Investigation of relative humidity effects on the response behavior of a pH indicator-based OWG vapor sensor

Abstract

The response of a pH indicator-based optical waveguide sensor was characterized with respect to the effects of relative humidity (RH) on the magnitude of the sensor response, and on the rate of response to both hydrochloric acid and ammonia/ammonium hydroxide vapors. Water vapor constitutes both a chemical and a systematic (optical) interference for the OWG sensor response to hydrochloric acid. Swelling of the polymer films upon exposure to water vapor results in a decrease in the loss of light at the polymer/air interface, resulting in an increase in the sensor signal. In addition, high RH conditions decrease the bromothymol blue indicator response to hydrochloric acid vapor. In contrast, the bromothymol blue indicator response to ammonia increases as the RH increases. High RH levels also increases the rate of diffusion (transport) of hydrochloric acid into (and out of) Nafion films, but does not affect the diffusion rate for poly(vinyl alcohol) polymer films. The RH does not appear to have any significant effect on the rate of transport of ammonia in any of the polymer films studied.