ABSTRACT

The azo-ligand, 1,5-dimethyl-2-phenyl-4-[(E)-(2,3,4-trihydroxylphenyl) diazenyl]-1,2-dihydro-3H-pyrazol-3-one (H₃L) and its Zn(II) and Cd(II) complexes have been synthesized and characterized based on stoichiometric, molar conductance, electronic and infra-red spectral studies. The results showed that H₃L reacted with the metals in 2:1 ratio. H₃L coordination was through the hydroxyl, azo and carbonyl groups to form [Zn(H₂L)₂]²⁺ and [Cd(H₂L)₂]²⁺ respectively. Solvent extraction studies on Zn(II) and Cd(II) using 1,5-dimethyl-2-phenyl-4-[(E)-(2,3,4-trihydroxylphenyl) diazenyl]-1,2-dihydro-3H-pyrazol-3-one were carried out with  CHCl₃. Effects of other extraction variables like, pH, salting-out agent, masking agent and acids were also investigated. Cd(II) was quantitatively extracted in 0.001 M HCl up to 100%; and 0.001 M of either thiocyanate, or 0.001 M tatrate masked Cd(II) up to 90%, under five minutes. Extraction of Zn(II) with H₃L/CHCl₃ was quantitative in 0.001 M HCl up to 96% under seventy minutes. In the same vein, 1 M cyanide and 1 M thiocyanate masked it up to 79% and 67% respectively. Cd(II) was successfully separated from Zn(II) following four-cycle extraction up to 96.5%  in 0.001 M HCl using H₃L/CHCl₃ in the presence of 1 M cyanide. Recovery of Zn(II) and Cd(II) from rubber carpet was up to 90% and 85% respectively under the established parameters. The extraction constant was established for both Zn(II) and Cd(II) complexes from the results obtained from pH, where the slope was 0.141 and 0.0516, and the extraction constant 7.316 and 3.899 respectively. Hence, H₃L is a promising extractant for Zn(II) and Cd(II) ions.

 

 

TABLE OF CONTENTS

Title Page                                                                                                        i

Approval Page                                                                                                            ii

Certification                                                                                                    iii

Dedication                                                                                                       iv

Acknowledgement                                                                                            v

Table of Contents                                                                                            vi

List of figures                                                                                                 xi

List of tables                                                                                                   xiii

Abstract                                                                                                          xiv

1. Introduction 1

1.1       The Solvent Extraction Process                                                                      2

1.2       Kinetics of Extraction                                                                                     3

1.3       Properties of Liquids                                                                                     4

1.4       Thermodynamics of Solutions                                                                        5

1.4.1    Ideal Mixtures and Solutions:                                                                        5

1.4.2    Non-Ideal Mixtures and Solutions                                                                 6

1.4.3    Scales of Concentration                                                                                  7

1.5       Solubility in Binary Systems                                                                          8

1.5       Measures of Effectiveness                                                                              9

1.6.1    Distribution Law                                                                                              9

1.6.2    Distribution Ratio                                                                                          11

1.7      Extraction Factor (D,)                                                                                      12

1.8       Quantitative Treatment of Solvent Extraction Equilibria                                14

1.9       Extraction Methods in Solvent Extraction                                                       18

1.9.1    Batch-Extraction Process                                                                                 18

1.9.2    Continuous Extraction                                                                                     23

1.9.3    Discontinuous Counter Current Extraction                                                   24

1.10     Classification of Inorganic Extraction System                                               25

1.10.1 Metal Chelate                                                                                                  25

1.10.2  Ion Association Complexes                                                                           30

1.10.3 Additive Complexes                                                                                       31

1.11 Factors that Influence Stability and Extractability of Metal Chelate Complexes                                                                    33

1.11.1 Effect of Acidity (pH)                                                                                    33

1.11.2 Effect of Organic Chelating Agent                                                                 34

1.11.3  Effect of Masking Agent                                                                               34

1.11.4 Effect of Variation of the Oxidation State of Metal                                                  35

1.11.5 Effect of Salting-Out Agent                                                                           35

1.11.6  Effect of the Stability of the Metal Chelate                                                  36

1.11.7  Influence of Organic Solvent                                                                        36

1.12     The Features of Ligand that Affects Chelate Formation                                37

1.13     Applications of Solvent Extraction                                                                 38

1.14     Drawbacks on Solvent Extraction                                                                   38

1.15     Scope of Study                                                                                               39

1.16     Aims and Objectives                                                                                       39

2.0       Literature Review                                                                                            41

