This method covers the determination of ammonia in drinking, surface, and saline waters, domestic and industrial wastes using APHA Standard Method 4500-NH3 G. Alkaline phenol and hypochlorite react with ammonia to form indophenol blue that is proportional to the ammonia concentration. The blue color formed is intensified with sodium nitroprusside. For details about the process see the colorimetry overview. Samples can be preserved by addition of 2 ml conc. H2SO4 per liter and refrigeration at 4 degrees C.
Calcium and magnesium ions may be present in concentration sufficient to cause precipitation problems during analysis. A 5% EDTA solution is used to prevent the precipitation of calcium and magnesium ions from river water and industrial waste. For sea water a sodium potassium tartrate solution is used. Sample turbidity and color may interfere with this method. Turbidity must be removed by filtration prior to analysis. Sample color that absorbs in the photometric range used will also interfere.