STUDY OF THE PERFORMANCE OF AN OPTOCHEMICAL SENSOR FOR AMMONIA

An optical sensor for ammonia based on ion pairing has been investigat ed. A pH-sensitive dye (bromophenol blue) was immobilized as an ion pa ir with cetyltrimethylammonium in a silicone matrix. The colour of the dye changes reversibly from yellow to blue with increasing concentrat ion of ammonia in the sample. The concentration of ammonia can be dete rmined by measuring the transmittance at a given wavelength. All measu rements were performed with a dual-beam, solid state photometer. The m easurement range is from 6 X 10(-7) to 1 X 10(-3) M (0.01 to 17 mu g m l(-1)) in 0.1 M sodium phosphate buffer, pH 8. The 90% and 100% respon se times at a now rate of 2.5 ml min(-1) are 4 min and 10 min, respect ively, for a change from 41.9 to 82.5 mu M ammonia, or 12 min and 48 m in, respectively, for change from 160 to 0 mu M ammonia. A continuous drift in signal baseline and ammonia sensitivity limited the measureme nt stability. The sensor was useful over a period of a few days. The s torage stability is more than 10 months (dry). No interference due to pH was observed in the range from pH 5 to pH 9. Sensor performance is seriously affected by amines and cationic detergents. The sensor could be sterilized with 3% hydrogen peroxide or dry heat (90 degrees C).