Chromatography and Rf Values

Chromatography questions hand out marks for a ruler and a fraction: measure two distances, divide, get a number between 0 and 1. Yet Paper 6 examiner reports record Rf values over 1, baselines drawn in ink, and solvent levels that drown the spots: all method errors, all preventable. Here is the technique exactly as the mark scheme wants it described.

The method, step by step

1. Draw a baseline in pencil about 2 cm from the bottom of the chromatography paper. Pencil, because ink would dissolve and travel with the samples.
2. Put a small spot of each sample on the baseline and let it dry. Concentrated spots beat large wet ones. A big spot smears.
3. Stand the paper in a beaker with solvent below the baseline. If the solvent starts above the line, the samples dissolve into the beaker instead of moving up the paper.
4. Cover the beaker (limits evaporation) and wait while the solvent rises by capillary action, carrying the dyes with it.
5. Remove the paper before the solvent reaches the top, and immediately mark the solvent front in pencil, because it becomes invisible once dry.
6. Let the chromatogram dry.

The principle behind it: substances that are more soluble in the solvent (and less attracted to the paper) travel further. A mixture separates into spots; a pure substance gives one spot only.

Reading a chromatogram

• Same dye, same height. Two spots at the same level (same Rf) in the same solvent are the same substance. This is how the paper asks “which dyes does ink X contain?”
• Pure vs mixture. One spot = pure; two or more = mixture. This links directly to identifying unknown substances.
• A spot on the baseline means that substance is insoluble in this solvent.

The Rf value

Rf = distance moved by the substance ÷ distance moved by the solvent front

Both distances are measured from the baseline: the substance’s distance to the centre of the spot, the solvent’s to the marked solvent front. Because the spot can never outrun the solvent carrying it, 0 < Rf < 1, and Rf has no units. Under fixed conditions (same solvent, same paper, same temperature) the Rf of a substance is constant, so measured values can be matched against data tables to identify a substance.

Worked numbers: spot at 4.5 cm, solvent front at 9.0 cm → Rf = 4.5 ÷ 9.0 = 0.50. Quote two decimal places unless told otherwise.

Locating agents

Amino acids, sugars and other colourless substances separate invisibly. After drying, the chromatogram is sprayed with a locating agent: a substance that reacts with the spots to make them visible. The 0620 mark is for the purpose (“to show the positions of colourless substances”), not for naming a particular chemical. Questions pair this with food-science contexts: which amino acids does a protein hydrolysate contain?

Worked exam question

A student uses paper chromatography to analyse a green food colouring. The solvent front moves 8.0 cm. A yellow spot moves 6.0 cm and a blue spot moves 2.4 cm. (a) Calculate the Rf value of the blue dye. [2] (b) State what the result shows about the green colouring. [1] (c) Explain why the baseline is drawn in pencil and why the solvent level must start below it. [2]

Model answer, mark by mark:

• (a) M1: Rf = 2.4 ÷ 8.0. M2: = 0.30 (no units). Show the working: the substitution mark survives an arithmetic slip.
• (b) M3: the green colouring is a mixture (of a yellow dye and a blue dye), because two spots means two substances.
• (c) M4: pencil is insoluble in the solvent; ink would dissolve and move up the paper with the samples. M5: if the solvent started above the baseline, the spots would dissolve into the solvent in the beaker rather than travel up the paper.

The mistakes that cost marks

1. Rf greater than 1. The fraction is substance ÷ solvent, in that order. 8.0 ÷ 2.4 = 3.3 is a flipped equation, and the error is unmissable because Rf must sit between 0 and 1.
2. Measuring to the top of the spot. Measure baseline to the centre of the spot. On a Paper 6 chromatogram printed at full size, top-of-spot readings fall outside the accepted range.
3. Units written after Rf. It is a ratio of two lengths: the centimetres cancel. “0.30 cm” loses the mark on most schemes.
4. Forgetting to mark the solvent front. In describe-the-method questions, “mark the solvent front immediately, before it dries invisible” is its own marking point.
5. “Use pen so you can see the line.” The reverse of the chemistry. Pencil, because graphite does not dissolve.

How examiners want it phrased

Student wording	Mark-scheme wording
”The colours spread up the paper"	"The solvent rises up the paper, carrying the dyes; more soluble dyes travel further"
"It only made one dot, so it’s fine"	"A pure substance gives a single spot on the chromatogram"
"The two inks match"	"The spots have the same Rf value in the same solvent, so they are the same substance"
"Spray it to see the spots"	"Spray with a locating agent to show the positions of the colourless substances”

Chromatography is a banker question on Paper 6 and a fixture of the wider experimental techniques topic. If your Rf answers have ever come out above 1, bring that script to a free trial lesson. Measurement habits are the fastest marks we fix.

Test yourself

Three questions, ruler optional. Answer before you click.

Q1 (2 marks). Write the equation used to calculate an Rf value, and explain why Rf has no units.

Show answer

• Rf = distance moved by the substance ÷ distance moved by the solvent front (both measured from the baseline) [1] • it is a ratio of two distances, so the units cancel [1]

Q2 (2 marks). State why the solvent front must be marked as soon as the paper is removed, and how a chromatogram shows that a substance is pure.

Show answer

• the solvent front becomes invisible once the paper dries [1] • a pure substance gives a single spot [1]

Q3 (3 marks). A mixture of amino acids is separated by paper chromatography, but no spots are visible on the dried paper. Describe how to make the spots visible and how to identify one of the amino acids.

Show answer

• spray the chromatogram with a locating agent [1] • this shows the positions of the colourless substances [1] • measure the Rf value of the spot and compare it with known Rf values for the same solvent (and conditions) [1]