This method is used for the determination of iron in water and wastewater. It is applicable across several ranges. See the flow diagram for specific range information.
Iron is reduced to the ferrous state by the addition of hydroxylamine. Excess acid is neutralized by the addition of buffer. An orange-red complex is then formed with 1,10-phenathroline reagent and measured at 510 nm.
Strong oxidizing agents such as cyanide and nitrite interfere. Phosphates, chromium and zinc in concentrations exceeding 10 times that of iron also interfere, as well as cobalt and copper in excess of 5 mg/L and nickel in excess of 2 mg/L. Bismuth, cadmium, mercury, molybdate, and silver all precipitate phenanthroline. The initial boiling with acid converts polyphosphates to orthophosphates and removes cyanide and nitrite. The addition of excess hydroxylamine eliminates errors caused by excessive concentrations of strong oxidizing agents. In the presence of interfering metal ions, a larger excess of phenanthroline is required to replace that complexed by the interfering metals.