Chapter 12: Experimental Techniques and Chemical Analysis — Part 3

01 Principles of Paper Chromatography

Chromatography separates mixtures of soluble substances based on their different solubilities.

• Stationary Phase: The chromatography paper.
• Mobile Phase: The solvent (e.g., water or ethanol) that travels up the paper.
• Separation: More soluble substances are more attracted to the mobile phase and travel further.

02 Method and Interpretation

• A pencil line (the baseline) is drawn 1 cm from the bottom of the paper. Pencil is used because ink would dissolve and interfere with the results.
• Spots of the sample are placed on the line, and the paper is stood in a small volume of solvent, ensuring the pencil line is above the solvent level.
• The solvent front is the furthest point reached by the solvent.
• Purity: A pure substance produces one spot; an impure substance produces multiple spots.
• Identification: Unknowns are identified by comparing their spots to known reference substances or by calculating R_f values.

03 Colourless Substances and R_f Values

• Locating Agents: Colourless substances (like amino acids) can be made visible using locating agents such as ninhydrin (colours them purple) or UV light (causes them to fluoresce).
• R_f Equation: The retention factor (R_f) is a ratio used to identify substances.
  • R_f = distance travelled by substance ÷ distance travelled by solvent.
  • Distances are measured from the baseline to the centre of the spot.