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. A portable fiber-optic sensor system for on-line high acidity ([H+] = 1--11 M) measurements is described. 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, with a performance of +/-2.0% with no re-calibration. 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. Sensors for the spectrometric determination of hydroxide ([OH-- ] = 1--10 M) are discussed. Such sensors consist 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 SiO2/ZrO2 with organic polymers were found to produce sensors with high mechanical stability and fast response. The performance of the sensors showed a relative standard deviation of less than 2%. The response time was short (5s), 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. The effect of salts, present in the acid analyte matrix, on the acid sensors was also investigated. To correct the salt effect on sensor response, a dual-sensor approach has been developed. A novel linear relationship between the partial derivatives of absorbance with respect to acid concentration and salt concentration was discovered. The dual-sensor approach to correct the salt effect was based on a set of non-linear equations derived from this relationship. The hysteresis of sensor response was found to be the major component of the analytical error in our optical sensor systems. A correlation between sensor response time and hysteresis was observed. Sensors with fast response times had a small hysteresis. A variety of techniques have been developed to reduce the hysteresis in our sensor response.