Iron(III) chloride

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Iron(III) chloride, also called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl₃ and with iron in the +3 oxidation state. The colour of iron(III) chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red. Anhydrous iron(III) chloride is deliquescent, forming hydrated hydrogen chloride mists in moist air. It is rarely observed in its natural form, the mineral molysite, known mainly from some fumaroles.

When dissolved in water, iron(III) chloride undergoes hydrolysis and gives off heat in an exothermic reaction. The resulting brown, acidic, and corrosive solution is used as a flocculant in sewage treatment and drinking water production, and as an etchant for copper-based metals in printed circuit boards. Anhydrous iron(III) chloride is a fairly strong Lewis acid, and it is used as a catalyst in organic synthesis.

Video Iron(III) chloride

Nomenclature

The descriptor hydrated or anhydrous is used when referring to iron(III) chloride, to distinguish between the two common forms. The hexahydrate is usually given as the simplified empirical formula FeCl₃?6H₂O. It may also be given as trans-[Fe(H₂O)₄Cl₂]Cl?2H₂O and the systematic name tetraaquadichloroiron(III) chloride dihydrate, which more clearly represents its structure.

Maps Iron(III) chloride

Structure and properties

Anhydrous iron(III) chloride adopts the BiI₃ structure, which features octahedral Fe(III) centres interconnected by two-coordinate chloride ligands. Iron(III) chloride hexahydrate consists of trans-[Fe(H₂O)₄Cl₂]⁺ cationic complexes and chloride anions, with the remaining two H₂O molecules embedded within the monoclinic crystal structure.

Iron(III) chloride has a relatively low melting point and boils at around 315 °C. The vapour consists of the dimer Fe₂Cl₆ (c.f. aluminium chloride) which increasingly dissociates into the monomeric FeCl₃ (D_3h point group molecular symmetry) at higher temperature, in competition with its reversible decomposition to give iron(II) chloride and chlorine gas.

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Preparation

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

2 Fe(s) + 3 Cl₂(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 iron ore in hydrochloric acid

   Fe₃O₄(s) + 8 HCl(aq) -> FeCl₂(aq) + 2 FeCl₃(aq) + 4 H₂O(l)

2. Oxidation of iron (II) chloride with chlorine

   2 FeCl₂(aq) + Cl₂(g) -> 2 FeCl₃(aq)

3. Oxidation of iron (II) chloride with oxygen

   4FeCl₂(aq) + O₂ + 4HCl -> 4FeCl₃(aq) + 2H₂O(l)

Small amounts can be produced by reacting iron with hydrochloric acid, then with hydrogen peroxide. The hydrogen peroxide is the oxidant in turning ferrous chloride into ferric chloride

Like many other hydrated metal chlorides, hydrated iron(III) chloride can be converted to the anhydrous salt by treatment with thionyl chloride. Conversion of the hydrate to anhydrous iron(III) chloride is not accomplished by heating, instead HCl and iron oxychlorides are produced. Similaryly, dehydration can be effected with trimethylsilyl chloride:

FeCl₃o6H₂O + 12 Me₃SiCl -> FeCl₃ + 6 (Me₃Si)₂O + 12 HCl

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Reactions

Iron(III) chloride undergoes hydrolysis to give an acidic solution. 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

It is a moderately strong Lewis acid, forming adducts with Lewis bases such as triphenylphosphine oxide; e.g., FeCl₃(OPPh₃)₂ where Ph is phenyl. It also reacts with other chloride salts to give the yellow tetrahedral [FeCl
₄]^-
ion. Salts of [FeCl
₄]^-
in hydrochloric acid can be extracted into diethyl ether.

Alkali metal alkoxides react to give the metal alkoxide complexes of varying complexity. The compounds can be dimeric or trimeric. In the solid phase a variety of multinuclear complexes have been described for the nominal stoichiometric reaction between FeCl₃ and sodium ethoxide:

FeCl₃ + 3 [C₂H₅O]^-Na⁺ -> Fe(OC₂H₅)₃ + 3 NaCl

Oxalates react rapidly with aqueous iron(III) chloride to give [Fe(C₂O₄)₃]^3-. Other carboxylate salts form complexes; e.g., citrate and tartrate.

