A Renewable-Reagent Fiber-Optic Sensor for Measurement of High Acidities

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Abstract
A renewable-reagent fiber-optic HNO3 sensor was developed for HNO3 measurement in the 0.1−10.0 M range. The HNO3 sensor employs a tubular Nafion cation-exchange membrane to extract acid species from an external HNO3 sample into an internal flowing reagent solution. In high-concentration HNO3 samples, incomplete HNO3 dissociation results in a significant concentration of neutral HNO3 species in addition to protons. As both neutrals and protons are potentially membrane-permeable species, various reagent compositions were tested to examine the contributions of both acid transfer mechanisms. Continuous reagent flow limited internal acid accumulation and transferred reagent to the sensor optical detection cell. All reagent compositions included cresol red as a colorimetric indicator, which was measured within the sensor detection cell. Careful fiber-optic alignment provided sufficient light throughput in a backscatter illumination mode to allow use of a photodiode array detector for visible spectral acquisition. The use of Ca2+ as a reagent countercation produced notable reductions in HNO3 sensor response to interferent cations and temperature changes. Sensor measurement of HNO3 samples in the tested concentration range produced average relative standard deviations of less than 0.4%. Control over reagent flow rate should allow for extension of the HNO3 sensor measurement range to 16.0 M HNO3.