Freon and halogenated hydrocarbon detection with electrochemical sensors

68

Sensors and Actuators,

BI (1990) 68-72

Freon and Halogenated Hydrocarbon Detection with Electrochemical Sensors HIROTAKA KOMIYA and SHINICHI KIMURA 6-1254-2 Shimizu, Higashiyamato,

Tokyo 189 (Japan)

Abstract

Experimental

A new method to detect various freons and other chlorinated hydrocarbons continuously at room temperature has been developed. The system for these gases consists of a heated filament and electrochemical sensors for HCl, HF and Cl,. Electrochemically inactive freons, such as R11 (CHCIF,) and CC1,F1, and a variety of chlorinated hydrocarbons can only be detected with an HF sensor and an HCI sensor via a pyrolytic process over a heated filament. It is found that the HF sensor can detect all the freons used in this experiment, including CF,, C2F6, CHF, and CF,Br, which could not be detected with the HCl sensor. The Cl* sensor detected only freon 13Bl of the freons used here.

Preparation of Gas Samples

Introduction It is extremely important to detect freon gas leaks, which cause the destruction of the ozone layer. Although a variety of freon gas leak-detection methods have been developed [ 1,2], we found that the more effective combination of a heated filament and an electrochemical sensor can continuously detect freons and other chlorinated hydrocarbons. This combination was initially proposed for the detection of benzene [31. Attempts have been made to quantify the products of a pyrolysis device using detector tubes; however, this method is not continuous [4]. It has been pointed out that the pyrolysis products of chlorinated hydrocarbons include chlorine and hydrogen chloride gas [5-71. In this study, three kinds of sensors based on a gas membrane galvanic cell for HCl, HF and Cl, were investigated in conjunction with the pyrolyser. This made it possible to detect not only freons but also other chlorinated hydrocarbons at threshold limit values (TLV) or lower concentrations. 0925-4005/90/$3.50

Freons of 1500 ppm, except CF, (5%) and C2F, (5%) (Takachiho Chemical Industry Co., Ltd.), were used. Various concentrations of freons in air were prepared by dynamic mixing of freons and synthetic air at various flow ratios through a blender (model SECB-202, Estec Co., Ltd.), which contained two mass-flow meters. Various concentrations of chlorinated hydrocarbons, except CH,Cl and C,H,Cl, were generated with diffusion tubes (Gastec Corp.). Pyrolyser

A pyrolyser (model No. 840, Gastec Corp.) was modified to meet the requirements for continuous measurement in this study. The pyrolyser has an internal capacity of 1 ml and an electrically heated platinum coil (about 600 “C). The construction of this heat decomposer is shown in Fig. 1. Sensors

Bionics Instruments’s HCl sensor (GS-4 1I), HF sensor (GS-720) and Cl, sensor (GS-101) were used in this study. These are based on the gas galvanic membrane technique and have the same structure as that previously [8] described. In all experiments, samples were introduced to a flow meter, a pyrolyser and an electrochemical sensor with a diaphragm pump as shown in Fig. 2. The

Power

source

Fig. 1. Structure of pyrolyser. 0

Elsevier Sequoia/Printed

in The Netherlands

69

Electrochemical Flow Meter Filament IN +v z! -=

flow rate O.Sllmin

sensor

10 A !

7.5ppm

HCI air balance

OUT

Adaptor Power source

Pump

Fig. 2. System configuration.

sensor was mounted vertically in a PVC flow block connected to a conventional flow system with Teflon tubes. The outer diameter of these tubes was 3 mm and the inner diameter was 2 mm. These sensors were exposed to air or mixtures of air and each of the chlorinated hydrocarbons. A total flow of 0.5 l/min was used throughout the experiments for freons. However, the flow was decreased to 0.2 l/min for the other chlorinated hydrocarbons.

