Structure and uses of green vitriol, Ferric chloride, Mohr's salt

Green Vitriol

Iron(II) sulfate (Iron(II) sulphate) or ferrous sulfate (ferrous sulphate) is the chemical compound with the formula (FeSO₄), known since ancient times as copperas. It is most commonly encountered as the blue-green heptahydrate.

Hydrates

Iron(II) sulfate can be found in various states of hydration, and several of these forms exist in nature.

• FeSO₄·H₂O (mineral: szomolnokite, relatively rare)
• FeSO₄·4H₂O (mineral: rozenite, white, relatively common, may be dehydratation product of melanterite)
• FeSO₄·5H₂O (mineral: siderotil, relatively rare)
• FeSO₄·6H₂O (mineral: ferrohexahydrite, relatively rare)
• FeSO₄·7H₂O (mineral: melanterite, blue-green, relatively common)

At 90 °C, the heptahydrate, also called green vitriol or copperas, loses water to form the colorless monohydrate, In its anhydrous, crystalline state, its standard enthalpy of formation is Δ_fH°_solid = -928.4 kJ·mol^-1 and its standard molar entropy is S°_solid = 107.5 J·K^-1·mol^-1. All mentioned mineral forms are connected with oxidation zones of Fe-bearing ore beds (pyrite, marcasite, chalcopyrite etc.) and related environments (like coal fire sites). Many undergo rapid dehydration and sometimes oxidation.

Uses

Industrially, ferrous sulfate is mainly used as a precursor to other iron compounds. It is a reducing agent, mostly for the reduction of chromate in cement.

Nutritional supplement

Together with other iron compounds, ferrous sulfate is used to fortify foods and to treat iron-deficiency anemia. Constipation is a frequent and uncomfortable side effect associated with the administration of oral iron supplements. Stool softeners often are prescribed to prevent constipation.

Colorant

Ferrous sulfate was used in the manufacture of inks, most notably iron gall ink, which was used from themiddle ages until the end of the eighteenth century. It also finds use in wool dyeing as a mordant.

Two different methods for the direct application of indigo dye were developed in England in the eighteenth century and remained in use well into the nineteenth century. One of these, known as china blue, involved iron(II) sulfate. After printing an insoluble form of indigo onto the fabric, the indigo was reduced toleuco-indigo in a sequence of baths of ferrous sulfate (with reoxidation to indigo in air between immersions). The china blue process could make sharp designs, but it could not produce the dark hues of other methods.

Ferrous sulfate can also be used to stain concrete and some limestones and sandstones a yellowish rust color.^[2]

Woodworkers use ferrous sulfate solutions to color maple wood a silvery hue.

Ferric Chloride

Iron(III) chloride, also called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl₃. 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 isdeliquescent, forming hydrated hydrogen chloridemists in moist air. It is rarely observed in its natural form, mineral molysite, known mainly from some fumaroles.

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

Reactions

Iron(III) chloride is a moderately strong Lewis acid, forming adducts with Lewis bases such astriphenylphosphine oxide, e.g. FeCl₃(OPPh₃)₂where Ph = phenyl.

Iron(III) chloride reacts with other chloride salts to give the yellow tetrahedral FeCl₄⁻ ion. Salts of FeCl₄⁻ in hydrochloric acid can be extracted intodiethyl ether.

When heated with iron(III) oxide at 350 °C, iron(III) chloride gives iron oxychloride, a layered solid and intercalation host.^{[citation needed]}

FeCl₃ + Fe₂O₃ → 3 FeOCl

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

2 FeCl₃ + 6 C₂H₅OH + 6 NH₃ → (Fe(OC₂H₅)₃)₂ + 6 NH₄Cl

Oxalates react rapidly with aqueous iron(III) chloride to give [Fe(C₂O₄)₃]³⁻. Other carboxylatesalts form complexes, e.g. citrate and tartrate.

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

Laboratory use

In the laboratory iron(III) chloride is commonly employed as a Lewis acid for catalysing reactions such aschlorination of aromatic compounds and Friedel-Crafts reaction of aromatics. It is less powerful thanaluminium 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.^[9] The mixture is filtered before use. The organic substance is dissolved in water, methanol orethanol, 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.

Mohr's Salt

Ammonium iron(II) sulfate, or Mohr's Salt, is a double salt of iron sulfate and ammonium sulfate, with the formula [NH₄]₂[Fe][SO₄]₂·6H₂O. Mohr's salt is preferred over iron(II) sulfate for titration purposes as it is much less affected byoxygen in the air than iron(II) sulfate, solutions of which tend to oxidise to iron(III). The oxidation of solutions of iron(II) is very pH dependent, occurring much more readily at high pH. The ammonium ions make solutions of Mohr's salt slightly acidic, which prevents this oxidation from occurring. The relevant equation for this is:

4 Fe²⁺ + O₂ + (4+2x) H₂O ⇌ 2 Fe₂O₃.xH₂O + 8 H⁺

The presence of protons keeps this equilibrium to the left, the Fe(II) side.

Mohr's salt is named after the German chemist Karl Friedrich Mohr, who made many important advances in the methodology of titration in the 19th century.