Re-examination of the determination of ammonium as the indophenol blue complex using salicylate

The most sensitive spectrophotometric procedure for the determination of ammonium is based on the conversion of ammonium into the intense blue indophenol complex (IPC) by means of salicylate and nitroprusside. Optimization of concentration, preparation and timing of the addition of reagents, reaction temperature and protection from light increased the conversion of ammonium into IPC by about 50% compared with existing methods. The absorbance was 0.60 at a concentration of 20 nmol of ammonium per millilitre of final solution (RSD of the manual procedure=0.6%; n=10) with a detection limit of 0.4 μM in samples.