solvent extraction

SOLVENT EXTRACTION : 

SOLVENT EXTRACTION BY:- SWAPNIL NIGAM

THE BEGINNING : 

THE BEGINNING First reported solvent extraction separation. was carried out in 1805 by “BUCHOLZ”. During the 19th century the extraction of neutral compounds into pure solvents was studied experimentally as well as theoretically by NERNST, PELIGOT and BERTHELOT

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INTRODUCTION In this technique, a solvent (known as the “extractive solvent” ) is brought in contact with another solvent (termed as the “solute bearing solvent”) in order to bring about the transfer of one or more solutes into the first solvent. Also known as LIQUID-LIQUID EXTRACTION.

BASIC PRINCIPLE : 

This technique is based on the relative solubility of an analyte in two immiscible liquids. The solute(or analyte) dissolves more readily and becomes more concentrated in the solvent in which it has a higher solubility. BASIC PRINCIPLE

GENERALISATIONS : 

The solute bearing solvent is the aqueous phase(i.e. water). This is so, because we need to produce neutral metal chelate and ion-association complexes that must be soluble in the extractive solvent( i.e. the organic phase). GENERALISATIONS

IMPORTANT TERMS : 

IMPORTANT TERMS DISTRIBUTION COEFFICIENT:- According to “Nernst Distribution(or Partition) Law” , for any solute A distributed between two immiscible phases ‘a’ & ‘b’, provided its molecular state in both phases is same & temperature is constant,

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Concentration in solvent ‘a’ Concentration in solvent ‘b’ KD Where, KD Distribution or Partition Coefficient

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The law stated is thermodynamically rigorous ,i.e., it takes no account of the activities of the various species involved & thus, is expected to be applied in very dilute solutions. Also, the law in its simple form is not applicable, when the distributing species undergo dissociation or association in either phase.

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DISTRIBUTION RATIO:- In practical application, we are concerned with the fraction of the total solute present in one or other phase, thus, its convenient to use “Distribution Ratio(D)”, i.e., D (CA)a (CB)b Where, CA denotes concentration of ‘A’ in all its form as determined analytically.

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SEPARATION COEFFICIENT:- If the solution contains two solutes ‘A’ and ‘B’, then it often happens that in conditions favouring complete extraction of ‘A’ , some ‘B’ is also extracted. The effectiveness of separation then increases with the magnitude of the “Separation factor(or coefficient)” which is related to the individual distribution ratios as follows;

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β DA DB β SEPARATION FACTOR Where,

THEORY : 

THEORY The greater the number of small extractions, the greater the quantity of solute removed. This means, For example, suppose we have Amount of solute in aqueous phase(xo)=300g Volume of aqueous phase = 100ml Volume of organic phase to be added = 200ml Distribution ratio of the particles = 0.5

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NOW, The total amount of solute (x n) left non-extracted in the aqueous phase (‘V’ ml) on adding the extractive organic phase (‘v’ ml) can be calculated using the formulae,

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xn xo D D V V V n WHERE, xo = amount of solute present before adding extractive solvent D = Distribution ratio of the solute particles n = Number of times the solvent is added

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CASE 1: If n=1, i.e., the extractive solvent is added in one complete go, then, from the formulae, we find that, x1 = 60g, that means 240g of the solute gets extracted. CASE 2: If n=2, i.e., if we add the extractive solvent in two parts each of 100 ml, then,

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v=100ml, and using the formulae, we find x2 to be 33g, which implies that, 267g of the solute has been extracted. Hence, from the above example it is clear that, it is more efficient to carry out small extraction with small equal portions of the extractive solvent, rather than using single large volume.

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The inorganic solutes, with which we are concerned, tend to be more soluble in water rather than organic solvents. Also, there occurs a large loss of electrostatic solvation energy if, inorganic solutes are directly expected to be extracted by organic solvents. Thus, for the extraction of inorganic solutes, we use appropriate reagents which can mask the water solubility of the inorganic ionic species by neutralizing their charge.

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The masking of the water solubility of the inorganic ionic species present in water can be done in two ways: By formation of a neutral metal chelate complex, or By ion association.

