Iron(II) nitrate explained

Iron(II) nitrate is the nitrate salt of iron(II). It is commonly encountered as the green hexahydrate, Fe(NO₃)₂·6H₂O, which is a metal aquo complex, however it is not commercially available unlike iron(III) nitrate due to its instability to air. The salt is soluble in water and serves as a ready source of ferrous ions.

Structure

No structure of any salt Fe(NO₃)₂·xH₂O has been determined by X-ray crystallography. Nonetheless, the nature of the aquo complex [Fe(H₂O)₆]²⁺ is well known and relatively insensitive to the anion. The Fe-O distances are longer for [Fe(H₂O)₆]²⁺ (2.13 Å) than for the ferric analogue [Fe(H₂O)₆]³⁺ (1.99 Å).^[1] Both [Fe(H₂O)₆]ⁿ⁺ complexes are high spin, which results in pale colors, paramagnetism, and weak Fe-O bonds.

Production

Iron(II) nitrate can be produced in multiple ways, such as the reaction of iron metal with cold dilute nitric acid:

If this reaction is conducted below -10 °C, nonahydrate is produced. It readily releases water to give the hexahydrate.^[2]

The above reaction can also co-produce ferric nitrate. Reacting iron(II) sulfate and lead nitrate under dilute ethanol and then evaporating the solution leads to the formation of the green crystals of the hexahydrate. A solution of iron(II) nitrate is produced by the ion-exchange reaction of iron(II) sulfate and barium nitrate, producing a concentration of up to 1.5 M due to the limited solubility of barium nitrate.^{[3] [4]}

The solution of the iron(II) nitrate-hydrazine complex is produced by the reaction of hydrazine nitrate and ferric nitrate at 40 °C with copper(II) nitrate as a catalyst:

If the compound is used in situ, the compound is produced by the reaction of iron(II) chloride and calcium nitrate:^{[5] [6]}

Reactions

The hexahydrate melts at 60 °C and then decomposes at 61 °C into iron(III) oxide rather than iron(II) oxide. A solution of iron(II) nitrate is much more stable, decomposing at 107 °C to iron(III), with the presence of nitric acid lowering the decomposition temperature. Concentrated nitric acid oxidizes iron(II) nitrate into iron(III) nitrate:

Uses

Iron(II) nitrate has no uses, however, there is a potential use for dye removal.

Notes and References

1. 10.1021/ic50168a006. Structure of Hexaaquairon(III) Nitrate Trihydrate. Comparison of Iron(II) and Iron(III) Bond Lengths in High-Spin Octahedral Environments. 1977. Hair. Neil J.. Beattie. James K.. Inorganic Chemistry. 16. 2. 245–250.
2. Book: John Newton Friend . Iron and Its Compounds . 1921 . C. Griffin, Limited . 175–176 . en.
3. Gus van Weert . Yuxing Shang . Iron control in nitrate hydrometallurgy by (auto) decomposition of iron (II) nitrate . Hydrometallurgy . 1993 . 33 . 3 . 255–271 . 10.1016/0304-386X(93)90066-M . Elsevier B.V. . en.
4. Ludmila G.Lavrenova . Spin-crossover in the complex of iron(II) nitrate with tris(3,5-dimethylpyrazol-1-yl)methane . Inorganica Chimica Acta . 2012 . 382 . 1–5 . 10.1016/j.ica.2011.11.030 . en.
5. Development of a PANI/Fe(NO3)2 nanomaterial for reactive orange 16 (RO16) dye removal . Analytical Methods . 2021 . 44 . 5309–5327 . 10.1039/D1AY01402A. Arfin . Tanvir . Bhaisare . Dipti A. . Waghmare . S. S. . 13 . 34714901 . 240228865 .
6. Synthesis and characterization of Fe(NO3)2-NiO composite as a photocatalyst for degradation of methylene blue dye under UV-irradiation . Optik . 2019 . 177 . 36–45 . 10.1016/j.ijleo.2018.10.014. Muthusamy . Keerthana . Muzaffar . Aqib . M . Basheer Ahamed . 2019Optik.177...36M . 125609537 .