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Solvent extraction

Solvent extraction, also known as liquid-liquid extraction, is  a separation methods in which one component or solute present in one phase is extracted into another phase by using a suitable solvent.  The method  can be used for the preparation, purification, and separation of chemical species.

 The common extraction solvents used are diethyl ether, Chloroform, Benzene, Carbon tetrachloride ( CCl4), Acetone, etc.

In simple Solvent extraction, the solute is partitioned between two immiscible phases(solvents).In most cases one of the phases is aqueous and the other phase is an organic solvent such as ether, or chloroform. Because the phases are immiscible, they form two layers with the denser phase on the bottom. The solute is initially present in one phase but after extraction, it is present in both phases.

Principle of solvent extraction

Solvent extraction is based on the fact that a particular solute distributes itself in a fixed ratio between two immiscible solvents.

A target solute may be soluble in two different solvents but the extent of solubility may be different, provided that solvents are immiscible with each other. When solutions of a particular solute in two immiscible solvents (say aqueous and organic solvents) are mixed, the solute distributes itself from one phase to another phase in a fixed ratio. This is the underlying principle of solvent extraction.

Solvent Extraction Methods

The aqueous solution of the given solute is taken in a separating funnel. It is mixed with the desired organic solvent. The funnel is closed, and its contents are shaken vigorously. It is then allowed to remain undisturbed for some time. Water and organic solvent will form separate layers, and the solid or liquid solute will be transferred from the aqueous layer to the organic layer as it is more soluble in an organic solvent. In the funnel, the solvent forms the upper layer while the water forms the lower layer.

The two layers can be recovered by opening the stop cock and collecting them in separate bakers. On evaporating the organic solvent, the solute can be recovered.

For example, benzoic acid from water may be extracted from its aqueous solution by using benzene. Then benzoic acid may be finally recovered from its solution by distilling off the organic solvent. This process can also be used to separate two organic liquids such as aniline and water.

Nernst Distribution or Partition Law.

Solvent extraction is governed by the Nernst Distribution or Partition Law which states that:

at constant temperature and pressure, a solute, S, will always be distributed in the same proportions between two particular immiscible solvents.

The ratio of the equilibrium concentrations  in the two phases defines a distribution or partition coefficient, KD,  which is given by the expression

•                  KD = [S]org
•                            [S]aq

Where

• [S]org denotes the concentration of substance dissolved in the organic solvent
• [S]aq denotes the concentration of the substance dissolved in the aqueous phase

Distribution Ratio

 The partition coefficient KD can also be referred to as the distribution ration D which is often quoted as a measure of how well-extracted a species or an analyte is.

The distribution ratio D is equal to the concentration of a solute in the organic phase divided by its concentration in the aqueous phase.

•       D =  Concentration of a solute in the organic phase
•           Concentration of the solute in the aqueous phase
•           
•                          Or
•                     D =  (CS)org
•                             (C)Saq

where (CS) represents the total concentration of all forms of the distributing solute S in each phase. If no interactions involving S occur in either phase, then D and KD would be identical. However, the value of D is determined by the experimental conditions and it can be adjusted over a wide range to suit the requirements of the analytical procedure.

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 solvent extraction, two immiscible liquids are shaken together. The more polar solutes dissolve preferentially in the more polar solvent, and the less polar solutes in the less polar solvent.

The Extraction Efficiency And Selectivity

The efficiency of an extraction depends on the value of the distribution ratio, D. For solvent extraction, it also depends on the relative volumes of the two liquid phases and for solid-phase extraction on the surface area of the sorbent. With solvent extraction, the percentage of a solute extracted, E, is given by the expression

•    E  =     100D
•           [D + (Vaq/Vorg)]

where Vaq and Vorg are the volumes of the aqueous and organic phases, respectively.

For solutes with small values of D, multiple extractions will improve the overall efficiency, and an alternative expression enables this to calculated

(Caq)n = Caq [Vaq / (DVorg + Vaq)]ⁿ

where C aq and (Caq)n are the amounts of solute in the aqueous phase initially and remaining after n extractions, respectively.

