Model for the response of an optical sensor based on absorbance measurements to HCl

A model for the response of an HCl optical sensor based on chemical reactions which produce a change in absorbance is presented. The model is based on the diffusion and kinetic processes involved in the sensing film. Parameters important to model include diffusion coefficients and concentrations of the different species, reaction rate constants and film layer thickness. A computer program is developed which includes all this relevant information in order to obtain the concentration of all the species in the layer as a function of time. The model response is compared with the quasi-reversible experimental response for an acidic gas sensor, usually used for CO₂, based on a membrane with an indicator. For the purpose of modelling the sensing film has been studied and characterised by different techniques and experiments. For this sensor the model reproduces very well the experimental change in absorbance, the difference of speed of the gas exposure and recovery transients, the quasi-reversibility and the reversibility dependence on the acid concentration. The model can also be applied to fully reversible sensors, as they are a particular case of quasi-reversibility, and to other similar sensors based on absorbance measurements and chemical reactions. Similar models can be applied to other systems based on optical, conductivity or mass change responses considering their respective processes controlling the response.