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Saturday, 26 September 2015

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

FSc Notes Chemistry Part 1 Chapter 2 Experimental Techniques in Chemistry

(3) Crystallization:

When ever we have a solution having a component which is soluble in the solvent at high temperature and upon cooling, the excess amount of it is thrown at as crystals; we use the process of crystallization for the separation of such components of a solution.
The process of crystallization involves the following steps.

(a) Choice of a solvent:

For the selection of a particular solvent for this process, first several solvents are tried and then the most suitable one of them is chosen. So we can say that solvent is chosen on hint and trial basic. An ideal solvent should have the following features.

  • It should dissolve a large amount of the substance at its boiling point and small amount at room temperature.
  • It should not undergo chemical reaction with the solute.
  • It should either not dissolve the impurities or the impurities along with the solute.
  • On cooling it should deposit well formed crystals of the pure compound.
  • It should be in-expensive.
  • It should be safe to use and should be easily removable.

The most commonly used solvents for crystallization are water rectified spirit (95% ethanol) absolute ethanol, diethyl ether, acetone, chloroform, carbon tetrachloride, acetic acid, petroleum ether etc. If none of them is suitable then a combination of two or move miscible solvents may be used. Water both should because in case of inflammable solvents.

(b) Preparation of the saturated solution:

After the selection of a suitable solvent the substance is dissolved in a little amount of solvent. Then it is heated at constant stirring. If necessary then more solvent is added at the boiling solution until all the solute has dissolved.

(c) Filtration:

The hat solution is then filtered either though normal or fluted filter paper. The filtration is done when the solution is hot because it avoids the premature crystallization of the solute on the filter paper. Thus usually a hot water funnel should be used.

(d) Cooling:

The hot filtered solution is then cooled. The cooling process is carried out it a moderate rate. So that medium sized crystals are formed with slow cooling we will got bigger crystals. Thus they will trap some considerable amount of the solvent which may contain the impurities and it also makes the process of drying more complicated.

(e) Collecting the Crystallize.

After the crystallization i.e. crystal formation they are separated from the mother liquor (solvent) by passing the solution through a Gooch crucible using a vacuum pump. Then when the crystal cake is rigid enough, it is pressed firmly with a cork to drain the left over liquid. Then these crystals are washed with cold solvent, several times. Then the solvent is allowed to evaporate and thus we get pure crystals of the particular sample.

(f) Drying of the crystallized substance:

The wet pure crystals are pressed between several folds of filter paper. This process is repeated several times due to which the moisture is absorbed by the filter papers and we get dry crystals. This process has a draw back. That is due to this process the crystals are crushed to fine powder. Similarly some times the fibers of the filter paper contaminate the product. Thus an alternate way for drying process is that the crystals are place in the oven provided the substance does not melt at 100 degree Celsius. So the most safe and suitable method for drying is the use of a vacuum desiccators. In this process the crystals are spread over a watch glass and placed in vacuum desiccators for several hours. The most common drying agents used in vacuum desiccators are CaCl2 silica gel or phosphorous pentoxide.

(g) Decolonization of undesired colors:

Sometimes during the preparation of a crude substance, the coloring matter affects the appearance of product and it may look colored. Such impurities are removed by boiling the substance in the solvent with sufficient quantity of finely powdered animal charcoal and then the hot solution is filtered. All the colored impurity is adsorbed by charcoal and then pre decolorized substance crystallizes out from the filtrate upon cooling.

Written by: Asad Hussain

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