PAG 09 - 9.3: Iodine test

What biological molecule does the iodine test detect?

Starch (a polysaccharide made of amylose and amylopectin).

Outline the method for the iodine test

1 - Place a few drops of the liquid sample onto a spotting tile (or into a test tube).

2 - Add 2–3 drops of iodine solution (iodine dissolved in potassium iodide solution).

3 - Gently mix and observe any colour change at room temperature.

What is a positive and negative result for the iodine test?

• Positive: Turns from orange-brown to blue-black.

• Negative: Remains orange-brown (or yellow-brown).

Why does iodine turn blue-black in the presence of starch?

Triiodide ions I^-3 slip inside the helical coil of amylose, forming a starch-iodine complex that absorbs light differently and appears blue-black.

How do you prepare a solid food sample before carrying out the Biuret or Iodine test?

Grind the solid food using a pestle and mortar, mix with distilled water to dissolve the biological molecules, and filter to obtain a clear liquid extract.