Solvent extraction, principle, technique, types of extraction techniques with figure, Factors affecting extraction efficiency,and applications.

Liquid- liquid extraction is a technique in which a solution (usually aqueous) is brought in contact with a second solvent (usually organic), essentially immissible with the first, in order to bring about a transfer of one or more solutes into the second solvent. The separation are convenient, rapid and simple. Mostly, they are carried out in separatory funnel. 

Principle

The technique of solvent extraction is based on Nernst's distribution law. According to this law "When a solute is equilibrated between two immiscible solvents in contact with each other, it gets distributed between the two solvents in fixed proportion ". The ratio of concentration of a solute in two solvents is called as distribution coefficient or partition coefficient. It is given by K=Co/Cw

                                                             

Where,

      Co= concentration of solute in organic phase

     Cw= concentration of solute in aqueous phase

The species to be separated is converted into a form, which can be extracted by other solvent. 

After conversion into suitable form, the solution is brought in contact with other solvent that is immiscible. It is equilibrated for sufficient time so that the species passes selectively into other solvent and can be analyzed subsequently. 

e.g. If a metal is to be extracted, it is converted into complex and then extracted into organic solvent. 

Technique

 Solvent extraction is carried out in three steps. 

1. Formation of distributable species:

If a metal ion is to be extracted from aqueous solution into organic solvent, it can not be directly brought in contact with organic liquid. Firstly, it is treated with suitable ligand to form a complex. 

M + nL → MLn

2. Distribution:

After formation of distributable species, immiscible solvent is added to this solution and shaken vigorously in order to establish equilibrium the concentrations of organic and aqueous phases follow Nernst's distribution law. 

(MLn)aq  ⇌ (MLn)org 

3. Interaction in organic phase: 

After crossing the phase boundary, the species may undergo polymerization, dissociation etc in the organic phase. This can be analyzed either in the same solvent or by re-extracting into aqueous solvent. 

Types of extraction techniques

For extraction of the solute from one phase to other, different methods are used. The most generally used techniques are discussed below. 

1. Batch extraction:

In this method the total volumes of organic and aqueous solvents are brought into contact with each other in single batch. This type of extraction is carried out in separatory funnel as shown in fig. The two liquids are put in contact with each other and shaken for sufficient time. After complete extraction, the lower layer is removed and then the upper layer is taken out. 

2. Back extraction or stripping:

Most of the times, the analysis is not carried out in organic phase. On the contrary, the species is brought back into aqueous phase. This can be achieved either by setting appropriate pH or by using another complexing agent to form a complex that is more soluble in water. 

3. Continuous extraction:

For complete extraction, it is necessary to have a large number of extractions. This is possible using continuous extraction process. For this, the extractant is added dropwise and removed dropwise from the solution of species as shown in Fig.

4. Counter-current extraction:

In this technique, two immiscible solvents are made to flow in opposite direction in contact with other. This technique is probably the most efficient among all. 

5. Soxhlet extraction:

This method is based on continuous extraction of desired constituent from a solid sample into an organic solvent. It is carried out in special type of apparatus called as soxhlet extractor. With this equipment, extraction can be continued up to 8 to 10 hours without break. Figure shows soxhlet extractor assembly. 

Factors affecting extraction efficiency

 In this process of extraction, three components are involved, two solvents and one solute. Nature of all three components affects efficiency of the method. 

1.  Nature of solute species:

Most of the times, it is the metal ion being separated using solvent extraction. It is converted into suitable chelate by adding ligand solution. Due to formation of chelate, the solubility of the species increases in organic solvent. Thus, extraction becomes possible. Hence any factor that enhances the stability of the chelate, increases efficiency of extraction. Such factors are-

1. Higher basic strength of chelating groups. 

2. Soft-base nature of the donating groups in ligands. 

3. Five or six membered ring formation in chelate that reduces ring strain. 

4. Resonating structures of the chelate.                        

2.  Nature of solvents:

   Out of the two solvents, one is generally water. Other solvent is called as extractant. The choice of solvent for extraction is governed by following considerations:

1. High distribution ratio of the solute and low distribution ratio of undesirable impurities in extracting solvent. 

2. Low solubility in aqueous phase. 

3. Low viscosity and large density difference from the aqueous phase. 

4. Low toxicity and flammability. 

5. Easy recovery of solute for further analysis. 

3.  Conditions of extraction:

Various conditions adjusted during extraction of one or more components in a mixture affect efficiency of extraction. One of the conditions is pH. By proper adjustment of pH, selective extraction of desired component can be achieved. pH is controlled by using buffers. 

