## 4 Applications of Piezoelectric Materials in Analytical Chemistry
### Detector for Air Pollutant Detection and Determination  
For over 60 years, analyzing SO\({}_{2}\) in the air has been a great concern. Oil refineries, paper and pulp mills, and the effluents released from a variety of other industries are all major sources of SO\({}_{2}\) emissions into the environment. Another ground-level pollution is the combustion of high-sulfur fuels in automobiles. As a result, there is a growing demand for new, effective, simple, and low-cost methods for measuring and controlling SO\({}_{2}\) pollution. Several scientific studies [144; 145; 146; 147; 148; 149; 150; 151; 152] have described the use of coated piezoelectric crystals as sensitive SO\({}_{2}\) detectors. Table 5 summarizes the various coating materials and their detection limits, as well as additional measurement settings. For SO\({}_{2}\), many coating materials have been studied. A novel detector was designed using triethanolamine and quadrol as the coating materials, which may detect trace quantities of SO\({}_{2}\)[153]. The primary characteristic of this system is the separation of the column effluent into two equal streams that fall on the opposite faces of a coated crystal simultaneously and directly. This design is expected to enhance sensitivity since the amount of the sample gas reacting with the coating at any specific time is greatly increased. Several experiments examined the effect of changing the temperature. With rising the temperature, the crystals' frequency also increased, according to [PERSON] et al. [145], especially from 100 \({}^{\circ}\)C to 200 \({}^{\circ}\)C. The influence of temperature increases just slightly from 25 \({}^{\circ}\)C to 40 \({}^{\circ}\)C at 40 Hz. According to these experiments, the temperature needs to be consistent during the reading, but an increase in temperature of 10 degrees Celsius is not critical. [PERSON] et al. [146] employed triethanolamine as a coating substance. They found that an uncoated 9 MHz crystal's temperature dependence changed by 71 Hz when the temperature was raised from 10 \({}^{\circ}\)C to 35 \({}^{\circ}\)C. Investigations were also conducted on the SO\({}_{2}\) desorption and adsorption of the coated material at various temperatures. Several diverse ways of applying the substrate to the piezoelectric crystal were examined. Spraying, dropping, and dipping techniques were among them. According to [PERSON] [154], the most important factor in coating the crystal is the capability to reproduce the coating procedure, not the amount of coating. For SO\({}_{2}\) detection, [PERSON] et al. [144] utilized cotton swabs to cover a crystal with ethylenedinitrotetraethanol. A center-covered 9 MHz crystal (340 Hz) was shown to be more sensitive to SO\({}_{2}\) as compared to the fully coated crystal (260 Hz). The scientists also found that the frequency variation caused by varying coating is predictable and constant for a center-coated crystal but not for a fully-coated crystal. Table 5 shows that NO\({}_{2}\) and moisture created significant interferences in the SO\({}_{2}\) assay when all coatings were utilized. Coated piezoelectric crystals cannot provide a reliable quantitative measurement of SO\({}_{2}\) in the presence of NO\({}_{2}\). Figure 32: Future society [134]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Coating Material** & **Coating Method** & **Sampling Method** & **Detection Limit ppm** & **Interference** & **Results** & **References** \\ \hline Ehylenediintriillo-tetraethanol & \begin{tabular}{c} Smearing with cotton \\ swabss \\ \end{tabular}  & Flow System in N\({}_{2}\) & 1 ppm & NO\({}_{2}\) & \begin{tabular}{c} Investigated the position \\ of the coating material \\ \end{tabular}  & [144] \\ \hline Triethanolamine & - & \ \begin{tabular}{c} Gas handling apparatus, \\ EPA method \\ \end{tabular}  & 25 ppm & NO\({}_{2}\) & \begin{tabular}{c} Investigation of the \\ response time and the \\ nature of the absorption \\ \end{tabular}