Determination of ammonia by solid-phase spectrophotometry

A simple, rapid and highly sensitive method was developed for the determination of trace amounts of ammonia. The method is based on the following steps: (1) ammonia is oxidized to nitrite ion with sodium hypochlorite in the presence of potassium bromide in an alkaline media; (2) the effect of remaining hypochlorite is masked by the addition of acetone; (3) the ion is reacted with sulfanilamide and N-1-naphthylethylenediamine in an acidic media; (4) the resulting reddish-purple compound is concentrated and fixed on a finely ground cation-exchange resin by batch operation; (5) the resin particles are filtered through a membrane filter in the presence of Zephiramine, leaving a colored thin-layer disc on the filter; and (6) the absorbance of this thin layer is measured directly with a spectrophotometer (545 nm). A calibration graph of good linearity was obtained up to 1.0 mug ammonia in a 10 mi of sample solution when the resin phase absorbance was measured against a resin thin layer, while linearity up to 0.8 mug of ammonia was obtained when the resin-phase absorbance was measured against a membrane filter. The detection limit was 5.3 ng (n = 5) and 24.0 ng (n = 5) in a 10 mi sample solution, respectively. The proposed method was applied to the determination of ammonia in a river-water sample. Recoveries of 93.6 similar to 107% were obtained.