12.3 Chromatography Revision Notes

1. Overview

Chromatography is a versatile analytical technique used to separate mixtures of soluble substances and identify the components present. It relies on the movement of a solvent (mobile phase) through a material like paper (stationary phase), separating substances based on their different solubilities and levels of attraction to the paper.

Key Definitions

Chromatography: An analytical method used to separate the components of a mixture based on their solubility in a solvent.
Solvent: The liquid (e.g., water, $H_2O (l)$, or ethanol, $C_2H_5OH (l)$) that carries the substances up the paper.
Solute: The substance that dissolves in the solvent to be separated.
Chromatogram: The final piece of paper showing the separated spots after the process is complete.
Baseline (Origin): The starting line drawn in pencil where the original mixture is placed.
Solvent Front: The highest point the solvent reaches on the paper.
Locating Agent: A chemical used to react with colorless substances to produce a colored product, making them visible.

Core Content

How Paper Chromatography Works

A baseline is drawn in pencil near the bottom of the chromatography paper.
Small spots of the mixtures/substances are placed on this line.
The paper is placed in a beaker containing a small volume of solvent, ensuring the baseline is above the level of the solvent.
As the solvent travels up the paper, the substances in the mixture dissolve and move at different speeds.
The more soluble a substance is in the solvent, the further up the paper it will travel.

📊A chromatography setup showing a beaker with a solvent at the bottom, paper suspended inside, a pencil baseline above the solvent level with various colored spots, and the solvent front near the top.

Interpreting Chromatograms

Identification: An unknown substance can be identified by comparing it to a known reference substance. If two spots travel the same distance from the baseline in the same solvent, they are likely the same substance.
Pure vs. Impure:
- A pure substance will only produce one spot on the chromatogram.
- A mixture (impure substance) will separate into two or more spots.

Extended Content (Extended curriculum only)

Separating Colorless Substances

Some substances, such as amino acids or sugars, are colorless and cannot be seen on a chromatogram.

The chromatography is performed as usual.
The paper is dried and then sprayed with a locating agent (e.g., ninhydrin for amino acids).
The locating agent reacts with the colorless spots to turn them into colored spots, or the paper is viewed under Ultraviolet (UV) light.

The $R_f$ Value

The $R_f$ (retention factor) value is a ratio used to identify substances accurately. It is constant for a specific substance in a specific solvent.

Formula: $$R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}$$

Worked Example: A spot of red dye moved $4.0 \text{ cm}$ from the baseline. The solvent front moved $10.0 \text{ cm}$ from the baseline. Calculate the $R_f$ value.

$$R_f = \frac{4.0 \text{ cm}}{10.0 \text{ cm}} = 0.4$$ (Note: $R_f$ values have no units and are always less than 1.0)

Key Equations

Equation	Meaning	Units
$R_f = \frac{d_{substance}}{d_{solvent}}$	$R_f$ = Retention factor; $d$ = distance measured from the baseline	None (Ratio)

Note on Solvents: Common solvents used include:

Water: $H_2O (l)$
Ethanol: $C_2H_5OH (l)$
Propanone: $CH_3COCH_3 (l)$

Common Mistakes to Avoid

❌ Wrong: Drawing the baseline in ink.
✓ Right: Always use pencil for the baseline. Ink contains dyes that will dissolve in the solvent and interfere with the results.
❌ Wrong: Letting the solvent level start above the baseline.
✓ Right: The solvent level must be below the baseline, otherwise the samples will wash off into the solvent rather than moving up the paper.
❌ Wrong: Calculating an $R_f$ value greater than 1.
✓ Right: The substance can never travel further than the solvent; ensure the smaller number is divided by the larger number.

Exam Tips

Command Word - "State": If asked to "state" how many substances are in a mixture, simply count the number of vertical spots produced by that sample.
Command Word - "Explain": If asked to explain why a substance didn't move, mention it is insoluble in that specific solvent.
Measuring Distances: Always measure from the center of the spot to the baseline.
Real-world Context: Be prepared for questions involving food coloring, ink analysis in forensics, or checking the purity of a synthesized drug in a laboratory.
Typical Values: $R_f$ values are usually given to 2 decimal places (e.g., 0.65). If a spot stays on the baseline, its $R_f$ is 0.