OCR A Level Chemistry A: Chromatography and qualitative analysis (H432) – Revision Notes

These notes condense section 6.3.1, on chromatography and qualitative analysis, from the H432 Chemistry A specification from OCR (Version 3.1, May 2026, for first assessment in 2017). They cover learning outcomes (a), (b)(i)–(ii) and (c)(i)–(vii), with the PAG6 and PAG7 links. H432 is the full A level. Module 6 is examined, according to the specification, in component 02 (Synthesis and analytical techniques) as well as component 03 (Unified chemistry). Full teaching and longer worked examples are in the study guide.

Other useful links: the OCR chemistry hub, the printable checklist, the free diagnostics and the practice questions.

Key definitions

• Stationary phase: the part that does not move. In TLC, a solid (silica or alumina) on a plate. In GC, a high-boiling liquid on a solid support in a column.
• Mobile phase: the part that moves and carries the mixture. In TLC, a solvent. In GC, an unreactive carrier gas such as helium or nitrogen.
• Adsorption: how a component is held on the surface of the solid in TLC.
• Rf value: the ratio of how far a component travels to how far the solvent front travels, both from the baseline.
• Retention time: the time from injection of the sample until a component reaches the detector.
• External calibration curve: a graph of peak area against concentration, made from standards run separately from the sample.

6.3.1(a): TLC in brief

Point	What to remember
One-way	Developed once, in one direction, with one solvent
Baseline	In pencil, above the solvent level
Solvent front	Marked as soon as the plate comes out
Colourless spots	Ultraviolet lamp or iodine vapour
Measure	Baseline to the centre of each spot
Rf range	0 to 1, no units
Polar compound on silica	Adsorbed strongly, so low Rf
Identify	Match Rf with a reference on the same plate

Method box: Rf from a chromatogram

1. Measure baseline to solvent front.
2. Measure baseline to the centre of each spot.
3. Divide each spot distance by the front distance.
4. Compare with references on the same plate.

Worked reminder. Solvent front 5.60 cm, spot centre 1.96 cm. Rf = 1.96 ÷ 5.60 = 0.35.

A mixture lane that shows the reference spot plus extra spots contains impurities, or an unfinished reaction. A single spot suggests one component in that solvent, but a second solvent could still split it.

6.3.1(b)(i): Retention time

• Longer in the column if the component is more soluble in the liquid stationary phase or less volatile.
• Changes with the column, oven temperature and carrier gas flow rate.
• Identify a peak by matching it to a pure standard run under the same conditions.
• Two compounds can share a retention time, so a match is evidence, not proof.

6.3.1(b)(ii): Amounts and proportions

Task	Method
Proportion of a component	Its peak area ÷ total peak area × 100
Which component is most abundant	Largest area, not tallest peak
Exact concentration	External calibration curve
Diluted sample	Multiply the graph reading by the dilution factor

Peak integration values are supplied. The percentage method assumes the detector responds equally to every component; a calibration curve for that compound removes the assumption.

Method box: external calibration

1. Prepare standards of known concentration that cover the expected range.
2. Run each one under identical conditions; record peak areas.
3. Plot peak area (y) against concentration (x); draw the line of best fit.
4. Run the sample; read its concentration from the line.
5. Correct for any dilution.

Worked reminder. A calibration line fits area = 850 × concentration (mmol dm⁻³). A sample diluted by a factor of 4 gives an area of 1955. Concentration read = 1955 ÷ 850 = 2.3 mmol dm⁻³. Original = 2.3 × 4 = 9.2 mmol dm⁻³.

6.3.1(c): Organic tests at a glance

Spec	Looking for	Reagent	Conditions	Positive result	Negative result
(i)	C=C	Bromine water	Shake, room temperature	Orange → colourless	Stays orange
(ii)	C–X	AgNO₃(aq) in ethanol	Warm, water bath	White AgCl, cream AgBr, yellow AgI	No precipitate
(iii)	Phenol	NaOH(aq); Na₂CO₃(aq)	Room temperature	Neutralised by NaOH; no fizzing with carbonate	—
(iv)	C=O (aldehyde or ketone)	2,4-DNP	Room temperature	Yellow-orange precipitate	No precipitate
(v)	Aldehyde	Tollens’ (ammoniacal AgNO₃)	Warm, water bath	Silver mirror	No change
(vi)	1° or 2° alcohol, aldehyde	K₂Cr₂O₇/H₂SO₄	Warm, water bath	Orange → green	Stays orange
(vii)	Carboxylic acid	Na₂CO₃(aq)	Room temperature	Effervescence; CO₂ turns limewater milky	No fizzing

Equations to know

• Alkene: CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
• Haloalkane hydrolysis: R–X + H₂O → R–OH + H⁺ + X⁻
• Halide precipitation: Ag⁺(aq) + X⁻(aq) → AgX(s)
• Phenol: C₆H₅OH + NaOH → C₆H₅ONa + H₂O
• Aldehyde with Tollens’: RCHO + [O] → RCOOH, with Ag⁺ + e⁻ → Ag
• Dichromate: Cr₂O₇²⁻ (orange) is reduced to Cr³⁺ (green)
• Carboxylic acid: 2CH₃CH₂CH₂COOH + Na₂CO₃ → 2CH₃CH₂CH₂COONa + CO₂ + H₂O

The 2,4-DNP equation and the structure of its derivative are not required.

