A renewable-reagent fiber-optic sensor for measurement of high acidities

A renewable-reagent fiber-optic HNO(3) sensor was developed for HNO(3) measurement in the 0.1-10.0 M range. The HNO(3) sensor employs a tubular Nafion cation-exchange membrane to extract acid species from an external HNO(3) sample into an internal flowing reagent solution. In high-concentration HNO(3) samples, incomplete HNO(3) dissociation results in a significant concentration of neutral HNO(3) 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 Ca(2+) as a reagent countercation produced notable reductions in HNO(3) sensor response to interferent cations and temperature changes. Sensor measurement of HNO(3) 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 HNO(3) sensor measurement range to 16.0 M HNO(3).