Iron(II) hydroxide

Iron(II) hydroxide
Names
IUPAC name
Iron(II) hydroxide
Other names
Ferrous hydroxide, green rust
Identifiers
CAS Number
  • 18624-44-7 checkY
3D model (JSmol)
  • Interactive image
ChemSpider
  • 8305416 checkY
ECHA InfoCard 100.038.581 Edit this at Wikidata
PubChem CID
  • 10129897
UNII
  • 7JIM5W32UU checkY
CompTox Dashboard (EPA)
  • DTXSID8066393 Edit this at Wikidata
InChI
  • InChI=1S/Fe.2H2O/h;2*1H2/q+2;;/p-2 checkY
    Key: NCNCGGDMXMBVIA-UHFFFAOYSA-L checkY
  • InChI=1/Fe.2H2O/h;2*1H2/q+2;;/p-2
    Key: NCNCGGDMXMBVIA-NUQVWONBAV
  • O[Fe]O
Properties
Chemical formula
 Fe(OH)2
Molar mass 89.86 g/mol
Appearance green solid
Density 3.4 g/cm3 [1]
Solubility in water
 0.000052 g/100 g water (20 °C, pH 7)[2]
Solubility product (Ksp)
8.0 x 10−16[3]
Acidity (pKa) 17[4]
Hazards
Flash point Non-flammable
Related compounds
Related compounds
 Iron(II) oxide
Iron(III) hydroxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
Chemical compound

Iron(II) hydroxide or ferrous hydroxide is an inorganic compound with the formula Fe(OH)2. It is produced when iron(II) salts, from a compound such as iron(II) sulfate, are treated with hydroxide ions. Iron(II) hydroxide is a white solid, but even traces of oxygen impart a greenish tinge. The air-oxidised solid is sometimes known as "green rust".

Preparation and reactions

Iron(II) hydroxide is poorly soluble in water (1.43 × 10−3 g/L), or 1.59 × 10−5 mol/L. It precipitates from the reaction of iron(II) and hydroxide salts:[5]

FeSO4 + 2 NaOH → Fe(OH)2 + Na2SO4

If the solution is not deoxygenated and iron not totally reduced in Fe(II), the precipitate can vary in colour starting from green to reddish brown depending on the iron(III) content. Iron(II) ions are easily substituted by iron(III) ions produced by its progressive oxidation.

It is also easily formed as a by-product of other reactions, a.o., in the synthesis of siderite, an iron carbonate (FeCO3), if the crystal growth conditions are imperfectly controlled.

Structure

Fe(OH)2 is a layer double hydroxide (LDH) easily accommodating in its crystal lattice ferric ions (Fe3+
) produced by oxidation of ferrous ions (Fe2+
) by the atmospheric oxygen (O2).

Related materials

Green rust is a recently discovered mineralogical form. All forms of green rust (including fougerite) are more complex and variable than the ideal iron(II) hydroxide compound.

Reactions

Under anaerobic conditions, the iron(II) hydroxide can be oxidised by the protons of water to form magnetite (iron(II,III) oxide) and molecular hydrogen. This process is described by the Schikorr reaction:

3 Fe(OH)2 → Fe3O4 + H2 + 2 H2O

Anions such as selenite and selenate can be easily adsorbed on the positively charged surface of iron(II) hydroxide, where they are subsequently reduced by Fe2+. The resulting products are poorly soluble (Se0, FeSe, or FeSe2).

Natural occurrence

Iron III hydroxide staining caused by oxidation of dissolved iron II and precipitation, Perth, Western Australia.

Iron dissolved in groundwater is in the reduced iron II form. If this groundwater comes in contact with oxygen at the surface, e.g. in natural springs, iron II is oxidised to iron III and forms insoluble hydroxides in water.[6] The natural analogue of iron(II) hydroxide compound is the very rare mineral amakinite, (Fe,Mg)(OH)2.[7][8]

Application

Iron(II) hydroxide has also been investigated as an agent for the removal of toxic selenate and selenite ions from water systems such as wetlands. The iron(II) hydroxide reduces these ions to elemental selenium, which is insoluble in water and precipitates out.[9]

In a basic solution iron(II) hydroxide is the electrochemically active material of the negative electrode of the nickel-iron battery.

