adding and mixing its solution in incipient quantity to the gel. The impregnated gel was then dried at room temperature in fumes-free atmosphere and then impregnated with mercuric cyanide.

Detector tubes of ammonia, hydrogen cyanide, cyano­gen chloride, phosgene and sulphur mustard gas were made by fi lling 0.2 g of the respective impregnated silica gel in glass tubes of 4.0 mm i .d. and 6.0 mm o. d. Porous plugs used to retain the bed of the impregnated sil ica gel in the glass tubes were made of stainless steel mesh, size 60 BSS. In sulphur mustard detector tubes an additional glass am­poule of 2% aqueous sodium hydroxide solution was placed. Both the ends of the glass tubes were then sealed.

Shelf life of the detector tubes was also investigated. For this, the tubes were stored at 50°C and evaluated peri­odically for their efficiency to detect the toxic gases with­out reduction in their detection limit.

Gas Mixture Generation Air-gas mixtures for a�monia, hydrogen c�anide, cy­

anogen chloride, phosgene and mustard gases were gen­erated and their detector tubes evaluated. A parallel evaluation of the detector tubes from Mis Drager, Germany was also carried out. The procedures followed for the generation of air-gas mixtures for various agents are sum­marized here.

1 Ammonia

Air at a flow nite of 50 mllmin was passed through 1 % aqueous solution of liquor ammonia (Fig. I ). The concen­tration of ammonia in the generated gas mixture was deter­mined using acid-base titration. For this a known volume of gas was passed through a known volume of Nil 00 H2S04. Un-neutralized H2S04 was titrated against Nil 00 sodium hydroxide and the concentration was computed. This was found to be 2.25 to 2.75 mgll which was further diluted to requirements.

2 Hydrogen cyanide Hydrogen cyanide was generated by static and dynamic

methods and used for the evaluation of detector tubes for this gas (Fig 2). Hydrogen cyanide was generated by pass­ing nitrogen gas through 30% aqueous solution of potas­sium cyanide, kept in a bubbler. The potassium cyanide due to hydrolysis forms hydrogen cyanide which was swept away by incoming nitrogen gas. In order to know the concentration of hydrogen cyanide, it was bubbled through a 0. 1 molar sodium hydroxide trap for a fixed period of time. The concentration of cyanide in the trap was deter­mined using ion selective electrode. Orion ion-analyser 90 I was used for this purpose. For different concentration of hydrogen cyanide, it was further diluted with nitrogen. The Stainless steel tube for passing nitrogen and 30% aqueous potassium hydroxide bubbler were kept in a water bath maintained at 30°e. This ensured the reproducibility of hydrogen cyanide gas-nitrogen mixture.

3 Cyanogen chloride Cyanogen chloride from the cylinder was diluted in an

aspirator bottle, and used to evaluate the detector tubes, after measuring the concentration 7.

4 Phosgene Phosgene from cylinder was diluted using the test rig

shown in Fig. 3 . From the cylinder, phosgene ( I ) was passed through a flow meter (2) and mixed with air/nitro­gen. Further dilution was made by passing a part of the gas-nitrogen mixture through flow meters for measuring

Am. 50ml/min. .. .. ..

Fig. I - Test set-up to generate ammonia gas

..

SINGH et at.: SILICA GEL DETECTOR TUBES 27

the phosgene concentration and evaluating the detector tubes. The phosgene concentration was determined by trapping the phosgene nitrogen mixture in a known volume of 30% methanolic sodium hydroxide solution (0.0 1 N). The remaining sodium hydroxide was back titrated against 0.0 1 N sulphuric acid and phosgene concentration was computed.

5 Mustard gas Mustard gas was generated using the standard method6

(Fig 4). Approximately 99.0% pure sulphur mustard, pre­pared by Process Technology and Development Division, Defence R&D Establishment, Gwalior, India was used for the generation of the air-sulphur mustard mixture. An improved dynamic diffusion system was used for the gen­eration of sulphur mustard calibrant gas mixtures from the head space above the test substance (sulphur mustard). Its vapour emerges through a diffusion tube. A metered carrier g�s (nitrogen) is passed over the outlet of the tube and mixed with vapour at constant rate. The generated concen­tration of the gas mixture was ascertained by trapping it in XAD-4 (Amberlite) sorbent tube followed by liquid (chlo­roform) desorption and was analysed by GC technique.