flow rate O.Sllmin 3.09

Characteristics of the Three Electrochemical Sensors The characteristics of the HCl, HF and Cl* sensors tested are shown in Fig. 3 and Table 1. Table 1 shows that the HCl and Cl, sensors are very specific, while the HF sensor is not specific, i.e., the HF sensor responds to all acid gases such as HCl, Cl,, SO* and NO*. The HCl and Cl* sensors are insensitive not only to combustible gases such as hydrogen (H,) and carbon monoxide (CO), but also to SO2 and NOz, to which conventional electrochemical sensors are crosssensitive. In the case of the HCl sensor, hydrogen sulfide ( H2 S) was the only interferent, and chlorine (Cl*) at the 1 ppm level did not generate an output signal. On the other hand, when HCl (15 ppm)

air balance

(‘3

10

Results and Discussion

7.5PP m HF

PA

T

flow rate O.fil/min 22ppm Ctz air balance

m

j(tmin+

(4

Fig. 3. (a) Repeatability of HCl sensor. (b) Repeatability of HF sensor. (c) Repeatability of Cl, sensor.

was applied to the Clz sensor, an output signal was not generated. The typical response curves of these three sensors are shown in Fig. 3. The response times of the Cl, and the HCl sensors were faster than that

TABLE 1. Cross-sensitivity of three electrochemical sensors G&S

Concentration

HF sensor

Cl, Sensor

HF Cl* HCl

9 PPm 1 PPm 3 PPm 15 PPm 29 PPm 60 PPm 10 PPm

9 PPm 9 PPm 6 PPm >9PPm 0 PPm 0 PPm 2 PPm 9 PPm 0 PPm 0 PPm 0 PPm 9 PPm

0 PPm 1 PPm OPpm 0 PPm 0 PPm OPpm 0 PPm 0 PPm 0 PPm 0 PPm 0 PPm 1Ppm

HZS NH, NO, soz HZ co IPA Brz

1mm 4% 1% 1% 1 PPm

HCI sensor 0 PPm 0 PPm 15 PPm >lSppm 0 PPm

0 Ppm 0 PPm 0 PPm 0 PPm 0 PPm 0 PPm

70

of the HF sensor. The Cl2 sensor was especially fast, with a response time to 90% level (t& within 20 s. The response time of the HF sensor, however, was three minutes. It was confirmed that these sensors remained sensitive for six months. The repeatability figures for the HCl, Cl2 and HF sensors were & 5%, f 1% and &-lo%, respectively. Response to Freons The response obtained when the sensor was exposed to a flow of freon 12 ( 150 ppm) at a rate of 0.5 l/min is shown in Fig. 4(a). The HCl sensor showed good repeatability ( f 10%) when the flow rate was kept constant. A significant reduction of flow rate O.Sl/min 150ppm R-l 2 air balance pyrolyser off

flow rate 50001jpm

O.lSl/min CF4 air balance pyrolyser ojf

the output signal could be observed when the pyrolyser was turned off. This proves that the pyrolyser functioned effectively on freon 12, which is an electrochemically inactive gas. The response of the HF sensor to freon 12 was slower than that of the HCl sensor. The Cl1 sensor did not show any response to freon 12. It is concluded that the product formed by pyrolysis of freon 12 is HCl, not Clz. This is why the Cl1 sensor could not detect freon 12. The sample flow rate might a&t the sensitivity of the sensor, as shown in Fig. 5. With the HCl sensor system, linearity up to 500 ppm was obtained with freon 12. These data are shown in Fig. 6. In addition to freon 12, freons 11, 22, 113, 13B1, 14, 116 and 23 were also examined. The results are shown in Table 2. Of all of these gases, freon 11 generated the largest output signal in the HCl sensor. Freons 14, 116, 23 and 1381, however, could not be detected. The HF sensor could detect all freons tested in this experiment. The Cl, sensor displayed excellent performance only with respect to freon 13Bl. Its repeatability was f 1% and its response time was much shorter than that of the other two sensors. The final value was obtained within 30 s. The Cl* sensor, however, did not show any response to other freons. We concluded from this finding that the component of pyrolysis of freon 13Bl is Br2, which can be detected with the Cl, sensor.

40PA

Sample

R12

I\

(b) flow

rate

5Oppm

0.2llmin

CH3Br air balance pyrolyser off

I

10

(cl Fig. 4. (a) Effect of the pyrolyser for HCI sensor. (b) Effect of the pyrolyser for HF sensor. (c) Effect of the pyrolyser for Cl, sensor.

