Paper Chromatography

Introduction

Paper chromatography is the simplest form of chromatography, and it can be carried out with little more than a strip of paper, a beaker and a solvent. Yet every detail of the set-up matters: where the line is drawn, what it is drawn with, how deep the solvent is and when the paper is removed. Getting these details right is the difference between clear, separated spots and a useless smear.

Core explanation

The equipment. A rectangle of chromatography paper (a smooth, absorbent paper similar to filter paper), a beaker or tank with a lid, a solvent such as water or ethanol, a pencil, a ruler and a fine glass capillary tube or dropper for applying the samples.

The baseline. A horizontal line is drawn in pencil about 1–2 cm above the bottom edge of the paper. Pencil is used because graphite does not dissolve in the solvent; a line drawn in ink would itself separate into dyes and spoil the result.

Spotting the samples. A tiny drop of each sample is placed on the baseline using a capillary tube, leaving a gap between samples so that they do not merge. The spots are kept small and concentrated: a small spot gives a sharp result, while a large spot spreads into a blurred band. Several samples, including known reference substances, are often run side by side on the same paper.

Running the chromatogram. The paper is lowered into the beaker so that its bottom edge dips into the solvent, but the solvent level is below the baseline . If the spots were under the surface, they would simply dissolve into the solvent in the beaker instead of travelling up the paper. A lid is placed on top so that the air inside becomes saturated with solvent vapour, which stops the solvent evaporating from the paper and keeps it rising evenly.

The rising solvent. The solvent soaks upwards through the tiny gaps between paper fibres by capillary action . As it passes the baseline it dissolves the samples and carries them upwards, each at its own rate.

Finishing. Before the solvent reaches the top edge, the paper is removed and the position of the solvent front is marked immediately in pencil, because it becomes invisible once the paper dries. The paper is then left to dry. The finished chromatogram shows the baseline, the solvent front and a set of spots above each starting point.

Step-by-step reasoning

The logic behind each step:

1. Pencil baseline — pencil will not dissolve or move. 2. Baseline above solvent — the samples must be carried up, not washed away. 3. Small spots — sharp, separate results. 4. Lid on — prevents evaporation, so the solvent rises steadily. 5. Mark the solvent front — it is needed for measurements later.

Visual explanation

Picture the paper hanging in a beaker. At the bottom is 1 cm of solvent. Just above it runs a grey pencil line with four small dots. Higher up, a faint damp edge marks the solvent front, and between the line and the front sit columns of coloured spots.

Real-world analogy

Setting up the chromatogram is like lining runners up on a starting line before a race. They all begin level on the same line, the "track" (paper) is the same for everyone, and the race is stopped before anyone runs off the end so that everyone's position can be measured.

Real-world example

In school laboratories, paper chromatography is used to compare the inks in different black pens. Because each brand uses its own blend of dyes, the pattern of spots acts like a fingerprint, and an unknown ink sample can be matched to the pen that wrote it.

Why?

Why is a lid placed over the beaker? Without it, solvent would evaporate from the surface of the paper as it rises. The solvent front would then move unevenly and slowly, the spots could spread, and the results would be harder to read and to repeat.

Common misconception

"More sample gives a better result, so the spot should be large." A large spot overloads the paper, producing long, overlapping streaks. Small, concentrated spots, sometimes built up by adding a drop, letting it dry and adding another, give the clearest separation.

Worked example

Question: A student draws the baseline in blue biro and places it 0.5 cm above the paper's edge, then pours 1 cm of solvent into the beaker. Identify two mistakes and their effects.

Reasoning: Biro ink dissolves in many solvents, and the baseline is below the solvent level.

Answer: First, the biro line would separate into its own dyes and confuse the result; pencil should be used. Second, the baseline is under the solvent, so the samples would dissolve into the beaker instead of rising; the baseline should be above the solvent level.

Quick check

1. Why is the solvent front marked as soon as the paper is removed? Answer: Because it becomes invisible once the solvent dries, and its position is needed for measurements.

Exam focus

"Describe how to carry out paper chromatography" is a very common question. Mention the pencil baseline, small spots, solvent below the baseline, a lid, removing the paper before the solvent reaches the top and marking the solvent front. Always give a reason for each point when asked to explain.

Advanced insight

Some chromatography uses thin-layer plates — glass or plastic coated with a thin layer of silica — instead of paper. They run faster and give sharper spots. Choosing the right solvent is also a skill: if every substance stays near the baseline, the solvent is too weak; if all run to the front together, it is too strong.

Summary

In paper chromatography, small spots of the samples are placed on a pencil baseline near the bottom of the paper. The paper stands in a covered beaker with the solvent below the baseline. The solvent rises by capillary action, carrying the substances upwards at different rates. The paper is removed before the solvent reaches the top, the solvent front is marked, and the paper is dried to give a chromatogram.

Practice questions

1. Give the reason the baseline is drawn in pencil rather than ink. Answer: Pencil (graphite) is insoluble in the solvent, so it does not move; ink would dissolve and separate into its own dyes. 2. What would happen if the solvent level were above the baseline? Answer: The samples would dissolve into the solvent in the beaker rather than being carried up the paper. 3. Explain how the solvent moves up the paper. Answer: By capillary action, soaking upwards through the narrow gaps between the paper fibres. 4. Why should the spots on the baseline be kept small? Answer: Small spots give sharp, well-separated results; large spots spread out into streaks that overlap.