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Experimental Techniques and Chemical Analysis - Chromatography

Grade 12A LevelChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Chromatography is an analytical technique used to separate and identify the components of a mixture based on their different solubilities in a mobile phase and their affinity for a stationary phase.

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The Stationary Phase is a fixed material (such as chromatography paper or a silica-coated TLCTLC plate) that slows down the movement of components.

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The Mobile Phase is a solvent (liquid or gas) that moves through the stationary phase, carrying the components of the mixture with it.

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Components with higher solubility in the mobile phase and lower affinity for the stationary phase travel faster and further up the chromatogram.

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A pure substance will yield only one spot on a chromatogram, whereas a mixture will separate into multiple spots.

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If the substances being separated are colorless (e.g., amino acids), a locating agent such as ninhydrin or UVUV light is used to make the spots visible.

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The RfR_f (Retention Factor) value is a ratio used to identify substances; it is characteristic for a specific substance under a given set of conditions (same solvent and stationary phase).

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Gas-Liquid Chromatography (GLCGLC) is used for volatile organic compounds, where the mobile phase is an inert carrier gas like HeHe or N2N_2 and the stationary phase is a high-boiling point liquid on an inert solid.

📐Formulae

Rf=distance moved by the substancedistance moved by the solvent frontR_f = \frac{\text{distance moved by the substance}}{\text{distance moved by the solvent front}}

Distance moved by substance=Rf×Distance moved by solvent front\text{Distance moved by substance} = R_f \times \text{Distance moved by solvent front}

💡Examples

Problem 1:

In a paper chromatography experiment to identify a food dye, the baseline was drawn 1.0 cm1.0\text{ cm} from the bottom of the paper. After the experiment, the solvent front had reached a point 11.0 cm11.0\text{ cm} from the bottom of the paper. A specific blue dye spot was found 8.5 cm8.5\text{ cm} from the bottom of the paper. Calculate the RfR_f value for the blue dye.

Solution:

Distance moved by solvent=11.0 cm−1.0 cm=10.0 cm\text{Distance moved by solvent} = 11.0\text{ cm} - 1.0\text{ cm} = 10.0\text{ cm} Distance moved by dye=8.5 cm−1.0 cm=7.5 cm\text{Distance moved by dye} = 8.5\text{ cm} - 1.0\text{ cm} = 7.5\text{ cm} Rf=7.510.0=0.75R_f = \frac{7.5}{10.0} = 0.75

Explanation:

To calculate the RfR_f value, we must measure the distance from the baseline (starting line), not the edge of the paper. Subtracting the 1.0 cm1.0\text{ cm} offset gives the actual travel distance for both the solvent and the solute.

Problem 2:

A mixture containing two amino acids, AA and BB, is analyzed using TLCTLC. The RfR_f values for AA and BB in the chosen solvent are 0.420.42 and 0.680.68 respectively. If the solvent front travels 15.0 cm15.0\text{ cm}, calculate the distance between the two spots on the chromatogram.

Solution:

Distance for A=0.42×15.0 cm=6.3 cm\text{Distance for } A = 0.42 \times 15.0\text{ cm} = 6.3\text{ cm} Distance for B=0.68×15.0 cm=10.2 cm\text{Distance for } B = 0.68 \times 15.0\text{ cm} = 10.2\text{ cm} Separation=10.2 cm−6.3 cm=3.9 cm\text{Separation} = 10.2\text{ cm} - 6.3\text{ cm} = 3.9\text{ cm}

Explanation:

The distance moved by each component is found by multiplying the RfR_f value by the solvent front distance. The separation is the absolute difference between these two calculated distances.