The Iodine Test for Starch

Add a drop of orange-brown iodine solution to a slice of potato and it turns an intense blue-black almost instantly. The iodine test is one of the quickest and most dramatic tests in chemistry and biology. It detects starch, it can show whether a leaf has been photosynthesising, and it can track enzymes digesting starch in real time. It also has a surprisingly elegant explanation at the molecular level.

Purpose

• To test foods and solutions for the presence of starch.
• To use the test to follow the digestion of starch by amylase.
• (Extension) to show that leaves make starch in the light.

Principle

“Iodine solution” in school labs is really iodine dissolved in potassium iodide solution. Iodine (I₂) on its own is barely soluble in water, but it combines with iodide ions to form soluble triiodide ions, I₃⁻, which give the solution its orange-brown colour.

Starch is a mixture of two glucose polymers (see starch vs glycogen vs cellulose):

• Amylose: unbranched chains that coil into a helix, with about six glucose units per turn.
• Amylopectin: a branched polymer.

The inside of the amylose helix is a narrow, relatively non-polar channel. Iodine and iodide species slip inside and line up in a long chain along the helix axis. This arrangement changes how the iodine absorbs light: it absorbs strongly across much of the visible spectrum, especially orange-red light, so the complex appears an intense blue-black.

Branched amylopectin forms only short helical segments, so it gives a weaker red-purple colour. Glycogen, even more branched, gives red-brown. Cellulose, whose chains are straight rather than helical, gives no colour.

Two practical consequences:

• Heat breaks the complex. Warm a blue-black starch–iodine mixture and the colour fades as the helix loosens; cool it and the colour returns. So the test should be done at room temperature.
• Short chains don’t give the colour. As amylase breaks starch into maltose and short chains, the blue-black colour disappears, which is why the test can track digestion.

Equipment and materials

• Iodine solution (iodine in potassium iodide), in a dropper bottle
• Spotting tile (white dimpled tile)
• Test tubes, rack, droppers or pipettes
• Samples: starch suspension (1%), glucose solution, distilled water (control), potato, bread, rice, apple, cucumber, flour, sugar, crushed cereal
• For the enzyme extension: amylase solution (about 1%), water bath at about 37 °C, stopwatch, syringes or graduated pipettes
• Mortar and pestle
• Eye protection

Safety

• Wear eye protection. Iodine solution is an irritant and stains skin and clothing.
• Wash off any splashes on skin promptly.
• Amylase solution can cause allergic reactions (it’s a known respiratory sensitiser in powder form). Use ready-made solutions, avoid creating dust, and wash hands afterwards.
• Do not eat any food used in the lab.
• For the leaf extension, which involves boiling ethanol, use a water bath heated by a kettle, never a naked flame, because ethanol is highly flammable.

Method

Part A: testing foods

1. Place small samples of each food, or a few drops of each solution, into separate dimples of a spotting tile.
2. Crush solid foods with a little distilled water first if they’re dry.
3. Add 1 to 2 drops of iodine solution to each sample.
4. Record the colour immediately.
5. Include distilled water as a negative control and starch suspension as a positive control.

Part B: following starch digestion by amylase

6. Put one drop of iodine solution into each dimple of a clean spotting tile.
7. Place 5 cm³ of starch suspension and 5 cm³ of amylase solution in separate test tubes in a 37 °C water bath for 5 minutes, so they reach the same temperature.
8. Pour the amylase into the starch, mix, and start the stopwatch.
9. Every 30 seconds, use a dropper to transfer one drop of the mixture into the next dimple of iodine.
10. Continue until the iodine stays orange-brown. Record the time taken.
11. (Optional) Repeat at different temperatures (e.g. 20, 30, 40, 50 and 60 °C) or pH values to investigate how conditions affect enzyme activity.

Results

Part A: typical food results

Sample	Colour with iodine	Starch present?
Distilled water	Orange-brown	No
Starch suspension	Blue-black	Yes
Glucose solution	Orange-brown	No
Potato	Blue-black	Yes
Bread, rice, flour	Blue-black	Yes
Table sugar	Orange-brown	No
Ripe apple	Orange-brown or faint blue	Little (converted to sugars as it ripens)
Unripe banana	Blue-black	Yes
Cucumber	Orange-brown	Very little

The ripening results are a nice real-world observation: as fruits ripen, enzymes convert stored starch into sugars, making them sweeter. Pair the iodine test with Benedict’s test for reducing sugars on unripe and ripe bananas to see the change both ways.

Part B: example enzyme results

Time (s)	Colour of drop
30	Blue-black
60	Blue-black
90	Purple
120	Red-brown
150	Orange-brown (starch gone)

The colour passes through purple and red-brown as starch chains get shorter, until no chains long enough to form the complex remain. The time to reach orange-brown gives a measure of the enzyme’s rate. For a rate, calculate 1000 ÷ time (in seconds) and plot against temperature to show the enzyme’s optimum.

Extension: do leaves make starch?

Plants store the glucose from photosynthesis as starch. To test a leaf:

1. Boil the leaf in water for about 30 seconds to kill it and break down cell membranes.
2. Place it in ethanol in a test tube, and stand the tube in hot water (from a kettle, with no flames nearby) to remove the green chlorophyll.
3. Rinse the leaf in warm water to soften it.
4. Spread it on a white tile and add iodine solution.

A leaf kept in the light turns blue-black; a leaf kept in the dark for 48 hours stays orange-brown. With a variegated (green and white) leaf, only the green parts turn blue-black, showing that chlorophyll is needed for photosynthesis.

Sources of error and limitations

• Temperature: hot samples give weak or no colour. Always test at room temperature.
• Colour judgement: subjective, especially for intermediate colours. Use a white tile and a colour chart.
• Timing in Part B: sampling every 30 seconds gives a resolution of only 30 seconds. Shorter intervals give better data.
• Carry-over: a dropper not rinsed between samples can transfer starch. Use a clean dropper or rinse with distilled water.
• Not quantitative: the test shows presence, not amount, unless combined with a colorimeter and standards (see colorimetry).
• Other substances: very strong colours in food can disguise the result; add more water to dilute.

Key takeaways

• Iodine–iodide solution (orange-brown) turns blue-black with starch.
• The colour comes from iodine species lining up inside the amylose helix.
• Branched polysaccharides give weaker colours; sugars and cellulose give none.
• The test tracks amylase digestion: the blue-black colour disappears as starch chains are broken down.
• Test at room temperature, use controls, and combine with Benedict’s test for a full carbohydrate analysis. For the bigger picture, see carbohydrates: sugars, starch and fibre.