Pencil is used in paper chromatography instead of pen because pencil lead, primarily graphite, is insoluble in the solvents typically used, ensuring it does not interfere with the separation process, whereas pen ink contains soluble dyes that would migrate and contaminate the chromatogram.
The Core Principle: Avoiding Interference
In paper chromatography, the aim is to separate components of a mixture based on their differing affinities for the stationary phase (the paper) and the mobile phase (the solvent). For accurate results, nothing else should move or react during the separation. This is where the choice between pencil and pen becomes critical.
Why Pencil is Preferred
The "lead" in a pencil is actually a mixture of graphite (a form of carbon) and clay. Graphite is an extremely stable and inert material.
- Insolubility: Graphite is virtually insoluble in common chromatography solvents like water, ethanol, acetone, or various organic solvents. When the solvent front moves up the paper, the pencil line remains fixed, serving as a stable baseline from which the sample components travel.
- No Contamination: Because the pencil line does not dissolve or move, it does not introduce any new compounds into the separation. This ensures that only the components of the sample being analyzed migrate and become visible on the chromatogram.
- Accurate Baseline: The starting line, or baseline, is crucial for measuring the retention factor (Rf value), which is a key characteristic used to identify separated compounds. A stable, non-migrating baseline ensures precise measurements.
Why Pen Ink is Problematic
Pen inks, particularly those found in ballpoint and felt-tip pens, are formulated to be soluble in various solvents to allow for smooth writing and drying. This solubility is precisely what makes them unsuitable for chromatography.
- Soluble Dyes: Pen inks are typically composed of a mixture of dyes and pigments, solvents, and other additives. Many of these dyes are designed to be soluble in common organic solvents or water, which are the same types of solvents used in chromatography.
- Migration and Contamination: If a starting line were drawn with a pen, as the solvent moves up the paper, it would dissolve the ink. The different colored dyes in the pen ink would then separate and travel up the paper alongside the sample components. This creates several issues:
- False Positives: The separated ink dyes could be mistaken for components of the sample, leading to incorrect identification.
- Obscured Results: The ink trails could overlap with the actual sample spots, making it difficult or impossible to distinguish and measure the separated compounds accurately.
- Inaccurate Rf Values: The presence of migrating ink would compromise the integrity of the experiment, leading to unreliable data.
Comparison: Pencil Lead vs. Pen Ink
| Feature | Pencil Lead (Graphite & Clay) | Pen Ink (Dyes, Solvents, Additives) |
|---|---|---|
| Composition | Carbon (graphite), clay | Organic dyes, pigments, water, organic solvents, resins |
| Solubility in Solvents | Negligible; virtually insoluble | Highly soluble in many common chromatography solvents |
| Effect on Separation | None; remains fixed as a stable baseline | Migrates with solvent front, contaminates chromatogram |
| Result | Clear, accurate separation of sample components | Blurred, contaminated results; difficulty in identification |
Best Practices in Paper Chromatography
To ensure successful and accurate chromatographic separation:
- Use a Pencil: Always use a light pencil line for the baseline and any initial markings.
- Minimal Markings: Keep all pencil markings minimal to avoid excess graphite that could potentially interfere, although the risk is low.
- Spotting: Apply sample spots as small and concentrated as possible directly on the pencil line.
- Correct Solvent: Choose a solvent system appropriate for the compounds being separated.
By understanding the fundamental differences in solubility and composition between pencil lead and pen ink, it becomes clear why pencil is the only suitable tool for marking in paper chromatography experiments.
