Pencil is used in paper chromatography because it is insoluble in the solvent used in the process (typically water or a mixture of solvents), ensuring the baseline remains intact and doesn't interfere with the separation of compounds.
Here's a more detailed explanation:
- Insolubility is Key: The most crucial reason is that pencil lead (composed primarily of graphite, a form of carbon) is insoluble in the solvents commonly used in paper chromatography.
- Baseline Integrity: A line is drawn with a pencil to mark the origin where the sample mixture is spotted. If the line were drawn with ink (which contains soluble dyes), the ink would dissolve in the solvent and travel up the paper, potentially contaminating the sample and disrupting the separation process. The pencil line, however, remains stationary and does not interfere.
- Accurate Separation: Using an insoluble substance for the baseline ensures that only the components of the sample mixture are being separated and analyzed, leading to more accurate and reliable results.
- No Interference: Because the graphite is inert in the solvent, it doesn't react with the sample or the chromatography paper, preventing unwanted chemical reactions that could skew the results.
In summary, using a pencil in paper chromatography provides a stable, insoluble baseline that doesn't interfere with the separation of compounds, leading to accurate and reliable results.
