Maharashtra Board Class 11 Chemistry Chapter 3 Basic Analytical Techniques PDF Download

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.

Teacher's Note

When we buy salt from the market, it may have sand mixed in it. We can separate the sand using simple filtration. This is how we get pure salt.

Exam Trick

Remember: Filtration separates solid from liquid. It is like making chai - you use a sieve to separate tea leaves from the liquid.

Points to Remember

Filtration is used to separate solid impurities from liquid.
The solid left on the filter paper is called residue.
The liquid collected is called filtrate.
Filtration under suction is faster than simple filtration.

Purification of Solids

A solid substance may contain two types of impurities:

1. Those which are soluble in the same solvent as the main substance

2. Those which are not soluble in the same solvent as the substance

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. 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.

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.

Teacher's Note

Filtration under suction is used in hospitals to separate plasma from blood very quickly. This method is much faster than simple filtration.

Exam Trick

Remember: Buchner funnel has holes in the bottom. This helps the liquid to fall down quickly when we create suction.

Points to Remember

Suction filtration uses a vacuum pump to speed up separation.
Buchner funnel has a porous plate at the bottom.
A safety bottle prevents the filtrate from being sucked into the pump.
Crystals form on the filter paper during suction filtration.

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:

Step 1: Preparation of Saturated Solution

A saturated solution is a solution which cannot dissolve additional 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 impurities, as they are in small proportion.

Step 2: Hot Filtration

The above solution is quickly filtered while hot. Filtration under suction allows rapid filtration. Undissolved impurities get removed in this process as residue.

Step 3: 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.

Step 4: Filtration

The crystals 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 be used 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.

Teacher's Note

When you heat sugar in water and then cool it, sugar crystals form. This is crystallization. We use the same method to purify impure chemicals in the lab.

Exam Trick

Remember: Crystallization happens because hot solution can hold more solid. When we cool it, the solid cannot stay in the liquid and comes out as crystals.

Points to Remember

Crystallization is used to purify solid compounds.
The solvent must dissolve the compound better when hot.
Mother liquor is the liquid left after crystals separate.
The solvent should be easy to remove.
Trial and error method helps choose the right solvent.

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