Humidity and pH sensor based on sulfonated poly-{styrene–acrylic acid} polymer.

Solid polymer electrolytes have been developed to interact with several different substances. In particular, humidity sensors and ionic conductivity have recently attracted increased attention due to their applications in food quality control and storage as well as environment humidity for air conditioning systems. In this work we report the preparation and characterization of sulfonated poly-{styrene–acrylic acid} to assess studies as humidity and pH sensors. Copolymers poly-{styrene–acrylic acid} were synthesized employing different styrene/acrylic acid monomer ratios. The copolymers were sulfonated using H2SO4 in CH2Cl2 as described elsewhere. Conductivity measurements were performed at different values of relative humidity (RH%). In a 10–100 RH% range the conductivity showed linear dependence with values ranging from 5.7×10−2 S to 1.0×10−1 S. The pH vs. λmax dependence can be observed at the maximum absorption band with changes from 435 nm (pH 2.6) to 460 nm (pH 12.0). However, in the pH 5.5 to 9.0 range, the λmax is constant and close to 444 nm. These results suggest the possible use of this material as a sensor in food quality control.