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

Abstract 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 HNO 3 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 HNO 3 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 HNO 3 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.