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Paper chromatography
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Chromatography separates the dyes in an ink and can identify unknown substances using R_f values.
How it works
A spot of ink is put on a pencil line near the bottom of the paper. The paper stands in a solvent below the line.
The solvent moves up the paper (the mobile phase). The paper is the stationary phase.
Different dyes travel different distances.
The solvent moves up the paper (the mobile phase). The paper is the stationary phase.
Different dyes travel different distances.
R_f values
R_f = distance moved by spot ÷ distance moved by solvent
Both are measured from the pencil line. R_f is always between 0 and 1 and has no unit.
Both are measured from the pencil line. R_f is always between 0 and 1 and has no unit.
Reading a chromatogram
A pure substance gives one spot.
If an unknown spot has the same R_f as a known substance in the same solvent, they are probably the same substance.
The line is drawn in pencil because pencil does not dissolve in the solvent.
If an unknown spot has the same R_f as a known substance in the same solvent, they are probably the same substance.
The line is drawn in pencil because pencil does not dissolve in the solvent.
A spot moves 3.0 cm. The solvent front moves 7.5 cm. Find the R_f value.
- R_f = spot distance ÷ solvent distance
- R_f = 3.0 ÷ 7.5
- = 0.4
Answer: 0.4
R_f = distance moved by spot ÷ distance moved by solvent. Pure substances give one spot. Matching R_f values suggest the same substance.
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