Ammonium iron(III) sulfate

Ammonium iron(III) sulfate, NH₄Fe(SO₄)₂·12 H₂O, or NH₄[Fe(H₂O)₆](SO₄)₂·6 H₂O, also known as ferric ammonium sulfate (FAS) or iron alum, is a double salt in the class of alums, which consists of compounds with the general formula AB(SO₄)₂ · 12 H₂O.^[2] It has the appearance of weakly violet, octahedrical crystals. There has been some discussion regarding the origin of the crystals' color, with some ascribing it to impurities in the compound,^[3] and others claiming it to be a property of the crystal itself.^[4]

Ammonium iron(III) sulfate

Names
IUPAC name Ammonium iron(III) sulfate
Other names Ferric ammonium sulfate Ferric alum
Identifiers
CAS Number	anhydride: 10138-04-2 Y dodecahydrate: 7783-83-7 Y
3D model (JSmol)	anhydride: Interactive image
ChemSpider	anhydride: 55405 Y
ECHA InfoCard	100.030.335
EC Number	anhydride: 233-382-4
PubChem CID	dodecahydrate: 61485
UNII	anhydride: LUX2X1H1IC Y dodecahydrate: 65390568Z5 Y
CompTox Dashboard (EPA)	anhydride: DTXSID8051417
InChI InChI=1S/Fe.H3N.2H2O4S/c;;2*1-5(2,3)4/h;1H3;2*(H2,1,2,3,4)/q+3;;;/p-3 Y Key: XGGLLRJQCZROSE-UHFFFAOYSA-K Y anhydride: InChI=1/Fe.H3N.2H2O4S/c;;2*1-5(2,3)4/h;1H3;2*(H2,1,2,3,4)/q+3;;;/p-3 Key: XGGLLRJQCZROSE-DFZHHIFOAY
SMILES anhydride: [Fe+3].[O-]S(=O)(=O)[O-].[O-]S([O-])(=O)=O.[NH4+]
Properties
Chemical formula	FeNH4(SO4)2•12H2O
Molar mass	482.25 g/mol (dodecahydrate)
Appearance	Pale violet octahedral crystals
Odor	weak ammonia-like
Density	1.71 g/cm3
Melting point	39 to 41 °C (102 to 106 °F; 312 to 314 K)
Solubility in water	1240 g/L
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Irritant
NFPA 704 (fire diamond)	[1] 1 0 0
Related compounds
Other anions	Ammonium iron(III) citrate Ammonium chloride
Other cations	Ammonium aluminium sulfate potassium aluminium sulfate
Related compounds	Ammonium iron(II) sulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

FAS is paramagnetic,^[5] acidic and toxic towards microorganisms.^[6] 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 and ammonium sulfate. Iron(II) in ferrous sulfate is oxidized to ferric sulfate by addition of sulfuric and nitric acid. 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,^[7] tanning,^[7] production of dyestuffs,^[7] and as an etching agent in the production of electronic components.^[8] It has been used in a wide area of applications, including adiabatic refrigeration equipment,^[9] biochemical analysis,^[10] and organic synthesis.^[11]

Gallery

• 
  Crystals of ferric ammonium sulfate
• 
  Crystals of ammonium iron(III) sulfate after 16 days in the air