Iron(III) chloride

Iron(III) chloride, generically called ferric chloride, is an industrial scale commodity hydrogen chloride mists in moist air.

When dissolved in water, iron(III) chloride undergoes organic synthesis.

Chemical and physical properties

Iron(III) chloride has a relatively low melting point and boils at around 315 °C. The vapour consists of the dimer Fe₂Cl₆ (compare chlorine gas.^[1]

Reactions

Iron(III) chloride is a moderately strong triphenylphosphine oxide, e.g. FeCl₃(OPPh₃)₂ where Ph = phenyl.

Iron(III) chloride reacts with other diethyl ether.

When heated with iron(III) oxide at 350 °C, iron(III) chloride gives iron oxychloride, a layered solid and intercalation host.

FeCl₃ + Fe₂O₃ → 3 FeOCl

In the presence of base, alkali metal alkoxides react to give the dimeric complexes:

2 FeCl₃ + 6 NH₄Cl

tartrate.

Iron(III) chloride is a mild hydrazine convert iron(III) chloride to complexes of iron(II).

Structure

Iron(III) chloride adopts the BiI₃ structure, which features octahedral Fe(III) centres interconnected by two-coordinate chloride ligands.

Preparation and production

Anhydrous iron(III) chloride may be prepared by union of the elements:^[2]

2 Fe(g) → 2 FeCl₃(s)

Solutions of iron(III) chloride are produced industrially both from iron and from ore, in a closed-loop process.

1. Dissolving pure iron in a solution of iron(III) chloride
   Fe(s) + 2 FeCl₃(aq) → 3 FeCl₂(aq)
2. Dissolving hydrochloric acid
   Fe₃O₄(s) + 8 HCl(aq) → FeCl₂(aq) + 2 FeCl₃(aq) + 4 H₂O
3. Upgrading the chlorine
   2 FeCl₂(aq) + Cl₂(g) → 2 FeCl₃(aq)

Alternatively, iron(II) chloride can be oxidised with sulfur dioxide:

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

Like many other hydrated metal chlorides, hydrated iron(III) chloride can be converted to the anhydrous salt by refluxing with thionyl chloride.^[3] The hydrate cannot be converted to anhydrous iron(III) chloride by only heat, as instead HCl is evolved and iron oxychloride forms.

Uses

Industrial

In industrial application, iron(III) chloride is used in floc of iron(III) hydroxide, or more precisely formulated as FeO(OH)^-, that can remove suspended materials.

Fe³⁺ + 4 OH⁻ → Fe(OH)₄⁻ → FeO(OH)₂⁻·H₂O

Another important application of iron(III) chloride is etching copper(II) chloride in the production of printed circuit boards.^[5]

FeCl₃ + Cu → FeCl₂ + CuCl

FeCl₃ + CuCl → FeCl₂ + CuCl₂

Iron(III) chloride is used as catalyst for the reaction of ethylene with PVC.

H₂C=CH₂ + Cl₂ → ClCH₂CH₂Cl

Laboratory use

In the laboratory iron(III) chloride is commonly employed as a 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 ethanol, then the neutralised iron(III) chloride solution is added—a transient or permanent coloration (usually purple, green or blue) indicates the presence of a phenol or enol.

Other uses

Anhydrous Iron(III) chloride is sometimes used as a drying reagent in certain reactions.

Iron(III) chloride 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.

Iron(III) chloride is commonly used by knife craftsmen and sword smiths to stain blades, as to give a contrasting effect to the metal, and to view metal layering or imperfections.

Iron(III) chloride is necessary for the etching of photogravure plates for printing photographic and fine art images in intaglio and for etching rotogravure cylinders used in the printing industry.

Iron(III) chloride is also used in veterinary practice to treat overcropping of an animal's claws, particularly when the overcropping results in bleeding.

Precautions

Iron(III) chloride is toxic, highly corrosive and acidic. The anhydrous material is a powerful dehydrating agent. In secondary/high schools all around the world, where Design and technology is a subject taught, Ferric Chloride used for PCB etching is usually diluted with water. Despite this, hands and other surfaces that have contacted it should still be washed immediately after one finishes with it.

See also

• iron(II) sulfate
• aluminium chloride

References

Further reading

1. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
2. The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
3. D. Nicholls, Complexes and First-Row Transition Elements, Macmillan Press, London, 1973.
4. A.F. Wells, 'Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.
5. J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
6. Handbook of Reagents for Organic Synthesis: Acidic and Basic Reagents, (H. J. Reich, J. H. Rigby, eds.), Wiley, New York, 1999.