Alcoholic hydroxyl group
Phenolic hydroxyl group
Liebermann's Nitroso Reaction
Add a crystal of sodium nitrite to 2 mL of concentrated sulfuric acid, and shake until dissolved. Add 0.1 g of the unknown.
A blue color indicates the presence of a phenol. Another indication of the presence of the phenol is the change of color of the solution to red, when it is poured into 20 mL of ice water, and the return of the blue color when the mixture becomes alkaline after the addition of 10% sodium hydroxide solution.
Practical Chemistry by O.P. Pandey, D.N. Bejpai, and S.Gir Pub. by S.Chand