A sensor for pH based on an optical reflective device coupled to the swelling of an aminated polystyrene membrane

We have developed a low cost pH sensor based on an optical reflective device (ORD) coupled to a toughened, lightly crosslinked aminated polystyrene membrane. Protonation of the amine group causes the polymer to swell. This is accompanied by a decrease in the turbidity of the membrane. As a result, reflected intensity measured by the ORD is a function of pH. The resulting pH sensor is inexpensive and has modest power requirements. The sensor is stable, provided the membrane is not allowed to dry out and crack. Changing ionic strength affects response both by changing the turbidity of the membrane due to refractive index effects and also by reducing the extent to which the protonated polymer swells. Increasing temperature leads to a larger change in turbidity.