The following example demonstrates the use of these formulae.

Example 1

A water sample contains 10 mg each of a halogenated pesticide and an ionic herbicide which are to be separated by extraction of the pesticide into methylbenzene. Given that the pesticide distribution ratio, D, for methylbenzene/ water is 50, calculate the extraction efficiency for

• one extraction of 20 cm3 of water with 10 cm3 of methylbenzene

(ii) one extraction of 20 cm3 of water with 30 cm3 of methylbenzene

(iii) three extractions of the same 20 cm3 of water with 10 cm3 portions of methylbenzene (30 cm3 in total)

• Substitution of the values for D, Vaq and Vorg in equation give
• E  =  100 X 50
•       [50+(20/10)] = 96.15%

(ii) Substitution of the values for D, Vaq and Vorg in equation gives

• E = 100 X 50
•       [50+(20/30)]  = 98.68%

(iii) Substitution of the values for D, Vaq, Vorg, Caq and (Caq)n in equation gives

• (C aq)₃ = 10 X [20/((50 X10) + 20)]³
• (Caq)₃ = 10 X [0.03846]³ = 5.6896 X 10^-4 mg
•          E = 99.99%

An extraction of 99.99%, as achieved in (iii), would be considered quantitative, although the lower efficiencies obtained in (i) and (ii) might be acceptable in the context of a defined analytical problem. It is clear that increasing the volume of organic solvent, or extracting with the same volume divided into several smaller portions, increases the overall efficiency of an extraction.

NB the ionic herbicide has a negligibly small distribution ratio, being very polar and highly water soluble.

Percentage extraction in solvent extraction

Percentage extraction can be defined as the amount of solute extracted out of per hundred parts of solute distributed in a pair of immiscible solvents. Mathematically, it can be expressed as shown below:

Note: In a given quantity of solvent, the amount of organic substance extracted depends upon the number of extractions. The larger the number of extractions, the greater is the amount of substance extracted. Therefore, it is better to extract several times with smaller quantities of solvent than once with the entire solvent provided. Normally, the solvent is recovered and reused in the extraction process.

Characteristics Of A Good Extraction Solvent

The characteristics of a good extraction solvent are stated below:

1. The solvent should be able to dissolve at least one component to a large extent than the rest of the components in the mixture.
2. The solvent should be miscible with the liquid to be extracted but not with the other components of the mixture or react with the solute.
3. The reaction taking place should be stable and irreversible. Reversible reactions can bring back the dissolved components in their previous form and the extraction will not be completed successfully.
4. The boiling point of the solvent should be low enough ( well below the melting point of the solute) such that it can be evaporated easily after collection.
5. The compound formed after the reaction should be easily separated from the extracted compound so that it can be reused.
6. The density of the compound should be different from the required component to help the separation readily.
7. It should have a favorable temperature coefficient
8. It should be inexpensive and cost-effective.
9. The solvent should not be toxic , flammable or corrosive as it can harm the extraction instruments.

Unfortunately, few solvents are known to meet this  criteria. Some solvents are not toxic but flammable (e.g., diethyl ether, petroleum ether, hexanes). Some are not flammable but toxic (e.g., dichloromethane, chloroform). whereas , some solvents are both toxic and flammable (e.g., benzene).

Conditions For Extraction

1. Solvent– A solvent with a similar polarity to that of the analyte should be chosen. Lower viscosity is beneficial for faster diffusion rates in the solvent phase. Ideally, the solvent should be nontoxic, nonreactive, nonflammable, environmentally friendly and affordable.
2. Particle size– Grinding before solvent extraction promotes an increase of the contact area between the solvent and the solid matrix. Extraction efficiency increases with decreasing particle size. However, smaller particle sizes also result in lower drainage rates and may create problems if liquid circulation is hindered.
3. Sample Humidity– the water in the solid material can compete with the extraction solvent for the solute’s dissolution, so in most cases the material is dried under conditions that do not cause degradation of the compounds.
4. Temperature– the solubility and diffusivity of the material being extracted increases with increasing temperature, hence improving the rate of extraction. An upper temperature limit should be set to avoid undesirable degradation reactions and to ensure the solvent remains in a liquid state.
5. Solvent agitation– Agitation of the solvent increases turbulent diffusion and material transfer rates.
6.  PHThe extractability of metal complexes is greatly influenced by the acidity of an aqueous phase, so it necessary to assure optimum concentration of H⁺ions for maximum extraction.

