Millon’s Test- Definition, Principle, Procedure, Result, Uses


Millon’s test is an analytical test used for the detection of the amino acid tyrosine, which is the only amino acid containing the phenol group. Tyrosine is one of the 20 standard amino acids that are used to synthesize proteins in living organisms. Tyrosine has a hydroxyl group attached to the benzene ring, which makes it a phenolic compound. Phenolic compounds are organic compounds that have at least one hydroxyl group attached to an aromatic ring.

Millon’s test is a specific test for tyrosine, but it is not a specific test for protein as it also detects the phenolic group present in other compounds as well. Therefore, while performing Millon’s test, it is essential that other tests like the Biuret test and Ninhydrin test also be performed. The Biuret test detects the presence of peptide bonds in proteins, while the Ninhydrin test detects the presence of free amino groups in amino acids and proteins.

The test was discovered by and named after the French Chemist Auguste Nicolas Eugene Millon. Millon was a pioneer in organic chemistry and biochemistry and made significant contributions to the study of proteins, carbohydrates, and uric acid. He also developed several other analytical tests for organic compounds, such as Millon’s reaction for aldehydes and ketones, and Millon’s base for nitro compounds.

Millon’s test is based on the principle of nitrification of the phenol group in tyrosine, which then forms complexes with heavy metals like mercury. The reagent used for the test is called Millon’s reagent, and it consists of mercuric nitrate and mercurous nitrate that is dissolved in concentrated nitric acid. In the test, the phenol group on the tyrosine molecule is nitrated by the nitric acid present in the reagent. The nitrated tyrosine then combines with the mercury ions in the solution to form a red-colored precipitate or solution.

Millon’s test can be performed on pure tyrosine or on proteins that contain tyrosine residues. In some proteins containing tyrosine, the initial reaction between mercuric nitrate results in a white or yellow colored precipitate. After the addition of nitric acid and heating, however, the residue turns red in color. Both of these results are considered positive results and indicate the presence of tyrosine in the solution.

Millon’s test is useful in the detection of tyrosine-containing proteins in a given sample. It can also help in the differentiation of tyrosine from other amino acids. The test is commonly used in biochemistry and food analysis to detect casein protein and protein found in raw meat.