What’s the Difference Between Ferrous and Ferric Iron?

Written by Craig "The Water Guy" Phillips

Ferrous iron (Fe²⁺) is dissolved iron with a +2 charge: water containing it comes out of the tap clear, then turns orange or brown as it meets air. Ferric iron (Fe³⁺) is oxidized iron with a +3 charge: it is already in solid particles, so the water looks rusty or cloudy right away. The difference matters for treatment, because a water softener can remove small amounts of ferrous iron but not ferric iron, which has to be filtered out.

Ferrous vs. Ferric Iron at a Glance

What Is Ferrous Iron?

Ferrous iron is iron that has lost two electrons (Fe²⁺). It stays dissolved in water that has little oxygen, which is why it is common in wells and deep groundwater. Because it is dissolved, a glass of well water with ferrous iron looks clear when you first pour it. Leave it on the counter and it slowly turns yellow, orange or brown as the iron oxidizes.

The name follows the chemistry convention for lower charge: -ous means the lower oxidation state (+2) and -ic means the higher one (+3). The same pattern shows up in ferrous sulfate (FeSO₄) and ferric sulfate (Fe₂(SO₄)₃), and in ferrous oxide (FeO) and ferric oxide (Fe₂O₃, the main part of rust).

What Is Ferric Iron?

Ferric iron has lost three electrons (Fe³⁺). In household water it does not stay dissolved; it forms tiny particles of iron oxide and hydroxide. That is why ferric iron shows up as rusty, cloudy or orange water straight from the tap, settles to the bottom of a glass, and leaves red-brown stains on fixtures and laundry.

Ferric iron in a home usually comes from ferrous iron that has already oxidized, for example after it sits in a pressure tank, mixes with air, or meets chlorine. It can also come from corroding iron or steel pipes.

How Ferrous Iron Turns Into Ferric Iron

When dissolved ferrous iron meets an oxidizer such as oxygen in the air, chlorine or hydrogen peroxide, it loses another electron and becomes ferric iron, which then comes out of solution as particles. Higher pH speeds this up. Iron filters use this on purpose: they oxidize ferrous iron into ferric particles and then trap those particles in a media bed that is backwashed to the drain.

How to Tell Which Iron You Have

The simplest check is the glass test: fill a clear glass with cold water straight from the tap. If it is clear at first and turns orange after sitting, you have ferrous iron. If it comes out rusty or cloudy right away, you have ferric iron (often along with some ferrous). We explain the full method in how to know if iron is ferrous or ferric . For an actual number in ppm, use a lab test or a test kit; see DIY methods to test for high iron .

Treatment Differences: Ferrous vs. Ferric Iron

For most wells, the practical rule is: up to 3 ppm of clear-water (ferrous) iron, a SoftPro softener can handle it along with hardness. Above 3 ppm, or if you see ferric iron, rotten-egg odor or manganese, put an iron filter in front of the softener. The SoftPro AIO Iron Master is an air-injection filter with Katalox media that handles up to 30 ppm of iron, up to 7 ppm of manganese and hydrogen sulfide, and raises pH on water as low as 5.8. Note that a softener does nothing for sulfur odor.

Learn more in how an iron filter for well water works , the best setup for high iron in well water , or browse our iron filters for well water . If iron has already built up in your softener or plumbing, see how to flush iron buildup from your system .

Is Ferrous or Ferric Iron Harmful in Water?

Neither form is considered a health hazard at the levels found in household water. The EPA lists iron under its secondary (non-enforceable) standards at 0.3 mg/L because above that level it causes metallic taste, discolored water and stains. The practical problems are stains on fixtures and laundry, buildup in pipes, water heaters and appliances, and resin fouling in softeners.

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