Ferric chloride test

The ferric chloride test is used to determine the presence or absence of phenols in a given sample (for instance natural phenols in a plant extract). Enols give positive results as well. The bromine test is useful to confirm the result, although modern spectroscopic techniques (e.g. NMR and IR spectroscopy) are far superior in determining the identity of the unknown. The quantity of total phenols may be spectroscopically determined by the Folin-Ciocalteau assay.

Technique

The sample is dissolved in water, or a mixture of water and ethanol, and a few drops of dilute ferric chloride solution is added. The formation of a red, blue, green, or purple coloration indicates the presence of phenols. Where the sample is insoluble in water, it may be dissolved in dichloromethane with a small amount of pyridine.

Chemistry

Phenols form a violet complex with Fe(III), which is intensely colored. This is the basis for the test.

Clinical use

The ferric chloride test can be used to detect metabolites in urine in case of inborn error of metabolism.

See also

• Prussian blue assay, a similar test for total phenol determination