An optical sensor for measurement of gaseous ammonia

A simple optical fibre chemical sensor based on a nafion - crystal violet f ilm for the determination of gaseous ammonia in air is described. The film was prepared from a methanolic solution mixture containing 1.0 x 10(-3) mol L-1 crystal violet and 1.0 x 10(-3) mol L-1 nafion (in sulphonate groups) and determinations of ammonia concentration in air were achieved using refl ectance measurements at 580 nm. Ammonia seems to react irreversibly with th e film, since it cannot be released by flushing the film with dry air. Howe ver, a short pulse of humid air, followed by a stream of dry air can revers e the response of the film, providing a signal recovery greater than 90% at 580 nm. The film did not respond to carbon dioxide even at concentrations higher than 350 ppm, but it responds on exposure to 5 ppm of hydrogen hydro chloride and nitrogen dioxide. The influence of the relative humidity to th e sensor response is also presented and response mechanism to ammonia in dr y and humid air is suggested. A repeatability of 3.5%, expressed as the rsd of 6 measurements with a 20 ppm NH3 standard, was achieved. The sensitivit y of the sensor is tuneable by altering the exposure time to ammonia gas. D etection limits of 1.40 ppm and 0.46 ppm and linear responses up to 50 and 10 ppm in dry air were achieved with exposure times of 30s and 5 min, respe ctively.