e.g. Dithizone can be used as extracting agent for selective extraction of various metals in chloroform under pH conditions as-

Metal ion     Optimum pH

             Cu(II)                 1

             Hg(II)                 1-2

             Ag(II)                 1-2

             Sn(II)                 6-9

             Co(II)                 7-9

             Ni(II)                    8

             Zn(II)                 8.5-11

             Pb(II)                 8.5-11

 4.  Synergistic effect:

In some cases, when a mixture of reagents is used for extraction, the extraction efficiency is increased. Such effect is called as synergistic effect. A mixed Complex is formed in this case of dithizone in combination with pyridine or 1,10-phenanthroline. 

e.g. Complex of Ni(II)  with Dithizone in the presence of 1,10-phenanthroline get extracted according to equation-

Ni² + 2H₂Dz + phen → [Ni(HDz)₂ (phen)] + 2H+

5.  Number of extractions:

In the same volume of extractant, if large number of extractions are carried out taking small volume of extractant every time, the efficiency of extraction increases. 

This has been discussed in details. 

Let us assume that one of the two phases is water and extracting phase is organic phase. The ratio of molar concentrations of solute in two solvents is called as distribution ratio (D) given by-

  D=Co 

       Cw 

Co= concentration of solute in organic phase

Cw= concentration of solute in water

Let V= total volume of solution from which solute is to be extracted (generally water)

v= portion of extractant taken every time for extraction (ml)

n= number of extractions

X₀= total weiɡht of solute in water(ɡ)

xn= weiɡht of solute remaininɡ after 'n' extraction in water(ɡ),then

                                 n

Xn= x₀[V/V+Dv]

   

It should be noted that by increasing the number of extractions and decreasing the column taken for each extraction, one can decrease the amount of unextracted solute, thereby increasing the efficiency of extraction. Efficiency of extraction can be determining percentage of solute extracted. 

It is given by- 

  Percentage efficiency= X₀-Xn  ×100

                                             X₀

Applications

1. Determination of iron using 8-hydroxyquinoline :

Fe(II) can be extracted from aqueous solution with 1% of 8-hydroxyquinoline in chloroform. The Complex is extracted quantitatively in chloroform. It is a dark-coloured Complex that absorbs at 470 nm. It is analyzed spectrophotometrically. 

2. Determination of lead using dithizone:

Lead can be analyzed at very low concentration level using dithizone reagent. Solvent extraction is carried out in chloroform. The method is highly sensitive. 

3. Determination of nickel using dimethylglyoxime (DMG):

Nickel forms a red Complex with DGM that can be extracted in chloroform. The yellow Complex absorbs at 366 nm and 465 nm. Hence it can be estimated spectrophotometrically. 

4. Determination of copper using diethyldithiocarbamate :

Sodium salt of diethyldithiocarbamate (Na-DDC) reacts with copper in weakly acidic or ammoniacal solution to produce a brown colloidal suspension. It may be extracted with an organic solvent like chloroform, carbon tetrachloride etc. Analysis is carried out spectrophotometrically at 435 nm. 

5. Separation of lanthanides:

Similar to ion exchange technique, solvent extraction is also widely used method for separation of lanthanides. For separation of lanthanides, they are extracted with tri-n-butyl phosphate (TBP) in the presence of nitrate ions. Extraction is carried out in continuous countercurrent apparatus. 

6. Separation of Fe³+ and Mg²+ :

Mixture of Fe³+ and Mg²+ is shaken with ethyl acetate as an extractant in strongly acidic conditions. Fe³+ gets selectively extracted in ethyl acetate leaving behind Mg²+. The aqueous phase containing  Mg²+ is evaporated to concentrate Mg²+. It is neutralized and pH is adjusted to 10 using ammonia buffer and determined complexometrically using FBT as an indicator. The ethyl acetate solution containing Fe³+ is shaken with water to re-extract iron in aqueous phase. The Fe³+ is reduced to Fe²+ and estimated by redox titration with K₂Cr₂O₇.

 

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Separation techniques, chromatography its classification and paper chromatography, types of paper chromatographic techniques with chromatography diagram and applications.

