Cobalt(II) chloride

Physical and chemical characteristics

The most familiar distinction is between the anhydrous salt, which is typically blue, and the common hexahydrate, CoCl2·6H2O, which appears pink. This reversible color change on gaining or losing water makes the material strongly hygroscopic. In solution and in many coordination complexes cobalt(II) adopts an octahedral coordination geometry and participates readily in ligand exchange and redox chemistry under suitable conditions.

Preparation and reactions

Laboratory preparation typically involves dissolving cobalt metal, oxide or carbonate in hydrochloric acid, followed by crystallization to give hydrated forms. Heating the hydrate removes water to yield the blue anhydrous salt; exposure to moisture returns it to the pink hydrate. Cobalt(II) chloride can form complexes with ammonia, water and other ligands and reacts with bases to give cobalt(II) hydroxide and related species.

Uses and examples

Because of its pronounced and reversible color change with humidity, cobalt(II) chloride has been widely used as a moisture indicator in desiccants, humidity test papers and some laboratory applications. Other uses include its role as a reagent in chemical synthesis, as a precursor in electroplating or catalyst preparation, and in academic demonstrations of coordination chemistry.

Safety, handling and notable facts

Cobalt salts can be toxic and may cause skin sensitization or other adverse health effects; some cobalt compounds are regarded as hazardous and must be handled with appropriate protective measures and waste controls. In many jurisdictions their use as consumer humidity indicators has been reduced or regulated for safety reasons. The element name "cobalt" derives from a historical term for troublesome ores, reflecting the metal's long association with colored pigments and minerals.

Quick reference

• Formula: CoCl2
• Common hydrated form: hexahydrate (pink)
• Anhydrous form: blue, hygroscopic
• Typical applications: humidity indicators, reagents, catalysts (chloride ions and coordination chemistry)
• Further reading and suppliers: see technical and safety sources (material safety data)