Upper secondary · 15 min

The iodine test: starch, glycogen and enzyme digestion

Use iodine to distinguish polysaccharides from simple sugars, then watch the blue-black colour track amylase digestion.

Goal

Link the iodine colour to helical chain length, and use the colour ladder (blue-black → red-violet → brown) to follow enzymatic breakdown.

Apparatus and reagents

Starch, glycogen and glucose solutions, Lugol’s iodine (I₂/KI), α-amylase or saliva, vitamin C, test tubes and a warm-water bath.

Procedure

Select the starch sample: iodine turns it an intense blue-black instantly.
Replay heating then cooling: colour vanishes near 80 °C and returns on cooling — a reversible helix unwinding.
Compare glycogen and glucose with the same iodine dose: red-violet versus unchanged yellow-brown.
Run short then long amylase digestion, and finish with vitamin C on a blue sample.

What to observe

Blue-black means long amylose helices; red-violet means shorter branched segments; yellow-brown means iodine with nothing to wrap around.
Digestion slides the colour through red-violet before reaching iodine’s own colour; vitamin C removes colour even without digestion.

Explanation

Amylose forms α-1,4-linked helices (~6 glucose units per turn) that host linear I₃⁻/I₅⁻ polyiodide chains; the charge-transfer complex absorbs near 600 nm, giving blue-black. Amylopectin and glycogen hold only short helical runs (red-violet). Amylase cleaves internal α-1,4 bonds → shorter dextrins → maltose/glucose; ascorbic acid decolourises by reducing I₂ to I⁻ (I₂ + C₆H₈O₆ → 2 I⁻ + C₆H₆O₆ + 2 H⁺).

History of the experiment

The deep blue-black colour was noticed almost as soon as iodine existed: Gay-Lussac, who named the new element in 1813, described the starch reaction in 1814, and it became a standard test within a decade. For a century it served mainly to find free iodine, until Bodo and later Kashima showed the colour comes from

\ce{I3-}

–

\ce{I5-}

chains trapped inside starch's amylose helix. The same colourimetry underlies the iodometric titrations that still quantify oxidants today.

Chemists behind it

Joseph Louis Gay-Lussac