Locating Agents for Colourless Substances Flashcards

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What is a locating agent in chromatography?

A locating agent is a chemical or method used to make colourless spots visible on a chromatogram.

Why do we need a locating agent for some substances in chromatography?

Some substances are colourless and cannot be seen, so a locating agent helps us see where they have moved on the paper or plate.

At which stage of a chromatography experiment is a locating agent usually applied?

It is used after the chromatogram has been run and dried, so it can show the positions of the separated substances.

How is a chemical locating agent used on a paper chromatogram?

The dry chromatogram is sprayed or dipped in the locating agent, then warmed if needed, and coloured spots appear where the substances are.

What does the locating agent ninhydrin show up in chromatography?

Ninhydrin reacts with amino acids and some proteins to give coloured, usually purple, spots.

How does iodine vapour act as a locating agent?

Iodine vapour sticks to many organic substances on the chromatogram and makes them appear as brown spots.

Why do you need to mark spots quickly when iodine vapour is used as a locating agent?

The brown spots can fade as the iodine leaves, so you must circle the spots in pencil before they disappear.

How are ultraviolet (UV) lamps used as locating agents in thin-layer chromatography (TLC)?

The TLC plate is viewed under a UV lamp, and the colourless spots show up as dark patches against a glowing background.

Why do some TLC plates glow under UV light?

They contain a fluorescent dye that glows under UV light, except where the separated substances block the light and appear as dark spots.

Give two examples of locating agents used for colourless substances in chromatography.

Examples include chemical sprays like ninhydrin or iodine vapour, and ultraviolet (UV) light with fluorescent TLC plates.