Detection of gases The generated air-gas mixtures of known concentrations

of ammonia, hydrogen cyanide, cyanogen chloride, phos­gene and sulphur mustard were sucked separately for each gas mixture over the bed of impregnated silica gel in the detector tubes made using the procedure described in ex­perimental procedure. The produced colour changes were observed for the detection of the gases in the air-gas mix­tures.The specific colour change for the detection of the specific gas is described in Table I . However, in case of sulphur mustard detection, the detector tube was heated at 60°C for 2 min after drawing the air-gas mixture, breaking the glass ampoule containing sodium hydroxide solution already placed in the tube, to run sodium hydroxide solu­tion over the adsorbent bed and colour change was ob­served, i.e. from off white to blue.

The suction of the air-gas mixtures through the detector tubes was accomplished in two ways: (a) by passing the dynamically generated air-gas mixtures at a flow rate of 1 Ipm for one minute and (b) by sucking air-gas mixtures 1 0

HeN GAS -

Fig. 2 - Test set-up to generate hydrogen cyanide gas [ I : Nitro­gen cylinder; 2: coiled copper tube (damping capillary); 3: Flow meter; 4: Bubbler containing 30% aqueous potassium cyanide]

times at a flow rate of 1 00 ml per stroke using a suction hand pump. The method (a) was used for detection and determination of sensitivity of hydrogen cyanide, phos­gene and sulphur mustard detector tubes. Method (b), however, was used in case of ammonia and cyanogen chloride detector tubes. In order to determine the sensitiv­i ty/ detection limits of the detector tubes, the air-gas mix­tures were diluted till these were found to be detectable with the detector tubes. The detector tubes, however, were not calibrated to detect the various concentrations to enable the user to know the concentration of the gas present in the environment. The developed detector tubes can detect the gases up to the detection l imits given in Table 2 or more than the detection limits; below these limits the detector tubes do not show the colour changes. The quantities of the impregnants impregnated on silica gel corresponding to the achieved detection limits were recorded and described in Table 2.

The indigenous and imported detector tubes were com­paratively evaluated for their detection limits.

Measurement of air way resistance through the detec­tor tubes

Air way resistance through the indigenous and imported tubes was measured at 0. 1 , 0.6 and 1 .0 lpm using the test rig Fig.5. and the data are given in Table 3 .

Air was sucked through the detector tube (5) a t the given flow rate through flowmeter (4) using the leakage valve (2), control valve (3) and the vacuum pump ( 1 ) . The air-way resistance/ pressure drop is noted in mm WG or mm Hg using water or mercury U- tube manometer (6).

10

VENT Fig. 3 - Test set-up for generation of phosgene [ I : Phosgene cyl­inder; 2: 5,8, 1 0 & 13 Control valve; 3: 6 & 14 Flow meter; 4: Ni­trogen cylinder; 7: Mixing chamber; 9: Scrubber; 12 : Methanolic NaOH trap; 1 5 : Phosgene detector tube]

28 J SCI IND RES VOL 58 JANUARY 1 999

Results and Discussion Shelf life study indicated that the tubes (except hydro­

gen cyanide) could be stored at room temperature for 2 y w.ithout deterioration, and reduction in the detection limit. Hydrogen cyanide could be stored at room temperature for 1 6 months. After this period drastic reduction in the detec­tion limit was observed.

Gas mixtures were generated for various gases and were used to evaluate the indigenous and imported detector tubes. The results (Table 3) indicate that indigenous detec­tor tubes except hydrogen cyanide could detect the gas concentration verY near to the detection limit of imoorted

detector tubes. Indigenous hydrogen cyanide detector tubes could detect 5 ppm compared to imported tubes at 2 ppm hydrogen cyanide concentration. The lower sensitivity of indigenous tubes can be attributed to the less resistance of the detector tube, and bed length and particle size of ad­sorbent used in detector tube. These parameters seemed to be responsible for the escape of the gas being sucked over the impregnated silica gel. However, this gets compensated due to the low air way resistance of the indigenous detector tubes and the ease with which one can operate the suction pump while using the indigenous detector tubes.

flOW METER flOW CON1'ROl VALVES

DFFUSION VESSElS

GAS GEtERATOR

EQULBRATOI aweER

DETECTOR ME

Fig. 4 - Functional diagram of dynamic diffusive calibrant gas generator

Table I - Detection of gases through impregnated silica gel

S No Detector tube 1mpregnant Optimum quantity of colour change impregnant (wt%)

Ammonia Bromophenol blue 0. 1 25 pale yellow to purple blue

2 Hydrogen cyanide Mercuric chloride 0.377 pale yellow to pinkish red

Thymol blue 0.0225

3 Cyanogen chloride 4-benzylpyridine 7.0 pale yellow to pink

Barbituric acid 1 .9

4 Phosgene 4( 4-nitobenzyl)-pyridine 2.3 pale yellow to brick red

N-phenylbenzylamine 0.92

sodium carbonate 3.05

5 Sulphur mustard 4(4-nitobenzyl)-pyridine 4.04 off white to blue Mercuric cyanide 0.45

�.

