The iodine test is a simple chemical test used to detect the presence of starch in a given sample. It is a type of colorimetric test that uses an iodine solution (Lugol’s Reagent), which is a solution of iodine and potassium iodide in water.

When iodine solution comes into contact with starch, it forms a complex known as the starch-iodine complex. This complex has a characteristic blue-black color that is easily visible, indicating the presence of starch.

The iodine test is commonly used in various fields, including in the food industry to detect starch in food products, in biology to detect starch in plant tissues, and in medical settings to test for conditions such as iodine deficiency. It is a quick and easy way to determine the presence of starch in a sample.

Principle of Iodine Test

The principle of this test is based on the reaction between iodine and starch, which results in the formation of a dark blue or black color.

Iodine forms a charge transfer (CT) complex with the helical structure of starch molecules. This CT complex causes a shift in the absorption spectrum of the iodine, resulting in a color change that is visible to the naked eye. The intensity of the color is directly proportional to the amount of starch present in the sample.


Step 1: Iodine is added to the sample being tested, which contains starch. In this step, the iodine forms a complex with the helical structure of the starch molecules.

I2 + starch → starch-iodine complex

Step 2: The iodine-starch complex causes the formation of a blue-black color. The intensity of the color is directly proportional to the amount of starch present in the sample.

starch-iodine complex → blue-black color

Step 3: If no starch is present in the sample, the iodine remains in its original state, with no color change.

I2 + sample → no color change 

Preparation of Reagent (Lugol’s Reagent)

To prepare the reagent, first dissolve 10 grams of potassium iodide in a small amount of hot water, as it takes longer to dissolve in cold water. Then, add 2.5 grams of iodine to the solution and mix thoroughly. Next, add enough distilled water to make a final volume of 1 liter. It is important to store the solution in a sealed glass container in a dark place, as it can deteriorate when exposed to light. Keep the reagent stored until it is needed.

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  • Lugol’s Reagent (Iodine solution)
  • Sample to be tested (liquid or solid)
  • Dropper or pipette
  • Test tubes or a plate
  • Distilled water
  • Test tube stand
  • Water bath
  • Vortex mixer


  1. Take two test tubes labeled test tube I and test tube II.
  2. Add 1-2 grams of solid or 1-2 mL of liquid sample to test tube I.
  3. Add 1-2 mL distilled water to test tube II.
  4. Add 2-3 drops of Lugol’s iodine to both tubes and mix thoroughly on a vortex mixer.
  5. Observe the color that develops in both test tubes.
  6. Heat the test tubes in a water bath until the color disappears.
  7. Allow the test tubes to cool down completely.
  8. Observe the color in both test tubes again.


  • If a blue-black color appears (positive iodine test).
  • If there is no change in color (negative iodine test).
Iodine test by Lugol's reagent

Interpretation of the Results

The appearance of a blue-black color confirms the presence of starch (positive iodine test). The intensity of the blue-black color formed depends on the amount of starch present in the sample. However, the original yellow color confirms the absence of starch (negative iodine test).

Applications of Iodine Test

  1. Detection of starch in various samples.
  2. Differentiating starch from monosaccharides, disaccharides, and other polysaccharides.
  3. Distinguishing between starch, glycogen, and other carbohydrates.
  4. Identifying hyper or hypothyroidism in patients through blood iodine testing.
  5. Testing the process of photosynthesis in plants.
  6. Basis of iodometric titrations with the starch indicator for quantitative analysis.
  7. This test is also used to detect reducing sugars.


  • Color variations in samples containing high levels of starch are not clearly discernible.
  • The iodine test is qualitative, indicating the presence or absence of starch, but not its concentration.
  • Starch hydrolyzes under acidic conditions, making the iodine test unsuitable for such samples.
  • Performing the iodine test on samples with very dark colors is impractical since the resulting color changes may not be detectable.

Concepts Berg

What is a Radioactive Iodine Uptake Test?

A radioactive iodine uptake test is a diagnostic test. It measures the amount of iodine absorbed by the thyroid gland. It helps evaluate thyroid function. Therefore, it can identify conditions such as hyperthyroidism or thyroid nodules.

What are the signs of iodine deficiency?

Signs of iodine deficiency include weakness, fatigue, hair loss, dry skin, sensitivity to cold, cognitive issues, and pregnancy complications.

What are the steps of the iodine test? 

Label test tube I and test tube II. Add 1-2 grams of solid or 1-2 mL of liquid sample to test tube I, and 1-2 mL of distilled water to test tube II. Mix both tubes with 2-3 drops of Lugol’s iodine, and observe the color. Heat until the color disappears, cool, and observe the color again.

When is the iodine test positive or negative? 

Positive or negative iodine test depends on changes in color. The appearance of a blue-black color confirms a positive iodine test. However, no color change in the original yellow color indicates a negative iodine test.

Why is iodine used for starch tests? 

When iodine solution i.e. Lugol’s reagent comes in contact with amylose starch, it forms a charge transfer complex. This complex absorbs light in the visible region and a blue-black color is observed.

Is iodine red or yellow? 

Iodide color depends upon the nature of the solvent and concentration. Its color varies from yellow to red for different solvents and concentrations.

What does iodine test positive for? 

Iodide test is an identification test for starch. A positive iodine test is also used to distinguish between monosaccharides and disaccharides from polysaccharides.

How to read iodine test results? 

The appearance of a blue-black color reads a positive iodine test. Whereas, no change in color reads a negative iodine test.

What is the biochemical basis of the iodine test? 

Mixing of iodine solution and starch solution gives rise to a charge transfer complex. This complex gives blue-black color observations.

Can an iodine test be used to detect reducing sugar? 

The iodide test is used for the detection of reducing sugars. The color of the iodine solution disappears when it comes in contact with reducing sugar. Therefore, the disappearance of color confirms a positive iodine test for reducing sugars.

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