A different point of view: a continuous tracing of acid-base titration with fiber-optic sensor

Our main aim was to investigate variations in the refractive index of a solution during chemical reaction. The variation of refractive index was measured through the entire acid--base titration process, especially near the equivalence point. To determine the equivalence point, the conductivity of the solution and the refractive index were measured simultaneously. In this preliminary study, sodium hydroxide and hydrochloric acid were used as the base and the acid solution, respectively. To measure the refractive index variation, a fiber-optic refractive index sensor was designed. The fiber-optic probe was dipped into the solution and acted as a refractive index sensor according to Fresnel's fundamental reflection law. A conductometer, a lab-made fiber-optic refractive index sensor ensemble, and a lab-made optical drop counter were each connected to a computer via an analog digital converter and the data acquisition was performed with the LabVIEW program. The equivalence point was derived easily from the refractive index data for the sodium hydroxide solution with different molarities of hydrochloric acid. In our opinion, the measurement of the variation of the refractive index during this kind of chemical reaction is more sensitive than the conductometric measurement.