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Saturday, 26 September 2015

FSc Notes Chemistry Part 1 Chapter 2 Experimental Techniques in Chemistry Lecture 1

FSc Notes Chemistry Part 1 Chapter 2 Experimental Techniques in Chemistry

Introduction:

Basically the analytical chemistry is the science of characterization. By the term “chemical characterization”, we mean that a substance must be known both qualitatively as well as quantitatively.
By qualitative analysis we mean to determine that which elements are present in a sample substance.
By qualitative analysis we mean to determine the relative amount of each element present in the sample.

A complete quantitative analysis mainly consists of the following four steps
  • Obtaining sample for analysis
  • Separation of the desired component
  • Measurements & calculations of the results.
  • making the conclusion from the analysis.

Among all the analytical analysis, the separation of various components of a mixture, is an essential step. We use various techniques for this purpose some important techniques used for separation of the components of a mixture are as follow.
  1. Filtration
  2. Evaporation
  3. Crystallization
  4. Sublimation
  5. Solvent extraction
  6. Distillation
  7. Chromatography.

(1) FILTRATION:

When ever we have a mixture where one move components are in liquid from but in the solid form, then we separate this solid component by the process of filtration. Depending upon the nature of the solid substance in the mixture. We can use several types of filters i.e. filter media. But usually we use either a “filter paper” or “filter crucible”. They can be explained as.
Filter Paper:
Here a filter paper is folded and formed in the form of a cone. Then it is placed inside a glass funnel. A beaker is placed then the stem of the funnel is placed inside the beaker in such a way that it should completely touch one inner wall (side) of the beaker. It is essential because in this way, the filtrate (liquid that posses through the filter paper) , runs down the side of beaker with out splashing. Filtration by glass funnel and filter paper is a slow process. Here the mixture is poured onto the filter paper, the solvent passes through the filter paper, while the in soluble solid part is left behind on the filter paper. In this process, the folding of the filter paper is necessary and important. For folding of the filter paper the following steps should be kept in mind.
  • The paper should be folded twice along the diameter of the paper while the second fold should be such that edges do not quite match.
  • The paper should be opened on the slightly larger section.
This provide a cone with three folds thickness halfway around and one thickness the other halfway around, and an apex angle very slightly greater than 60 degree. Then the paper is inserted into 60 degree funnel, moistened with water and firmly pressed down.
As filtration by filter paper is a very slow process, but by using “fluted filter paper” the rate of filtration can be increased. A fluted filter paper is formed by folding the ordinary filter paper in such a way that it gets a fan like arrangement with alternate elevations and depressions at various folds.

Filter Crucible:

The best and convenient way for the filtration of a precipitate is by suction through a crucible. Usually we use two types of crucibles which are

  1. Gooch Crucible
  2. Sintered glass Crucible
Gooch Crucible
It is made of porcelain having a bottom pulp or filter paper cut to its size. This is useful for the filtration of those precipitates which need to be ignited at high temperature. If its perforations are covered with asbestos mat then those solutions that react with paper , e.g concentrated HCl. And KMnO4.
Sintered Glass Crucible:
It is a glass crucible with a porous glass disc sealed into the bottom. It is very convenient to use because no preparation is needed as with the Gooch crucible.

(2) EVAPORATION:

The technique by which the dissolved component of a solution is separated by heating the solution at the boiling point of the solvent is called evaporation. This process usually used for the separation of salts from their aqueous solutions. For example if we want to separate the dissolved NaCl from its aqueous solution, we use this process. We heat the solution placed in a beaker. At 100co the water starts boiling and thus water starts its vaporization. Finally all the water goes to gaseous state leaving behind solid NaCl. The NaCl is wet at this stage, so it is placed in between the filter papers. The filter papers absorb the moisture and finally we get dry NaCl. Thus by the process of evaporation or vaporization we have seen the separation of a salt from its aqueous solution. This process cannot be used for the separation of all sort of dissolved substances in a solvent at is good and suitable only for those dissolved components of a solution which are stable at the billing point of the solvent and do not get decomposed.

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