2.1       History of Solvent Extraction                                                                        41

2.1.1    Early Models on Solvent Extraction                                                              44

2.2       The Solvent Extraction of Zinc                                                                      44

2.2.1    Previous Works on Solvent Extraction of Zinc                                              46

2.3    The Solvent Extraction of Cadmium                                                                 50

2.3.2    Previous Works on Solvent Extraction of Cadmium                                       52

2.4       The Chemistry of Ligand Formation                                                              57

3.0       Methods and Materials                                                                                               59

3.1       Description of Apparatus                                                                                 59

3.2       Preparation of Metal Stock Solution                                                              59

3.3       Synthesis of 4-Amino Antipyrine-Pyrogallol                                                  60

3.4.      Complexes of the Ligand .                                                                             61

3.4.2    Stiochiometry of the Complexes                                                                   61

3.5       Extraction Procedure                                                                                       61

3.5.1    Extraction from Buffer Solution                                                                   62

3.5.2    Extraction from Acid Media                                                                          62

3.5.3    Extraction in Salting-Out Agent                                                                      63

3.5.4    Extraction in Complexing Agent                                                                   63

3.6       Measurement of Distribution Ratio                                                                64

3.7       Spectrophotometric Analysis of the Metal Ion                                               64

3.7.1    Cadmium (II) Analysis                                                                                   64

3.7.2    Zinc (II) Analysis                                                                                             64

3.7.3    Calibration Curve                                                                                            65

3.8       Separation Procedures                                                                                     65

3.9       Extraction from the Industrial Material                                                          65

4.0       Results and Discussions                                                                                 67

4.1       Electronic Spectra                                                                                           67

4.2       IR Spectra                                                                                                       71

4.3       Metal-Ligand Mole Ratio                                                                               77

4.4       The Molecular Formula of the Ligand and the Complexes                            77

4.5       The Properties of the Ligand and the Metal complexes                                 78

4.5.2    Solubility Test Data                                                                                         79

4.5.3    Dissociation and Protonation Constant of the Ligand                                   79

4.6       Equilibration Time                                                                                          81

4.6.1    The Effect of pH Buffer on Extraction of Zn(II) and Cd(II)                                   82

4.6.2    Effect of Acidity                                                                                           84

4.6.3    Effect of Salting-Out                                                                                      86

4.6.4    Effect of Masking Agent                                                                                89

4.6.5     Metal Separation                                                                                            91

4.6.6     Determination of Metal from Real Material                                                  92

4.7       Quantitative Treatment of Solvent Extraction Equation                                93

Conclusion                                                                                                      94

REFERENCE                                                                                                            95

 

 

 

CHAPTER ONE

•                               INTRODUCTION

During the years 1900 to 1940, solvent extraction was mainly used by the organic chemist for separating organic substances. Since in these systems, the solute, (desired component) often exist in only one single molecular form, such system are referred to as non- reactive system¹. However, it was also discovered that mainly weak acids could complex metals in the aqueous phase to form complex soluble in organic solvent. This is an indication that organic acid may be taken from the aqueous or the organic phase; such system is referred to as reactive system. This has become a tool for analytical chemist, when the extracted metal complex showed a specific colour that could be identified spectrometrically.

Solvent extraction is a process whereby two immiscible liquids are vigorously shaken in an attempt to disperse one in the other so that solutes can migrate from one solvent to the other². When the two liquids are not shaken the solvent to solvent interface area is limited to the geometric area of the circle separating the two solvents. However as the two liquids are vigorously shaken the solvents become intimately dispersed in each other. The dispersal is in the form of droplets. The more vigorous the shaking the smaller the droplets will be. The smaller the droplets are, the more surface area there is between the two solvents. The more the surface area between the two solvents, the smaller the linear distance will be that molecules will travel to reach the other solvent and migrate into it. The shorter the linear distance travelled by the molecules, the more rapid will be the extraction. The fundamental reason for molecules to migrate from one phase into another is solubility. The molecules will preferentially migrate to the solvent where they have the greatest solubility. If the molecules are very polar they will generally favour the aqueous phase. If the molecules are non-polar they will favour the organic phase. The key concept to take away at this point is that the process of solvent extraction requires that the chemist adjust the solution conditions so that the radionuclide of interest is in the proper oxidation state and the solution pH is adjusted so that the appropriate complexing agent will form a neutral complex that will easily migrate into the organic phase based on those chemical conditions¹.

