Optical sensors for the determination of concentrated hydroxide.

An optical sensor system has been developed for the determination of concentrated strong bases ([OH-] = 1-10 M). The base sensors consist of SiO2/ZrO2-organic polymer composites and doped high-pKa indicators. Films were obtained by spin-casting these composite materials on glass substrates and were used as sensor elements for the spectrometric determination of hydroxide. The hydrophilic nature of the mixed oxide SiO2/ZrO2 and its chemical stability 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 response. The composite sensors showed a relative standard deviation of less than 2%. The response time of a SiO2/ZrO2-Nafion composite (sensor 2) was short (5 s), and a small hysteresis was observed during reproducibility measurements 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.