Cobalt(II) chloride

Solid forms Anhydrous cobalt chloride has the cadmium chloride structure () (Rm) in which the cobalt(II) ions are octahedrally coordinated. At about 706 °C (20 degrees below the melting point), the coordination change to tetrahedral. The vapor pressure has been reported as 7.6 mmHg at the melting point. The anhydrous salt, which is hygroscopic, sublimes under flowing hydrogen chloride. Solid hexahydrate •6 contains the neutral molecule trans- and two molecules of water of crystallization. Claims of the formation of tri- and tetrahydrates have not been confirmed. Solutions Cobalt chloride is fairly soluble in water. Under atmospheric pressure, the mass concentration of a saturated solution of in water is about 54% at the boiling point, 120.2 °C; 48% at 51.25 °C; 35% at 25 °C; 33% at 0 °C; and 29% at −27.8 °C. ==Preparation==

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: : Aqueous solutions of cobalt chloride can be prepared from cobalt(II) hydroxide or cobalt(II) carbonate and hydrochloric acid according to this idealized equation: : 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. :•6 + 12 → + 6 + 12 HCl ==Reactions==

Cobalt(II) chloride is a precursor to many other cobalt compounds. 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 (): : With triphenylphosphine (), a tetrahedral complex results: : Salts of the anionic complex CoCl42− can be prepared using tetraethylammonium chloride: : + 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: : Precursor to organocobalt derivatives with the norbornyl anion Reaction of the anhydrous compound with sodium cyclopentadienide gives cobaltocene . Reaction of 1-norbornyllithium with the ·THF produces tetrakis(1-norbornyl)cobalt(IV) — a rare example of a stable transition metal/saturated alkane compound, == Moisture indication ==

is due to cobalt chloride. When hydrated the colour changes to a light pink/purple. 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). ==Health issues==

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''. 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. In 2005–06, cobalt chloride was the eighth-most-prevalent allergen in patch tests (8.4%). ==Other uses==

• 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. • Cobalt chloride is an established chemical inducer of hypoxia-like responses such as erythropoiesis. Cobalt supplementation is not banned and therefore would not be detected by current anti-doping testing. Cobalt chloride is a banned substance under the Australian Thoroughbred Racing Board. • Cobalt chloride is one method used to induce pulmonary arterial hypertension in animals for research and evaluation of treatment efficacy. ==References==