Determination of water-soluble ammonium ion in soil by spectrophotometry

A sensitive spectrophotometric procedure has been developed for the determination of water-soluble ammonium ion based on the formation of a coloured complex by the reaction of ammonium ion with salicylate in the presence of sodium hypochlorite and sodium nitroprusside. The suitable basicity range for the formation of the coloured complex is 0.006–0.023 M NaOH and the stoicheiometric ratio is 1 : 2 (NH4+:salicylate).The wavelength of maximum absorbance is 697.5 nm and the system obeys the Lambert-Beer law between 0 and 2 µg ml–1 of ammonium ion. A molar absorptivity of ε=1.5 × 104 l mol–1 cm–1 and a Sandell's sensitivity of 9.3 × 10–4µg cm–2 have been obtained.The detection limit of ammonium ion is 10 µg l–1. The procedure has good selectivity and hence can be applied to the direct spectrophotometric determination of trace amounts of ammonium ion in soils.