SINGH et al.: SILICA GEL DEfECTOR TUBES 29

The air way resistance results (Table 3) showed that the indigenous detector tubes offer a better flow rate to con­t�minated air compared to foreign counterparts which may be attributed to the ceramic porous plugs, particle size (40-60 mesh silica) and larger beds of the impregnated silica gel and other adsorbents such as zeolites.

2

Fig. 5 - Test-rig for measurement of air way resistance through detector tubes

Relevance to chemical Industry The relevance of toxic chemical monitoring need not be

overemphasized as industrial workers can not be exposed to a hazardous environments. The study is of importance to the industries manufacturing dyes, polymers(plastics), pesticides and other petrochemicals. Early, timely detec­tion of the toxic chemicals in these i ndustries can force/make the workers gear up for appropriate physical protection. The detector tubes developed indigenously are economically viable and can be manufactured with an investment of Rs 5.0 lakhs capital. Two indigenous fabri­cators exist for the supply of residual vapour detection kit. The kit includes a suction pump to suck the contaminated air through the tube to confirm the pollutants. The kit costs around Rs 4500 and can be supplied by the fabricators whose particulars can be obtained from the Director of the institution. Facilities are available at the author, s labora­tory for evaluating the cyanogen chloride, hydrogen cya-

Table 2 - Comparative detection limits of detector tubes

S No Detector tube Source Detection limit lLV Sensitivity (mgll) (mgll)

Ammonia DRDE, Gwalior 0.00497 0.01775

2. Ammonia Drager, Germany 0.002 13 25.0 ppm (USA 199 1 )

3 Hydrogen Cyanide DRDE, Gwalior 0.00500 0.0 1 1 30

4 Hydrogen Cyanide Drager. Germany 0.00 1 1 3 10.0 ppm (USA 1989)

5 Cyanogen Chloride DRDE. Gwalior 0.00522 0.000768

6 Cyanogen Chloride Drager. Germany 0.003 1 3 0.3 ppm (USA 1 990)

7 Phosgene DRDE. Gwalior 0.00500 0.00041 3

0. 1 ppm (USA 1 99 1 )

8 Sulphur Mustard DRDE. Gwalior 0.00045 0.000003 (USA Federal Register)

Table 3 - Comparative air way resistance/pressure drop of detector tubes

S No Detector tube Source Bed height Pressure drop @ air flow rate (lpm) (cm)

0. 1 0.6 1 .0 (mm WG)

(cm Hg)

I Ammonia DRDE. Gwalior 2.0 33.0 2. 1 3.3

2 Ammonia Drager. Germany 7.0 1 80.0 6. 1 9.3

3 Hydrogen Cyanide DRDE. Gwalior 2.0 33.0 I .3 2.0

4 Hydrogen Cyanide Drager. Germany 7.0 1 33.0 5 . 1 7.9

5' Cyanogen Chloride DRDE. Gwalior 2.0 44.0 0.9 1 .3

6 Cyanogen Chloride Drager. Germany 3.2 88.0. 3.3 5.2

7 Phosgene DRDE. Gwalior 2.0 33.0 1 .5 3.5

Phosgene Drager. Germany 9.0 39.0 8.7 20.0

8 Sulphur Mustard DRDE. Gwalior 2.0 47.0 1 .9 2.9

30 J SCI IND RES VOL 58 JANUARY 1 999

nide, phosgene, sulphur mustard, nerve gas and ammonia detector tubes.

Conclusion The study enabled us to develop the impregnation pro­

cedure of commercial silica gel with suitable composition of impregnants, for detecting ammonia, hydrogen cyanide, cyanogen chloride, phosgene, sulphur mustard and nerve gases below their TL V s .

. Comparative evaluation of the detector tubes indicates that the air-way resistance offered by the indigenously developed tubes is significantly lower than the drager detector tubes. However, sensitivity seems to be two times lower while perfonning the detection test with ease using less air for suction.

Acknowledgment Authors are thankful to Dr R Vaidyanatha Swamy,

Director, Defence Research and Development Estab­lishment, Gwalior for providing facilities and useful sug-

gestions. Authors also thank Mr. G R Khanwilkar for secretarial help.

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4 Olthoff Uwe, Eichardt Annette, Sauerbrey Werner, Pelzing Karl & Czepuck Andreas, Cere East) Pat DD 299, 765, 07 May 1992; Chern Abstr, 118 ( 1 993) 93594q.

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