Development of dissolved CO_2 sensor for sea-water using ion conducting glass-based transducer

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Potentiometric dissolved CO_2 sensor was explored based on ion conducting glass which has ionic conductivity and water-resistance. Li_3Zr_2Si_2PO_<12> (Li^+ion conductor) was prepared by a sol-gel method. The gel was calcined in the range from 500℃ to 1000℃ and pressed into discs in the range from 400Kg/cm^2 to 600 Kg/cm^2. By XRD measurement, the halo patterns due to amorphous state were observed in the calcined samples below 650℃, and the peaks due to ZrO_2 or SiO were observed in the calcined samples above 650℃. FT-IR spectrum of the calcined sample at 1000℃ was different from those of the other samples. Ionic conductivity at room temperature of the calcined samples at 500℃ and 650℃ were ca. 10^<-7> S・cm^<-1> and ca.10^<-6> S・cm^<-1>, respectively. The calcined sample at 1000℃ was not ionic conductor. It was shown that the pressed disc at 550 Kg/cm^2 of the calcined sample at 650℃ has the best water-resistance of the other discs by measurement of the weight-loss of the disc after immersed in water for several weeks. Sensor elements were obtained by combining the discs with an auxiliary phase of metal carbonates. The EMF was found to follow the Nernst's equation of dissolved CO_2 in the range of 87-1308 ppm in phosphate buffer solution (pH 7.0) at 30℃. The observed Nernst slope by using of the pressed disc at 400 Kg/cm^2 of the calcined sample at 500℃ indicated n=4. On the other hand, the observed slopes by using of the pressed disc at 550Kg/cm^2 of the calcined sample at 650℃ indicated n=4-2. Further experiments, which apply to sea-water, will be made in the future.