Iron(III) chloride

Iron(III) chloride

General
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 hexahydrate: 270.30 g/mol
Appearance	brown-black solid hexahydrate: yellow solid solution: brown
CAS number	[7705-08-0] hexahydrate: [10025-77-1]
Properties
Density and phase	2.80 g/cm³ 40% solution: 1.4 g/ml
Solubility in water	92 g/100 ml (20 °C)
In other solvents: acetone Methanol Ethanol 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 (pK_a)	?
Viscosity	40% solution: 12 c P
Structure
Molecular shape	?
Coordination geometry	octahedral
Crystal structure	hexagonal
Dipole moment	? D
Hazards
MSDS	External MSDS
Main hazards	Very corrosive
NFPA 704
Flash point	? °C
R/S statement	R: 22, 34 S: 26,28
RTECS number	LJ9100000
Supplementary data page
Structure & properties	n, ε_r, 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 an iron-based salt of chemical formula FeCl₃. It is very deliquescent and it fumes in moist air with hydrolysis and when dissolved in water, it evolves a great deal of heat and produces a brown, acidic solution. This corrosive liquid is used in treating sewage and drinking water, and to etch copper-based metals (such as those found in electrical 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 FeCl₃ which in fact has the structure [FeCl₂(H₂O)₄]Cl.2H₂O (compare chromium(III) chloride).

When heated, FeCl₃ melts then boils at around 315 °C. The vapour contains dimers of Fe₂Cl₆ (compare aluminium chloride) which slowly decompose to give FeCl₂ and Cl₂.

Chemical properties

Iron(III) chloride is a moderately strong Lewis acid which with Lewis bases such as triphenylphosphine oxide forms stable adducts such as FeCl₃(OPPh₃)₂ where Ph = phenyl. With chloride ion several anionic complexes are known, but the most stable contain 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 the oxychloride FeOCl is formed. 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 cause reduction of FeCl₃ to complexes of iron(II).

Preparation and production

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

2 Fe( s) + 3 Cl₂( g) → 2 FeCl₃( s)

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

Dissolving pure iron in iron(III) chloride solution:

Fe(s) + 2 FeCl₃(aq) → 3 FeCl₂(aq)

Dissolving iron ore in hydrochloric acid:

Fe₃O₄(s) + 8 HCl(aq) → FeCl₂(aq) + 2 FeCl₃(aq) + 4 H₂O

Upgrading the iron(II) chloride with chlorine:

FeCl₂(aq) + Cl₂(g) → FeCl₃(aq)

Hydrated FeCl₃ can be dehydrated to the anhydrous salt by heating with thionyl chloride.

Uses

Iron(III) chloride is probably the most widely used etching material^[1]. It is commonly 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.

Another industrial application 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 most commonly used 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^[7] 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 sword smiths to stain blades, as to give a contrasting effect to the metal, and also to view metal layering or imperfections.

Precautions

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

Suppliers/Manufacturers

Sigma-Aldrich
Strem
Fisher
VWR
Alfa