Required Practical: Chromatography

Purpose of Chromatography

Chromatography is a technique used to separate mixtures into their individual components. It helps chemists identify what substances are present in a mixture and check the purity of a sample. For example, it can separate different coloured dyes in ink or food colouring.

The main reasons for using chromatography are:

• To separate mixtures into individual substances
• To identify the components of a mixture by comparing them to known substances
• To analyse the purity of a substance by checking if it contains one or more components

Required Practical Setup

To carry out paper chromatography, you need:

• A strip of chromatography paper
• A solvent (such as water or ethanol) that will carry the substances up the paper
• A sample mixture to test
• A pencil to draw a baseline
• A capillary tube or a fine pipette to apply the sample
• A beaker or container to hold the solvent

Steps to prepare the chromatography:

1. Draw a horizontal pencil line about 2 cm from the bottom of the chromatography paper. This is the baseline where you will place the sample spots.
2. Use a capillary tube to place a small, concentrated spot of the sample mixture onto the baseline. Allow it to dry.
3. Pour a small amount of solvent into the beaker. The solvent level should be below the baseline on the paper to prevent the spots from dissolving directly into the solvent.
4. Attach the chromatography paper to a glass rod or pencil and suspend it so the bottom of the paper dips into the solvent, but the spots remain above the solvent level.

Safety note: Work in a well-ventilated area when using volatile solvents like ethanol to avoid inhaling fumes.

Running the Chromatogram

Once the setup is ready, the solvent will start to move up the paper by capillary action. As it passes through the spot of the sample, the different components in the mixture will dissolve in the solvent and travel at different speeds depending on their solubility and how strongly they stick to the paper.

Key points during the run:

• Allow the solvent to rise up the paper until it is near the top, but do not let it reach the edge.
• Remove the paper from the solvent before the solvent reaches the top to avoid losing separated spots.
• Immediately mark the position of the solvent front with a pencil. This is important for later analysis.
• Let the paper dry before analysing the spots.

For example, if you separate black ink, you might see different coloured spots appear at different heights, showing the different dyes present.

Interpreting Results

After the chromatogram has dried, you can analyse the results by measuring how far the solvent and each spot have travelled. This helps to identify the substances in the mixture.

Steps to interpret:

• Measure the distance from the baseline to the solvent front (the highest point the solvent reached).
• Measure the distance from the baseline to the centre of each spot.
• Calculate the Rf value for each spot using the formula:

Rf = (distance travelled by substance) / (distance travelled by solvent front)

• Compare the Rf values to those of known substances run under the same conditions.
• If the sample contains only one spot, it is likely pure. Multiple spots indicate a mixture.

For example, if a sample shows two spots at the same heights as two known substances, it suggests the sample contains those two substances.

Learning example:

Suppose the solvent front travelled 12 cm from the baseline. One spot travelled 8 cm and another spot travelled 6 cm. By comparing these distances to known substances run in the same solvent, you can identify the components of the mixture.