65. Paper Chromatography

1. What is Chromatography?

Chromatography is an analytical technique used to separate the different substances in a mixture so they can be identified. Paper chromatography is specifically used to separate different dyes in an ink.

2. Method for Paper Chromatography

  1. Preparation: Draw a baseline near the bottom of a piece of filter paper using a pencil (pencil is used because it is insoluble and won't move with the solvent).

  2. Sampling: Add a spot of the ink sample onto the pencil line.

  3. Setup: Place the paper into a beaker with a shallow amount of solvent (e.g., water or ethanol).

    • Crucial: Ensure the solvent level is below the pencil line so the ink doesn't wash away.

    • Tip: Place a lid on the beaker to prevent the solvent from evaporating.

  4. Process: The solvent seeps up the paper, carrying the dyes with it. Different dyes move at different rates, separating them out.

  5. Result: Once the solvent nearly reaches the top, remove the paper and let it dry. The finished pattern of spots is called a chromatogram.

3. The Two Phases

The separation happens because substances distribute themselves between two phases:

  • Mobile Phase: The substance that the molecules can move in (always a liquid or gas). In this experiment, it is the solvent.

  • Stationary Phase: The substance the molecules cannot move in (usually a solid). In this experiment, it is the paper.

4. Why Substances Separate

The distance a substance travels depends on its solubility in the mobile phase and its attraction to the stationary phase:

  • Faster movement: A substance that is more soluble in the solvent spends more time in the mobile phase.

  • Slower movement: A substance that is less soluble or more attracted to the paper spends more time in the stationary phase.

5. Calculating Rf Values

The Rf value (Retention Factor) is a ratio used to identify substances. It is independent of how long the experiment runs.

  • Formula: Rf = Distance traveled by substance / Distance traveled by solvent

  • Example: If a dye moves 6 cm and the solvent moves 10 cm, the Rf value is 6 / 10 = 0.6.

6. Identifying Pure vs. Impure Substances

  • Pure substances: Will only produce a single spot on the chromatogram.

  • Impure substances (mixtures): Will separate into multiple spots.

Note: The Rf value of a substance will change if you change the solvent (mobile phase) or the paper (stationary phase).