Form 1:
Classification of substances

Menu|Table of content

Chromatography

Chromatography is a method of separating components of a solution mixture by passing it through a medium where the different components move at different rates.

The medium through which the solution mixture is passed is called absorbent material.

Paper chromatography is a method of separating colored dyes by using paper as the absorbent material.

Since dyes are insoluble/do not dissolve in water, ethanol and propanone are used as suitable solvents for dissolving the dye.

Practically, a simple paper chromatography involve placing a dye/material on the absorbent material, adding slowly a suitable soluble solvent on the dye/material using a dropper, the solvent spread out on the absorbent material carrying the soluble dye away from the origin.

The spot on which the dye is initially/originally placed is called baseline. The farthest point the solvent spread is called solvent front.

The farthest a dye can be spread by the solvent depend on:

(i) Density of the dye-the denser the dye, the less it spread from the basely ne by the solvent.

(ii) Stickness of the dye-some dyes sticks on the absorbent material more than other thus do not spread far from baseline.

Experiment:

To investigate the colors in ink

Procedure

Method 1

Place a filter paper on an empty beaker.

Put a drop of black/blue ink in the centre of the filter paper.

Wait for about one minute for the ink drop to spread.

Using a clean teat pipette/dropper add one drop of ethanol/propanone.

Wait for about one minute for the ink drop to spread further.

Add about twenty other drops of ethanol waiting for about one minute before each addition.

Allow the filter paper to dry

Experiment:

To investigate the colors in ink

Procedure

Method 2

Cut an 8 centimeter thin strip of a filter paper.

At about 3cm on the strip, place a drop of ink.

Place the filter paper in a 10cm length boiling tube containing 5cm3 of ethanol.

Ensure the cut strip of the filter paper just dips into the ethanol towards the ink mark.

Cover the boiling tube.

Wait for about twenty minutes.

Remove the boiling tube and allow the filter paper to dry.

Set up of apparatus

Method 1

Set up of apparatus

Method 2

Explanation

When a drop of ink is placed on an absorbent material it sticks. On adding an eluting solvent, it dissolves the dye and spread out with it. The denser and sticky pure dye move least.

The least dense/sticky pure dye move farthest. A pure dye will produce the same chromatogram/spot if the same eluting solvent is used on the same absorbent material.

Comparing the distance moved by a pure dye with a mixture, the coloured dyes in a mixture can be deduced as below:

Example 1

The chromatogram of pure dyes A, B ,C and a dye mixture D is shown below

Determine the pure dyes present in D. On the diagram show:

(i)the solvent front
(ii) Baseline
(Iii) the most soluble pure dye
<< Sublimation/deposition | Solvent extraction >>