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In counter current extraction the substance which is to be extracted is distributed between two different solvent depends upon their distribution coefficient.
This can be best performed by Craig apparatus.
There are a number of glass tubes (r = 0, 1, 2, 3, ----) in the Craig apparatus and they are designed such that the solvent which are lighter in density can be transferred from first tube to second tube and then forward.
The tubes are driven or shaken electromechanically.
The lower phase which have higher density is called as stationary phase and the solvent which have the lighter density is called the mobile phase.
The substance to be extracted is present in the stationary phase.
Equal quantity of stationary phase is present in all the tubes.
The mobile phase (solvent have less density) is transferred into the first tube (tube =0) shake properly so that extraction take place and then phases are allowed to separate down.
The mobile phase (or upper phase) then transferred to next tube no 1and repeat the same process and transfer it into further tube like tube no 2, 3, 4 and so on after every shaking at the fresh mobile phase to tube no 0.
The substance having higher distribution ratio separate more quickly compare to lower distribution ratio.
Now a day’s Craig apparatus is less frequently used because of the availability of more recent chromatographic techniques but it has more significant to clear the concept of extraction to the students.
It is very difficult to construct and operate a Craig apparatus having more than 100 test tubes.
Advantages:
1) A unit quantity of the plant material can be extracted using a small volume of solvent.
2) It is carried out at room temperature, thus the thermolabile constituents are not exposed to heat.
3) The drug is pulverised under wet conditions, so the heat generated during comminution is neutralised with water. Thus, the thermolabile constituents are not exposed to heat.
4) The CCE extraction procedure is more efficient and effective than the continuous hot extraction method.
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