Solvent extraction has been used predominantly for the isolation and pre-concentration of a single chemical species prior to its determination³; it may also be applied to the extraction of group of metals or classes of organic compounds, prior to their determination by techniques such as atomic absorption or chromatography. Solutes have differing solubilities in different liquids due to variation in the strength of the interaction of solute molecules with those of the solvent. For this reason, the choice of solvent for extraction is governed by the following⁴:

1. A high distribution ratio for the solute and a low distribution ratio for undesirable impurity.
2. Low solubility in the aqueous phase.
3. Sufficient low viscosity and sufficient density difference from the aqueous phase to avoid the formation of emulsion.
4. Low toxicity and flammability.
5. Ease of recovery of solute from the solvent for subsequent analytical processing. Thus the boiling point of the solvent and the ease of stripping by chemical reagents merit attention when a choice is possible. Sometimes, mixed solvent may be used to improve the above properties; and salting-out agent may also improve extractability.

1.1            The Solvent Extraction Process

There are five general steps that are involved in the solvent extraction process². They rely on the fact that the solution conditions have been optimized to maximize the extraction for one radionuclide over the others: The first step is to ensure that the proper complexing agents have been added to the aqueous phase so that the extractable complex is of sufficiently low charge density, so that the transfer of the radionuclide to the organic phase will be maximized. In the second step, the equilibration process occurs by shaking of the separatory funnel. Unless otherwise specifically noted in a particular method, the amount of time that the two phases are shaken during this step is about two minutes. The initial organic phase is separated and set aside. Step three involves a process known as re-extraction¹. The original aqueous phase is extracted with a fresh aliquot of the organic phase of the same volume as the first. This improves the efficiency of extraction of the radionuclide of interest. After step two is repeated the two organic phases are combined. The aqueous phases are discarded at this point unless they are needed for analysis of radionuclide not extracted. In step four the combined organic phases are equilibrated with a solution of aqueous phase that is of the same composition as the original sample solution, but without any sample. This step helps to ensure that any interfering materials that may have been extracted are re-distributed back to the aqueous phase, while the radionuclide of interest remains in the organic phase. This phase known as the wash is then discarded. The final step is to strip the radionuclide of interest back into an aqueous phase using a pH and lower concentration of complexing agent so that migration back to the aqueous phase is favourable

1.2                  Kinetics of Extraction

It is important to investigate the rate at which the solute is transferred between the organic and aqueous phase. In some cases, by an alteration of the contact time, it is possible to alter the selectivity of the extraction. For instance, the extraction of palladium or nickel can be very slow because the rate of ligand exchange at these metal centres, which is much lower than the rates for iron or silver complexes³.

 

 

1.3                                      Properties of Liquids

If the externally imposed conditions of pressure and temperature permit a substance to be in the liquid state of aggregation, it possesses certain general properties; that is, it flows under the influence of forces and is characterized by its fluidity, or viscosity. A liquid has a surface, and is characterized by a surface tension; the volume of a liquid does not change appreciably under pressure; it has a low compressibility and shares this property with matter in the solid (crystalline, glassy, or amorphous) state^5,6. The particles of a liquid do not possess long-range order. Although over a short range, 2 to 4 molecular diameters, there is some order in the liquid, this order dissipates at longer distances. A particle in the liquid is free to diffuse and, in time, may occupy any position in the volume of the liquid, rather than being confined at or near a lattice position, as in the crystalline solid, the particles in a liquid are in close proximity to each other (closely packed) and exert strong forces on their neighbours⁷. The close packing of the molecules of a substance in the liquid state results in a density much higher than in the gaseous state and approaching that in the solid state. The density depends on the temperature. Many liquids used in solvent extraction are polar. Their polarity is manifested by a permanent electric dipole in their molecules, since their atoms have differing electronegativities.

When non-polar liquids are placed in an electric field, only the electrons in their atoms respond to the external electric forces, resulting in some atomic polarization. This produces a relative permittivity (dielectric constant) ε, which is approximately equal to the square of the refractive index. Polar molecules, however, further respond to the external electric field by reorienting themselves, which results in a considerably larger relative permittivity. Therefore, the ionic dissociation of electrolytes strongly depends on the relative permittivity of the solvent that is used to dissolve them

 1.4                        Thermodynamics of Solutions

Thermodynamics is the branch of science dealing with the energetics of substances and processes. It describes the tendency of processes to take place spontaneously the effects of external conditions, and the effects of the composition of mixtures on such processes^1,6. Thermodynamics is generally capable of correlating a variety of data pertaining to widely changing conditions by relatively simple formulae. One approach to such a correlation involves the definition of a hypothetical ideal system and the subsequent consideration of deviations of real systems from the ideal one.

