Experimental Techniques in Chemistry

Analytical Chemistry: The branch of chemistry that deals with the quantitative and qualitative analysis of substances is called Analytical Chemistry.

Question 2.1: What is difference between quantitative and quantitative analysis? What are various steps involved in quantitative analysis?

Answer: 

Qualitative analysis: The analysis in which elements are identified is called qualitative analysis. 

Quantitative analysis: The analysis in the form of which relative amounts of elements are determined is called quantitative analysis. 

There are four main steps involved in quantitative analysis. 1. Obtaining a sample for analysis 2. Separation of desired constituent 3. Measurement and calculations of results 4. Drawing conclusions from analysis

Question 2.2: Define filtration. How do we select the filter media?

Answer: 

The process of separating solids or suspended particles from liquids by using a filter medium is called filtration.

Question 2.3: Describe filtration carried out through filter paper.
Answer: The apparatus used for this purpose consist of filter paper, funnel, beaker glass rod and stand.

Filter paper: Filter paper should be large enough to hold the whole precipitate. Filter paper is folded twice. First fold should be along diameter and second one along its radius. The filter paper is then opened in a way that three folds of paper are one side and one on other side to form a cone with apex angle greater then 60o as shown in figure. Then paper may be soaked with water and firmed pressed along funnel. The rate of filtration can be increased by making fluted paper. The filter paper with fan like arrangement is called fluted filter paper.

Funnel: 

The funnel of glass should be large enough for its rim to extend 1 to 2 cm above the top of filter cone. The stem of the funnel should be several inched long so that it can extend down to receiving beaker.

Filtration: 

The funnel is fitted on the stand. A receiving beaker is placed blow it. The stem of the funnel should touch the side of beaker in order to avoid splashing. The liquid containing precipitate or solid particles is taken in another beaker. The lower end of a glass is rod is directed to the center of filter paper and the liquid is poured on the glass rod. The process of filtration can be run smoothly if the stem is full of liquid as long as there is liquid in conical portion. The process of filtration is aided by gentle suction as liquid passes through the stem.

Question 2.4: Describe the filtration through filter crucibles. OR 

Compare the Gooch Crucible and Sintered glass Crucible. OR

What is difference between Gooch Crucible and Sintered glass Crucible?

Answer: 

This is another way to separate solid material from liquid. Two types of crucible are commonly used. 1. Gooch Crucible 2. Sintered glass Crucible

1.Gooch Crucible: 

It is made of porcelain with perforated bottom. The perforations of bottom are covered with paper pulp or a filter paper cut to its size. Its perforation can also be covered with asbestos mat for the solution which can react with paper. The quick filtration can be done by placing the Gooch crucible in suction filtering apparatus. It is also very useful to separate the very hot precipitate.  

2. Sintered glass Crucible: 

It is made of glass crucible with porous glass disc sealed into its bottom.This porous glass disc needs not to be covered with the filter paper or asbestos. The liquid passes down to receiving flask and precipitates stay in the crucible. It is very useful to filter the solutions that react with paper e.g. Conc. HCl and KMnO₄ solution.

Question 2.5: What is meant by crystallization?

Answer: It is a process in which a pure substance is separated in form of crystals of well defined geometrical shapes from a hot saturated solution by cooling it very slowly. 
Question 2.6: Describe the steps involved in the crystallization of a pure substance.
Answer: The process of crystallization involves following steps.

1. Choice of solvent:
i. It should dissolve a large amount of substance at its boiling points and  only small amount at the room temperature.
ii. It should not react chemically with the solute.
iii. It should not dissolve the impurities or if impurities are soluble in it, they should not crystallize from impurities along with solute.
iv. It should deposit well formed crystals of the pure solute when its saturated solution is cooled.
v. it should not be expensive.
vi. It should be safe to use and should easily be removed.

2. Preparation of saturated solution: 

The substance is dissolved in a small amount of suitable solvent at an elevated temperature with constant stirring. Add more solute and heat directly on water bath, until the boiling point is reached. If the solute settle down a bit, add more solvent to the boiling solution until the whole of the solute has dissolved.

3. Filtration: 

Insoluble impurities in the hot saturated solution are removed by its filtration. If the filtration of hot saturated solution is not done under hot conditions, premature crystallization of solute takes place on the filler paper or in the funnel stem.

4. Cooling: 

After filtering the hot saturated solution is cooled at moderate rate to get crystals of medium size. Slow cooling gives big crystals which may contain impurities originally preset in the solvent. These impurities may complicate the drying process.

5. Collecting the Crystals: 

When crystallization is complete, the crystals are separated from the mother liquor by filtration through Gooch crucible. Suction of mother liquor is done by using vacuum pump until the filter cake is rigid enough. The cake is then pressed firmly with a cork to remove the left over mother liquor. The crystals are washed repeatedly with cold solvent. The mother liquor is concentrated by evaporation and then cooled to get a fresh crop of crystals.

6. Drying of the Crystals: 

The crystals are dried by pressing them between folds of filter paper repeatedly but disadvantage is that the crystals are crushed to powder and get contaminated with the filter paper fiber. The best way to dry the crystals is to place them in vacuum desiccator for few hours. The desiccators contains drying agent such as CaCl_{2, P2O5} or silica gel which absorbs moisture from the crystals. Alternatively, the crystals are dried in an oven provided the substance does not melt or decompose on heating at 100 ^οC.

7. Decolorization of crystals: 

The crude crystals may appear colored due to absorption of coloring matter or resinous products. The colored impurities can be removed by dissolving them in solvent and boiling the solution with fine animal charcoal. The solution becomes colorless because charcoal absorbs colored impurities. 

Question 2.8: Define sublimation. How sublimation carried out? Name some substances which can be purified by sublimation.
Answer: 

Definition: Sublimation is a process in which a solid directly changes to its vapors on heating and the vapors directly changes to solid on cooling with out farming liquid phase. Example This process is often used to purify solids such ammonium chloride, iodine, naphthalene and benzoic acid. The substance to be purified by sublimation is placed on watch glass covered with inverted, funnel and heated on a sand bath very slowly. The funnel is plugged with a cotton swabs and wet cotton is wrapped round its. The solid substance changes to vapor. The vapour rise up and condense on the inner side i of funnel. The pure solid substance deposited on the walls of the funnel is scratched by means of spatula.

Question.2.8: Explain solvent extraction of solute from a given solution.
Answer: 

Solvent Extraction: The process in which solute is separated from solution by mixing it with another solvent in which the solute is more soluble and added solvent is immiscible with the solution.