Iron(II) hydroxide is an inorganic compound used in diverse fields, from water chemistry to industrial synthesis. It is often found where iron is present under limited oxygen conditions. The compound is also known by its older name, ferrous hydroxide, which references the iron atom’s oxidation state.
Determining the Chemical Formula
The chemical formula for Iron(II) Hydroxide is \(\text{Fe}(\text{OH})_2\). This formula is determined by the charges of the constituent ions, ensuring the compound maintains electrical neutrality. The Roman numeral (II) indicates that the iron atom (Fe) exists as a cation with a positive two charge (\(\text{Fe}^{2+}\)). The hydroxide portion is a polyatomic anion (\(\text{OH}^-\)) carrying a negative one charge. To balance the \(+2\) charge of a single iron(II) ion, two hydroxide ions are required. The parentheses indicate that the subscript two applies to the entire hydroxide group.
Fundamental Chemical Characteristics
When prepared under oxygen-free conditions, the pure compound is a white, gelatinous precipitate. Trace amounts of oxygen cause it to appear as a pale green solid, sometimes called “green rust.” Iron(II) hydroxide is classified as a base because it can accept protons or release hydroxide ions in solution. It has a molar mass of approximately 89.86 grams per mole and exhibits very low solubility in water. The compound precipitates readily when an iron(II) salt is mixed with a strong base, which is a common laboratory preparation method.
Oxidation and Stability
Iron(II) hydroxide is unstable and highly sensitive to oxidation when exposed to oxygen, especially in aqueous solution. Oxygen initiates a rapid reaction where the iron ion transitions from the \(+2\) oxidation state (\(\text{Fe}^{2+}\)) to the more stable \(+3\) oxidation state (\(\text{Fe}^{3+}\)). This process accelerates significantly as the pH of the solution increases. As it oxidizes, it converts into Iron(III) Hydroxide (\(\text{Fe}(\text{OH})_3\)) or a hydrated iron(III) oxide. This chemical change is observed by a progressive color shift from white or pale green to a final reddish-brown color, characteristic of common rust. Researchers must handle the compound under anoxic conditions, often using inert gases like nitrogen or argon, to prevent its rapid transformation.
Real-World Uses
Despite its instability, Iron(II) hydroxide is utilized in several practical applications due to its reducing properties. In environmental engineering, it is used in water treatment processes. It acts as a reducing agent to remove toxic contaminants, such as selenate and selenite ions, by converting them into insoluble, elemental selenium that precipitates out. The compound also serves as a component in electrochemical systems, specifically within the negative electrode of nickel-iron batteries. Here, the iron(II) hydroxide undergoes a reversible oxidation to iron(III) hydroxide during the battery’s charging cycle. It is also used as a precursor chemical in the manufacture of other iron compounds, including pigments and dyes.