Paper Chromatography
Difficulty: Easy | Time: 15–30 minutes | Visual Impact: High
Historical Context
Chromatography was invented by the Russian botanist Mikhail Tsvet in 1900. He was separating plant pigments by passing solutions through a glass column packed with chalk, and noticed that different pigments traveled at different speeds, creating distinct colored bands. He named the technique chromatography from the Greek chroma (color) and graphein (to write) — though today chromatography separates colorless compounds far more often than colored ones.
Paper chromatography was developed in 1944 by Martin and Synge, who later received the Nobel Prize in Chemistry for the work. It was instrumental in the identification of amino acids, sugars, and peptides in the postwar era, before modern instrumental methods were available. Today it is used primarily for teaching because it requires no instrumentation — but the same underlying principle governs high-performance liquid chromatography (HPLC) systems that cost tens of thousands of dollars.
Materials
- Coffee filter or chromatography paper — cut into strips 3 cm × 15 cm
- Water-soluble markers (black or brown reveal the most colors) or food coloring
- Solvent: water, or 70% isopropanol (rubbing alcohol) for different separation
- Tall jar, beaker, or glass
- Pencil and ruler
Procedure
- Cut strips from the coffee filter
- Using pencil (not pen — pen ink smears), draw a line 2 cm from the bottom edge
- Use a marker to make a dot or short line on the pencil line — keep the mark small
- Pour 0.5–1 cm of solvent into the jar (water for water-based inks; isopropanol for permanent markers)
- Hang the strip so its bottom edge touches the solvent but the ink dot is above the liquid surface
- Watch the solvent front climb — it will carry pigments at different rates
- Remove the strip when the solvent front is 1–2 cm from the top; let it dry
- Observe the separated bands
Comparison experiment: try the same marker with water vs. isopropanol as solvent. The band pattern often changes dramatically.
The Science
Chromatography separates compounds based on how strongly they interact with two competing phases: a stationary phase (the paper, made of cellulose fibers with water molecules clinging to them) and a mobile phase (the solvent traveling up the paper).
Each dye molecule is constantly partitioning between the two phases: - A molecule that prefers the stationary phase (more polar, more attracted to water on the paper) moves slowly - A molecule that prefers the mobile phase (more soluble in the solvent) moves quickly
After the solvent has traveled a fixed distance, each component ends up at a reproducible position. This position is expressed as the Rf value (retardation factor):
\[R_f = \frac{\text{distance traveled by compound}}{\text{distance traveled by solvent front}}\]
Rf values range from 0 (stayed at the origin) to 1 (traveled with the solvent front). Each compound has a characteristic Rf for a given stationary phase and solvent — making Rf a simple identification tool.
Black marker ink typically contains 3–5 distinct dyes: blue, yellow, red, and sometimes orange, which the eye integrates into apparent black. Chromatography unmixes them. Food colorings labeled as a single color are often mixtures as well.
Tips
- Keep the ink dot small — large blobs separate poorly
- Use pencil for the baseline, never pen
- A lid or plastic wrap on top reduces evaporation and keeps the solvent front moving steadily
- Dry the strip flat and photograph it before the bands fade