Demonstration of Thenards Blue

Chemicals needed

aluminum chloride, hexahydrate

cobalt(II)-chloride Xn    R 22    S 22-24/25

One can use instead of the chlorides the corresponding oxides/hydroxides. If one uses silicon dioxide instead of aluminum chloride smalt results, boric acid also yields a colorant.

Equipment needed

test tube (heat resistant)
burner (preferably gas-)

Test procedure

• Ventilation!
• One adds in a test tube two spatula-tips of aluminum chloride(hexahydrate) and one spatula-tipful of cobalt(hexahydrate) and mixes them by swaying the tube back and forth.
• Vigorously heat the mixture with the burner. There is soon formed, beginning at seperate points, a strong deep blue coloration, water and hydrochloride acid are vaporized.

Disposal

• Dispose of the residue as solid heavy metal waste.

Elucidation

One can formulate the formation of Thenards Blue as follows:

2 AlCl3 + CoCl2 + 4 H2O --> CoAl2O4 + 8 HCl

Photos

Formation of Thenards Blue

Literature

derived from "9. Nachweis als Thenards Blau", p. 425, [7]