Overview
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Have you ever seen a purple colour appear when testing for proteins or amino acids in the lab?
That’s often due to the Ninhydrin Test. It’s a simple and important chemical test used to detect the presence of amino acids and proteins in a sample. When Ninhydrin reacts with these compounds, it produces a deep purple or blue colour, which helps scientists know if amino acids are present. This test is widely used in biology labs, which helps scientists know if amino acids are present. This test is widely used in biology labs, forensic investigations, and even in food and drug testing. In this article, you’ll learn how the test works, how to perform it, and where it’s used in real life.
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The test used for the detection of amines, amino acids and proteins is known as ninhydrin test. Ninhydrin is a chemical having chemical formula C9H6O4 and its IUPAC name is 2, 2-dihydroxyindane-1, 3-dione. When ninhydrin is added to a test solution of the sample containing amines, amino acids and proteins, the solution changes colour to deep blue which indicates the presence of these substances. The chemical reaction taking place in the solution where amines, amino acids and proteins reacts with ninhydrin are as follows-
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In the ninhydrin test, the amino group which belongs to a free amino acid undergoes a chemical reaction with ninhydrin which acts as an oxidising agent and undergoes reduction itself. Amino acid reacts with ninhydrin and undergoes oxidative deamination which results in the liberation of carbon dioxide, ammonia, aldehyde and hydrindantin which is the reduced form of ninhydrin.
The ammonia produced in this step reacts with another ninhydrin molecule to form diketohydrin which is also known as Ruhemann’s complex. This diketohydrin is responsible for the deep blue colour in the solution which indicates the presence of amines, amino acids and proteins. But in the case of amino acids, for example proline, the complex formed is yellow in colour and in the case of asparagine, the colour of the complex is brown.
To perform the Ninhydrin Test, you’ll need the following materials and chemicals:
The procedure for the ninhydrin test is as follows-
Area |
Use |
Biochemical analysis |
Helps detect amino acids and proteins in biological or lab samples |
Forensic science |
Used to reveal fingerprints on paper, clothes, or other porous surfaces |
Pharmaceutical testing |
Checks purity of drugs containing amino acids |
Food chemistry |
Estimates protein content in food products |
Limitation |
Explanation |
Doesn’t work on all compounds |
Some amino acids without a free amino group may not give a colour change |
False positives |
Other compounds like ammonia or amines can also react and give a similar result |
Cannot identify specific amino acids |
It only shows the presence, not the exact type of amino acid |
Needs careful heating |
Overheating can spoil the results or make colours unclear |
We hope this article was helpful in clearing all your doubts and queries and helped in your exam preparation. Need more help with your studies? You can also check out other Chemistry topics on the Testbook website. Get help from the experts to prepare for the exams with selective study material, mock tests and valuable insights all designed to help you score better marks in the exam. Download the free Testbook App to grab some exclusive offers now.
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