In many cases, indeed, such deviations are relatively small and can be ignored in a first approximation. Such examples include a gas under low pressure or a dilute solution of a solute in some solvent. In many other instances (unfortunately in many that pertain to practical solvent extraction), such an approximation is far from being valid, and quite incorrect estimates of properties of the real systems can result from ignoring the deviations from the ideal.

1.4.1                         Ideal Mixtures and Solutions

1. One Liquid Phase: Consider two liquid substances that are rather similar, such as benzene and toluene or water and ethylene glycol. When n_A moles of the one are mixed with n_B moles of the other, the composition of the liquid mixture is given by specification of the mole fraction of one of them⁶. It can be deduced that, the energy or heat of the mutual interactions between the molecules of the components is similar to that of their self interactions, because of the similarity of the two liquids, and the molecules of A and B are distributed completely randomly in the mixture. In such mixtures, the entropy of mixing A and B attains its maximal value per mole of mixture.

The molar heat of mixing of such a mixture, Δ_M H_AB, is zero, since no net change in the energies of interaction takes place on mixing. Therefore, the molar Gibbs energy of mixing, in the process that produces an ideal mixture, is⁷:

Δ_MG_AB = Δ_M H_AB -TΔ_M S_AB = RT [x_A ln x_A + x_B ln x_B ] ……… (1)

The solute and the solvent are not distinguished normally in such ideal mixtures, which are sometimes called symmetric ideal mixtures.

2. Two Liquid Phases: Consider now two practically immiscible solvents that form two phases, designated by ׳ and ׳׳. When these two liquid phases are brought into contact, the concentrations (mole fractions) of the solute adjust by mass transfer between the phases until equilibrium is established and the chemical potential of the solute is the same in the two phases⁷

.

(It is the difference in the chemical potentials of the solute that is the driving force for the mass transfer.)

1.4.2                       Non-Ideal Mixtures and Solutions

For most of the situations encountered in solvent extraction the gas phase above the two liquid phases is mainly air and the partial (vapour) pressures of the liquids present are low, so that the system is at atmospheric pressure¹. Under such conditions, the gas phase is substances in the gas phase (fugacity). Equilibrium between two or more phases means that there is no net transfer of material between them, still there is a dynamic exchange. This state is achieved when the chemical potential µ, (as inequality of the activities) of a substance in two phases that causes some of the substance to transfer from the one (higher) to the other phase, (until equality) is achieved⁸. The activity of a pure liquid or solid substance is defined as unity. In any mixture, whether ideal or not, the activity of a component A, a_A, is related to its chemical potential by:

µ_A  = µ _A^ᵒ =  RT ln a _A ………….. (3)

Therefore, if component A is the solvent in a solution, and it obeys Raoult’s law when the solution is dilute in all solutes, then a_A = x_A under these conditions. Otherwise, the solution is non-ideal, and the deviation from the ideal is described by means of the activity coefficient, or by the ratio of the vapour pressures, p_A/p*_A (neglecting interactions in the vapour phase, otherwise the fugacities have to be employed. As the solution is made more dilute and approaches the dilute ideal solution and the pure solvent, the activity coefficient f_A, approaches unity⁸:

The activity coefficients f_A and f_B may be smaller or larger than unity; hence, the excess chemical potentials µ_A^E and µ_B^E may be negative or positive. Depending on the sign of µ_A^E, the solution is said to exhibit positive or negative deviations from Raoult’s law.

1.4.3                                 Scales of Concentration

The composition of a mixture need not be given in terms of the mole fractions of its components. Other scales of concentration are frequently used, in particular, when one of the components, say, A, can be designated as the solvent and the other (s), B, (C, . . . ) as the solute (or solutes).

Fig. 1.1 Concentration scales.

