Fibre optic pH sensors, for use in acid-base titrations, have been developed which use two wavelengths, in one case from two LEDs and in the other an internally generated reference replaces one of the LEDs, to sense the change in absorption of an indicator dye and provide a reference channel. A description of the construction and calibrated response of these inexpensive sensing devices is given.
K. T.v. Grattan,
R. K. Selli,
"PH Sensor Using A LED Source In A Fibre Optic Device", Proc. SPIE 0798, Fiber Optic Sensors II, (14 October 1987); doi: 10.1117/12.941111; https://doi.org/10.1117/12.941111