2.1.2—Iodine test for starch

The iodine test is a qualitative test for starch. Iodine solution starts orange-brown; if starch is present, the mixture turns blue-black because iodine interacts with the centre of starch molecules.

1. Place the sample in a clean test tube or on a white spotting tile.
2. Add a few drops of iodine solution in potassium iodide to the sample.
3. Mix and observe against a light background: blue-black indicates starch; the iodine colour remains orange-brown when starch is not detected.

Iodine is supplied in potassium iodide solution because iodine is not sufficiently soluble in water alone. The positive colour is evidence of a starch–iodine complex, so the test identifies starch rather than all carbohydrates.

A blue-black result supports the presence of starch but does not measure its concentration. Use the same sample volume, reagent amount and observation background when comparing samples; do not treat a negative iodine result as proof that no other carbohydrate is present.