Test for proteins | Chemical Methods of Identification in Labs
Proteins are the essential biomolecules present in the living organisms. They give structural support and rigidity to the organisms. There are many proteins like albumin, globulin, casein, gelatin, peptone etc. The tests for proteins include
- Coagulation test
- Biuret test
- Neumann’s test
- Half saturation test
- Aldehyde test
- Millon’s test
- Ninhydrin test
- Xanthohydrin test
- Arginine test
- Isoelectric test
Coagulation test: A sample mixed in water when boiled leads to coagulation. This indicates the presence of proteins like albumin and globulin.
Biuret test: The organic sample is mixed with an equal volume of NaOH(10%) and 2 drops of copper sulfate. The formation of blue precipitate indicates protein.
Neumann’s test: Sample when mixed with CPR indicator, turns to pink color. To this when drops of 1% acetic acid are added slowly, the color changes to the yellow indicating presence of gelatin.
Half saturation test: Take sample solution with and add Ammonium sulfate powder till the solution saturates to form a precipitate. Filter and perform the biuret test with the filtrate. Lack of violet, purple and pink color indicates albumin. While the presence of any of those colors indicate globulin.
Aldehyde test: To the test solution add few drops of Millon’s reagent and few drops of formalin. Mix the solution and add concentrated sulfuric acid from the side of the test tube. Formation of the violet ring at the junction indicates that albumin is present.
Ninhydrin test: To the test solution add a 1/4th volume of Ninhydrin solution and boil and cool. Formation of a blue color indicates the presence of protein.
Xanthohydrin test: Mix the sample solution with concentrated nitric acid, boil and cool. Then add 40% sodium hydroxide drops. Formation of orange color solution indicates protein.
Arginine test: To the test solution add sufficient sodium hydroxide solution. Then boil and cool the solution. Add a few drops of Lead acetate solution. Lack of formation of black precipitate indicates the presence of protein Albumin.
To the same sample added with sodium hydroxide, mix with 5 drops of α-Naphthol and few drops of fresh sodium hypobromite solution to get the red color. This indicates the presence of the protein gelatin.
Isoelectric test: To the given sample solution, 2 drops of Bromocrystal indicator and few drops of 1% acetic acid is added till the formation of green color. This is formed at the pH of 4.6.
Millon’s test: Take a sample of the test solution and add an equal volume of Millon’s reagent. Then boil and cool the solution and add few drops of Sodium nitrite (NaNO2). Formation of red color precipitate indicates the presence of protein.
Hellers test: When concentrated acids like nitric acid are added from the side of the test tube, a white ring formed at junction indicates the presence of albumin.