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Name: Gary
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: Around 1999

I am using cobalt chloride as a chemical indicator for water. Is there a chemical reaction that causes blue cobalt chloride test paper to change from blue to pink, if so can you briefly explain. Also, when heated it changes from pink back to blue - what is this process?

Dear Gary,

Cobalt chloride forms a hydrate, as does many inorganic salts. The formation of the hydrate, wherein one or more water molecules are incorporated into the anhydrous salt to form a new crystal structure, can cause a color change as the water molecules become incorporated into the metal cation's coordination sphere.

The reaction is
Co Cl2 + 6 H2O --> CoCl6 . 6 H2O

where the (.) symbol indicates that the water molecules are intact within the new compound. Hydration reactions are all reversed by heating to drive off the water of hydration.

best regards,
robert topper

The blue form is cobaltous chloride dihydrate. The pink form is cobaltous chloride hexahydrate, [CoCl2(H2O)4].2H2O. When the dihydrate is exposed to water or moist air, it forms the pink hexahydrate, When the hexahydrate is heated, it loses four molecules of water, making the blue dihydrate.

Richard Barrans Jr., Ph.D.

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