Qualitative tests for identifying the presence of phenol often involve specific reactions that highlight its characteristic properties. Here are the main common qualitative tests for phenol:
- Litmus test
- Ferric chloride test
- Libermann’s test
- Bromine water test
- Phthalein dye test
Litmus test:
Phenol turns blue, litmus paper red. This shows that phenol is acidic in nature. Carboxylic acid also gives this test. Compared to carboxylic acid, phenol is weakly acidic and does not give an effervescence with aqueous sodium carbonate.
Ferric chloride test:
The Ferric Chloride Test involves adding aqueous ferric chloride solution to a sample. The appearance of a characteristic color change, ranging from violet to green, brown, or blue, indicates the presence of phenols. This test is specific to phenols and offers a quick and reliable qualitative identification.
The chemical reaction is given below.
Libermann’s test:
Phenol reacts with concentrated sulfuric acid and sodium nitrite to form a yellow quinone monoxime complex. A deep blue indophenol complex is formed with excess phenol and sulfuric acid. Dilution may produce a red indophenol complex and treatment with sodium hydroxide results in a deep blue sodium salt solution of indophenol.
Bromine Water Test:
Phenol undergoes an electrophilic substitution reaction with bromine. The brown color of bromine vanishes when bromine water is added to an aqueous phenol solution, resulting in a white precipitate identified as tribromophenol.
Phthalein dye test:
Heating phenol with phthalic anhydride in concentrated sulfuric acid produces a colorless condensation compound called phenolphthalein. Further reaction with dilute sodium hydroxide produces a pink-colored fluorescent compound called fluorescein. Phenolic compounds display distinct characteristic colors, visible against a white background.
The chemical reaction is given below.