The molality m_B [in mol/(kg solvent)] and molarity c_B [in mol (L solution)^-1] of solute B, which has a molar mass M_B 0.100  kg mol^-1 and a molar volume V_B 0.050  L mol^-1, in the solvent A, which has M_A = 0.018  kg mol^-1 and V_A 0.018  L mol^-1 (water), are shown as a function of the mole fraction x_B of the solute. Note that the molarity tends toward a maximal value (1/V_B), whereas the molality tends toward infinity as x_B increases toward unity. The differences between the standard chemical potentials of a solute, in the two liquid phases employed in solvent extraction also depend on the concentration scale used⁹.

1.5     Solubilities in Binary Systems

Binary systems are systems that involve two components, that is, two substances that can be added individually^10,11. The phase rule states that at a given temperature and pressure, when only a single liquid phase is present, there will be one degree of freedom, and we can choose the composition of this phase, made up from the two components, at will. This does not preclude the phenomenon of saturation, where beyond a certain amount of solute in the liquid mixture a new phase appears. When two phases are present at the given temperature and pressure in the binary system, there are no longer any degrees of freedom, and the compositions of the phases are fixed¹⁰. The new phase may be a solid or a second liquid. Generally, the solid phase is the pure solid solute, but in rare circumstances, it is a solid solution of the two components in each other; occasionally, it is a pure solid solvate of the original solute by the solvent. When a second liquid phase separates out, it is generally a saturated solution itself, rather than a pure liquid. We then have a solvent-rich dilute solution of the solute and a solute-rich concentrated solution of it. As an example, consider a solution of phenol in water at 25 ᵒC¹¹. At this temperature, pure phenol is a solid (its melting point is 40.9 ᵒC), but when equilibrated with water at 25 ᵒC, the saturated aqueous layer contains 8.66% by mass of phenol, and the phenol-rich layer contains 28.72% by mass of water. However, on a mole fraction basis, both layers appear to be water-rich phases, the one having x_water = 0.982 and the other (the “phenol” phase) having x_water = 0.678. The solubility of a solute in a solvent is given by the composition of the saturated solution at a given temperature and pressure¹¹. The solubility may be expressed on any of the concentration scales: the molar, the molal, the mole fraction, the mass fraction (wt%), or the volume fraction scales. The pressure is of significance, only if the solute is a gas. For liquid and solid solutes, however, the temperature is the only variable that ordinarily needs to be specified.

1.6     Measures of Effectiveness

1.6.1   Distribution Law

18989. Nernst derived the distribution law in 1898⁹. He related it to the distribution of a solute in the organic phase and in the aqueous phase. That is, the distribution of a solute between two immiscible liquids in equilibrium as the basis for calculating the distribution of a substance between two phases¹⁰. Take for instance, if solute A is distributed between two immiscible liquids, the equilibrium expression of it will be:

……. (4)

So that;

…… (5)

By this, Nernst distribution or partition law states that the concentration of the solute in organic to the concentration of the solute in aqueous phase is a constant, provided that the temperature is constant and that the molecular state of the solute is the same in the two liquids. Constant K, also known as distribution constant or partition coefficient can be abbreviated with P and are expressed in terms of moles and volume¹². Plotting a graph of organic concentration against aqueous concentration will give a straight line graph. The values of the slope ( K_D ) is a reflection of the relative solubility of the solute in the two phases.

Limitations of Nernst Distribution Ratio

1. Nernst distribution ratio equation is valid only with pure solvents⁹. In practise the solvents are always saturated with molecules of the other phase, say water in the organic phase.
2. The law does not apply when the distribution species undergoes polymerisation, dissociation, or association in either of the two phases^13,14. Here our primary interest is in the fraction of the solute in one or other phases regardless of its mode of polymerisation, dissociation, association or interaction with other dissolved species. That is, it takes no account of the activities of the various species involved and for this reason will be expected to be applied only in a very dilute solution, where the ratio of the activity approaches unity( neglect of activity Corrections)
3. Participations of the distributing solute in chemical interactions in either or both of the two solvent phases.
4. It is not thermodynamically rigorous but useful in approximation.
5. The solute may be differently solvated in the two solvents, but if the mutual solubilities of the solvent are small, say less than one percent and the activity factors of the system are constant, the law will hold; but if the solute is strongly solvated, or at high concentration (mole fraction, > 0.1 ) or if the ionic strength of the aqueous phase is large ( > 0.1M ) or changes^9,3.
6. For aqueous electrolytes, the activity factor varies with the ionic strength of the solution. This has led to the use of constant ionic medium method^3,4.
7. It did not consider when there is an alteration of conditions, say pH, masking agent, e.t.c, during extraction.

