What are the Basic Color reactions of Amino acids?

The color reactions of amino acids are qualitative biochemical tests used to detect and identify amino acids and proteins based on their functional groups.

These chemical tests produce specific color changes when amino acids react with certain reagents—making them essential tools in biochemistry laboratories, clinical pathology, and food science.

Understanding these reactions helps students and researchers distinguish between different amino acids, detect peptide bonds, and identify aromatic, sulfur-containing, and guanidinium-group-bearing amino acids.

This guide covers all major color reactions of proteins and amino acids, including the Ninhydrin test, Biuret test, Xanthoproteic test, Sakaguchi test, Millon’s test, Hopkins-Cole test, Lead Acetate test, and Pauly’s test.

Laboratory Safety Precautions

Several reagents used in the following color reaction tests—including concentrated nitric acid, lead acetate, sodium hydroxide, and sodium hypochlorite—are hazardous chemicals. Always follow standard laboratory safety protocols:

• Wear appropriate personal protective equipment (PPE): lab coat, safety goggles, and gloves.
• Avoid direct contact with skin, eyes, or mucous membranes.
• Do not inhale chemical powders or vapors.
• Dispose of chemical waste according to your institution’s guidelines.
• Always wash hands thoroughly after completing experiments.

Color Reactions of Amino acids

Experiment 1: Ninhydrin Test

Principle

The Ninhydrin test is a general test for all α-amino acids and proteins. Ninhydrin (triketohydrindene hydrate) reacts with the free α-amino group of amino acids to produce a blue-purple color known as Ruhemann’s purple. This reaction is positive for all primary amino acids. Proline and hydroxyproline (secondary amines) yield a yellow color instead of purple. The test is widely used in paper chromatography, food science, and forensic analysis to detect amino acids and fingerprints.

Positive Result: Blue-purple (Ruhemann’s purple) for most amino acids; yellow for proline.

Procedure

• Dissolve the vial contents of “gelatin” in 100 mL water (gentle heating may be used; avoid boiling).
• Dissolve “arginine,” “glutamic acid,” “glycine,” “cysteine,” and “tryptophan” each in 200 mL water separately.
• Dissolve “ninhydrin” reagent in 200 mL ethanol.
• Set up test tubes and add 2.0 mL (34 drops) of ninhydrin reagent to each.
• Add 2 drops (0.1 mL) of each test solution and mix well.
• Heat in a boiling water bath for 2 minutes and observe color change.
• Test albumin, gelatin, unknown compound, and at least two amino acids.
• For cystine or tyrosine: add a small amount of powder directly to the ninhydrin tube and boil.

Experiment 2: Biuret Test for Protein

Principle

The Biuret test is a general test for peptide bonds and is used to detect proteins in biological fluids. When copper (II) sulfate (CuSO₄) reacts with two or more peptide bonds in an alkaline solution (NaOH), a purple or violet color is produced with an absorption maximum at 550 nm. The color results from the formation of a coordination complex between Cu²⁺ ions and the carbonyl oxygen and amide nitrogen atoms of the peptide bonds. The test is named after the compound biuret, formed by heating urea to 108°C, which structurally resembles a peptide bond.

Note: Individual amino acids give a negative result (blue color). Dipeptides show a faint violet; tripeptides and larger proteins give a distinct purple-violet color.

Positive Result: Purple/violet colour

Procedure

• Use the biuret reagent and each solution prepared in Experiment 1.
• Set up test tubes and add 1.0 mL (17 drops) of biuret reagent to each.
• Add 1.0 mL of the test solution to each tube; mix well.
• Observe and record any color change over 10 minutes.
• Test albumin, gelatin, the unknown compound, and two amino acids.

Experiment 3: Solubility of Amino acids

Principle

The solubility of amino acids in water depends on their R-group (side chain) polarity. Most amino acids are freely soluble in water due to their zwitterionic (dipolar ion) nature at physiological pH. However, tyrosine (aromatic hydroxyl group) and cystine (disulfide-bonded dimer of cysteine) are sparingly soluble in cold water due to their relatively nonpolar or bulky structures.

Procedure

• Add 2.0 mL (34 drops) of water to a clean test tube.
• Add a small spatula tip of tyrosine powder to the water and shake well.
• Note whether tyrosine dissolves. If not, gently heat and observe solubility.
• Record: freely soluble / sparingly soluble / insoluble.
• Repeat the procedure with cystine.

Expected Result: Both tyrosine and cystine are sparingly soluble in cold water; solubility increases slightly on heating.

Experiment 4: Lead Acetate Test for Cysteine and Cystine 

Principle

This test detects sulfur-containing amino acids — specifically cysteine and cystine. When heated with a strong alkali (NaOH), the sulfur group in cysteine and cystine is liberated as sulfide ions (S²⁻). These sulfide ions react with lead acetate to form lead sulfide (PbS), a black/dark brown precipitate, confirming a positive result.

