Optical sensing of high acidity using a sol–gel entrapped indicator

Nuclear fuel reprocessing requires a real-time monitoring of nitric acid concentration in order to optimize extraction yields of valuable uranium and plutonium actinides. Thus, optical fiber sensors using doped sol-gel coatings have been developed to measure HNO3 concentrations in an extensive range 1-10 N. The transmission probes containing the reagent chromoxane cyanine R (CCR) have been tested in pure, plutonium-charged and real process HNO3 solutions. The sensors response exhibits a modification of the absorption properties and a broadening of the acidity range when compared to the response of free CCR in solution. Analytical performances are promising in the separate media, as they result in a short response time, a reversibility without hysteresis, a reproducibility better than 0.3 N for a 5 N HNO3 concentration and a lifetime of several months. As leaching of the indicator is the main responsible factor of sensors ageing, a careful control of the sol-gel parameters is still under study in order to reduce the porosity of the sensing films. (C) 1998 Elsevier Science S.A. All rights reserved.