Chromatography Notes

Chromatography

Chromatography is a technique used to separate mixtures into their individual components, similar to separating mixed lollies. It works by using a stationary phase and a mobile phase.

Key Concepts

• Stationary Phase: A fixed material, such as filter paper in paper chromatography, that acts as the "race track."

• Mobile Phase: A liquid or gas that moves through the stationary phase, carrying the mixture. It acts as the "driver."

• Separation Process: The components of the mixture "race" through the stationary phase at different speeds due to their varying attractions to the stationary and mobile phases.

Polarity and Separation

Polarity is the "stickiness" of a molecule, influencing how it interacts with other molecules.

• Polar Substances: Like magnets with positive and negative ends, they prefer to stick to other polar substances (e.g., water).

• Non-Polar Substances: More balanced, without strong "sticky" ends, and prefer to mix with other non-polar substances (e.g., oil).

Role of Polarity in Chromatography

The separation in chromatography occurs due to the different attractions between the components of the mixture and the stationary and mobile phases.

• Attraction to Stationary Phase: A component with similar polarity to the stationary phase will stick to it more strongly and move slowly.

• Attraction to Mobile Phase: A component with similar polarity to the mobile phase will dissolve better in it and be carried along more quickly.

This difference in "stickiness" causes the components to separate into different spots or bands on the stationary phase.

Rf Value (Retention Factor)

The Rf value is a ratio that indicates how far a substance traveled compared to the solvent (mobile phase).

Formula

The Rf value is calculated as:

Rf = \frac{Distance\ travelled\ by\ the\ substance}{Distance\ travelled\ by\ the\ solvent\ front}

• Distance travelled by the substance: Measured from the baseline to the middle of the separated spot.

• Distance travelled by the solvent front: Measured from the baseline to the top edge of where the solvent reached.

Important Characteristics of Rf Values

• Range: Always between 0 and 1 because a substance cannot travel further than the solvent.

• Uniqueness: Under the same conditions (stationary and mobile phases), the Rf value is unique for a specific substance.

Usefulness in Identifying Unknowns

The "fingerprint" quality of Rf values is useful in chemistry for identifying unknown substances.

1. Run Knowns: Perform chromatography on known substances ("standards") and calculate their Rf values.

2. Run Unknowns: Perform chromatography on an unknown substance under the same conditions.

3. Compare Fingerprints: Compare the Rf value of the unknown substance to the Rf values of the known substances. If the Rf values match, it indicates the unknown substance is likely the same as the known substance.

Solvent Influence

Changing the mobile phase (solvent) will likely change the Rf value of a substance due to different interactions with the new solvent.

Summary of Chromatography

Chromatography is a separation technique using stationary and mobile phases to separate mixtures based on the different affinities of their components for each phase.

• Polarity plays a crucial role.

• "Like dissolves like" is a key principle.

• The Rf value quantifies substance travel and aids in identifying mixture components.

Example: Separating Colours in Black Ink (Paper Chromatography)

Setup

• Stationary Phase: Filter paper (cellulose with -OH groups, making it polar).

• Mobile Phase: A polar solvent such as water.

Experiment

1. Place a small spot of black ink near the bottom of the filter paper (baseline).

2. Place the paper in a beaker with a shallow layer of water (below the ink spot).

3. Water (mobile phase) moves up the paper (stationary phase) by capillary action.

Role of Polarity in Separation

Black ink contains different coloured pigments, each with unique polarity.

• Highly Polar Pigments (e.g., Blue): Strong attraction to both polar paper and polar water. If it sticks more to the paper, it will move slowly, resulting in a low Rf value.

• Less Polar Pigments (e.g., Yellow): Weaker attraction to the polar filter paper but stronger attraction to the moving water. It will be carried along more easily, resulting in a high Rf value.

Result

Black ink separates into different coloured spots at different heights on the paper.

• More polar pigments are closer to the baseline (low Rf).

• Less polar pigments are further up the paper (high Rf).

Example Rf Calculation

• Solvent front traveled 10 cm from the baseline.

• Blue spot traveled 2 cm from the baseline.

• Yellow spot traveled 8 cm from the baseline.

Rf_{blue} = \frac{Distance\ travelled\ by\ blue\ pigment}{Distance\ travelled\ by\ solvent\ front} = \frac{2 \text{ cm}}{10 \text{ cm}} = 0.2

Rf_{yellow} = \frac{Distance\ travelled\ by\ yellow\ pigment}{Distance\ travelled\ by\ solvent\ front} = \frac{8 \text{ cm}}{10 \text{ cm}} = 0.8

The more polar blue pigment has a lower Rf value (0.2), while the less polar yellow pigment has a higher Rf value (0.8).