Iodine is used in Thin Layer Chromatography (TLC) as a visualization agent when the compounds being separated are not visible under UV light.
Understanding TLC and Visualization
Thin Layer Chromatography (TLC) is a technique used to separate non-volatile mixtures. After the sample is run on the TLC plate, the spots of the separated compounds need to be visualized.
- UV Light Visualization: Many organic molecules contain chromophores, which absorb ultraviolet (UV) light. These compounds will appear as dark spots under a UV lamp.
- Iodine Staining: When a UV lamp is not available, or the compounds do not absorb UV light effectively, iodine staining can be used to visualize the spots.
How Iodine Works in TLC
- Iodine Vapors: Iodine crystals are placed in a closed container, and over time, they will sublime (turn directly from a solid to a gas) creating iodine vapors.
- Interaction with Compounds: The TLC plate is then placed in the container with the iodine vapors.
- Complexation: The iodine molecules interact with the organic compounds on the TLC plate, forming a complex. This interaction temporarily stains the spots, making them visible to the naked eye. The spots appear as yellowish-brown.
- Temporary Staining: The iodine complex is usually not permanent and will fade over time as the iodine sublimes away again. Therefore, the spots should be marked immediately after staining.
When to Use Iodine
According to the provided reference, iodine staining is an alternative method when:
- A UV light source is not available.
- The organic compounds on the TLC plate do not have chromophores that allow for visualization under UV light.
Practical Insight
- Iodine staining is a universal visualization method, meaning it can be used for a wide variety of compounds.
- The use of iodine is especially helpful when separating compounds which do not have chromophores or do not fluoresce under UV light.
- Iodine staining is a destructive method that can interfere with further analysis using methods that involve mass spectrometry.
Summary
Iodine is used in TLC to make the separated compounds visible when they are not visible under UV light. It works by forming temporary colored complexes with organic compounds.
