Chemistry - Basic Analytical Techniques

Introduction :

There has been a systematic development

in the techniques used for analysis of chemical

substances. In this chapter we are going to

look into basic analytical techniques, namely,

purification and separation techniques.

Chemical substances occur in nature in impure stage. Also, when synthesized in the laboratory they are obtained in crude and impure form.

Before investigating their composition and

properties it is essential to obtain them in the pure form. Methods of purification and separation of compounds depend on the

difference in their physical properties.

Purification of solids :

A solid substance may contain two types of

impurities.

(i) which are soluble in the same solvent as the main substance and

(ii) which are not soluble in the same solvent as the substance.

Separation of mixture of water and sand by simple filtration

The second type of impurity can be separated easily using a suitable solvent to dissolve the main compound when the impurities remain undissolved and can be separated by a simple process called filtration.

This process is similar to separating tea leaves

from a decoction of tea, or sand from mixture

of sand and water. Filtration is carried out with the help of a filter paper cone placed in a funnel as shown in the Figure. A circular piece of filter paper is folded to form a cone and fitted in the funnel. The funnel is fixed on a stand and a beaker kept below it. The paper is made moist, the solution to be filtered is poured on the filter paper.

The insoluble part remaining on the filter paper is called residue and the liquid collected in the beaker is called filtrate.

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.The assembly for filtration under suction consists of a thick wall conical flask with a side arm. The flask is connected to a safety bottle by rubber tube through the side arm. The safety bottle is used to prevent sucking of the filtrate into suction pump. A special porcelain funnel called Buchner funnel is fitted on the conical flask with the help of a rubber cork as shown in Figure.

The Buchner funnel has a porous circular

bottom. A circular filter paper of correct size

is placed on the circular porous bottom of the

Buchner funnel and the funnel is placed on the flask. It is moistened with a few drops of water

or solvent. Suction is created by starting the

pump and filtration is carried out. Crystals are

collected on the filter paper and filtrate in the

flask.

Setup for filtration under suction

Crystallization :

When a crude solid is made of mainly one substance and has some impurities, it is purified by the process of crystallization. It is done in four steps:

(i) Prepartion of saturated solution : A

saturated solution is a solution which cannot

dissolve addtional quantity of solute. A

saturated solution of the crude solid is prepared by boiling it in a small but sufficient quantity of a suitable solvent. On doing so the main solute forms an almost saturated solution, but the solution is not saturated with respect to the soluble impurties, as they are in small proportion.

(ii) Hot filtration : The above solution is

quickly filtered while hot. Filtration under

suction allows rapaid filtration. Undissolved

impurities get removed in this process as

residue.

(iii) Cooling of the filtrate : The hot filtrate

is allowed to cool. Solubility of a substance

decreases with lowering of temperature. As a

result, the filtrate becomes supersaturated with

respect to the main dissolved solute. The excess

quantity of the dissolved solute comes out of the solution in the form of crystals. The dissolved impurities, however, do not supersaturate the solution, as their quantity is small. These continue to stay in the solution in dissolved state even on cooling. The separated crystals are, therefore, free from soluble impurities as well.

(iv) Filtration : The crytals of the pure

substance are separated by filtration. The

filtrate obtained is called mother liquor. The

crystals so formed are free from soluble as well

as insoluble impurities.

Choice of the solvent : 

The solvent to beused for crystallization must have following properties :

1. The compound to be crystallized should be least or sparingly soluble in the solvent at room temperature but highly soluble at high temperature.

2. Solvent should not react chemically with

the compound to be purified.

3. Solvent should be volatile so that it can be

removed easily.

Water, ethyl alcohol, methyl alcohol, acetone,

ether or their combinations are generally used

as solvent for crystallization. The choice is

done by trial and error method.

Fractional crystallization :

Two or more substances in a mixture 

can be separated by fractional crystallisation 

process. Fractional crystallisation is a process 

wherein two or more soluble substances 

having widely different solubilities in the same solvent at room temperature are separated by 

crystallization. Mixture of two solutes A and B are dissolved in a suitable hot solvent to prepare a saturated solution. The saturated solution is filtered to remove dust particles and then allowed to cool. As the solution cools, the 

solute which is less soluble crystallizes out first. The crystals are filtered, washed with solvent and dried. The mother liquor is concentrated by evaporating the solvent. The second solute crystallizes from the mother liquor. These crystals are filtered to obtain the separated and purified second component.