Ammonia-sensitive polymer matrix employing immobilized indicator ion pairs

Abstract

A new sensor material for monitoring ammonia in aqueous solution is presented. It is based on lipophilic ion pairs consisting of an anionic pH indicator and a quaternary ammonium cation. The ion pairs are homogeneously distributed inside a layer of a silicone elastomer. Exposure of the indicators to ammonia leads to de-protonation of the hydroxy group, and this causes the strong absorbance of the basic form of the indicator. The process is fully reversible. The sensor can easily be manufactured from plastic materials and responds to ammonia over the concentration range of 0.015 to 20 mg l^–1 with a detection limit of 15 µg l^–1, and is not affected by pH because the silicone matrix is proton-impermeable. The sensor is intended for use as a probe to monitor ammonia in surface waters.