Chromatography
The last technique, and the only one that separates components without heating anything, dissolving anything away, or filtering anything out. It uses a race.
Chromatography
Chromatography is used to separate and identify, one from another, the components present in a mixture — solid or liquid — that holds non-volatile components. Various methods exist; the method carried out using paper (cellulose) is called paper chromatography.
The two phases
Put a little water in a petri dish and dip one end of a dry strip of filter paper into it. A flow of water particles can be observed moving up the paper strip from the bottom. The same upward flow can be seen if acetone, ether or ethyl alcohol is used in place of water. Those two things have names.
The stationary phase
The paper strip is the stationary phase. It does not move; everything else moves through it.
The mobile phase
The solvent travelling through the paper is the mobile phase.
What decides how far a component travels
When a small amount of the mixture whose components are to be separated is added to the paper, the components in the mixture dissolve in the solvent and are drawn upwards with the solvent flow. How far each is drawn up is decided by the strength of its attraction to the stationary phase.
Strongly attracted to the paper
If one component of the mixture is strongly attracted to the stationary phase, its speed of travelling up through that phase is reduced.
Weakly attracted to the paper
A component that shows less attraction to the stationary phase travels upwards through it quickly.
Because of this difference in the speeds at which components travel through the stationary phase, the components in the mixture are separated from one another.
Activity 3.3.4 — separating the pigments in chlorophyll
Method
Grind a few spinach leaves well using the pestle and mortar in the laboratory. Put the crushed pulp into a piece of thin silk cloth and squeeze it to collect a chlorophyll extract onto a watch glass. Cut a strip of chromatography paper, filter paper or roneo paper. A little above one end of the strip, place a drop of the chlorophyll extract taken up in a capillary tube. The solvent will evaporate and the chlorophyll will remain on it; place another drop on the same spot. Tie a thread to the end opposite the chlorophyll spot. Put a liquid such as acetone, kerosene or petrol into a boiling tube and close it with a stopper, letting the solvent saturate the tube. Then attach a hook to the stopper, hang the paper strip from it and lower it so that one end touches the liquid, keeping the strip clear of the walls of the tube. Leave it a while, take the strip out and observe it.
Figure to be added
The observation
The components of several colours can be seen to have separated. From this it can be concluded that chlorophyll holds components that differ from one another.
Where it is used
- Testing whether toxic chemicals have got into water
- Checking whether harmful substances have been added to foods
- Discovering the active chemical compounds present in plants