OPTOCHEMICAL SENSOR FOR AMMONIA BASED ON A LIPOPHILIZED PH INDICATOR IN A HYDROPHOBIC MATRIX

An optical sensor for the determination of ammonia in water based on i on pairing has been investigated. A pH-sensitive dye is immobilized as an ion pair in a silicone matrix. The colour of the dye changes from yellow to blue depending on the concentration of ammonia in the sample solution. This change is reversible. The concentration of ammonia can be determined by measuring the transmittance at a given wavelength. A ll measurements were performed with a dual-beam optical meter. The mea surement range was from 5.9 x 10(-7) to 1 x 10(-3) M (0.01 to 17 mg/l) in 0.1 M phosphate buffer of pH 8. The detection limit was 10 mu g/l. The response times at a flow rate of 2.5 ml/min were 4 min for t(90) and 10 min for t(100) at a change from 41.9 to 82.5 mu M ammonia and 1 2 min for t(90) and 48 min for t(100) at a change From 160 to 0 mu M a mmonia. The operational lifetime of the ammonia sensor was limited to a period of a few days only. A continuous decrease in baseline signal and relative signal change was observed over the whole measurement. Th e storage stability was more than 10 months (dry). With respect to pos sible application of the ammonia sensor to environmental analysis, the influence of pH, typical interferences, such as amines and various de tergents on the sensor response was investigated. No interference due to pH was observed in the range from pH 5 to pH 9. With methyl-and eth ylamine the response was not completely reversible. The sensor was aff ected by cationic detergents, but not by anionic or neutral detergents .