Analytical chemistry consist of qualitative as well as quantitative chemical analysis. In a sample, the component to be analysed is called as 'Analyte'. Quantitative chemical analysis consists of two steps:

1. Separation and preconcentration of the analyte.

2. Analysis of the sample.

Separation techniques used are of different types depending on the nature of analyte. Difference in some of the property on the components in a mixture is used as a basis for separation of the components. This property may be difference in size that is used in sieving fine material to separate it from coarse 1 hour may be separation by distillation based on difference in boiling points of the two liquids in a mixture. Various separation techniques like sublimation ,crystallization, etc are used frequently in Chemistry laboratory.

Chromatography is one of such methods used frequently in analytical chemistry. The word chromatography is derived from Greek words meaning "colour" (Chroma) and "write"(Graphy). This technique was first used by Russian botanist M.Tswett in 1906 for separation of coloured plant pigments on column of alumina.

Chromatography is defined as "physical method of separation, in which the components are separated by distribution between two phases". The phase that has large surface area and remains steady is called as 'Stationary Phase' while the other moves through the stationary phase called as 'Mobile phase'.Mixture of various component is equilibrated between two phases. Separation of these components takes place either by the mechanism of 'adsorption' or 'partition'.

Classification of chromatography

Depending on the principle of separation chromatographic techniques are divided into two main types:

A)  Adsorption chromatography

In this method separation of components takes place on the basis of difference in their adsorption properties on solid stationary phase. Here, one phase is always solid. Depending on the mobile phase, adsorption chromatography is classified into two types:

1.  Liquid-Solid Chromatography (LSC): in this technique the stationary phase is solid while mobile phase is liquid. Different components adsorb to different extent on the stationary phase.

The less adsorbed component is taken away first by the mobile liquid phase, while is strongly adsorbed component is eluted afterwards.

e.g.Column chromatography, ion exchange chromatography, thin layer chromatography (TLC) etc.

2. Gas-Solid Chromatography (GSC): In this method the stationary phase in solid while mobile phase is gas. It is carried out at every high temperatures to that the analyte remains in gaseous phase.

  B) Partition chromatography

In this technique one of the phases is always liquid. Different components get dissolved in the liquid to different extent and thus get separated from each other. Depending on the other phase, it can be classified into two types:

1. Gas-Liquid Chromatography (GLC): when stationary phase is liquid and mobile phase is gas the chromatographic method is called as GLC. The stationary phase is liquid immobilized on solid support like silica gel.

2.  Liquid-Liquid Chromatography (LLC): When both stationary  as well as mobile phases are liquids, the technique is called as LLC. The two liquids are immiscible with each other. Generally, one liquid is organic while other is inorganic. Difference between solubilities of the components in these two liquids act as a basis of separation.

e.g. Paper chromatography, solvent extraction etc.

Table:Types of chromatography

Stationary phase	Mobile phase	Name of method	Example
Solid	Liquid	Liquid-solid chromatography	Ion-exchange, TLC
Solid	Gas	Gas-solid chromatography	GSC
Liquid	Liquid	Liquid-liquid chromatography	Paper chromatography,HPLC , solvent extraction
Liquid	Gas	Gas-liquid chromatography	GLC

PAPER CHROMATOGRAPHY

Principle

It is a type of chromatography in which mixture of substance is distributed between two liquids. One Liquid is held in the fibres of a paper called as stationary phase while the other is moving liquid called mobile phase. In paper chromatography, the stationary phase is generally moving liquid called as mobile phase. In paper chromatography, the stationary phase is generally water. It is immobilized in the fibres of the filter paper. Mobile phase is generally an organic solvent immissible with water.

Various components when distributed between water and other liquid, get dissolved in these two liquids in different proportions. When mobile phase moves over stationary phase, equilibrium is established at every stage of partition. They get separated on the basis of difference in partity coefficients of various components,.

Technique

The technique used in paper chromatography consists of following steps-

1. Preparation of sample solution and application on paper: 

Paper chromatography assembly consists of a jar containing the mobile liquid. The liquid may be a pure solvent or a mixture of two solvents in fix proportion. This liquid is placed at the bottom of a jar. A filter paper strip is cut in proper size and the mixture is applied in the form of a small spot near one end of the strip. The paper used is generally a Whatman filter paper no.1, no.2 etc depending on the rate of flow required. 

