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• Physical Properties

  Two forms occur, a rose-red powder (more stable) and a bluish-green powder less stable than the red form; rhombohedral crystals; density 3.597 g/cm3; decomposes on heating; practically insoluble in water 3.2 mg/L; Ksp 1.0x10–15; soluble in acids and ammonia; insoluble in dilute alkalis.
  
  The addition of alkali metal hydroxides to solutions of cobalt(II) salts results in the precipitation of cobalt(II) hydroxide in either a blue or pink form depending upon the conditions. The pink form is the more stable of the two and is obtained when a suspension of the blue form is allowed to stand or is warmed. Cobalt(II) hydroxide is amphoteric, dissolving in alkalis to form blue solutions of the [Co(OH)4]2~ ions. In the presence of alkali, suspensions of Co(OH)8 are oxidized by air to the brown CoO(OH), this oxidation being brought about rapidly by oxidants such as hypochlorite, bromine water or hydrogen peroxide. The pink Co(OH)2 (density 3.597) has the brucite (Mg(OH)2) structure in which the cobalt atoms are surrounded by six hydroxides. The blue form is more disordered, but its structure is not known for certain.;

• Uses

  Cobalt(II) hydroxide is used as a drier for paints and varnishes and is added to lithographic printing inks to enhance their drying properties. Other applications are in the preparation of cobalt salts; as a catalyst; and in storage battery electrodes. ;

• Preparation

  Cobalt(II) hydroxide is obtained as a precipitate when an alkaline hydroxide is added to an aqueous solution of cobalt(II) salt:
  CoCl2 + 2NaOH → Co(OH)2 + 2NaCl
  Co(NO3)2 + 2NaOH → Co(OH)2 + 2NaNO3 ;

• Reactions

  Thermal decomposition to cobaltous oxide, CoO, occurs at 168°C in a vacuum.
  Cobalt(II) hydroxide is oxidized by air and other oxidizing agents, forming cobalt(III) hydroxide, Co(OH)3. Reactions with mineral acids produce corresponding Co²⁺ salts. ;

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