Millon Reaction

Millon Reaction (Millon), a color test for proteins: purplish-red coloring of the protein precipitate (the liquid may also become colored) when heated with M. reagent. The latter is prepared by dissolving 40 g of mercury in 57 cm³ of concentrated HNO₃, first at room temperature, and then heating on a water bath; the solution is diluted with two volumes of water and the liquid is decanted from the settled precipitate. An aqueous solution of mercury acetate can be used, to which before use several drops of a 1% solution of KNO₂ or NaNO₂ and a little diluted CH₃COOH are added if the liquid is not sufficiently acidic. - M. reagent gives a color with almost all phenols, and the M. reaction in proteins is due to the presence of tyrosine (oxyphenyl-alanine) HO.C₆H₄.CH₂CH(NH₂).COOH in them. The protein gelatin, which does not contain tyrosine, does not give the Millon reaction. Chlorides, H₂O₂, alcohol, and large amounts of mineral salts interfere with the reaction. Alkaline liquids are previously neutralized with nitric acid or acetic acid. The Millon reaction, like other color tests, is not applicable for detecting protein in urine.