What is Ferrous Hydroxide?
Ferrous hydroxide is an inorganic iron compound in its +2 oxidation state (iron II). It is a pale green or white-greyish solid, practically insoluble in water, which forms when ferrous iron reacts with hydroxides in aqueous solution.
Unlike its counterpart, ferric hydroxide (Fe(OH)3, iron in the +3 state), ferrous hydroxide is highly unstable in the presence of oxygen and tends to oxidise rapidly, transforming into brownish-red ferric compounds. This oxidation property is precisely what makes it so valuable in industrial applications.
Chemical Formula of Ferrous Hydroxide
Molecular formula: Fe(OH)2
IUPAC name: Dihydroxidoiron(II) / Iron(II) hydroxide
CAS number: 18624-44-7
Molar mass: 89.86 g/mol
Physical state: Solid (powder or gel)
Colour: Pale green to white-greyish (oxidises to brown-red on contact with air)
Physical and Chemical Properties
| Property | Value |
|---|---|
| Density | 3.4 g/cm3 |
| Solubility in water (25°C) | Practically insoluble (Ksp ≈ 4.87 × 10⁻¹⁷) |
| Melting point | Decomposes before melting |
| pH of saturated solution | ~9.4 |
| Acid-base character | Weak base (amphoteric under extreme conditions) |
| Air stability | Very low – oxidises to Fe(OH)3 and FeOOH |
Fundamental Chemical Reactions
Formation of Ferrous Hydroxide
Ferrous hydroxide forms when a base is added to a ferrous salt solution:
FeCl2 + 2 NaOH → Fe(OH)2 + 2 NaCl
FeSO4 + 2 KOH → Fe(OH)2 + K2SO4
Oxidation: From Ferrous to Ferric
In the presence of oxygen and water, ferrous hydroxide oxidises rapidly:
4 Fe(OH)2 + O2 + 2 H2O → 4 Fe(OH)3
This reaction, known as the ‘green rust sequence’, is fundamental in water treatment, iron corrosion and desulphurisation processes.
Formation of Oxyhydroxide (FeOOH)
Under partial oxidation conditions, Fe(OH)2 can transform into iron oxyhydroxide (goethite):
Fe(OH)2 + O2 → FeOOH + H2O
Key Differences: Ferrous vs Ferric Hydroxide
| Characteristic | Ferrous Hydroxide Fe(OH)2 | Ferric Hydroxide Fe(OH)3 |
|---|---|---|
| Fe oxidation state | +2 (ferrous) | +3 (ferric) |
| Colour | Pale green / white-greyish | Brownish-red / orange |
| Air stability | Very low (oxidises) | High (stable) |
| Water solubility | Practically insoluble | Practically insoluble |
| Formation | Reducing or O2-free environments | Oxidising environments, with O2 |
| Main application | Intermediate step in treatments | Desulphurisation, pigments, catalysis |
Industrial Applications of Ferrous Hydroxide
1. Wastewater Treatment
Ferrous hydroxide is used as a coagulant and precipitating agent in water treatment. As it oxidises to ferric hydroxide in the process, it forms flocs that capture contaminants, heavy metals, phosphorus and organic matter, facilitating their removal by sedimentation or filtration.
2. Iron Oxide Pigment Synthesis
Ferrous hydroxide is the starting precursor in the synthesis of yellow (FeOOH) and black (Fe3O4) iron oxide pigments. Control of Fe(OH)2 oxidation is critical to obtaining pigments with the desired colour coordinates and particle size.
3. Industrial Catalysis
In certain catalytic processes, Fe(OH)2 acts as a precursor to reduced iron catalysts, used in ammonia synthesis (Haber-Bosch Process) and Fischer-Tropsch processes for synthetic fuel production.
4. Heavy Metal Removal
In the treatment of mine effluents and acid mine drainage (AMD), ferrous hydroxide — together with ferric hydroxide — precipitates and co-precipitates heavy metals such as lead, cadmium, zinc and arsenic, making it a fundamental tool in environmental remediation.
Safety and Handling
Ferrous hydroxide has a relatively benign safety profile:
- Not classified as a hazardous substance under CLP Regulation.
- Not carcinogenic or mutagenic.
- In aquatic environments it can slightly affect pH; controlled waste management is recommended.
- Standard PPE (gloves, protective goggles) is recommended during handling.
Conclusion
Ferrous hydroxide, although less well known than ferric hydroxide, is a compound of great industrial relevance. Its role as a precursor in pigment synthesis, its function in desulphurisation cycles and its utility in water treatment make it a key component in multiple industries. At PROMINDSA, our deep knowledge of iron compounds allows us to develop products such as MICRONOX BIOX that make the most of the unique properties of these minerals.
Do you have questions about which type of iron compound is most suitable for your industrial process? Consult our specialists.