7.Pressure

 Pressure does not affect the reaction much, as most of the solvent extractions are carried out under the atmospheric pressure.

8. Stripping

Stripping is the removal of the extracted solute from the organic phase for further processing or analysis. This is  necessary where further separation steps are required, it is necessary to remove the solute from the organic layer to more stable medium.

9.Backwashing

Backwashing is an auxiliary technique used with batch extractions to influence quantitative separations of elements. With the proper conditions, most of the impurities can be removed by this backwashing operation, with negligible loss of the main component, thereby attaining a selective operation.

Methods Of Extraction 

1. Batch extraction

Batch extraction is the simplest and most commonly used method, consists of extracting the solute from one immiscible layer in to other by shaking the two layers until equilibrium is attained, after which the layers are allowed to settle before sampling.  This is commonly used on the small scale in chemical laboratories using a separatory funnel.

The batch extractions may also be used when the distribution ratio is large.

1. Continuous extraction

Continuous extraction, makes use of a continuous flow of immiscible solvent through the solution of both phases.

Continuous extractions are particularly applicable when the distribution ratio is relatively small.

Continuous extraction consist of distilling the extracting solvent from a boiler flask and condensing it and passing it continuously through the solution being extracted.

The extracting liquid separates out and flows back into the receiving flask, where it is again evaporated and recycled while the extracted solute remains in the receiving flask. When the solvent cannot easily be distilled, a continuous supply of fresh solvent may be added from a reservoir.

1. Counter-current extractions

Counter-current  extraction is used primarily for fractionation purposes. The separation is achieved by virtue of the density difference between the fluids in contact. In vertical columns, the denser phase enters at the top and flows downwards while the less dense phase enters from the bottom and flows upwards.

The choice of method to be employed will depend primarily upon the value of the distribution ratio of the solute of interest, as well as on the separation factors of the interfering materials

Application of solvent extraction

It is one of the most important separation techniques used in environmental, clinical, and industrial laboratories.

• Public drinking water supplies are routinely monitored for trihalomethane, which is known for carcinogenicity. These are extracted from their aqueous matrix by liquid-liquid extraction using pentane as extracting solvent.

Soxhlet Extractor 

A Soxhlet Extractor has three main sections: A percolator (boiler and reflux) which circulates the solvent, a thimble (usually made of thick filter paper) which retains the solid to be laved, and a siphon mechanism, which periodically empties the thimble.

Assembly of Soxhlet Extractor

1. Thesource material containing the compound to be extracted is placed inside the 
2. The thimble is loaded into the main chamber of the Soxhlet 
3. The extraction solvent to be used is placed in a distillation 
4.    The flask is placed on the heating element.
5.   The Soxhlet extractor is placed atop the flask.
6.   A reflux condenser is placed atop the extractor

Operation

The solvent is heated to reflux. The solvent vapour travels up a distillation arm, and floods into the chamber housing the thimble of solid.

The condenser ensures that any solvent vapour cools, and drips back down into the chamber housing the solid material.

The chamber containing the solid material slowly fills with warm solvent. Some of the desired compound dissolves in the warm solvent.

When the Soxhlet chamber is almost full, the chamber is emptied by the siphon.

The solvent is returned to the distillation flask. The thimble ensures that the rapid motion of the solvent does not transport any solid material to the still pot.

This cycle may be allowed to repeat many times, over hours or days. During each cycle, a portion of the non-volatile compound dissolves in the solvent.

After many cycles the desired compound is concentrated in the distillation flask.

The advantage of this system is that instead of many portions of warm solvent being passed through the sample, just one batch of solvent is recycled.

After extraction the solvent is removed, typically by means of a rotary evaporator, yielding the extracted compound.

The non-soluble portion of the extracted solid remains in the thimble, and is usually discarded.