Iodine Test for Carbohydrates

Title

Lab Report: Iodine Test for Starch

Objective

To perform the iodine test in order to detect the presence of starch and distinguish it from other carbohydrates.

Principle

• The iodine test is a biochemical method used to identify certain polysaccharides, such as starch, dextrin, and glycogen, by their reaction with iodine.
• When iodine solution is added to these polysaccharides, a color change occurs due to the formation of charge transfer complexes between polyiodide ions and the helical structure of the polysaccharide.
• The specific color produced depends on the type of polysaccharide present.

Materials and Reagents

• 1% solutions of glucose, sucrose, and starch
• Cellulose sample (e.g., a small ball of cotton)
• Distilled water (control)
• Dilute Lugol’s iodine solution (prepared by diluting standard Lugol’s iodine about five times with distilled water)
• Four clean, dry test tubes
• Graduated droppers or pipettes (1–2 mL)
• Test tube stand and holder

Procedure

1. Label four test tubes and place them in the test tube stand.
2. Using separate droppers, add 1 mL of each test sample (glucose, sucrose, starch, and distilled water) to the respective tubes.
3. Place the cellulose sample in a separate spot for testing.
4. Add a few drops of dilute iodine solution to each test tube and to the cellulose sample.
5. Observe and record any color changes in each sample.

Observations

• The test tube containing starch solution develops a deep blue-black color rapidly.
• The glucose, sucrose, cellulose, and distilled water samples retain the original color of the iodine solution, showing no significant color change.
• Starchy foods (such as potato, bread, or rice) also produce a blue-black coloration with iodine.

Results and Interpretation

• A positive iodine test is indicated by a blue-black color, confirming the presence of starch (specifically, the amylose component).
• Other carbohydrates, such as glucose, sucrose, and cellulose, do not produce this color change, indicating a negative result.
• The test distinguishes starch from other carbohydrates due to the helical structure of amylose, which allows polyiodide ions to form colored complexes.

Conclusion

The iodine test is a simple and effective method for detecting starch in a sample. Only polysaccharides with a helical structure, like amylose in starch, react with iodine to produce a blue-black color. Other carbohydrates, including cellulose, do not react due to their different structural arrangements.

Key Terms

• Starch: A polysaccharide composed of amylose (linear) and amylopectin (branched) glucose units.
• Lugol’s Iodine: A solution of iodine and potassium iodide in water, used as the test reagent.
• Charge Transfer Complex: The interaction between the amylose helix and polyiodide ions, resulting in a visible color change.
• Negative Control: A sample (distilled water) expected to show no reaction, confirming the specificity of the test.

References

• Source lecture on the iodine test for polysaccharides.