Locating Agent
Summary: A locating agent is a chemical used in paper chromatography to make colourless substances visible on the chromatogram. It reacts with the separated spots to produce coloured compounds that can be seen. Ninhydrin is a common locating agent for amino acids. Tags: igcse chemistry definition practical-skills chromatography Created: 2026-07-14 Last Updated: 2026-07-23
A locating agent is a chemical substance applied to a paper chromatogram to make colourless substances visible. Many organic compounds (amino acids, sugars) are colourless and cannot be seen on the chromatogram after separation. The locating agent reacts with these invisible spots to produce coloured compounds.
How Locating Agents Work
- The chromatogram is run as usual in a suitable solvent
- The paper is dried
- The locating agent is sprayed onto or dipped onto the paper
- The paper is often gently heated (e.g. in an oven or with a hairdryer)
- The locating agent reacts with the separated substances → coloured spots appear
- The positions and colours of the spots can be recorded
Common Locating Agents
| Locating Agent | Used For | Colour Produced |
|---|---|---|
| Ninhydrin | Amino acids and proteins | Purple / blue-violet spots |
| Iodine vapour | Many organic compounds | Brown spots (iodine adsorbs to compounds) |
| UV light | Fluorescent compounds | Spots glow under UV lamp |
Why Locating Agents Are Needed
- Many substances separated by chromatography are colourless in solution
- Unlike coloured dyes/inks, they cannot be seen with the naked eye
- A locating agent chemically reacts with the substance to make it visible
- IGCSE does NOT require knowledge of specific locating agents — only the concept that they exist and are used for colourless substances
IGCSE Syllabus Context
The syllabus states: “Describe how paper chromatography is used to separate mixtures of soluble colourless substances, using a suitable solvent and a locating agent. Knowledge of specific locating agents is not required.”
This means you need to know WHAT a locating agent does, but NOT the names of specific ones.
Key Facts
- Locating agents make colourless substances visible on chromatograms
- Applied by spraying or dipping after the chromatogram has been run
- Often followed by gentle heating
- React with the separated substances to produce coloured compounds
- Used because many organic compounds are colourless in solution
- Knowledge of specific locating agents is NOT required for IGCSE
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.