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FSc Notes Chemistry Filtration Crystallization Choice of solvent

           FSc Notes Chemistry Filtration Crystallization

Analytical Chemistry: 

The branch of chemistry which provides a complete characterization of a substance. A complete characterization means, the quantitative and qualitative analysis of the substance.

Qualitative analysis: 

The detection and identification of elements in a compound is called qualitative analysis.

Quantitative analysis: 

The determination of amount or percentage of different elements in a compound is called quantitative analysis.
A complete quantitative determination generally consists of four steps:
  • Obtaining a sample for analysis.
  • Separation of desired constituents.
  • Measurement and calculation of the results.
  • Drawing conclusion from that analysis.


The process by which insoluble particles are separated from liquid is called filtration.


The liquid which is obtained after passing through the filter medium is called filtrate.


The solid substance which is present at the filter paper during filtration is called residue.

Filter Medium: 

The porous substance used for filtration is called filter medium. Filter paper, Gooch crucible, sintered crucible are different filter mediums.


Separation of excess solid at low temperature in the from of crystals from a saturated solution prepared at high temperature is called crystallization.
Principle: Solute should be soluble in suitable solvent at high temperature and when solution is cooled down, the excess amount of solute comes out of the solution in the form of crystals.
This technique is used for separation of different substances on the basis of their solubilities. The process of crystallization involves the following steps:

1. Choice of Solvent:

The solvent is chosen on hit and trial basis and it is necessary to try a number of solvents before arriving at a conclusion. An ideal solvent should have following characteristics.
  1. It should dissolve a large amount of the solute at its boiling point and only only a small amount of solute at the room temperature.
  2. It should not react chemically with the solute.
  3. It should either not dissolve the impurities or the impurities should not crystallize from it along with the solute.
  4. On cooling it should deposit well formed crystals of the pure compound.
  5. It should be inexpensive.
  6. It should be safe to use and should be easily removable.
Solvents mostly used for crystallization:
  1. Water
  2. Rectified Spirit (95% ethanol)
  3. Absolute Ethanol (100%)
  4. Diethyl Ether
  5. Acetone
  6. Chloroform
  7. Carbon Tetrachloride
  8. Acetic Acid
  9. Petroleum Ether
If none of the solvents is found suitable for crystallization, then a combination of two or more miscible solvents may be employed. If this solvent is inflammable then precaution should be taken while heating the solution so that it does not catch fire. In such cases, water bath or sand bath is used for heating purpose.

2. Preparation of the Saturated Solution:

After selecting a suitable solvent, the substance is then dissolved in a minimum amount of solvent and is heated directly or on a water bath with constant stirring. Add more solvent to the boiling solution if necessary until all the solutes has dissolved.

3. Filtration:

The insoluble impurities in the saturate solution are then removed by filtering the hot saturated, through a normal or a fluted filter paper. This avoids the premature crystallization of eh solute on the filter paper or in the funnel stem. If necessary, hot water funnel should be used for this purpose.

4. Cooling:

The hot filtered solution is then cooled at a moderate rate so that medium size crystals are formed. In slow cooling bigger crystals are obtained but have considerable amount of solvent carrying impurities in it.

5. Collecting the crystals: 

When the crystallization is complete, the mixture of crystals and the mother liquid is filtered through a filter paper or Gooch crucible using a vacuum pump. The solution remaining after formation of crystals is called mother liquid. Full suction is applied in the order to drain the mother liquor from the crystals as effectively as possible. When the filter cake is rigid enough it is pressed firmly with cork to drain the left over liquid. The crystals are the washed with a small portion of cold solvent and the process is repeated several times. The mother liquid is quite often concentrated by evaporation and cooled to obtained a fresh crop of crystals. The process of crystallization appears to be very simple yet the success of operation lies in the amount or the percentage of crystallized product obtained from the crude substance.

6. Drying of the Crystallized Substance: 

Drying of crystals can be done by three ways:
  1. Pressing it between several folds of filter papers and repeating the process several times dries the crystallized substance. This process has the disadvantage that the crystals are crushed to a fine powder and sometimes the fibers of the filter paper contaminate the product.
  2. Alternatively the crystals are dried in an oven provided the substance does not melt or decompose on heating at 100 C.
  3. A safe and reliable method of drying is through a vacuum desiccators. In this process the crystals are spread over a watch glass and kept in a vacuum desiccators for several hours. The drying agent usually used in a desiccators are CaCl2, silica gel or phosphorous penta oxide P2O5.

7. De-colorization of Undesirable Colors: 

Sometimes during preparation of a crude substance, the coloring matter or resinous products affect the appearance of product and it may appear colored. Such impurities are conveniently removed by boiling the substance in the solvent with the sufficient quantity of finely powdered animal charcoal and then filtering the hot solution. The colored impurities are absorbed by animal charcoal and the pure decolorized substance crystallized out from the filtrate on cooling.