Oxidation

Iron(III) chloride is a mild oxidising agent, for example, it is capable of oxidising copper(I) chloride to copper(II) chloride.

FeCl₃ + CuCl -> FeCl₂ + CuCl₂

It also reacts with iron to form iron(II) chloride:

2 FeCl₃ + Fe -> 3 FeCl₂

Reducing agents such as hydrazine convert iron(III) chloride to complexes of iron(II).

Decomposition

FeCl₃ can be decomposed by electrolysis:

2FeCl₃ o 3H₂O -> 6HCl + Fe₂O₃

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Uses

Industrial

Iron(III) chloride is used in sewage treatment and drinking water production. In this application, FeCl₃ in slightly basic water reacts with the hydroxide ion to form a floc of iron(III) hydroxide, or more precisely formulated as FeO(OH)^-, that can remove suspended materials.

[Fe(H₂O)₆]³⁺ + 4 HO^- -> [Fe(H₂O)₂(HO)₄]^- + 4 H₂O -> [Fe(H₂O)O(HO)₂]^- + 6 H₂O

It is also used as a leaching agent in chloride hydrometallurgy, for example in the production of Si from FeSi (Silgrain process).

Another important application of iron(III) chloride is etching copper in two-step redox reaction to copper(I) chloride and then to copper(II) chloride in the production of printed circuit boards.

FeCl₃ + Cu -> FeCl₂ + CuCl

FeCl₃ + CuCl -> FeCl₂ + CuCl₂

Iron(III) chloride is used as 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.

H₂C=CH₂ + Cl₂ -> ClCH₂CH₂Cl

Laboratory use

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 iron(III) chloride solution is added--a transient or permanent coloration (usually purple, green or blue) indicates the presence of a phenol or enol.

This reaction is exploited in the Trinder spot test, which is used to indicate the presence of salicylates, particularly salicylic acid, which contains a phenolic OH group.

This test can be used to detect the presence of gamma-hydroxybutyric acid and gamma-butyrolactone, which cause it to turn red-brown.

Other uses

• Used in anhydrous form as a drying reagent in certain reactions.
• Used to detect the presence of phenol compounds in organic synthesis; e.g., examining purity of synthesised Aspirin.
• Used in water and wastewater treatment to precipitate phosphate as iron(III) phosphate.
• Used by American coin collectors to identify the dates of Buffalo nickels that are so badly worn that the date is no longer visible.
• Used by bladesmiths and artisans in pattern welding to etch the metal, giving it a contrasting effect, to view metal layering or imperfections.
• Used to etch the widmanstatten pattern in iron meteorites.
• 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.
• Used to make printed circuit boards (PCBs).
• Used in veterinary practice to treat overcropping of an animal's claws, particularly when the overcropping results in bleeding.
• Reacts with cyclopentadienylmagnesium bromide in one preparation of ferrocene, a metal-sandwich complex.
• Sometimes used in a technique of Raku ware firing, the iron coloring a pottery piece shades of pink, brown, and orange.
• Used to test the pitting and crevice corrosion resistance of stainless steels and other alloys.
• Used in conjunction with NaI in acetonitrile to mildly reduce organic azides to primary amines.
• Used in an animal thrombosis model.
• Used in energy storage systems
• Historically it was used to make direct positive blueprints.
• A component of modified Carnoy's solution used for surgical treatment of keratocystic odontogenic tumor (KOT)

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Safety

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

Although reports of poisoning in humans are rare, ingestion of ferric chloride can result in serious morbidity and mortality. Inappropriate labeling and storage lead to accidental swallowing or misdiagnosis. Early diagnosis is important, especially in seriously poisoned patients.

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See also

• Iron(II) sulfate
• Aluminium chloride
• Verhoeff's Stain

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Notes

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References

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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.

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