Flow injection indirect amperometric detection of ammonium ions using a clinoptilolite-modified electrode

A copper ion-doped clinoptilolite-modified carbon paste electrode was evalu ated as an amperometric sensor for non-electroactive ammonium cations in fl ow injection analysis. The electrochemical activity of copper was prevented in electrolyte solutions containing size-excluded cations such as tetrabut ylammonium, while the presence of non-size-excluded cations such as ammoniu m gave rise to appreciable responses. Consequently, the injection of ammoni um samples at the electrode surface (into a carrier stream made of tetrabut ylammonium bromide solution) resulted in peak currents proportional to the concentration of the analyte NH4+. After optimisation of the experimental c onditions, very reproducible results (standard deviations < 1%, n = 10) wer e obtained, linear calibration was achieved in the 2 X 10(-5) M to 1 X 10(- 3) M concentration range, and the detection limit was 5 X 10(-6) M. The eff ect of interfering cations was also discussed and the sensor's performance was evaluated for the analysis of ammonium in synthetic ground water. (C) 1 999 Elsevier Science S.A. All rights reserved.