Plotting a graph of organic concentration (C₁) against aqueous concentration (C₂) will give three behaviour or partition isotherms:

 

 

C₁    c          a      b

 

 

 

C₂

Fig 1.2  Deviation from Nernst distribution law.

Where a = ideal behaviour, b = solute association, dimerization, e.t.c. and c = solute approaching saturation^13,14. This occurs due to the factors listed above and can be corrected when the partition law from thermodynamic point of view is obeyed to the letter.

1.6.4        Distribution Ratio

The principle of solvent extraction entails a separating funnel containing two layers of liquid, water (S_aq) and organic solvent (S_org). The organic solvent may be more or less dense than water, but whichever way, the solute A, will dissolve in only one of the two liquids, and eventually distributes itself between the two phases¹⁷. When the distribution reaches equilibrium, the solute will be at concentration [A]_aq in the aqueous layer and at concentration [A]_org in the organic layer. Making the distribution ratio of the solute to be:

      (6)

The distribution ratio is defined as the ratio of the total analytical concentration of the substance in the organic phase to its total analytical concentration in the aqueous phase (usually measured at equilibrium), irrespective of whether the organic phase is lighter or heavier¹². For a metal species M it can be written as the ratio of concentration of all species containing M in organic phase to concentration of all species containing M in aqueous phase. When M is present in various forms of different complexes in the aqueous phase and in the organic phase, [M]_t refers to the sum of the concentrations of all M species in a given phase (subscript t = total M). This is different from distribution constant, K_D which is useful only for single specified species; but D involve sum of species of all kinds as indicated by the index. In solvent extraction, a distribution ratio (D_R  or D) is often quoted as a measure of how well extracted a species is¹⁵. Therefore, abnormal distribution of solute in either of the phase, shows variation from normal distribution ratio. This may be that the same molecular species is not present in both phases, they can change form via: association, dissociation or polymerization. Although, when chemical forms of solute are the same in both phases, the value of D_C and D, become the same.

Values of D can range from < one to > 10,000.  During an initial extraction, solution conditions that favour D_R for the radionuclide of interest of at least 100%, would mean that greater than 99% of the radionuclide would end up in the organic phase. Performing a second extraction with fresh organic phase on the sample solution, and then combining the two organic phases would provide greater than 99.99% removal of the radionuclide of interest from the aqueous phase. This ratio is based on specific solution conditions such as pH.

`1.7                     Extraction Factor (D,)

This is the ratio of the total mass of solute in the extract, to that in the other phase. It is the product of the (concentration) distribution ratio and the appropriate phase ratio¹⁵. It is synonymous with the concentration factor or mass distribution ratio, this latter term being particularly apt. The term concentration factor is often employed for the overall extraction factor in a process or process step.

1.7.1                                       Efficiency of Extraction (E)

The fraction of the total quantity of a substance extracted (usually by the solvent) under specified conditions,

……..(7)

Where, Q, is the mass of A extracted and Q’, is the total mass of A present at the start. If the concentration of the solute in the organic phase is [A]_orgV_org and the concentration in the aqueous phase is [A]_aqV_aq, therefore ;

Dividing through by [A]_aqV_aq, gives:

 

Multiplying both sides by V_aq/V_org;;

Therefore, efficiency of extraction depends on the relative volumes of the liquid phases and on the distribution ratio, D.  Equal volume in both phases, therefore:

E = D/(1 + D)      …….(9)

1.7.2                                   Percentage Extraction

This is related to distribution ratio because it is less sensitive to distribution ratio, at very high or low values of distribution ratio. It is the more commonly used term for expressing the extraction efficiency by analytical chemist. It is related to D in the following equation;

% extraction   =

but when V_org = V_aq, then the equation will become: %E = 100D / D + 1

It may also be seen from equation (44) that at extreme values of “D”, “E” becomes less sensitive to changes in “D”. For example, at a phase volume ratio of unity, for any value of D below 0.001, the solute may be considered quantitatively retained in the aqueous phase whereas for D values from 500 to 1000, the value of “E” changes only from 99.5 to 99.9%. That is the change will also be negligible in relation to percentage extracted.

