What are the techniques for visualising colourless substances in chromatography?

Visualising colourless substances in chromatography can be achieved through techniques like UV light exposure, iodine vapour, or ninhydrin spray.

Chromatography is a technique used to separate mixtures into their individual components. However, some substances are colourless and cannot be seen with the naked eye. In such cases, different techniques are used to visualise these substances.

One common method is the use of ultraviolet (UV) light. Many substances absorb UV light and re-emit it as visible light, a phenomenon known as fluorescence. By shining UV light on the chromatogram, these substances will glow, making them visible. This method is particularly useful for substances like amino acids, which naturally fluoresce under UV light.

Another technique is the use of iodine vapour. The chromatogram is placed in a closed container with a few iodine crystals. The iodine sublimes, forming a vapour that reacts with the substances on the chromatogram, staining them a yellow-brown colour. This method is often used for substances like lipids and alkaloids.

Ninhydrin spray is another method used to visualise colourless substances, particularly amino acids. When ninhydrin reacts with amino acids, it forms a purple compound called Ruhemann's purple. By spraying the chromatogram with a solution of ninhydrin, any amino acids present will turn purple, making them visible.

In addition to these, there are other methods like the use of specific dyes that react with certain substances to produce a visible colour. The choice of method depends on the nature of the substances being analysed and the type of chromatography being used.