Ammonium iron(III) sulfate, NH₄Fe(SO₄)₂·12 H₂O, or NH₄ e(H₂O)₆SO₄)₂·6 H₂O, also known as ferric ammonium sulfate (FAS) or iron alum, is a

double salt A double salt is a salt that contains two or more different cations or anions. Examples of double salts include alums (with the general formula ) and Tutton's salts (with the general formula ). Other examples include potassium sodium tartrate, ammo ...

in the class of

alum An alum () is a type of chemical compound, usually a hydrated double salt, double sulfate salt (chemistry), salt of aluminium with the general chemical formula, formula , where is a valence (chemistry), monovalent cation such as potassium or a ...

s, which consists of compounds with the general formula AB(SO₄)₂ · 12 H₂O. It has the appearance of weakly violet, octahedrical

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

s. There has been some discussion regarding the origin of the crystals' colour, with some ascribing it to impurities in the compound, and others claiming it to be a property of the crystal itself. FAS is paramagnetic, acidic and

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

towards

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s. It is a weak oxidizing agent, capable of being reduced to Mohr's salt, ferrous ammonium sulfate.

Preparation

FAS can be prepared by crystallization from a solution of

ferric sulfate Iron(III) sulfate (or ferric sulfate), is a family of inorganic compounds with the formula Fe2(SO4)3(H2O)n. A variety of hydrates are known, including the most commonly encountered form of "ferric sulfate". Solutions are used in dyeing as a morda ...

and ammonium sulfate. Iron(II) in ferrous sulfate is oxidized to ferric sulfate by addition of

sulfuric Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

and

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...

. Upon addition of ammonium sulfate to the solution and damping in of the solution, ferric ammonium sulfate crystals precipitate. Equations for these conversions ignore the degree of hydration of the material. Oxidation: 6 FeSO₄ + 2 HNO₃ + 3 H₂SO₄ = 3 Fe₂(SO₄)₃ + 2 NO + 4 H₂O Synthesis: Fe₂(SO₄)₃ + (NH₄)₂SO₄ = 2 NH₄Fe(SO₄)₂

Uses

Areas of use for FAS include waste water treatment,''Wiley Encyclopedia of inorganic chemistry'': Volume 4, p. 1704:

tanning Tanning may refer to: *Tanning (leather), treating animal skins to produce leather *Sun tanning, using the sun to darken pale skin **Indoor tanning, the use of artificial light in place of the sun **Sunless tanning, application of a stain or dye t ...

, production of

dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution an ...

stuffs, and as an etching agent in the production of

electronic components An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not ...

. It has been used in a wide area of applications, including adiabatic refrigeration equipment, biochemical

analysis Analysis ( : analyses) is the process of breaking a complex topic or substance into smaller parts in order to gain a better understanding of it. The technique has been applied in the study of mathematics and logic since before Aristotle (38 ...

and

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

Gallery

Ammoniumeisenalaun.jpg, Crystals of ferric ammonium sulfate Ammonium iron(III) sulfate dodecahydrate in moist air, 2015-10-22 (1).jpg, Crystals of ammonium iron(III) sulfate after 16 days in the air

References

{{DEFAULTSORT:Ammonium Iron(Iii) Sulfate Ammonium compounds Iron(III) compounds Sulfates Double salts