 

1.7.4                                          Separation Factor

            Separation factor β, is defined as the ratio of the two solutes in a system¹⁸. It indicates the tendency for one solute to be separated more readily from aqueous phase into organic phase than the second solute; that is, it describes the effectiveness of separation of two solutes. Two species can only be separated with one extraction when the values of their corresponding distribution ratio are grossly different. Therefore, if the corresponding distribution ratio of A₁ and A₂ are D₁ and D₂, then the separation factor becomes:

β   ………(10)

Where, A and B represent the respective solutes.

In those systems where one of the distribution ratios is very small and the other relatively large, complete separations can be quickly and easily achieved. If the separation factor is large and the smaller distribution ratio is sufficiently large, then less separation of both components occurs. It is then necessary to apply various techniques to suppress the extraction of the undesired component. Moreover, two solutes whose distribution ratio differs by a constant factor would be separated most efficiently if the product, D_AD_B is unity. This principle can be illustrated thus; solute A_org and A_aq with distribution ratios 10¹ and 10³ respectively, if present in equal quantity, then single extraction would remove 99.9% of A_org and 90% of A_aq, Much more efficient extraction would be obtained if, using the same factor of 100 between the distribution ratios, the two distribution ratio were 10¹ and 10^-1. In this case, the respective fraction extracted would be 90% and 10%

1.8 Quantitative Treatment of Solvent Extraction Equilibria

Chelate complexes of many metals are known ^19-21 and with a given chelating agent, the properties of the complexes change from one metal to another. This chelate is a complex that is composed of a central metal atom and one or more multidented ligand¹⁸. In solvent extraction, the extraction of metal ion, M⁺ with an organic reagent HL, forming a chelate ML_n soluble in an organic solvent is expressed by the equilibrium:

+ nHL_org                    ML_n,org + n …….(11)

This, maybe treated quantitatively on the basis of the following assumptions ²².

1. The reagent and the metal complex exist as simple unassociated molecule in both phases.
2. Solvation plays no significant part in extraction processes, and
3. The solutes are unchanged molecules and their concentration are generally so low that the behaviour of their solutions only depart little from ideality.

The distribution ratio, that is; the ratio of the amount of metal extracted as complex into the organic phase to that remaining in all forms in the aqueous phase, is given as;

The extraction constant expression can be written as:

 

For instance, the extraction of an aqueous solution of copper ion with a chloroform solution of 8-hydroxyquinoline (oxime) forms the copper-oxime chelate which is extracted into chloroform; and the proton released increases the acidity of the aqueous phase²³.

From equation (11), if several simplifying assumption are made:

1. The concentration of chelate species other than ML_n are negligible,
2. The concentration of hydroxyl or other anion coordination complexes are negligible,

• The reagent HL and the chelate ML_n exist as simple undissociated molecules in the organic phase, then;

Substituting D, from

into equation (12) gives:

 

For a given reagent and solvent, the extraction of metal chelate depends only upon: pH and the concentration of reagent in organic phase, but independent of the initial metal concentration. In practice, a constant and excess of reagent is used to ensure that all the metal complexes exists as ML_n and D is dependent of pH, that is;

….(15)

From the equation, distribution ratio varies inversely as the exponential power of hydrogen ion concentration. The logarithm of equation (15) gives:

Log D  =  log K_ex  +  npH ………………(16)

Already we have that; D  =  (E/100-E), with logarithm equation of²⁴ ;

Log D = log E – log (100-E)  ………………. (17)

Combining equation (16) and (17) will give;

Log E – log (100 – E) = log K_ex + npH  …………. (18)

The distribution of metal in a given system of the above type is a function of the pH alone. The equation, represent the family of sigmoid curves when E is plotted against pH, with the position of each uniquely depending upon n. Some theoretical extraction curves for divalent metals showing how the position of the curves depends upon the magnitude of K_ex are depicted below:

Fig 1.4 A graph of % of metal extracted into organic phase against pH

It is evidence that a ten-fold change of reagent concentration is exactly off-set by a ten-fold change in hydrogen ion concentration (by a change of a single unit of pH) which is much easier to effect in practice. If pH_1/2 is defined as the pH value at 50% extraction, (E_% = 50), we see from above equation that;

pH_1/2 = 1/n log K_ex   …..(19)

The difference in pH_1/2 values of two metal ions in a specific system is a measure of ease of separation of the two ions. If the pH_1/2 values are sufficiently far apart, then excellent separation can be achieved by controlling the pH of extraction²⁴. It is often helpful to plot the extraction curves of metal chelates if one takes as criterion of successful single-stage separation of two metals by pH control, a 90% extraction of one with a maximum of 1%

