Surface water typically contains up to 0.7 mg/L of iron. Drinking water typically contains up to 0.3 mg/L of iron, but this level may increase significantly if plumbing fixtures contain iron. In well-oxygenated, non-acidic waters, iron exists mainly in the ferric form (Fe3+) and will precipitate as iron oxide hydroxide (FeO(OH)). However, anoxic water may have high levels of dissolved ferrous iron (Fe2+), which could precipitate in heating/cooling systems or other equipment after exposure to air. The Iron (II) method measures the ferrous (Fe2+) form of iron.

The HI96770-03 are high-quality reagents that are pre-measured, allowing for users to achieve fast and accurate colorimetric measurements. These reagents use an adaptation of Standard Methods for the Examination of Water and Wastewater, 23rd Edition, 3500-Fe B, Phenanthroline Method. In aqueous solution, reactive ferrous iron (Fe2+) reacts with 1,10-phenanthroline to form an orange-red complex.