Solution-Class-11-Science-Chemistry-Chapter-3-Basic Analytical Techniques-Maharashtra Board

(a) Fractional crystallization

(a) Distillation under reduced pressure

(b)  Chromatography

(b) Fractional distillation

(d)  All of the above

The technique of crystallization is used for the purification of solid organic compounds.

Residue : The solid (insoluble part) that remains behind on the filter paper after filtration is called the residue.

Filtrate : The liquid which passes through the filter paper and gets collected in the beaker is called the filtrate.

The condenser has a jacket with two outlets through which water is circulated. When the vapours of the liquid pass through the condenser, they get cooled and condense back into the liquid.

A moistened filter paper sticks to the funnel preventing any solid from escaping from the sides of the funnel.

In paper chromatography, the water trapped in the fibres of the paper acts as the stationary phase.

If when the upper outlet of the condenser is connected to the tap, the condenser will never be completely filled with water, due to which the vapours of the liquid passing through it will not be cooled efficiently.

A completely filled condenser provides maximum cooling resulting in maximum recovery of the purified liquid.

The materials used as stationary phase in chromatography are alumina and silica gel.

Properties of solvents for solvent extraction are :

• The organic solvent selected should be such that the organic compound (solute) to be separated is more soluble in it.
• The organic solvent should be immiscible with water.
• The organic solvent should not chemically react with the organic compound (solute) present in the aqueous solution.
• The density of the solvent must be different than that of the aqueous phase which facilitates the quick settling of the two layers.

Spotting of mixture is done above the level of mobile phase to ensure that the solvent does not touch the spots before separation begins. If the solvent touches the spots in the beginning itself, it causes the spots to smear and become useless.

(a) Stationary phase : The stationary phase is a solid or liquid supported on a solid which remains fixed in a place. The substances to be separated are selectively adsorbed or retained on the stationary phase.

(b) Saturated solution : A saturated solution is a solution which cannot dissolve additional quantity of the solute.

(a) Solvent extraction : Solvent extraction is a method of separating compounds from a solution by shaking with an immiscible solvent on the basis of their solubility.

(b) Distillation : Distillation is a process of heating a liquid to convert into vapour and then cooling the vapour to get back the liquid.

The solvent to be used for crystallization must have following properties :

• The compound to be crystallized should be least or sparingly soluble in the solvent at room temperature but highly soluble at high temperature.
• Solvent should not react chemically with the compound to be purified.
• Solvent should be volatile so that it can be removed easily.

Depending on the stationary phase chromatography is classified into two types :

• Adsorption chromatography
• Partition chromatography

Adsorption chromatography is further classified into two types :

• Column chromatography
• Thin layer chromatography

Principle of adsorption chromatography : This type of chromatography is based on the principle of differential adsorption. Different solutes are adsorbed to different extent on the stationary phase depending upon their ability of adsorption.

Principle of partition chromatography : In this type of chromatography, the separation of the components takes place by the continuous differential partitioning of the components between the stationary and mobile phase.

• In column chromatography, as the mixture moves down the column, the different components of the mixture get adsorbed at different regions on the stationary phase depending upon the degree to which they get adsorbed.
• The component with greater adsorption ability will be adsorbed at the top and the others will the adsorbed at different distances thus forming different bands.

Colourless compounds can be visualised by different ways :

• If the compounds separated on the TLC plate or the paper strip are not coloured, but have a property of fluorescence they can be visualized under UV light.
• The TLC plate can be kept in a chamber containing a few iodine crystals. The iodine crystals are adsorbed by the components and the spots appear brown.
• The TLC plate or the developed paper strip with separated components can also be sprayed with a suitable agent. The spots become coloured andthus are visible. For example, amino acids can be visualized by spraying the plate with a solution of ninhydrin.

Filtration under suction : When filtration is carried out using a vacuum pump it is called filtration under suction. It is a faster and more efficient technique than simple filtration.

Process for filtration under suction :

• The Buchner funnel is fitted on the conical flask with the help of the rubber cork (rubber bung).
• The filter paper fitted on the Buchner funnel is first moistened with water or a solvent, so that it remains fixed on the funnel.
• The solution to be filtered is poured on the filter paper.
• The vacuum pump is started to create suction.
• The insoluble part in the solution remains on the filter paper. This is called the residue.
• The liquid collects in the flask. This is called the filtrate