What is chromatography?
Chromatography is a technique of separating a mixture into its components and it also enables the identification of the components.
Explain why a pencil is used to draw the baseline and not a pen?
The graphite of pencil does not dissolve in the solvent while the dyes in the ink will dissolve in the solvent and produce spots which will interfere with the results.
Explain why the level of solvent should be below the pencil line / base line.
This willl lead to immediate dissolving of the spot and no separation will be obtained.
Explain why the glass tank containing the chromatography paper and the solvent is covered with a lid.
The lid prevents evaporation of the solvent which is volatile.
What is the simplest type of chromatography and what it is used for?
The simplest type of chromatography is paper chromatography. It is used to separate pigments fro plants, dyes from inks, amino acids from protein and also identify poisons and drugs in blood or urine sample.
Consider the separation and identification of dyes in black ink using the ascending paper chromatography method:
1) A pencil line is drawn about 2 cm from the end of the paper (baseline/ pencil line).
2) A tiny spot of black ink is placed on the pencil line using a capillary tube. The spot is allowed to dry, and the black ink is dripped for 3 times on the same spot so that it becomes concentrated.
3) The chromatography paper is suspended into a glass tank containing the solvent ethanol. The level of the solvent should be about 1 cm below the baseline. Other spots of suspected dyes are also placed on the pencil line.
4) The solvent travels up the chromatography paper and when it reaches nearly to the top of the paper, the chromatography paper is removed and allowed to dry. The result obtained is called a Chromatogram.
Solubility of solvent:
When the solvent comes in contact with the mixture and the other dyes, they are separated according to the solubility of the components in the solvent. A dye which is the least soluble in the solvent travels a shorter distance, while a dye which is more soluble in the solvent travels a longer distance.
How to deduce the number of components present in a mixture?
The number of spots formed in paper chromatography indicates the number of components present in the mixture.
Pure substance:
A pure substance always forms one spot,R while a mixture will form more than one spot.
Rf Values (Retention Factor)
It is the ratio of the distance travelled by the substance to the distance travelled by the solvent.
What is the substance used to render colourless/ invisible spots on the chromatography paper visible?
A locating agent is used to render the spots visible.
1) Iodine :- Forms brown spots
2) Ninhydrin :- Produces bright blue spots