THEORY:
Fractional crystallization is a fairly successful method of separating the components of a mixture of two or more soluble solids with different solubilities. It is based on the fact that when a solution containing dissolved solids with varying solubilities is gradually crystallized, the one with lower solubility will crystallize out first, and the other remains in the mother liquor.
APPARATUS :
1) Test tubes
2) Porcelain basin
3) Beaker
4) Filter stand
5) Funnel
6) Filter paper.
CHEMICALS:
1) KNO3
2) Cu(NO3)2
Suppose the given mixture contains KNO3 and Cu(NO3)2 In the ratio 5:1 by wt. (approx)
Solubility of KNO3 = 32 gm/100 ml at 25°C
…………Cu(NO3)2 = Very high at 25°C
PROCEDURE:
i) Take about 10 gms of the given mixture containing KNO3 and Cu(NO3)2 in a test tube, and add about 10 ml of water to it.
ii) Heat till the mixture completely dissolves.
iii) Cool the solution rapidly under the tap water and then keep it under Ice cold water for 5-10 minutes. White needle-like crystals of potassium nitrate separate out first. The mother liquor is blue, and contains the copper nitrate which is more soluble than potassium nitrate.
iv) Filter, and collect the filtrate in a clean porcelain basin.
v) Wash the residue of KNO3 with a little ice cold water to remove copper nitrate, and reject the washings.
vi) Take the crystals of KNO3, and recrystallize them (if necessary) in a basin. Dry the crystals between folds of filter paper.
vii) Evaporate the mother liquor (filtrate) in a basin to the crystallization point, when we get crystals of copper nitrate. Better do the evaporation on a water bath by placing the basin over a beaker half filled with boiling water.
Note that the separation is only approximate. Only repeated recrystallizations give good results. Also, try with the mixture of NaCl and Na2S₂O3.
Precipitation, and Separation of Precipitate