What Is Manganese Hydroxide?
Manganese hydroxide is the hydroxide of manganese.
Known manganese hydroxides include manganese hydroxide (II), manganese hydroxide (III), and manganese hydroxide (IV). The CAS registration number for manganese hydroxide (II) is 18933-05-6. Manganese hydroxide (II) is naturally obtained as chimamanite (pyrochroite).
Manganese hydroxide (II) can be synthesized by adding potassium hydroxide solution to a dilute aqueous solution of manganese chloride, heating with hydrogen, and cooling to form crystals.
Uses of Manganese Hydroxide
Manganese hydroxide is white, insoluble in water, and is used in Winkler’s method for the determination of dissolved oxygen in water.
Trivalent manganese hydroxide is a gray, brown, or black powder that can be used for dyes and pigments. Tetravalent manganese hydroxide is a brownish-black or black powder and is used as a drying agent for pigments, varnishes, and dyes.
Properties of Manganese Hydroxide
Manganese hydroxide (II) is insoluble in water. It is readily oxidized in the air. Oxidation produces trivalent and tetravalent hydroxides, which easily turn brown.
Manganese hydroxide (II) is slightly soluble in water. Its solubility in water at room temperature is 0.0005 g/100cm3. On the other hand, manganese hydroxide (II) dissolves readily in dilute acids and ammonium salt solutions. The solubility product is Ksp = 1.6 × 10-13. The solubility product is somewhat larger than that of other heavy metal hydroxides, which are also more basic.
Manganese hydroxide (II) is virtually insoluble in dilute alkaline aqueous solutions. However, it does dissolve in hot concentrated alkaline solutions to form manganese (II) acid salts.
Structure of Manganese Hydroxide
The chemical formula for manganese hydroxide (II) is Mn(OH)2 with a molar mass of 88.9527 g/mol. It is also called manganese (II) oxide monohydrate.
Manganese hydroxide (II) has a hexagonal, cadmium iodide-type structure. It has a density of 3.26 g/cm3 and lattice constants a = 3.34 Å and c = 4.68 Å.
Manganese (IV)-containing hydroxides include Mn3O4∙nH2O, a brown powder, and MnO2∙nH2O, a black powder; Mn3O4∙nH2O can also be written as 2MnIIO∙MnO2∙nH2O.
Other Information on Manganese Hydroxide
1. Reaction of Manganese Hydroxide (Iii)
Mn(OH)2 precipitates to Mn2O3∙nH2O by air oxidation; Mn2O3∙nH2O is dried at 100°C to Mn2O3∙H2O. Mn2O3∙H2O can also be written as MnO(OH). is 87.94.
Manganese hydroxide (III) has two transformations: orthorhombic (α-form) and monoclinic (γ-form). α-form has a density of 4.2-4.4 g/cm3 and γ-form has a density of 3.26 g/cm3.
Manganese hydroxide (III) occurs naturally as suimanganite. Manganese hydroxide (III) is not obtained in the form of Mn(OH).
2. Related Compounds of Manganese Hydroxide
Manganese hydroxide is sometimes compared to iron hydroxide. Iron hydroxide is the hydroxide of iron and exists as iron hydroxide (II) or iron hydroxide (III), depending on the oxidation number of iron. The chemical formula for iron hydroxide (II) is Fe(OH)2, and that for iron hydroxide (III) is Fe(OH)3.
Manganese hydroxide (II) is basic and reducing, although it is less reducing than iron (II) hydroxide. The standard redox potential of iron (II) hydroxide is E° = -0.556 V, while that of manganese hydroxide (II) is E° = -0.25 V.