Iron(III) chloride

2007 Schools Wikipedia Selection. Related subjects: Chemical compounds

Iron(III) chloride
Iron(III) chloride hexahydrate
Systematic name Iron(III) chloride
Other names ferric chloride
iron trichloride
molysite mineral form
Flores martis
Molecular formula FeCl3
Molar mass 162.21 g mol−1
hexahydrate: 270.30 g mol−1
Appearance green-black by reflected light,
purple-red by transmitted light
hexahydrate: yellow solid
aq. solutions: brown
CAS number [7705-08-0]
hexahydrate: [10025-77-1]
Density and phase 2.80 g cm−3
40% solution: 1.4 g ml−1
Solubility in water 92 g/100 ml (20 °C)
In other solvents: acetone
Diethyl ether
63 g/100 ml (18 °C)
highly soluble
83 g/100 ml
highly soluble
Melting point 306°C (579 K)
Boiling point 315°C (588 K), partial
decomposition to FeCl2+Cl2
Acidity (pKa)  ?
Viscosity 40% solution: 12 c P
Molecular shape  ?
Coordination geometry octahedral
Crystal structure hexagonal
Dipole moment  ? D
MSDS External MSDS
Main hazards Very corrosive
NFPA 704

R/S statement R: 22, 34 S: 26,28
RTECS number LJ9100000
Supplementary data page
Structure & properties n, εr, etc.
Thermodynamic data Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Other anions Iron(III) fluoride
Iron(III) bromide
Other cations Iron(II) chloride
Related metals Manganese(II) chloride
Cobalt(II) chloride
Ruthenium(III) chloride
Except where noted otherwise, data are given for
materials in their standard state (at 25°C, 100 kPa)
Infobox disclaimer and references

Iron(III) chloride, generically called ferric chloride, is the chemical compound with the formula FeCl3. The crystals appear a dark green colour by reflected light, but by transmitted light they appear purple-red as its colour depends upon the viewing angle. It is deliquescent, fuming in moist air due to the evolution of HCl, which hydrates, giving a mist. When dissolved in water, FeCl3 undergoes hydrolysis and gives off a great deal of heat as it is an exothermic reaction. The resulting brown, acidic solution, which is corrosive, is used as a coagulant in treating sewage and drinking water and as an etchant for copper-based metals (such as those found in printed circuit boards) and stainless steel.

Anhydrous iron(III) chloride is a fairly strong Lewis acid, and it is used as a catalyst in organic synthesis. The brownish-yellow hexahydrate is a common commercial form of ferric chloride, a yellowish species described by the formula [FeCl2(H2O)4]Cl.2H2O (compare chromium(III) chloride).


FeCl3 adopts the BI3 structure, which features octahedral Fe(III) centres and three-coordinate chloride ligands. FeCl3 has a relatively low melting point, and boils at around 315 °C. The vapour consists of the dimer Fe2Cl6 (compare aluminium chloride) that is increasingly dissociated into the monomer at higher temperature, in competition with the reversible decomposition to give FeCl2 and Cl2.

Chemical properties

Iron(III) chloride is a moderately strong Lewis acid which forms adducts with Lewis bases such as triphenylphosphine oxide, e.g. FeCl3(OPPh3)2 where Ph = phenyl. FeCl3 reacts with chloride salts to give the yellow tetrahedral FeCl4 ion. Solutions of FeCl4 in hydrochloric acid may be extracted into diethyl ether.

When heated with iron(III) oxide at 350 °C, FeCl3 gives the oxychloride FeOCl. In the presence of base, iron(III) chloride may undergo replacement of chloride, for example to produce an alkoxide:

FeCl3 + 3 C2H5OH + 3 NH3 → Fe(OC2H5)3 + 3 NH4Cl

Carboxylate salts such as oxalate, citrate or tartrate react easily with aqueous FeCl3 to form stable complexes such as [Fe(C2O4)3]3−.

Iron(III) chloride is also a mild oxidising agent, capable (for example) of oxidising copper(I) chloride to copper(II) chloride. Reducing agents such as hydrazine convert FeCl3 to complexes of iron(II).

Preparation and production

Anhydrous iron(III) chloride may be prepared by union of the elements:

2Fe( s) + 3Cl2(g) → 2FeCl3( s)

The solution of iron(III) chloride is produced on industrial scale via two methods, from iron and from ore, in a closed-loop process.

  • Dissolving pure iron in iron(III) chloride solution:
Fe(s) + 2FeCl3(aq) → 3FeCl2(aq)
Fe3O4(s) + 8HCl(aq) → FeCl2(aq) + 2FeCl3(aq) + 4H2O
  • Upgrading the iron(II) chloride with chlorine:
FeCl2(aq) + Cl2(g) → FeCl3(aq)

Alternatively, ferrous chloride can be oxidized with sulfur dioxide:

32FeCl2 + 8SO2 + 32HCl → 32 FeCl3 + S8 + 16H2O

Hydrated FeCl3(H2O)n can be converted to the anhydrous salt by heating with thionyl chloride. The hydrate cannot be converted to FeCl3 by heat, instead HCl is evolved and FeOCl forms


Iron(III) chloride is probably most widely used for etching copper in the production of printed circuit boards. This occurs by the redox reaction

FeCl3 + Cu FeCl2 + CuCl followed by FeCl3 + CuCl FeCl2 + CuCl2

Iron(III) chloride is also used as a catalyst for the reaction of ethylene with chlorine, forming ethylene dichloride ( 1,2-Dichloroethane), an important commodity chemical, which is mainly used for the industrial production of vinyl chloride, the monomer for making PVC.

In another industrial application, ferric chloride is an alternative to iron(III) sulfate in water treatment, where FeCl3 is treated with hydroxide ion to form a floc of "iron(III) hydroxide" (more correctly formulated as FeO(OH)) that can remove suspended materials.

In the laboratory iron(III) chloride is commonly employed as a Lewis acid for catalysing reactions such as chlorination of aromatic compounds and Friedel-Crafts reaction of aromatics. It is less powerful than aluminium chloride, but in some cases this mildness leads to higher yields, for example in the alkylation of benzene:

The "ferric chloride test" is a traditional colorimetric test for phenols which uses a 1% iron(III) chloride solution that has been neutralised with sodium hydroxide until a slight precipitate of FeO(OH) is formed. The mixture is filtered before use. The organic substance is dissolved in water, methanol or ethanol, then the neutralised FeCl3 solution is added—a transient or permanent coloration (usually purple, green or blue) indicates the presence of a phenol or enol.

FeCl3 is sometimes used by American coin collectors to identify the dates of Buffalo nickels that are so badly worn that the date is no longer visible.

FeCl3 is also commonly used by knife craftsmen and swordsmiths to stain blades, as to give a contrasting effect to the metal, and also to view metal layering or imperfections.


Iron(III) chloride is toxic, highly corrosive and acidic. The anhydrous material is a powerful dehydrating agent.

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