Important distinction: This test differentiates cysteine/cystine from methionine. Alkali treatment does not liberate sulfur from methionine; therefore, methionine gives a negative result.

Positive Result: Black or dark brown precipitate (PbS)

Procedure

• Dissolve “lead acetate” vial contents in 50 mL water. Dissolve “NaOH” vial contents in 50 mL water; set aside 20 mL for Experiment 5.
• Dissolve cysteine in 30 mL NaOH solution (shake to ensure complete dispersal).
• Set up 4 test tubes: glycine solution (2.0 mL), cysteine solution (2.0 mL), unknown compound (2.0 mL), cystine solution (2.0 mL).
• Add 2 drops of lead acetate to each tube.
• Heat in a boiling water bath for 5 minutes and record observations.

Experiment 5: Sakaguchi’s Test for Arginine

Principle

The Sakaguchi test is specific for arginine, an amino acid bearing a guanidinium group in its side chain. Arginine reacts with alpha-naphthol and sodium hypochlorite (or bromine water) in alkaline conditions to produce a bright red color. This reaction is characteristic of the guanidino group and helps distinguish arginine from other amino acids. A blank/control tube (reagents only) must always be prepared, as ammonia and ammonium ions can also give a faint positive result.

Positive Result: Bright red colour

Procedure

• Prepare 4% NaOH by mixing 20 mL of 40% NaOH (from Experiment 4) with 180 mL water.
• Set up 5 test tubes: water (2.0 mL), arginine solution (2.0 mL), unknown compound (2.0 mL), glutamic acid (2.0 mL), and albumin solution (2.0 mL).
• Add 1.0 mL of 4% NaOH to each tube.
• Add 2 drops of alpha-naphthol reagent to each tube.
• Add 1.0 mL sodium hypochlorite reagent and mix well.
• Observe color changes over 5 minutes.

Q: Does albumin test positive? Yes — albumin contains arginine residues, so it should give a positive result.

Experiment 6: Xanthoproteic Test for Tyrosine and Tryptophan

Principle

The xanthoproteic test detects aromatic amino acids—primarily tyrosine (Tyr), tryptophan (Trp), and phenylalanine (Phe). Concentrated nitric acid (HNO₃) causes nitration of the aromatic ring, producing a yellow color (xantho = yellow in Greek). Upon addition of alkali (NaOH), the yellow color deepens to orange due to ionization of the nitro compound. Phenylalanine reacts poorly unless sulfuric acid (H₂SO₄) is used as a catalyst. Proteins containing these aromatic residues also test positive.

Positive Result: Yellow colour → turns orange upon adding NaOH

Procedure

• Set up 4 test tubes: tryptophan solution (2 drops), unknown compound (2 drops), glycine solution (2 drops), and tyrosine powder (~5 mg).
• With care, add 1.0 mL concentrated HNO₃ to each tube (point tubes away from yourself and others).
• Gently heat in a water bath until boiling.
• Cool slowly, then add 4% NaOH drop by drop until the solution becomes alkaline.
• Observe: Was there a color change before adding alkali? What is the final color of each solution?

Note: The appearance of a white precipitate after adding nitric acid indicates protein presence in the sample

Experiment 7: Identifying the Unknown Compound

Objective: Use the results from Experiments 1–6 to deduce the identity of the unknown compound.

Based on which tests give a positive result, cross-reference with the known amino acid characteristics to determine the identity of the unknown compound. For example:

• If positive for Ninhydrin + Sakaguchi → likely Arginine
• If positive for Ninhydrin + Xanthoproteic → likely Tyrosine or Tryptophan
• If positive for Lead Acetate → likely Cysteine or Cystine

Experiment 8: Millon’s Test for Tyrosine

Principle: Millon’s test is specific for tyrosine and tyrosine-containing proteins. Millon’s reagent (acidified mercuric sulfate) reacts with the hydroxybenzene (phenol) group of tyrosine to form a yellow precipitate, which turns red upon heating. This red colour is the positive indicator

Positive Result: Red colour (on heating)

Experiment 9: Hopkins-Cole Test for Tryptophan

Principle: This test is specific for tryptophan. The indole ring of tryptophan reacts with glyoxylic acid in the presence of concentrated sulfuric acid to produce a purple-colored ring at the interface of the two layers.

Positive Result: Purple ring at interface

Experiment 10: Pauly’s Test for Histidine and Tyrosine

Principle: Pauly’s test detects imidazole-containing (histidine) and phenol-containing (tyrosine) amino acids. Sulfanilic acid is diazotized with sodium nitrite and HCl to form a diazonium salt, which couples with histidine or tyrosine to give a blood-red color.

Positive Result: Blood-red colour

Summary table of Color Reactions of Amino Acids