General model for the steady-state response properties of fiber-optic ammonia gas sensors

A mathematical model is derived to describe the steady-state response properties of the fiber-optic ammonia sensor. Unlike our previous model, the present model permits any concentration of total ammonia nitrogen initially present in the internal solution of the sensor. A cubic equation is solved to give the magnitude of the nonprotonated form of the indicator dye under equilibrium conditions. For the first time, the effect of the initial ammonia nitrogen concentration in the internal solution is evaluated. In addition, the response equation is differentiated with respect to the sample ammonia concentration to give an expression that can be used to evaluate the effects of critical experimental parameters on the measurement sensitivity. This model is verified by comparing predicted and actual responses for several different fiber-optic sensor configurations. The model is used to generate surface maps that allow evaluation of the interrelationships between key experimental parameters such as the indicator concentration, sample ammonia concentration, internal ammonium chloride concentration and indicator acid dissociation constant. Results of this analysis indicate that optimal responses and maximal sensitivity require careful selection of values for these parameters.