Overview: Cobalt(II) hydroxide is an inorganic compound with the formula Co(OH)2. It contains cobalt in the +2 oxidation state coordinated to hydroxide anions and typically forms fine, insoluble solids. For a concise discussion of its chemical identity see chemical identity and for the cobalt ion itself consult cobalt(II) ion.

Appearance and structure

Co(OH)2 commonly appears as either a bluish or a reddish precipitate depending on preparation and hydration. Structurally it adopts a layered, brucite-like arrangement in which cobalt centers are octahedrally coordinated by hydroxide ligands; more on its ionic form is available at ionic properties. The color variations arise from subtle differences in particle size, hydration, surface adsorbates, and the presence of minor impurities such as chloride or basic salts (hydroxide chemistry).

Preparation and transformations

Typical laboratory synthesis is a precipitation reaction: adding a strong base (e.g., sodium or potassium hydroxide) to an aqueous cobalt(II) salt produces a blue-green hydroxide precipitate. When cobalt(II) chloride is used the product can appear red or pink under some conditions; see cobalt salts. Using ammonium hydroxide often gives a deep blue material. On heating Co(OH)2 loses water and converts to cobalt(II) oxide (CoO) or other oxides depending on temperature and atmosphere; general thermal behavior is summarized at thermal decomposition.

  • Solubility: the compound is sparingly soluble in water but reacts with acids to give soluble cobalt(II) salts.
  • Red vs. blue forms: preparation method and accompanying anions influence hue and may yield basic cobalt salts rather than a pure hydroxide.
  • Relation to other cobalt compounds: cobalt(II) hydroxide is a common precursor to oxides, basic carbonates and complex cobalt coordination compounds; additional background at basic preparation.

Uses and importance

Cobalt(II) hydroxide is employed as an intermediate in pigment manufacture, as a precursor for catalysts and battery electrode materials, and historically as a drying agent in some paint formulations. In materials research Co(OH)2 and derived oxides are studied for electrochemical applications such as supercapacitors and rechargeable batteries because of redox-active cobalt centers.

Safety and distinctions

Like many cobalt compounds, Co(OH)2 should be handled with care: inhalation or ingestion of cobalt materials can pose health risks and environmental concerns. It is distinct from rarer cobalt(III) hydroxide species and from mixed oxyhydroxides or basic salts that may form under particular conditions. For practical handling and disposal guidance consult reliable safety resources (hydroxide chemistry, chemical identity).