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BIO NMDCAT Carbohydrates
nmdcat.online June 27, 2026

The iodine test is a specific qualitative test used to detect the presence of starch. The characteristic blue-black color is a result of the

A. Oxidation of iodine by the aldehyde groups of starch
B. Formation of a covalent bond between iodine and glucose monomers
C. Trapping of polyiodide ions (I₃⁻, I₅⁻) within the helical structure of amylose
✓
D. Reduction of iodine to iodide by the reducing end of amylopectin

📝 Explanation

Amylose forms a left-handed helix. Iodine (as I₃⁻ or I₅⁻ ions) fits into the central hydrophobic channel of this helix. The resulting charge-transfer complex absorbs light strongly, giving a characteristic deep blue-black color.

📖 Additional Information

  • Oxidation of iodine by the aldehyde groups of starch
  • Formation of a covalent bond between iodine and glucose monomers
  • Trapping of polyiodide ions (I₃⁻, I₅⁻) within the helical structure of amylose
  • Reduction of iodine to iodide by the reducing end of amylopectin

Amylose forms a left-handed helix. Iodine (as I₃⁻ or I₅⁻ ions) fits into the central hydrophobic channel of this helix. The resulting charge-transfer complex absorbs light strongly, giving a characteristic deep blue-black color.

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