Plastic colorimetric film sensors for gaseous ammonia

The preparation and characterization of three different plastic thin-film colorimetric sensors for gaseous ammonia is described. In the film sensors, the neutral form of a pH-sensitive dye (Bromophenol Blue, Bromocresol Green or Chlorophenol Red) was encapsulated in a plastic medium, either poly(vinyl butyral) or ethylcellulose plasticized with tributyl phosphate. Each of these film optodes gave a reproducible and reversible response towards gaseous ammonia. The sensitivity of the film sensors towards ammonia was found to be strongly dependent upon the pK_a of the encapsulated dye. Thus, the film with Chlorophenol Red (pK_a = 6.25), proved to be very insensitive (operating range: 0.29% < %NH₃ < 100%), whereas the film with Bromophenol Blue (pK_a = 4.1), was much more sensitive (operating range: 0.0003% < %NH₃ < 0.11%). The sensitivity of a plastic film sensor decreased markedly with increasing operating temperature and the 90% response (15–38 s) and recovery (820-127 s) times were slow and activation-controlled.