1 I

0

0.1

0.2

0.3 Flow

0.4 rate

0.6

0.6 llmin

Fig. 5. Effect of flow rate on HCl sensor, which is used in conjunction with a pyrolyser.

71 TABLE 2. Sensitivity of three electrocheakal Gas

sensors

Concentration (ppm)

p Alppm HF sensor

R-11 (CC&F) R-12 (CCl,F,) R-22 (CHCIF,) R-113 (C&l,F,) R-14 (CF,) _ R-116 (C,F,) R-23 (CHF,) R-13Bl (CF,Br)

300 300 300

0.07 0.06 0.09

5z!z MOO 500 100

6.4 0.08 x lO-4 5 x IO-’ 0.12 3.8 x lo-’

HCl sensor

Cl, sensor

0.16 0.09 0.07 0.08 0 0 0 0

0 0 0 0 0 0 0

0.03

using the HF sensor. This means that the HF sensor responds not only to HCl but also to other acid gases that are generated during pyrolysis.

1d.f Sample RI 2 flow rate 0.5 llmin

Response to Chlorinated Hydrocarbons

I2 50.z

0

600

1000 R12

1600 Ppm

Fig. 6. Linearity of HCl sensor, which is used in conjunction with a pyrolyser.

Chemically and thermally stable freons 14, 116 and 23 could be detected only with the HF sensor. The response curve resulting when freon 14 at 5000 ppm was applied to the HF sensor is shown in Fig. 4(b). The effectiveness of the pyrolyser is also shown in this Figure, but the response time of over three minutes was very slow. Freon 23, which contains H instead of F, generated a larger output signal than freons 14 and 116. The degree of sensitivity to freons of the HCl sensor is proportional to the number of chlorine atoms:

Other chlorinated hydrocarbons listed in Table 3 and 4 were tested using the same system as that for the freons. The results are shown in Tables 3 and 4. It was found that Ccl, generated the largest output signal among the four tested chlorinated hydrocarbons with the HCl sensor. However, there was no significant difference among the three chlorinated hydrocarbons except CH,Cl when the HF sensor was used as the detector. The Cl2 sensor did not show any response at the TLV level of these chlorinated hydrocarbons. Other chlorinated hydrocarbons listed in Table 4 were tested only with the HCl sensor. The response of the HCl sensor to these chlorinated hydrocarbons increases in the order tetrachloroethylene < 1,l ,Ztrichloroethane < trichloroethylene < 1,I, 1-trichloroethane. TABLE 4. Sensitivity of HCI sensor to various chlorinated hydrocarbons

Ccl, F, > Ccl, F, > CHClF, (R-11) (R-12) (R-22) However, there was no significant difference in output signal levels among these freons when

Gas

Concentration (ppm)

~Alppm

CH,CCl, CH,ClCHCl, CI,C:CCl, ClCH:CCl,

200 220 200 123

0.35 0.13 0.18 0.18

TABLE 3. Sensitivity of three electrochemical sensors Gi3.S

cc14 CHCl, CH,Cl, CH,Cl

Concentration (ppm)

fiAlppm

HF sensor

HCI sensor

Cl, sensor

30 200 100 100

0.21 0.16 0.20 0.05

0.42 0.41 0.21 0.12

0 0 0 0

12

sors such as those for HCl, HF and Cl,. The HCl sensor showed good response to all freons except CF.,, C2Fa, CHF, and CF,Br, which do not contain Cl. These freons could be detected with the HF sensor after pyrolysis. This HF sensor can also detect all freons and chlorinated hydrocarbons, although its response is much slower than that of the other two sensors. The chlorine sensor showed good repeatability and quick response only to CF,Br.

100 00 60 70 60 50 40 30 20

References

10 0

60

100

Chlorinated

160

200

250

hydrocarbon

300 PPrn

Fig. 7. Linearity of HCl sensor, which is used in conjunction with a pyrolyser.

The response to various chlorinated hydrocarbons had a linear dependence on concentration except in the case of l,l,Ztrichloroethane, which was saturated at 100 ppm. For comparison, the response to HCl is also presented in Fig. 7.

Conclusloas It is confirmed that various freons and chlorinated hydrocarbons can be detected continuously and quantitatively at room temperature by combining a pyrolyser and electrochemical sen-

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