- Manganese(II) hydroxide, with the chemical formula Mn(OH)₂, is an inorganic compound consisting of divalent manganese cations (Mn²⁺) and hydroxide anions (OH⁻). It is typically encountered as a pale pink to white solid in its freshly prepared state, although it rapidly turns brown upon exposure to air due to oxidation to manganese(III) and manganese(IV) oxides or oxyhydroxides. Mn(OH)₂ is sparingly soluble in water, and in aqueous solution, it exists in equilibrium with dissolved Mn²⁺ ions, especially under basic conditions.
- This compound is most commonly prepared through the precipitation method, where a soluble manganese(II) salt such as manganese(II) chloride (MnCl₂) or manganese(II) sulfate (MnSO₄) is treated with a strong alkali like sodium hydroxide (NaOH). The reaction produces a gelatinous precipitate of Mn(OH)₂, which must be handled in an oxygen-free environment if the pure compound is to be isolated without oxidation.
- In terms of chemical properties, manganese(II) hydroxide is a weak base and acts as a precursor to other manganese compounds. It readily reacts with acids to form manganese(II) salts and water. Upon heating or exposure to oxidizing agents, Mn(OH)₂ decomposes to form various higher oxidation state manganese oxides, such as Mn₂O₃ or MnO₂, which have diverse applications in catalysis, batteries, and water treatment technologies.
- Mn(OH)₂ is of interest in environmental and geochemical studies because manganese is a biologically and geochemically active element. Manganese(II) hydroxide can form under reducing conditions in natural waters and sediments, playing a role in manganese cycling in aquatic ecosystems. When environmental conditions become more oxidative, Mn(OH)₂ is rapidly converted into insoluble oxides that scavenge heavy metals and organic pollutants, thus influencing water quality and sediment chemistry.
- In the field of electrochemistry, Mn(OH)₂ and related manganese compounds have been investigated for use in alkaline batteries and supercapacitors. Though Mn(OH)₂ itself is not widely used as a cathode material due to its instability in air, its oxidized derivatives like MnO₂ are key active materials in zinc–manganese dioxide batteries. Research also explores Mn(OH)₂ as a precursor in the synthesis of nanostructured manganese oxides for energy storage and catalytic applications.