Method box: identifying a carbonyl compound from its derivative

1. Add 2,4-DNP; collect the precipitate by filtration.
2. Recrystallise it from a minimum of hot solvent; filter and dry.
3. Measure the melting point.
4. Compare with melting point data for the derivatives of the possible compounds.

PAG7 technique points

• Fresh portion of unknown for each test.
• Water bath or electric heater for flammable liquids, never a naked flame.
• Tollens’ reagent made fresh.
• Record the observation, then the inference.

Must-know distinctions

• Carboxylic acid or phenol? NaOH neutralises both; carbonate fizzes only with the acid.
• Aldehyde vs ketone. Both give a 2,4-DNP precipitate. Only the aldehyde gives a silver mirror and turns dichromate green.
• Primary vs secondary alcohol. Both turn dichromate green. Distil off the product and test it: an aldehyde (silver mirror with Tollens’) means primary; a ketone means secondary.
• Secondary vs tertiary alcohol. Dichromate turns green with secondary; stays orange with tertiary.
• Alkene vs phenol with bromine water. Both decolourise it. Phenol also gives a white precipitate.
• Rf vs retention time. Rf is a ratio with no units; retention time is a time. Both depend on conditions.
• Peak area vs peak height. Area measures amount.

Quick self-test

1. A solvent front is 7.20 cm above the baseline. A spot centre is 3.24 cm above it. Calculate the Rf value.
2. A compound has Rf 0.55 in a system where the solvent front moves 6.80 cm. How far does its spot move?
3. A GC trace has three peaks with areas 620, 1550 and 930. What percentage of the mixture is the second component?
4. A calibration line fits area = 1600 × concentration in g dm⁻³. An undiluted sample gives an area of 2080. Find its concentration.
5. Name one reagent that distinguishes phenol from propanoic acid, and give the result for each.
6. Which of the tests in (c)(iv), (v) and (vi) give a positive result with propanal?
7. Equal amounts of 1-chloro-, 1-bromo- and 1-iodopentane are warmed with ethanolic silver nitrate. In which order do precipitates appear?
8. Why is the TLC baseline drawn in pencil?
9. Why are the organic test mixtures warmed in a water bath?
10. What do you see when 2-methylbutan-2-ol is warmed with acidified dichromate?
11. How do you confirm which compound a GC peak belongs to?

Answers

1. 3.24 ÷ 7.20 = 0.45
2. 0.55 × 6.80 = 3.74 cm
3. Total = 3100; 1550 ÷ 3100 × 100 = 50.0%
4. 2080 ÷ 1600 = 1.30 g dm⁻³
5. Sodium carbonate solution: propanoic acid fizzes (CO₂); phenol shows no effervescence.
6. All three: yellow-orange precipitate with 2,4-DNP, silver mirror with Tollens’, orange to green with dichromate.
7. Iodo first, then bromo, then chloro. The C–I bond is the weakest, so it is hydrolysed fastest.
8. Graphite is insoluble in the solvent; ink components would travel up the plate as extra spots.
9. Many organic liquids are flammable; a water bath keeps flames away from them.
10. No change; it stays orange. It is a tertiary alcohol and resists oxidation.
11. Run a pure standard under the same conditions and match the retention time.

Where marks are usually lost

• Measuring Rf distances from the plate’s lower edge, or to a spot’s top edge.
• Giving an Rf greater than 1, or with units.
• Reading “tallest peak” as “most abundant” when a broader, shorter peak has a larger area.
• Not multiplying by the dilution factor after reading a calibration graph.
• Reading off a calibration graph beyond the standards without saying it is an extrapolation.
• Writing “goes clear” instead of “orange to colourless” for bromine water.
• Giving only the halide colour for a haloalkane, without naming silver nitrate in ethanol or warming.
• Claiming 2,4-DNP shows an aldehyde specifically.
• Using carbonate to test for phenol and reporting a positive result as “fizzes”.
• Saying a ketone turns acidified dichromate green.

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 6.3.1 Chromatography and qualitative analysis, learning outcomes (a), (b)(i)–(ii) and (c)(i)–(vii), with their additional guidance, PAG6 and PAG7. Published by OCR.