SO2 sensors

The applicability of this system in the development of a crystalline SO2 sensor was studied in greater detail. While binding of gaseous substrates by nonporous crystalline materials may lead to the destruction of the long crystalline order, this did not happen in the case of 26. The crystallinity of the sensor material is maintained during the uptake and release of SO2, as was shown by time-resolved powder diffraction experiments (Figure 8). 

Figure 13.7 Outputs of CO2 and SO2 sensors with NASICON/p alumina electrolyte chains [88-91],...

Hence, a cation-conducting solid electrolyte can be used for a SO2 sensor. Figure 13.14 shows that the output of a sensor with an Ag P" alumina electrolyte [177, 178] and an Ag2SO4 electrode generates a response that is similar to those of sensors using Li2SO4-Ag2SO4 electrolytes [86, 87]. 

Figure 13,14 Outputs of SO2 sensors based on using Ag2SO4 in the electrolyte [177, 178] or auxilia electrodes [86, 87],...

Figure 13.17 Outputs of zirconia-based SO2 sensors based at 650°C [213—215],... 

Among mixed oxides employed in mixed potential sensors is ITO, this having been used for both NO [291] and CO [292-294] sensors. A further example of a doped oxide being used as an electrode is TiO2, which has been doped with tantalum for hydrocarbon sensors [295] or vanadium for SO2 sensors [296]. 

Nonequilibrium SO2 sensors have been reported using a sodium ion-conducting electrolyte (Na5DySi40i2) with sulfide electrodes. Among a large number of binary and mixed sulfides, CdS provides the best response [337, 338],... 

Shimizu. Y, Okimoto, M. and Souda, N. (2006) Solid-state SO2 sensor using a sodium-ionic conductor and a metal-sulflde electrode. Int. J. Appl. Ceram. Technol., 3 (3), 193-9. 

Souda. N. and Shimizu, Y. (2003) Sensing properties of solid electrolyte SO2 sensor using metal-sulfide electrode. J. Mater. Sci., 38 (21), 4301-5. 

Frechet-type polyether dendrons were complexed with Pt for use as SO2 sensor and with Ni for the Kharasch addition of CCI4 to methyl methacrylate [37]. The dendron wedges, when functionalized at their focal point, displayed adequate catalytic activity with easy recovery and good stability. 

A good survey of this field was presented by T. Takeuchi at the 2nd international meeting on chemical sensors in Bordeau, France, 1986. In Fig.7 some forms of the three types are presented along with the working principle, based on the paper mentioned. A SO2 sensor based on a similar principle has been developed by H. Torvela. The sensor produces a Nernstian response. 

In the case of applying sodium sulfate as the electrolyte, doping of a rare-earth element is essential in obtaining a suitable electrolyte for a SO2 sensor. Here, rare-earth doping also improves the electrical characteristics markedly, as previously mentioned in sect. 2. 

To maintain the SO2 reference gas circulation content at a known constant value is a critical barrier in using this type of SO2 sensor in a commercial application. In order to approach more practical sensing, devices using a solid (rather than a gaseous) SO2 reference have been investigated. For instance, a metal sulfate/metal oxide mixture can be used to provide such a stable SO2 content gas reservoir. 

One of the representative applications of this SO2 sensor is the measurement of volcanic emissions. In this case, hydrogen sulfide gas will coexist with SO2, and its effect upon the sensor has been tested. The result is shown in fig. 55 (Adachi and Imanaka 1991). With H2S gas coexistence lower than approximately lOOOppm, the EMF value deviates to a lower value in comparison to the calculated (dashed) line. In this measurement, H2S is mixed with air with a fixed SO2 gas content. Therefore, H2S can be expected to be oxidized by the oxygen in the test atmosphere. The relation between the EMF output and the SO2 content is replotted in fig. 56 (Adachi and Imanaka 1991) as EMF vs. the total amount of SO2. The total amount corresponds to the sum of SO2 gas initially introduced and the SO2 gas produced by the oxidation, assuming that the introduced H2S gas is completely oxidized. The... 

As a practical test for on-site application of the sensor for SO2 detection, a prototype probe sensor was fabricated. A sectional view of the probe sensor is illustrated in fig. 57 (Adachi and Imanaka 1991). In this type of probe, a mullite tube is utilized instead of the more expensive quartz tube. In order to prevent the chemical reaction between the sulfate electrolyte and the solid reference, an alumina plate was placed between them, as for the case of the SO2 sensor with the solid reference reservoir (sect. 5.6). The measurement was... 

Fig. 57. Sectional view of the SO2 sensor probe (Adachi and Imanaka 1991). (Reprinted by permission of the publisher, Kodansha Ltd.)...

Most of the recent progress in the detection of SO2 has been achieved with a sensor based on a solid electrolyte. Other solid electrolytes such as P-alumina (Itoh et al. 1984, Sugimoto and Kozuka 1988) and NASICON (Na+ Super Ionic Conductor) (Saito et al. 1983, 1984b, Maruyama et al. 1985) are also promising candidates for a practical SO2 sensor. 

Nevertheless, sensor cross-sensitivities can also bring new opportunities for gas measurement. Roberts et al. deployed an NO2 sensor in the plumes of Japanese volcanoes Aso and Miyakejima, and, noting that NO2 concentrations were much less than H2S, used the sensor s cross-sensitivity to measure H2S abundance. The sensor presented advantages over traditional H2S sensors because it did not exhibit a cross-sensitivity to SO2, and also because the time-response of the NO2 sensor (to H2S) is more similar to the SO2 sensor (see further discussion of measurement uncertainties below). In the same study Roberts et al. also co-deployed two CO sensors, both of which exhibited cross-sensitivities to H2. By co-analysing the two sensor outputs (which contrasted in their degree of cross-sensitivity), both CO and H2 abundances could be determined in the volcanic plume. 

Fig. 15.5 The importance of sensor response time illustrated for HCl and SO2 sensor pair. Left SO2 and HCl ppmv time series illustrated alongside a simulated slow SO2 time series using E3, S02 . The latter has an improved correlation to HCl compared to the standard SO2 time series. Right Scatter plot of HCl versus simulated slow SO2 time series used to derive an estimate for HCI/SO2 ratio...

UAVs can also be used to explore the temporal evolution of the volcano plume chemistry. A novel CMET (controlled meteorological) balloon system" has been used to follow the trajectory of the Kilauea (Hawaii) plume to perform quasi-Lagrangian studies of the plume evolution. Observations from the payload that included an electrochemical SO2 sensor as well temperature, pressure and humidity sensors" demonstrated correlation between SO2 and humidity in the near-source plume, but appear to show anti-correlation further downwind, potentially due to in-plume processing. 

Fig. 15.6 Upper. SO2 mixing ratio time series obtained in a dilute downwind volcanic plume using low-noise electronics, logging output from two nrai-identical miniature electrochemical SO2 sensors with high sensitivity (black) and lower sensitivity (purple), at 1 Hz and 0.1 Hz, respectively. Lower. Mixing ratio abundances of excess (plume peaks - background) CO and NO2 at a cross-roads in an urban environment (Orleans, France), measured at 1 Hz using low-noise electronics, with the high-sensitivity SO2 sensm used to detect NO2 (cross-sensitivity —120 %) alongside a high-sensitivity sensor for CO...

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