2.  Running of chromatogram: 

The paper is dried completely and dipped in the solvent placed in the jar. This liquid is called as eluent and the process is called as elution. Eluent used is generally a mixture of organic solvent and water in fixed proportion. Experimental arrangement is shown in fig. The eluent starts moving in upward direction by capillary action of the filter paper. Various components of the mixture move at different speeds.

3.  Locating various components: 

When solvent is mood through suitable height (about 15 to 18 cm), the paper is dried and various spots are visualised by physical or chemical method post op physical methods are fluorescence and radioactivity while chemical method consists of spraying suitable reagent called locating reagent (or visualizing reagent). On addition of locating reagent different spots are obtained on the paper. such paper with different spots corresponding to different components is called as chromatogram.

The movement of various components is expressed in terms of 'retardation factor' called as Rf value it is given by-

  Rf=   Distance travelled by component 

          Distance travelled by solvent front

 The Rf value lies between 0 and 1. Higher Rf value shows higher mobility of the component . It depends upon number of factors as-

1. Solvent employed

2. Medium used for separation

3. Quality of paper used

4. Nature of mixture

5. Temperature.

6. pH of the solution.

     For each component in a mixture, the Rf value is constant under given experimental conditions.

Thus various components can be identified from their Rf values by fingerprint method.

Types of paper chromatographic techniques

For developing a chromatogram various techniques are used in paper chromatography. Thus paper chromatography is classified into following types-

1. Ascending paper chromatography:

In this technique, the eluent is taken at the bottom of the jar and ellution take place in  upward direction as shown in Fig.(paper chromatography)

Flow of eluent it takes place due to capillary action against gravitational force and thus takes more time and results in better partition of the component.

2. Descending paper chromatography: 

In this technique the eluent is stored at the top and moves along the paper in downward direction. This takes less time as the movement is in the direction of gravitational force. In this technique, the components with lower Rf- value are resolved better than ascending method. The experimental arrangement is shown in Fig 

3. Radial paper chromatography:

This is rarely used method.Here, sample is applied at the centre of a circular paper and develop by a wick. Wick is bent Don word at 90°and dipped in the eluent . Elution takes place radially and thus called as radial paper chromatography.

4. Two dimensional paper chromatography: 

In this technique rectangular or square paper is used. Sample is applied at one of the corners. First development is made as in ascending chromatography. Then paper is turned at right angle and again development is carried out in the similar manner (Fig). For second development either same or different solvent can be used. This technique is very useful when the mixture contains large number of components with close Rf- values.

Applications

Paper chromatography is being used as a simple separation tool in qualitative as well as quantitative chemical analysis.

1. Qualitative direction of constituents:

In a mixture it is sometimes necessary to detect the constitution present. In such case the mixture is spotted on paper and chromatogram is run using suitable solvent. After separation of components, spots are visualized using suitable locating agent. From the colours of the spots, the constituent can be identified.

For example,

1. Metallic ions of group l (Pb²+,Aɡ+ and Hɡ²+) have been identified and separated using distilled water or alcohol as a solvent and 0.25 M K₂CrO₄ aqua solution as locating agent. Pb²+ gives yellow, Ag+ gives Orange red while Hg₂²+ gives Orange spot. 

2. Group II ions ( Bi³+,Cd²+, Cu²+ and Hg²+ ) can be analyzed quantitatively using n butyl alcohol and 3M HCL reagent and dithizone solution in CCL₄ as a locating reagent.

3. Group IV cations Ni²+ , Co²+ , Zn²+ and Mn²+ etc can be analyzed qualitatively using paper chromatography. Eluent used is acetone-water-HCL mixture and locating reagent is rubeanic acid.

B. Quantitative analysis of metals:

Constituent of mixture of metals are separated as discussed above.the paper is cut and various components are dissolved in in suitable solvent and analysed using colorimetry, spectrophotometry or any other analytical technique.

C. Quantitative analysis of non-metals:

Various non metals anions organic compounds etc can be separated using paper chromatographic technique. Separation and estimation of Cl-,Br-,I- has become possible by using paper chromatography.

D. Separation of amino acids:

Mixture of naturally occuring amino acids can be analyzed using two dimensional paper chromatography. Ninhydrin is used as a colour-developing agent. It gives purple colour with all  α-amino acids. Other compounds like aliphatic amines, some heterocyclic compounds and imino acids give yellow colour.

                     

                    

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