Influence of Moisture on Potentiometric Cl[sub 2] Gas Sensor Using a Na[sup +] Conducting Solid Electrolyte

Abstract

The influence of moisture in a test gas containing Cl 2 was investigated for a Cl 2 gas sensor using a NaCl-RuO 2 mixed measuring electrode and a composite Na ϩ conductor of sodium superionic conductor, 40 wt % (Na 2 O-Al 2 O 3 -4SiO 2 ). For dried Cl 2 gas, the electron number ͑n͒ for the reaction on the electrodes was 1.86 which was very close to the theoretical value n ϭ 2.0. When water vapor was added to the Cl 2 measuring gas, the electromotive force ͑emf͒ significantly decreased, since the Cl 2 gas concentration was reduced by the reaction with the H 2 O gas. Based on the calibration curve observed from the relationship between emf and Cl 2 concentration at 450°C under dry conditions, it is suggested that the sensor can determine parts per billion levels of Cl 2 gas under ambient conditions. Experimentally, the observed response time became faster with an increase in the flow rate and mainly depended on the exchange time of the test gas.Although many types of gas sensors have been investigated until now, potentiometric gas sensors using solid electrolytes have been reported to show rapid response, high sensitivity, and high selectivity for specific gases. In addition, their structures are more compact than that of the other detection methods. An oxygen gas sensor using an O 2Ϫ ion conductor has already been commercialized to control the air/fuel ratio in automobile exhausts. 1 Furthermore, the use of alkali ion conductors has been demonstrated for the detection of SO x , NO x , and CO 2 . 2-7 Chlorine gas has been used in large quantities for chemical production processes. Recently, it is desired for control and detection of chlorine and hydrogen chloride contained in the exhaust gas from an incinerator. It seems that the use of Cl Ϫ ionic conductors is suitable for the Cl 2 gas sensor. The MCl 2 -KCl (M ϭ Pb, Ba, or Sr͒ systems such as the 0.97PbCl 2 -0.03KCl and 0.97BaCl 2 -0.03KCl systems have been applied as solid electrolytes. 8-13 However, the gas sensors using chlorides cannot detect Cl 2 at concentrations lower than 10 ppm due to their poor sinterability and poor stability at high temperatures. On the one hand, Ag ϩ -␤-alumina and MgO-stabilized zirconia can also be used for this application. 14,15 While a high mechanical strength can be obtained for the solid electrolyte, the response of a sensor using Ag ϩ -␤-alumina is very slow even for high Cl 2 gas concentrations.In our previous study, it was reported that the Cl 2 gas sensor using a Na ϩ conductor with the composite material of polycrystalline sodium superionic conductor ͑NASICON͒ and glassy Na 2 O-Al 2 O 3 -4SiO 2 ͑40 wt %͒ with a NaCl-RuO 2 electrode can even detect in the low Cl 2 concentration region below 10 parts per million ͑ppm͒. 16,17 However, the electron number for the electrode reaction was about n ϭ 1.5 which did not agree with the theoretical number of n ϭ 2.0. We assumed that Cl 2 gas directly reacted with Na 2 O on the solid electrolyte. 17 Based on this assumption, we did not examine the existence of moisture in the test gas....

Sensing Techniques on Determination of Chlorine Gas and Free Chlorine in Water

“…The experimental data obtained from the dry condition test showed an electron number value of 1.86, which was comparable with the theoretical value thus showing that the operating environment of the sensor needs to be controlled. It is postulated that this type of Cl 2 sensor would be useful to detect ppb levels of Cl 2 gas under the environmental conditions found with exhaust gas from an incinerator [124].…”

A simplified way of sensing gases based on electrochemical cells

“…Sodium ion based gas sensors have already been reported [4,17] We can also monitor the anode side gas interaction [1]. The elimination of the anode (sodium metal) appears to be desirable because of safety reasons.…”