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Solvent Extraction

INTRODUCTION

In chemistry, liquid-liquid extraction,  also known as solvent extraction and partitioning, is used to

separate compounds based on their relative solubilities in two different immiscible liquids, usually water

(or water-based solution) and an organic solvent which is immiscible with water. It is an extraction of a

substance from one liquid phase into another liquid phase. The success of this method depends upon the

difference in solubility of a compound in various solvents and becomes a very useful tool if you choose a

suitable extraction solvent.

Solvent Extraction (SX)  is a useful method to separate a substance selectively from a mixture, or to

remove unwanted impurities from a solution.

Solvent Extraction while not commonly used for quantitative analysis, it is a very common method for

sample preparation and concentration. In addition, concepts covered are directly applicable tochromatography.

Liquid-liquid extraction is a basic technique in chemical laboratories,  where it is performed using

a separatory funnel. This type of process is commonly performed after a chemical reaction as part of

the work-up. 

In other words, this is the separation of a substance from a mixture by preferentially dissolving that

substance in a suitable solvent. By this process, a soluble compound is usually separated from an

insoluble compound.

Liquid-liquid extraction is possible in non-aqueous systems: In a system consisting of a molten metal in

contact with molten salt, metals can be extracted from one phase to the other.

Solvent extraction is used in nuclear reprocessing,  ore  processing, the production of fine organic

compounds, the processing of  perfumes and other industries.

GENERAL CONCEPTS

Solvent extraction (SX ) is the traditional term of liquid-liquid distribution that involves the distribution of

a solute between two immiscible liquid phases in contact with each other. The principle is illustrated in

Figure 2.1.

The vessel (a separatory funnel) contains two layers of liquids, that one is usually water (S aq) while the

other, an organic solvent (Sorg).

The organic phase often settles as the upper layer when it has a lower density than water, but the opposite

situation also occurs. A solute A, which initially is dissolved in one of the two liquids, eventually

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  C 1  and C 2  are the concentration at equilibrium of solute A in solvent 1 (organic) and in solvent 2

(water) respectively. The solute distributes itself between the two solvents so that its chemical

activity is the same in each phase.

If a second solute, B, is present, the distribution ratios for the various solutes are indicated by D A , D B ,

and so on. If D B is different from D A , A and B can be separated from each other by (single or multistage)

solvent extraction. D is also called the distribution coefficient or the extraction coefficient .

Depending on the system, the distribution ratio can be a function of temperature, the concentration of

chemical species in the system, and a large number of other parameters.

In summary, partition (K, P) or distribution coefficient (D) is the ratio of  concentrations of a compound in

the two phases of a mixture of two immiscible solvents at equilibrium.  Hence these coefficients are a

measure of differential solubility of the compound between these two solvents. The phrase "Partition

Coefficient" is now considered obsolete by IUPAC, and the appropriate alternative ("partition constant",

"partition ratio" or "distribution ratio") should be used as appropriate.

Separation factors

The SEPARATION FACTOR (SF)  is one distribution ratio divided by another, it is a measure of the

ability of the system to separate two solutes.

For instance if the distribution ratio for nickel (D Ni) is 10 and the distribution ratio for silver (DAg) is 200,

then the silver/nickel separation factor (SFAg/Ni) is equal to DAg/D Ni = SFAg/Ni = 20.

Solvent Extraction Videos

http://www.youtube.com/watch?v=vcwfhDhLiQU  

http://www.youtube.com/watch?v=cMrWdx1KdU8  

http://en.wikipedia.org/wiki/File:Separation02.ogg  

TYPES OF EXTRACTION

Transfer of a solute from one solvent to another. To extract a solute from a solution, it is always better to

use several small portions. Extractions form an important part of the purification procedure of many

compounds formed from reactions. Extractions are grouped in three (3) categories:

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1.  Extraction of an organic material with water. This is designed to remove such highly polarmaterials as in organic salts, strong acids or bases, and such low molecular weigh polar substances

as alcohols, carboxylic acids and amines. Water extractions are also used immediately followingextractions with either acid or base to ensure that all traces of acid or base have been removed.

2. 

Extraction of an organic material with a dilute acid, usually five or ten percent  HCl  . Theseextractions are intended to remove basic impurities as amines. These bases are converted to theircorresponding cationic salts by the acid used in the extraction.

2 RNH   +  HCl    

Cl  RNH 3

(water soluble salt)

A water extraction may be used immediately following the acid extraction to ensure that all traces

of the acid have been removed from the organic material

3. 

Extraction of an organic material with a dilute base  usually five percent sodium bicarbonate(dilute sodium hydroxide may also be used). This type of extraction is used to convert acids intotheir corresponding anionic salts. In the preparation of an ester, a sodium bicarbonate extraction

might be used to remove any excess carboxylic acid which might be present.

 RCOOH   +3

 NaHCO   

 Na RCOO  +   O H 2

 +2

CO  

The anionic salts, being highly polar, would be expected to be soluble in the aqueous phase. As a

result these acidic impurities are extracted from the organic material into the basic solution. A

water extraction may be used immediately following the basic extraction to ensure that all traces

of the base have been removed.

Example: An organic MTBE solution is extracted with aqueous sodium bicarbonate solution. This

 base removes benzoic acid as  benzoate but leaves non-acidic benzil (yellow) behind in the upper

organic phase. {http://en.wikipedia.org/wiki/File:Separation02.ogg }

The separatory funnel is the piece of apparatus used in the extraction procedure. After an organic solvent

has been shaken with an aqueous solution, it will be wet. A drying agent may be used to remove water

from the organic layer. A drying agent is an anhydrous inorganic salt which acquires water of hydration

when exposed to the moist air or wet solution, e.g. anhydrous Na2SO4, CaCl2, MgSO4

APPLICATION

Solvent extraction is used in nuclear reprocessing,  ore  processing, the production of fine organic

compounds, the processing of  perfumes and other industries.

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In an industrial application, this process is done continuously by pumping an organic and aqueous stream

into a mixer. This mixes the organic component with the aqueous component and allows ion transfer

 between them. The mixing continues until equilibrium is reached. Once the ion transfer is complete

(equilibrium is reached), the mixture flows into a vessel, where the organic and aqueous are allowed to

separate, similar to the way oil and water would separate after mixing them. Fresh material is

continuously fed into the mixer, and a two continuous streams is removed from the settler (one organic,and one aqueous).

CALCULATIONS

Question:

1.  Consider an aqueous solution which contains 8 g of solute A in 1 dm3 of water. Given the

 partition coefficient for solute A between water and ether is 66. Calculate the mass of

solute A extracted in

(i)  A single 100 cm3 portion of ether and

(ii)  Two successive 50 cm3 portion of ether

2.  (a) Define the term partition coefficient  

(b) List two conditions under which the partition coefficient remains constant

(c) Benzene is immiscible with water. When 50 cm3 of an aqueous solution

containing 0.5 g of an organic solute X is shaken with 5 cm3 of benzene, 0.2 g of

solute X is extracted in the benzene. Calculate a value for the partition coefficient

of X between benzene and water

(d) If aqueous layer from (c) above is shaken with another 5 cm3 of benzene, what

mass of solute X will be extracted in the benzene when equilibrium is reached? 

Solution: