Paper chromatography and investigating inks

You need chromatography paper, a pencil, ink samples, a capillary tube or pipette, a beaker, a suitable solvent and a support to hold the paper upright. Water is suitable for water-soluble inks. An ink that is insoluble in water needs a different solvent in which its dyes dissolve.

1. Draw a pencil baseline across the paper, about 1 cm from its bottom edge. Pencil does not dissolve in the solvent; an ink baseline could introduce extra dyes.
2. Place small, separated spots of each ink on the baseline. Label their positions in pencil. Known substances can be spotted alongside unknown samples for comparison.
3. Add a shallow layer of solvent to the beaker. Suspend the paper with its bottom touching the solvent, but keep the baseline and sample spots above the solvent level. Otherwise, the samples could wash into the liquid instead of travelling up the paper.
4. Allow the solvent to rise through the paper. Remove the paper before the solvent reaches the top.
5. Immediately mark the solvent front in pencil, before the solvent evaporates. Let the paper dry, then examine the spots and measure their distances.

Reading the pattern of spots

A pure soluble substance produces one spot. A mixture that separates produces two or more spots, showing that more than one substance is present. One spot is evidence of purity under the test conditions, rather than an absolute guarantee: different substances can sometimes travel together.

To identify a substance, compare the unknown with known substances run under the same conditions. Spots of the same colour at the same height provide evidence that they are the same substance. Comparing the whole pattern also helps you decide whether two inks contain the same dyes.

In an example chromatogram, known substance A produces one spot 20 mm above the baseline, and known substance B produces one spot 40 mm above it. Unknown ink X produces spots at both heights. X is therefore a mixture, and the matching positions support the conclusion that it contains A and B.

Using Rf values to compare substances

An Rf value, or retention factor, expresses how far a substance travelled compared with the solvent front:

$$R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}}$$

Measure both distances from the baseline, measuring to the centre of the substance spot. Because the distances are divided, their units cancel: Rf has no unit. Values lie between 0 and 1.

In the example chromatogram, the solvent front has travelled 60 mm. The upper spot has travelled 40 mm, so its Rf is $40/60 \approx 0.67$. This means the spot travelled about two-thirds as far as the solvent front. The lower spot has an Rf of $20/60 \approx 0.33$.

Rf values can be compared with reference values for known substances to help identify an unknown. The comparison must use the same conditions, especially the same solvent: changing the solvent can change how far a substance travels and therefore change its Rf value.

Investigating the solvent in ink by simple distillation

The ink investigation has two complementary parts. Chromatography separates the dissolved dyes; simple distillation separates and collects the solvent. For a water-based ink, the solvent collected is water.

A suitable school arrangement uses a conical flask containing about 20 ml of ink, anti-bumping granules, a bung carrying a thermometer and delivery tube, and a receiving test tube cooled in a beaker of ice. The flask is supported above a Bunsen burner using a tripod and gauze on a heat-resistant mat. Anti-bumping granules help the liquid boil smoothly.

Heat the ink until it boils, then reduce the heating to maintain a gentle boil. Water evaporates and its vapour passes along the delivery tube. Cooling turns the vapour back into liquid, which collects in the receiving tube as the distillate. Collect a small sample, then turn off the burner.

The coloured dyes remain in the flask, so the collected water should be colourless while the remaining ink stays coloured. This shows that the original ink contained a solvent as well as dissolved colouring substances. The vapour temperature can also be compared with the boiling point of the suspected solvent; water boils at about $100\,^{\circ}\mathrm{C}$ at normal atmospheric pressure.

Wear eye protection, secure hair and loose clothing, and allow hot apparatus to cool before handling it. Keep flammable chromatography solvents away from the Bunsen flame. Together, the two techniques reveal different parts of the ink's composition: its solvent and its dyes.

$$R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent front}}$$

Measure from the baseline to the spot centre and solvent front. Rf has no unit and lies between 0 and 1. Changing the solvent can change Rf.

Ink investigation

Chromatography: pencil baseline → small ink spots → solvent below baseline → allow solvent to rise → remove paper and mark solvent front → dry, compare and measure.

Simple distillation: gently boil water-based ink → water evaporates → vapour cools and condenses → collect colourless water. Coloured dyes remain in the flask.

Wear goggles, let hot apparatus cool and keep flammable solvents away from flames.