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)/(III) method can be used to distinguish between the ferrous (Fe²⁺) and ferric (Fe³⁺) forms of iron in a 2-step measurement process.

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