Optochemical sensor for HCl gas based on tetraphenylporphyrin dispersed in styrene–acrylate copolymers: Effects of glass transition temperature of matrix on HCl detection

The spectral changes of tetraphenylporphyrin (TPPH2)–polymer composites in the visible region were examined in order to detect HCl gas. For TPPH2, the absorbance of the Soret and Q bands were sensitive to ppm levels of HCl. The sensing mechanism were characterized basing the protonation reaction and sorption isotherm: TPPH2+2HClfilm⇄TPPH42+·2Cl− and [HClfilm]=a[HClgas]n. The equilibrium constant of the former was decreased with increase in the working temperature and in glass transition temperature (Tg) of polymer matrix. The number of active sites (a) for HCl sorption decreased with a decrease in the Tg. The response behavior was improved by using polymer with lower Tg.