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5.6.6Unit 5. Transition Metals and Organic Nitrogen Chemistry

Organic Techniques – Purity — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 43 of 43 Unit quiz

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Part 1 of 6Melting Point Testing

5.6.6 Organic Techniques - Purity

Melting range is a quick check of purity for solids.

Pure solid
  • A pure crystalline solid melts sharply over a narrow range of about 1 °C at a fixed temperature.
Impure solid
  • Impurities lower the melting point and make the solid melt over a wide range of temperatures.
Method
  • A small sample in a capillary tube is heated slowly in an oil bath or melting point apparatus while the range is recorded.
Identification
  • The measured range can be compared with a data book value to help identify the compound.
SampleMelting rangeInterpretation
PureNarrow, close to data book valueHigh purity
Slightly impureLower and widerContaminated
MixtureVery wideLittle product

Part 2 of 6Boiling Point and Chromatography

Boiling Point and Chromatography
Boiling point
  • A pure liquid boils at a fixed temperature at a given pressure, while a mixture boils over a range.
Thin-layer chromatography
  • A spot of the sample is placed on a coated plate and a solvent carries different components different distances.
One spot
  • A single spot suggests the sample is pure, though a second compound could have an identical Rf value.
Rf comparison
  • Comparing Rf values with a pure standard run on the same plate helps confirm the identity.

Part 3 of 6Running a TLC Plate

1
Draw a pencil line near the bottom of the plate and mark the positions where each sample will be spotted.
2
Spot small amounts of the product and any reference sample on the line using a capillary tube.
3
Place the plate upright in a tank with the solvent level below the pencil line and cover with a lid.
4
Remove the plate when the solvent front is near the top and mark the front at once with a pencil.
5
Visualise the spots with ultraviolet light or a developing agent and calculate Rf = distance moved by spot ÷ distance moved by solvent front.

Part 4 of 6Evaluating Purity

Spectroscopy
  • An IR or NMR spectrum with no unexpected peaks supports a pure product.
Yield above 100%
  • A percentage yield above 100% suggests that the product is wet or still contains solvent.
Repeat purification
  • If the melting range is still wide, recrystallise again and measure the melting point again.
Limitations
  • No single test proves purity, so more than one method is combined.

Part 5 of 6Worked Example

Question

  • On a TLC plate the solvent front moves 6.0 cm and the product spot moves 2.4 cm.
  • Calculate Rf.

  1. Rf is distance moved by the spot divided by the distance moved by the solvent front.
  2. Rf = 2.4 ÷ 6.0 = 0.40.

Answer Rf = 0.40

Part 6 of 6Exam Checklist & Tips

5.6.6 – Key Points for the Exam

  • Melting range The span of temperatures from the first liquid appearing to the last solid melting.
  • Rf value Distance travelled by a spot divided by the distance travelled by the solvent front.
  • Purity Absence of other substances in a sample.
  • Stationary phase The solid coating on a TLC plate that holds components back.

Common Mistakes

  • Stating that impurities raise the melting point; they lower it.
  • Drawing the pencil baseline below the level of the solvent so the sample dissolves into the tank.
  • Using ink for the baseline, as the dye runs with the solvent.
  • Concluding purity from one TLC spot without checking another solvent or method.

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