ChemistryChapter 19: Chromatography
Locating Agents and Two-Way Paper
Colourless spots need a locating agent (ninhydrin for amino acids is the named school example; iodine vapour or UV for some inks).
Definition
Colourless spots need a locating agent (ninhydrin for amino acids is the named school example; iodine vapour or UV for some inks). Two-way chromatography turns the paper 90 deg and uses a second solvent so overlapping spots can separate. Rf is still distance spot / distance solvent, and it depends on the solvent.
Overview
When one run is not enough:
Locating
If the mixture is colourless, you cannot see the spot until you develop it. Draw the solvent front in pencil before it dries away. Measure from the original pencil start line, not from the bottom of the paper.
Examples Amino acids + ninhydrin named · Do not use a pen on the start line · Wear gloves as the teacher says
Two-Way
Spot in a corner. Run, dry, turn, run with a different solvent. A single Rf in one solvent can match two substances; two solvents make a pair of Rf values. Still not a medical test.
Examples Overlapping plant pigments · Dry fully before the second run · Compare with known spots on the same paper
Explanation
Rf is never more than 1. If your spot went with the solvent front, the mix is too soluble in that solvent — try a different one (teacher choice).
In simple words
Chromatography identifies by comparison. It does not, by itself, give a chemical formula.
Key points
- locating agent
- two solvents, two directions
- Rf depends on solvent
- pencil start line
- front before it fades
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Locating Agents and Two-Way Paper
Sticky Notes · Quick Revision
Core Idea
- Colourless spots need a locating agent (ninhydrin for amino acids is the named school example; iodine vapour or UV for some inks).
Key Points
- locating agent
- two solvents, two directions
- Rf depends on solvent
Simple Check
- Explain locating agents and two-way paper in your own words and connect it to one chapter example.
How to Answer
- Define the idea, show the main rule or evidence, apply it, and check the final statement.
Common Mistake
- Chromatography identifies by comparison. It does not, by itself, give a chemical formula.
FBISE Exam Tip
- Use the exact term, show working where needed, and include units or labelled evidence.
60-second revision
- locating agent
- two solvents, two directions
- Rf depends on solvent
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