Verification of urea as biuret

Chemicals needed

Fehling solution

urea

sodium carbonate (or hydrate) Xi   R 36    S (2)-22-26

sodium hydroxide solution 7.5%  C    R 35     S (1/2)-26-37/39-45

Equipment needed

test tube
burner (preferably alcohol-)

Test procedure

• Ventilation recommended. Safety goggles!
• A couple of spatula-tipfuls of urea (one can readily use the spherical form) and two spatula-tipfuls of sodium carbonate into a test tube.
• Heat the mixture until it melts, gas strongly develops, the test tube is misted over with a white coating, and the smell of ammonia is present.
• Let cool and then after the addition of about 2 ml of diluted sodium hydroxide heat again until the residue has dissolved.
• Add a couple of drops of Fehling solution. A red-violet coloration, usually rather weak, occurs.

Disposal

• The diluted solution can be disposed of into the drainage.

Elucidation

Through the thermic decomposition of urea biuret, inter alia, is formed. Together with Fehling solution this results in the characteristic coloration (biuret reaction).

Photos

Thermic decomposition of urea

Coloration due to biuret reaction

Literature

Derived from Versuch 1853 S, "Umwandlung des Harnstoffs beim Erhitzen", p. 323, [2]