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Abstract: A new optical sensor for fast screening of ammonium (NH4 +) ion was developed based on immobilisation of p-nitrobenzene-diazonium chloride (Riegler''s reagent) onto XAD-7 microbeads. In aqueous solution, the diazonium salt is unstable and will spontaneously decompose at room temperature but we have successfully stabilised this compound via immobilisation using physical adsorption to create a solid-state Riegler''s reagent for NH4 + ion determination with a storage period of one month. The quantification of NH4 + ion concentration was possible by using reflectance spectrophotometry method with an optical fibre probe. The solid-state Riegler''s reagent based optical sensor yielded a wide linear response range for NH4 + ion of 10–60ppm and a fast response time of 2min when compared with many commonly used coloured reagents. The limit of detection (LOD) of the optical NH4 + ion sensor was 7.9ppm NH4 + ion with optimum response at pH 7. The response of the sensor was reproducible (4.2–5.7% relative standard deviation, n =3) and can be regenerated using buffer pH 1. Except for Fe3+ ions, most common ions showed no serious interference. The sensor was also used for the determination of NH4 + ion in river water and the results obtained were comparable to those obtained by a standard titrimetric method. [Copyright &y& Elsevier]

Abstract: One of the major problems in the analysis of ammonia (NH3) using spectrophotometric method is highly coloured water sample. A reflectometric NH3 sensor based on a facile single-step immobilisation of cobalt(II) (Co2+) ion onto high capacity Dowex HCR-W2 microspheres has been developed to enable the direct NH3 determination in highly coloured water samples. The combination of Co2+ ion-immobilised microspheres via cation exchange chemical reaction and reflectometric technique has improved the optode sensing performance with respect to linear response range and response time. Immobilised Co2+ ion, when complexed with NH3 gave a blue colouration of hexamminecobaltate(II) ([Co(NH3)6]2+) complex at pH 13. The NH3 sensor reached maximum colour intensity in ∼6min at ambient conditions. The linear calibration curve was 60–290mM NH3 with a detection limit of 16mM NH3, which is suitable for direct determination of NH3-enriched leachate water samples. A good reproducibility between 5.4 and 5.9% relative standard deviation (RSD) was obtained with the probe and the sensing performance can be regenerated using buffer pH 1. The effect of potential interference ions has been investigated during the quantification of NH3 in heavily polluted sewage water. The results obtained were in good agreement with those produced by acidimetric reference method. [Copyright &y& Elsevier]