Which is more efficient, simple or multiple extraction? why?
Answer:
Multiple extraction is more efficient.
Why?
read this section: One big batch of solvent or several smaller batches? in:
ref: http://en.wikipedia.org/wiki/liquid-liqu...
Multiple extraction is known as an elegant way to increase the extraction yield.
multiple extraction. different solvents dissolve different components of a material extracted. it also depends on the type of extractive you want to remove or isolate from your sample. organic solvents such as alcohol or benzene can extract fats or lipid components. water removes subs that are readily soluble to it. specific acids extract carbohydrates.
Multiple extraction is always more efficient than simple one simple extraction, as explained below :
When a solute is being extracted from an aqueous phase using an organic solvent, a better recovery will be obtained by using two equal volumes of solvent than the recovery that would be obtained using all the solvent in one large volume.
This can be shown by experiment and can be explained by the following example.
Imagine that you have a litre of water containing 10 grams of a solute which has a distribution ratio of one when the solvent is nitrobenzene.
If the aqueous solution is shaken until a dynamic equilibrium is reached with one litre of nitrobenzene, then five grams of solute would be transferred. The aqueous raffinate will contain five grams of solute. 50% of the solute has been recovered.
Imagine that the same original aqueous mixture was shaken with 500 ml of nitrobenzene. When the system has been brought to equilibrium, the concentration of the solute in each phase is the same. As the organic to aqueous ratio is now 1:2 the aqueous phase will now contain 6.666 g of solute while the organic layer will have 3.333 g of solute.
If the two layers are now separated and the nitrobenzene extract is kept, then the aqueous layer (raffinate) can be treated further.
If the raffinate from this first extraction is extracted with 500 ml of new nitrobenzene, then 33.33% of the remaining solute will be extracted. This will be 2.222 g of solute in the new nitrobenzene extract. In total, 3.333 + 2.222 grams of solute for a total of 5.555 grams will have been extracted. This is a recovery of 55.56%. An additional 5.56% of product has been obtained.
If the nitrobenzene was to be used in four batches of 250 ml then the outcome is theoretically better still.
The first 250 ml will extract 2 grams.
The second 250 ml will extract 1.6 grams.
The third 250 ml will extract 1.28 grams.
The fourth 250 ml will extract 1.024 grams.
In total 5.904 grams is extracted; this is a recovery of 59%.
The process of using one large batch of solvent to extract solute is called a single extraction. The process of using several small batches of solvent to extract solute is called multiple extractions.
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