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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). 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.
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Locating Agents and Two-way Paper
What is it?
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.
In simple words
Locating Agents and Two-way Paper is a core idea in this chapter. Learn the meaning first, then the main parts, then one clear example.
Examples
- Amino acids + ninhydrin named
- Do not use a pen on the start line
- Wear gloves as the teacher says
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.
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.
Recommended for Class 9
Full school-level explanation
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.
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.
Remember
- pencil start line
- front before it fades
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.
Remember
- second solvent
- pair of Rf
Examples
- Overlapping plant pigments
- Dry fully before the second run
- Compare with known spots on the same paper
Remember
- locating agent
- two solvents, two directions
- Rf depends on solvent
- Rf <= 1
- compare with knowns
- pencil start line
- front before it fades
- second solvent
How the ideas connect
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).
Deeper understanding
Go beyond memorising Locating Agents and Two-way Paper. Connect the definition to reasons, relationships, exceptions, and exam wording.
How the ideas connect
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).
Exam focus
Quick revision
- locating agent
- two solvents, two directions
- Rf depends on solvent
- Rf <= 1
- LOCATING: If the mixture is colourless, you cannot see the spot until you develop it. Draw the solvent front i
- TWO-WAY: Spot in a corner. Run, dry, turn, run with a different solvent. A single Rf in one solvent can match
Chromatography identifies by comparison. It does not, by itself, give a chemical formula.
For long answers on Locating Agents and Two-way Paper: define clearly → explain the main parts → give one example → state one exam caution.
Exam Focus
Exam Focus
Important points
- locating agent
- two solvents, two directions
- Rf depends on solvent
- Rf <= 1
Important formulas
- locating agent
- two solvents, two directions
- Rf depends on solvent
Common exam traps
- Chromatography identifies by comparison. It does not, by itself, give a chemical formula.
- Chromatography identifies by comparison. It does not, by itself, give a chemical formula.
Long question structure
Definition → key parts → example → exam caution
Common Mistakes
Common Mistakes
- Mistake
Chromatography identifies by comparison. It does not, by itself, give a chemical formula.
- Mistake
Chromatography identifies by comparison. It does not, by itself, give a chemical formula.
- Mistake
Chromatography identifies by comparison. It does not, by itself, give a chemical formula.
Revision Zone
60-Second Revision
Quick cues from this topic — use them for last-minute revision.
locating agent
two solvents, two directions
Rf depends on solvent
Rf <= 1
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
Rf <= 1
- 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
- Formula
- Rf <= 1
- Example
- Amino acids + ninhydrin named
- Critical warning
- Chromatography identifies by comparison. It does not, by itself, give a chemical formula.
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Exam Practice
Define
Q1. Define Locating Agents and Two-way Paper.
Explain
Q2. Explain: LOCATING.
Example
Q3. Give one clear example of Locating Agents and Two-way Paper and say why it fits.
Mistake
Q4. State one common mistake students make about Locating Agents and Two-way Paper.
Exam note
Q5. Write a short exam note on Locating Agents and Two-way Paper: definition, two points, one example.
Sources & References
School-standard facts for Matric and Intermediate. Original explanations — not copied from textbooks.