Cobalt(II) chloride

The anhydrous compound can be prepared by heating the hydrated cobalt chlorides under flowing hydrogen chloride or by treating hydrated cobalt(II) acetate with acetyl chloride:^[6]

Co(O₂CCH₃)₂·(H₂O)₄ + 6 CH₃COCl → CoCl₂ + 6 CH₃CO₂H + 4 HCl

Aqueous solutions of cobalt chloride can be prepared from cobalt(II) hydroxide or cobalt(II) carbonate and hydrochloric acid according to this idealized equation:

CoCO₃ + 2 HCl + 3 H₂O → CoCl₂·(H₂O)₄ + CO₂

Cooling saturated aqueous solutions yields the dihydrate between 120.2 °C and 51.25 °C, and the hexahydrate below 51.25 °C. Water ice, rather than cobalt chloride, will crystallize from solutions with concentration below 29%. The monohydrate and the anhydrous forms can be obtained by cooling solutions only under high pressure, above 206 °C and 335 °C, respectively.^[9]

On rapid heating or in a closed container, each of the 6-, 2-, and 1- hydrates partially melts into a mixture of the next lower hydrate and a saturated solution—at 51.25 °C, 206 °C, and 335 °C, respectively.^[9] On slow heating in an open container, so that the water vapor pressure over the solid is practically zero, water evaporates out of each of the solid 6-, 2-, and 1- hydrates, leaving the next lower hydrate, at about 40°C, 89°C, and 125°C, respectively. If the partial pressure of the water vapor is in equilibrium with the solid, as in a confined but not pressurized container, the decomposition occurs at about 115°C, 145°C, and 195°C, respectively.^[9]

Dehydration can also be effected with trimethylsilyl chloride:^[11]

CoCl
₂•6H
₂O + 12 (CH
₃)
₃SiCl → CoCl
₂ + 6[(CH
₃)
₃SiCl]
₂O + 12 HCl

Cobalt(II) chloride is a precursor to many other cobalt compounds.^[12] The hexahydrate and the anhydrous salt are weak Lewis acids. The adducts are usually either octahedral or tetrahedral. It forms an octahedral complex with pyridine (C
₅H
₅N):^[13]

CoCl₂·6(H₂O) + 4 C₅H₅N → CoCl₂(C₅H₅N)₄ + 6 H₂O

With triphenylphosphine (P(C
₆H
₅)
₃), a tetrahedral complex results:

CoCl₂·6(H₂O) + 2 P(C₆H₅)₃ → CoCl₂[P(C₆H₅)₃]₂ + 6 H₂O

Salts of the anionic complex CoCl₄²⁻ can be prepared using tetraethylammonium chloride:^[14]

CoCl
₂ + 2 [(C₂H₅)₄N]Cl → [(C₂H₅)₄N)]₂[CoCl₄]

The tetrachlorocobaltate ion [CoCl₄]²⁻ is the blue ion that forms upon addition of hydrochloric acid to aqueous solutions of hydrated cobalt chloride, which are pink.

Treating anhydrous cobalt chloride with hydrogen fluoride gives cobalt(II) fluoride:^[15]

CoCl₂ + 2 HF → CoF₂ + 2 HCl

Precursor to organocobalt derivatives

Reaction of the anhydrous compound with sodium cyclopentadienide gives cobaltocene Co(C
₅H
₅)
₂. Reaction of 1-norbornyllithium with the CoCl
₂·THF produces tetrakis(1-norbornyl)cobalt(IV)^[16][17] — a rare example of a stable transition metal/saturated alkane compound,^[4] different products are obtained in other solvents.^[18]

Cobalt chloride is a common visual moisture indicator due to its distinct colour change when hydrated. The colour change is from some shade of blue when dry, to a pink when hydrated, although the shade of colour depends on the substrate and concentration. It is impregnated into paper to make test strips for detecting moisture in solutions, or more slowly, in air/gas. Desiccants such as silica gel can incorporate cobalt chloride to indicate when it is "spent" (i.e. hydrated).^[19]

Cobalt is essential for most higher forms of life, but more than a few milligrams each day is harmful. Although poisonings have rarely resulted from cobalt compounds, their chronic ingestion has caused serious health problems at doses far less than the lethal dose. In 1966, the addition of cobalt compounds to stabilize beer foam in Canada led to a peculiar form of toxin-induced cardiomyopathy, which came to be known as beer drinker's cardiomyopathy.^[20][21][22]

Furthermore, cobalt(II) chloride is suspected of causing cancer (i.e., possibly carcinogenic, IARC Group 2B) as per the International Agency for Research on Cancer (IARC) Monographs.^[23]

In 2005–06, cobalt chloride was the eighth-most-prevalent allergen in patch tests (8.4%).^[24]

• Invisible ink: when suspended in solution, cobalt(II) chloride can be made to appear invisible on a surface; when that same surface is subsequently exposed to significant heat (such as from a handheld heat gun or lighter) the ink reversibly changes to blue.^[25]
• Cobalt chloride is an established chemical inducer of hypoxia-like responses such as erythropoiesis.^{[citation needed]} Cobalt supplementation is not banned and therefore would not be detected by current anti-doping testing.^[26] Cobalt chloride is a banned substance under the Australian Thoroughbred Racing Board.^[27]
• Cobalt chloride is one method used to induce pulmonary arterial hypertension in animals for research and evaluation of treatment efficacy.