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Iron(III) chloride, generically called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl<sub>3</sub>. 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. Iron(III) chloride is deliquescent, fuming in moist air due to the evolution of HCl, which hydrates, giving a mist.
When dissolved in water, iron(III) chloride undergoes hydrolysis and gives off heat as the reaction is exothermic. The resulting brown, acidic, and corrosive solution is used as a coagulant 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.
Iron(III) chloride has a relatively low melting point and boils at around 315 °C. The vapour consists of the dimer Fe<sub>2</sub>Cl<sub>6</sub> (compare aluminium chloride) which increasingly dissociates into the monomeric FeCl<sub>3</sub> (D<sub>3h</sub> point group molecular symmetry) at higher temperature, in competition with its reversible decomposition to give iron(II) chloride and chlorine gas.[1]
Iron(III) chloride is a moderately strong Lewis acid, forming adducts with Lewis bases such as triphenylphosphine oxide, e.g. FeCl<sub>3</sub>(OPPh<sub>3</sub>)<sub>2</sub> where Ph = phenyl.
Iron(III) chloride reacts with other chloride salts to give the yellow tetrahedral FeCl<sub>4</sub><sup>−</sup> ion. Salts of FeCl<sub>4</sub><sup>−</sup> in hydrochloric acid can be extracted into diethyl ether.
When heated with iron(III) oxide at 350 °C, iron(III) chloride gives iron oxychloride, a layered solid and intercalation host. FeCl<sub>3</sub> + Fe<sub>2</sub>O<sub>3</sub> → 3 FeOCl
In the presence of base, alkali metal alkoxides react to give the dimeric complexes: 2 FeCl<sub>3</sub> + 6 C2H5OH + 6 NH<sub>3</sub> → (Fe(OC<sub>2</sub>H<sub>5</sub>)<sub>3</sub>)<sub>2</sub> + 6 NH4Cl
Oxalates react rapidly with aqueous iron(III) chloride to give [Fe(C<sub>2</sub>O<sub>4</sub>)<sub>3</sub>]<sup>3−</sup>. Other carboxylate salts form complexes, e.g. citrate and tartrate.
Iron(III) chloride is a mild oxidising agent, for example capable of oxidising copper(I) chloride to copper(II) chloride. Reducing agents such as hydrazine convert iron(III) chloride to complexes of iron(II).
Iron(III) chloride adopts the BI<sub>3</sub> structure, which features octahedral Fe(III) centres interconnected by two-coordinate chloride ligands.
Anhydrous iron(III) chloride may be prepared by union of the elements:[2]
2 Fe(s) + 3 Cl<sub>2</sub>(g) → 2 FeCl<sub>3</sub>(s)
Solutions of iron(III) chloride are produced industrially both from iron and from ore, in a closed-loop process.
Alternatively, iron(II) chloride can be oxidised with sulfur dioxide: 32 FeCl<sub>2</sub> + 8 SO<sub>2</sub> + 32 HCl → 32 FeCl<sub>3</sub> + S<sub>8</sub> + 16 H<sub>2</sub>O
Hydrated iron(III) chloride can be converted to the anhydrous salt by heating with thionyl chloride. The hydrate cannot be converted to anhydrous iron(III) chloride by only heat, as instead HCl is evolved and iron oxychloride forms.
In industrial application, iron(III) chloride is used in sewage treatment and drinking water production, where FeCl<sub>3</sub> in slightly basic water reacts with the hydroxide ion to form a floc of iron(III) hydroxide, or more correctly formulated as FeO(OH)<sup>-</sup>, that can remove suspended materials. Fe<sup>3+</sup> + 4 OH<sup>-</sup> → Fe(OH)<sub>4</sub><sup>-</sup> → FeO(OH)<sub>2</sub><sup>-</sup>.H<sub>2</sub>O
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.[3] FeCl<sub>3</sub> + Cu → FeCl<sub>2</sub> + CuCl FeCl<sub>3</sub> + CuCl → FeCl<sub>2</sub> + CuCl<sub>2</sub>
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. H<sub>2</sub>C=CH<sub>2</sub> + Cl<sub>2</sub> → ClCH<sub>2</sub>CH<sub>2</sub>Cl
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.[4] 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.
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 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 there the Ferric Chloride used for PCB etching is usually diluted with water but you still need to wash your hands and anything that’s touched it right after your finished with it unless otherwise told so.