Ferric chloride is FeCl3, iron(III) chloride: iron carries a +3 charge, and our 40% grade is a dark brown solution. Ferrous chloride is FeCl2, iron(II) chloride: iron carries a +2 charge, and its solutions are typically greenish. Both are iron chloride salts, but they do different jobs.
Two iron chlorides, one letter apart
"Iron chloride" is not one chemical. Iron forms two common chlorides, and the difference is the charge on the iron atom.
- Ferric chloride is FeCl3. The iron is in the +3 state, so three chloride ions balance it. Its systematic names are iron(III) chloride and iron trichloride.
- Ferrous chloride is FeCl2. The iron is in the +2 state, so two chloride ions balance it. Its systematic names are iron(II) chloride and iron dichloride.
The older names follow a simple rule: -ic marks the higher charge (ferric, +3) and -ous the lower (ferrous, +2).
So what is the name of FeCl2 and FeCl3?
FeCl3 is ferric chloride, also called iron(III) chloride or iron trichloride. FeCl2 is ferrous chloride, also called iron(II) chloride or iron dichloride. In a specification or a safety data sheet, the CAS number removes any doubt: 7705-08-0 for ferric chloride and 7758-94-3 for ferrous chloride.
Is iron chloride a salt?
Yes. Both ferric and ferrous chloride are salts: ionic compounds built from positively charged iron ions and negatively charged chloride ions. Both are highly soluble in water, and industry handles them mainly as liquid solutions rather than dry solids.
They are also acidic salts. When the iron ions meet water they hydrolyse, which releases acidity. Both solutions are strongly acidic; the value depends on concentration and grade. Our 40% ferric chloride grade is specified at pH below 1.0 (1% solution). That hydrolysis is also the reason ferric chloride works as a coagulant, as explained below.
Iron chloride colour: why FeCl3 is brown and FeCl2 is green
Colour is the quickest way to tell the two apart.
FeCl3 colour
Pure, dry (anhydrous) ferric chloride forms dark crystals. Reference data describe them as red by transmitted light and green by reflected light, sometimes appearing brownish-black. In practice, almost nobody in water treatment sees the dry solid. The commercial product is a solution, and ferric chloride solutions range from yellow-brown to dark brown; the US EPA design manual describes the commercial solution as orange-brown. Our 40% grade is a dark brown liquid.
FeCl2 colour
Pure anhydrous ferrous chloride is described as white crystals that may show a green tint. Ferrous chloride solutions are typically greenish. Our ferrous chloride grade is a greenish liquid.
Why the colour can drift
Ferrous iron tends to oxidise to ferric iron when it is exposed to air. A ferrous solution that has been open for a long time can therefore shift from green towards yellow-brown. A colour change on its own is not a test result. If the iron state matters to your process, check the agreed specification and the batch analysis, and refer an unexpected appearance to the supplier before use.
Side-by-side comparison
| Property | Ferric chloride | Ferrous chloride |
|---|---|---|
| Formula | FeCl3 | FeCl2 |
| Other names | Iron(III) chloride, iron trichloride | Iron(II) chloride, iron dichloride |
| Iron oxidation state | +3 | +2 |
| CAS number | 7705-08-0 | 7758-94-3 |
| Molar mass | 162.20 g/mol | 126.75 g/mol |
| Iron share of the pure salt (calculated) | about 34.4% | about 44.1% |
| Colour of the solution | Yellow-brown to dark brown | Greenish |
| Our grade | 40% ± 1% FeCl3 solution | Up to 30% FeCl2 solution |
| Specific gravity of our grade | 1.40 to 1.45 | 1.25 to 1.35 |
| Main uses for our grade | Phosphorus removal, sludge conditioning, turbidity removal, municipal wastewater treatment | Coagulation, hydrogen sulphide scavenging, refinery de-oiling, oil-production flooding |
The iron share is calculated from the molar masses: iron (55.85 g/mol) divided by the molar mass of the salt. It describes the pure compound, not the solution.
Uses: different charge, different job
Ferric chloride uses
Ferric chloride is one of the most widely used coagulants in water and wastewater treatment. When it is dosed into water, the Fe3+ ion hydrolyses to ferric hydroxide flocs. These flocs destabilise and collect fine particles, colour and organic matter, which then settle or filter out. The same chemistry makes ferric chloride a standard tool for chemical phosphorus removal, because Fe3+ precipitates dissolved phosphate. It is also used to condition sludge before dewatering.
For drinking-water use, many utilities and regulators require treatment chemicals to meet NSF/ANSI/CAN 60. Our ferric chloride is listed under NSF/ANSI/CAN 60, with a listed maximum use level of 100 mg/L. To compare it with the other common coagulants, see ferric chloride vs PAC vs alum.
Ferrous chloride uses
Ferrous chloride also coagulates, but the +2 ion behaves differently. It is used where its own chemistry is an advantage. One example is sulphide control: ferrous iron binds dissolved sulphide, which helps limit hydrogen sulphide. In refining and oil production it is used for de-oiling and related process duties. Ferrous chloride serves these specific duties; it is not a drop-in replacement for ferric chloride.
Which one do you need?
Choose by the job, not by the name.
- For coagulation in drinking water or municipal wastewater, and for phosphorus removal, ferric chloride is the usual choice. See our phosphorus removal guide.
- For sulphide control or refinery and oilfield duties, ferrous chloride is often the better fit. See our ferrous chloride page.
- If you are unsure, a jar test on your own water will show which product, and which dose, performs best.
Product specifications and the technical data sheet for our 40% grade are on the ferric chloride 40% (CHIMIFLOC FR 4014) page. Before you order, see our buyer's checklist for ferric chloride 40% solution. For hazard and handling information, see the safety data sheet (on request).
References
- PubChem, National Library of Medicine. Ferric chloride (CID 24380): formula, molar mass, synonyms, colour and form (checked 6 October 2026).
- PubChem, National Library of Medicine. Ferrous chloride (CID 24458): formula, molar mass, synonyms, colour and form (checked 6 October 2026).
- NIST Chemistry WebBook. Iron trichloride, CAS 7705-08-0 (checked 6 October 2026).
- NIST Chemistry WebBook. Iron dichloride, CAS 7758-94-3 (checked 6 October 2026).
- US Environmental Protection Agency. Nutrient Control Design Manual, EPA/600/R-10/100, August 2010: chapter 3, chemical phosphorus removal; forms of metal salts (checked 6 October 2026).
- NSF International. Drinking water treatment chemicals listings, NSF/ANSI/CAN 60, company search "chimi" (checked 6 October 2026).