A POLYPYRROLE-BASED AMPEROMETRIC AMMONIA SENSOR

An ammonia sensor is described in this work. The sensing membrane is a thin layer of oxidized polypyrrole (PPy) on a platinum substrate. Thi s sensor is used as the working electrode in a conventional three-elec trode system for amperometric measurement of ammonia in aqueous soluti ons in the potential range of +0.2 to +0.4 V (vs. Ag/AgCl). Contact wi th ammonia causes a current to flow through the electrode. This curren t is proportional to the concentration of free ammonia in the solution and ammonium ions do not contribute to the measured signal. The signa l is due to reduction of PPy by ammonia with subsequent oxidation of P Py by the external voltage source. The sensor is able to detect ammoni a reproducibly al the mu M level. The main interference is the doping effect of small anions such as Cl- and NO3-, also giving a response on PPy at the mM level. This anionic response can, to a certain degree, be reduced by covering the polymer surface with dodecyl sulfate. The s ensor gradually loses its activity when exposed to ammonia concentrati ons greater than 1 mM. The sensor has been tested by the flow injectio n analysis technique.