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Chelation complexes These complexes are often termed as INNER COMPLEXES, when uncharged. In these complexes the central metal ion coordinates with the poly-functional organic base to form a stable ring. For example, copper(II) acetylacetonate or iron(III) cupferrate.

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CH3 CH3 C C O O CH2 CH3 CH3 C C O CH ½ Cu2+ CH3 CH3 C C O O CH Cu/2 CH3 CH3 C C O O CH Cu/2 Copper(II) acetylacetonate H O H

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Factors which affect the stability of the chelate complexes: Basic strength of the chelating group (more is the basic strength, more is the stability). Nature of the donor atoms of chelating agent (soft-base type donor atoms form most stable complexes with the small group of class B metal ions, i.e., soft acid. Eg. Dithizone used for extraction of Pb2+.)

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Size of the ring (five or six membered conjugated chelate rings are more stable, since they have minimum strain). Resonance & steric effect (more are the resonance structures of the chelate ring, more will be its stability. Eg. Copper(II) acetylacetonate is more than copper chelate of salicylaldoxime. Also, the steric hindrance must be minimum).

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CRITICAL INFLUENCE OF pH ON SOLVENT EXTRACTION OF METAL CHELATES: The quantitative treatment of the solvent extraction of the neutral metal chelate can be done on the basis of the following assumptions;

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Solvation plays no significant part in the extraction process. The solutes are uncharged particle & their concentrations are so low that the solutions do not deviate much from ideality. The reagent and the metal complex exist as undissociated molecules in both phases.

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The formation of neutral metal(M) chelate complex, from a chelating reagent HR, takes place according to the following equation; Also, the dissociation of the chelating reagent HR in the aqueous phase is given by the equation; Mn+ n R- MRn HR R- H+

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Now, the above equilibria can be expressed in terms of the following thermodynamic constants; Dissociation Constant (K), & Partition Coefficient (p). in the following manner as described in the equations

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Dissociation constant of the K Mn+ R- R w w w n c complex H+ M H n r reagent

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Partition Coefficient of the p MRn w MRn o c complex HR HR r reagent

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The Distribution Ratio(D) ,i.e., ratio of the amount of metal extracted into the organic phase as complex to that remaining in all forms in the aqueous phase is; D MRn o MRn w w Mn+

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The equation for Distribution Ratio can be further reduced to the form, D o HR w HR n K Where, K K c r K p c r * p * ( ) n

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And the % of solute extracted is given by; D K * w HR If the reagent concentration remains virtually constant, then log ( ) E log ( ) 100-E - log ( ) K* n pH

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Thus, we observe that the distribution of the metal in the given system is a function of pH alone. The equation of the % of solute extracted represents sigmoid curves, when E is plotted against pH. The slope of the curve depends upon ‘n’. If pH1/2 is defined as the pH value at 50% extraction, i.e., E%=50, then,

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pH1/2 n -1 log ( ) K* The difference in pH1/2 values of two metal ions in a system is a measure of the ease of separation of the two ions. If the values are far apart, excellent separation can be achieved by controlling pH. The pH1/2 values may be altered using a competitive complexing agent.

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Ion-association complex These complexes are formed when the species to be extracted associates with oppositely charged ions to form neutral extractable species. Such complexes can clusters with increasing concentration, particularly in an organic solvent of low dielectric constant. Some types of ion-association complexes that have been recognized are:

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Those formed from the reagents yielding large organic ions, eg. Tetraphenylarsonium[(C6H5)4As+], which tend to form large ionic clusters with oppositely charged ions, like ReO4- . They do not have a hydration shell & thus, disrupt the water structure, due to which the tend to be pushed into the organic phase. Those involving a cationic or anionic

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chelate complex of the metal ion. Thus, the chelating reagent consists of two uncharged donor atoms. Eg. 1:10 phenanthroline forms cationic complexes. Those in which solvent molecules are directly involved in the formation of ion- association complex. Eg. ethers, esters etc.

EXTRACTION REAGENTS : 

EXTRACTION REAGENTS

INSTRUMENTS : 

INSTRUMENTS

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Thank you