Doped thin-film sensors via a sol-gel process for high acidity determination

An optical sensor has been developed for high-acidity ([H+] = 1-11 M) measurements. The sensor is made of thin films of silica sol-gels doped with an acid indicator. Acid- and base-catalyzed methods to make the sol-gel films have been studied, and the properties of the sol-gel sensors prepared by these methods are discussed. The acid-catalyzed method was found to give more robust films and has been optimized to prepare thin films which are mechanically and chemically stable for a period of at least 3 months. The performance of the sensors resulted in a relative standard deviation of less than 2.5%. The response time is short (1 s), and a small hysteresis was observed during reproducibility measurements with 2-10 M HCl solutions.