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?
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.
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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Update: June 2012