Optical sensors in harsh environments : determination of concentrated strong acids and bases for process control

This dissertation describes the development of optical sensors for the determination of concentrated strong acid and alkali for process control in harsh environments. An overview of the Ph. D. research is given in Chapter 1, along with a summary of optical sensors, and Hammett acidity functions. Chapter 2 describes a portable fiber-optic sensor system for on-line high acidity ([H⁺] = 1 -11 M)measurements. The sensor consists of a thin-film silica support prepared through a sol gel process and doped with a low pKa indicator. The sensors are mechanically and chemically stable in acid for a period of at least 9 months. The performance of the sensors in this test period had a relative standard deviation of less than 2.0% with no recalibration. The sensor response time was short (1 - 5 s), and a small hysteresis was observed during reproducibility measurements with 2-10 M HCl solutions. The use of the sensor in solutions containing a large amount of salts and organic compounds is also discussed. In Chapter 3, sensors for the spectrometric determination of hydroxide ([OH^-]= 1 -10 M) are discussed. Such a sensor consists of a support prepared with a blend of organic polymers and SiO₂/ZrO₂, and an immobilized high pKa dye. The hydrophilic nature of inorganic oxides and their chemical stability in concentrated alkali make them attractive as support materials for sensors. The organic polymers provide a stronger mechanical support and better dye immobilization. Composites of SiO²ZrO² with organic polymers were found to produce sensors with high mechanical stability and fastresponse. The performance of the sensors showed a relative standard deviation of less than 2%. The response time was short (5 s), and a small hysteresis was observed in reproducibility measurements with 1 - 4 M NaOH solutions. The diffusion kinetics and hysteresis performance of the sensors were also evaluated. Chapter 4 describes the effect of salts, present in the acid analyte matrix, on the acid sensors. To correct the salt effect on sensor response, a dual-sensor approach has been developed. A novel linear relationship between (δ A/ δ C_salt)_Cacid and (dA/dC_acid)_{Csalt = 0} (A = absorbance of the indicator doped in the sensor film; C_acid = acid concentration; C; C_salt = salt concentration)was discovered. The dual-sensor approach to correct the salt effect was based on a set ofnon-linear equations derived from this relationship. Finally, in Chapter 5, the importanceof response hysteresis in our sensor systems is presented. The hysteresis of sensorresponse was found to be the major component of the analytical error in our opticalsensor systems. A correlation between sensor response time and hysteresis was observed.Sensors with fast response times had a small hysteresis. A variety of techniques havebeen developed to reduce the hysteresis in our sensor response.