See also

References

  1. ^ Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3.
  2. ^ CRC Handbook of Chemistry and Physics, 84th Edition, CRC Press, 2004, pg 4-62
  3. ^ Stumm, Werner; Lee, G. F. (February 1961). "Oxygenation of Ferrous Iron" (PDF). Industrial & Engineering Chemistry. 53 (2): 143–146. doi:10.1021/ie50614a030. Retrieved 17 November 2022.
  4. ^ Perrin, D. D., ed. (1982) [1969]. Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution. IUPAC Chemical Data (2nd ed.). Oxford: Pergamon (published 1984). Entry 130. ISBN 0-08-029214-3. LCCN 82-16524.
  5. ^ H. Lux "Iron(II) Hydroxide" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1498.
  6. ^ lenntech.com
  7. ^ "Amakinite".
  8. ^ "List of Minerals". 21 March 2011.
  9. ^ Zingaro, Ralph A.; et al. (1997). "Reduction of oxoselenium anions by iron(II) hydroxide". Environment International. 23 (3): 299–304. doi:10.1016/S0160-4120(97)00032-9.
  • v
  • t
  • e
Fe(-II)
  • H2Fe(CO)4
  • Na2Fe(CO)4
Fe(0)
  • Fe(CO)5
  • Fe2(CO)9
  • Fe3(CO)12
  • Fe(CO)3CH3COC2H2C6H6
Fe(I)
  • FeH
Organoiron(I) compounds
  • (C5H5FeCO)2(CO)2
    • Fe(0,II)
    • Fe3C
    Fe(II)
    • FeH2
    • Mg2FeH6
    • FeF2
    • FeCl2
    • Fe(ClO4)2
    • FeBr2
    • FeI2
    • FeO
    • Fe(OH)2
    • FeS
    • FeSO4
    • (NH4)2Fe(SO4)2·6H2O
    • FeSe
    • FeSeO4
    • Fe(NO3)2
    • Fe3(PO4)2
    • FeSi2
    • Fe(BF4)2
    • FeCr2O4
    • FeMoO4
    • FeTiO3
    • FeCO3
    • FeC2O4
    • Fe(C2H3O2)2
    • Fe(C3H5O3)2
    • FeC6H6O7
    • FeC12H22O14
    • FeI2(CO)4
    Organoiron(II) compounds
    • Fe(C5H5)2
    • Fe(C5H5)(CO)2I
    • Fe(C5H4P(C6H5)2)2
    • C4H4Fe(CO)3
    • C4H6Fe(CO)3
    Fe(0,III)Fe(II,III)
    • Fe3O4
    • Fe3S4
    Fe(III)
    • FeI3
    • FeBr3
    • FeCl3
    • FeF3
    • FeP
    • Fe(NO3)3
    • Fe(acac)3
    • FeOCl
    • [(C2H5)4N][O(FeCl3)2]
    • FeO(OH)
    • FePO4
    • Fe4(P2O7)3
    • Fe2(CrO4)3
    • Fe2(C2O4)3
    • Fe2O3
    • Fe2(SeO3)3
    • Fe2S3
    • Fe2(SO4)3
    • Fe(N3)3
    • NH4Fe(SO4)2·12H2O
    Organoiron(III) compounds
    • Fe(C5H5)2BF4
    • C6H8O7⋅xFe3+⋅yNH3
    • C
      54      H
      105      FeO
      6
    Fe(IV)
    • FeF4
    Fe(VI)
    • K2FeO4
    • BaFeO4
    Purported
    • v
    • t
    • e
    HOH He
    LiOH Be(OH)2 B(OH)3 C(OH)4 N(OH)3
    [NH4]+OH−     		O(OH)2 FOH Ne
    NaOH Mg(OH)2 Al(OH)3 Si(OH)4 P(OH)3 S(OH)2 ClOH Ar
    KOH Ca(OH)2 Sc(OH)3 Ti(OH)2
    Ti(OH)3
    Ti(OH)4     		V(OH)2
    V(OH)3     		Cr(OH)2
    Cr(OH)3     		Mn(OH)2 Fe(OH)2
    Fe(OH)3     		Co(OH)2 Ni(OH)2 CuOH
    Cu(OH)2     		Zn(OH)2 Ga(OH)3 Ge(OH)2 As(OH)3 Se BrOH Kr
    RbOH Sr(OH)2 Y(OH)3 Zr(OH)4 Nb Mo Tc(OH)4 Ru Rh(OH)3 Pd AgOH Cd(OH)2 In(OH)3 Sn(OH)2
    Sn(OH)4     		Sb(OH)3 Te(OH)6 IOH Xe
    CsOH Ba(OH)2 * Lu(OH)3 Hf Ta W Re Os Ir Pt Au(OH)3 Hg(OH)2 TlOH
    Tl(OH)3     		Pb(OH)2
    Pb(OH)4     		Bi(OH)3 Po At Rn
    FrOH Ra(OH)2 ** Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og
     
    * La(OH)3 Ce(OH)3
    Ce(OH)4     		Pr(OH)3 Nd(OH)3 Pm(OH)3 Sm(OH)3 Eu(OH)2
    Eu(OH)3     		Gd(OH)3 Tb(OH)3 Dy(OH)3 Ho(OH)3 Er(OH)3 Tm(OH)3 Yb(OH)3
    ** Ac(OH)3 Th(OH)4 Pa U(OH)2
    U(OH)3
    UO2(OH)2     		Np(OH)3
    Np(OH)4
    NpO2(OH)3     		Pu Am(OH)3 Cm(OH)3 Bk Cf Es Fm Md No