Optical sensors for the determination of concentrated hydroxide

An optical sensor system has been developed for the determination of concen trated strong bases ([OH-] = 1-10 M). The base sensors consist of SiO2/ZrO2 -organic polymer composites and doped high-pK(a) indicators. Films were obt ained by spin-casting these composite materials on glass substrates and wer e used as sensor elements for the spectrometric determination of hydroxide. The hydrophilic nature of the mixed oxide SiO2/ZrO2 and its chemical stabi lity in concentrated alkali made it attractive as support in the composites . The organic polymers in the composites either provided better mechanical stability and dye immobilization or enhanced OH- diffusion and sensor respo nse. The composite sensors showed a relative standard deviation of less tha n 2%. The response time of a SiO2/ZrO2-Nafion composite (sensor 2) was shor t (5 s), and a small hysteresis was observed during reproducibility measure ments with 1-4 M NaOH solutions. The sensors were found to be stable in 4 M NaOH during a 30-day durability test, showing a standard deviation of 3.0- 4.7%, The diffusion kinetics and hysteresis performance of the sensors were also evaluated.