- Cobalt(II) chloride (CoCl₂) is an inorganic compound composed of cobalt in its +2 oxidation state and chloride ions.
- It is most commonly encountered as a hydrated salt, especially the hexahydrate form (CoCl₂·6H₂O), which appears as bright pink crystals. In its anhydrous form, cobalt(II) chloride is a blue crystalline solid. The dramatic and reversible color change between the hydrated (pink) and anhydrous (blue) forms makes it a classic example of a hydration–dehydration indicator, widely used in chemical education to illustrate chemical equilibrium and water absorption.
- Cobalt(II) chloride is highly soluble in water, forming pink-colored aqueous solutions due to the presence of the hexaaquacobalt(II) ion [Co(H2O)6]2+[Co(H₂O)₆]^{2+}[Co(H2O)6]2+. When water is removed, either by heating or exposure to a desiccant, the complex shifts to the blue, anhydrous form. This property has led to its use as a humidity indicator in silica gel and other moisture-sensitive packaging. However, due to health concerns (including carcinogenic potential), its use in consumer products is increasingly regulated.
- In the laboratory and industry, CoCl₂ is an important starting material for synthesizing other cobalt compounds, including cobalt-based catalysts, pigments, and coordination complexes. It is commonly used in transition metal-catalyzed reactions, electroplating, and as a precursor to cobalt-containing dyes and inks. In coordination chemistry, cobalt(II) chloride readily forms a wide range of colored complexes with ligands such as amines, phosphines, cyanide, and halides. These complexes often exhibit interesting optical, magnetic, and structural properties.
- Cobalt(II) chloride can be prepared by reacting cobalt metal, cobalt(II) oxide, or cobalt(II) carbonate with hydrochloric acid, followed by crystallization. The hydration state can be adjusted depending on the application, and different crystalline hydrates (mono-, di-, and hexahydrate) can be obtained under controlled conditions.
- While cobalt is an essential trace element in human nutrition—primarily as a component of vitamin B₁₂ (cobalamin)—cobalt salts like cobalt(II) chloride are toxic in larger amounts. Prolonged or repeated exposure may lead to dermatitis, respiratory irritation, and systemic toxicity, and CoCl₂ is classified as a possible human carcinogen (Group 2B) by the International Agency for Research on Cancer (IARC). Therefore, proper personal protective equipment and ventilation are important when handling this compound.