Fig 1.5 A graph of % extracted against pH

extraction of the other for bivalent metals difference is less for tervalent metals. If pH is controlled by a buffer solution, then those metals with pH_1/2 values in this region, together with all metals having small pH_1/2 values, will be extracted. The pH_1/2 values may be altered (and the selectivity of extraction thus increased) by the use of a competitive complexing agent or of masking agent. Thus in the separation of mercury and copper by extraction with dithizone in carbon tetrachloride at pH 2, the addition of EDTA forms a water-soluble complex, which completely mask the copper leaving the mercury extraction unaffected. Cyanide raises the pH_1/2 values of mercury, copper, zinc and cadmium in dithizone extraction with carbon tertrachloride.

1.9                Extraction Methods in Solvent Extraction

                          When two immiscible phases come together discretely or continuously, they give rise to three common ways of performing solvent extraction based on the relative values of distribution ratio of the original mixture; batch, continuous, and countercurrent method of solvent extraction. Their equipments are available and they are convenience.

1.9.1                         Batch Extraction Process:

Perhaps one of the most frequent cases encountered in separation of a neutral organic compound(s) from a solution or suspension in aqueous phase by shaking with an organic solvent in which the compound is soluble and which is immiscible with water²². The solvent generally employed for extraction are diethyl ether or diisoprpyl ether, toluene, dichloromethane, and light petroleum. The solvent selected will depend upon solubility of the substance to be extracted in that solvent and upon the ease with which the solvent can be separated from the solute.

Solvent selection should be based on small scale trials. A little quantity of the suspension or solution to be extracted is placed in a small test tube and shaken with an equal volume of diethyl ether; when dissolution of suspended material clearly indicated that the solvent would be satisfactory. If the solution to be extracted is homogeneous initially, then the ether solution is removed with a dropper pipette on to a watch glass and the ether is allowed to evaporate, to determine whether material has been extracted. This helps to differentiate between organic liquid extracted and traces of water simultaneously removed during the extraction process²². If extraction with diethyl ether proves unsatisfactory, the experiment is repeated with a fresh sample of reaction mixture using dichloromethane; as the extraction solvent, or another trial with other solvents in the same way until a suitable solvent is selected.

An illustration of bulk batch extraction techniques of aqueous solution with diethyl ether is as follows; a separator funnel is selected of about twice the volume as that to be extracted and mounted in a ring on a stand with a firm base. The barrel and plug of the stopcock are dried with a linen cloth and in the case of glass stopcock lightly treated with a suitable lubricant (Apiezon or silicon grease). The solution and the extraction solvent are introduced into the funnel and the latter stoppered. The funnel with the stopper firmly held in position is then shaken gently (so that the excess vapour pressure will be developed slowly), inverted and the stopcock opened in order to relieve the excess pressure²². The stopcock is again closed, the funnel again shaken and the internal pressure released. When the atmosphere inside the funnel is saturated with ether vapour, the further shaking develops little or no additional pressure. At this stage, the funnel is vigorously shaken for 2-3 ether layers, and then return to the stand in order to allow the mixture to settle.

 

Fig 1.6 A separating funnel

When the phases separate out, the lower aqueous layer is run off and separated as completely as possible. The residual ethereal layer is then poured out through the upper neck of the funnel; contamination with any drops of the aqueous solution still remaining in the stem of the funnel is thus avoided. The aqueous solution may now be returned to the funnel and the extraction being repeated, using fresh ether on each occasion until the extraction is complete. Not more than three extractions are usually required; but the exact number will naturally depend upon the partition coefficient of the substance between water and ether. The completeness of it can be determined by evapourating a portion of the last extraction on the water bath and noting the amount of residue. The combined ethereal solutions are dried with an appropriate reagent, and the ether removed on a water bath^3,22. The residual organic material is further purified depending upon its properties and the organic impurities removed in the extraction by: chromatography, recrystalisation, or distillation. It is also important to retain the aqueous solution until the final purified product is isolated, so that incorrect observations on the solubility characteristics of the required product do not lead to premature discarding of the product. This method of doing more than one extractions and later, mixing the extracts together is regarded as successive extraction.

Occasionally emulsions are formed in the extraction of aqueous solution by organic solvents, thus rendering a clean separation impossible. This is liable to occur when the aqueous phase is alkaline, and when dichloromethane is the extracting solvent.

Successive Extraction

Successive extraction is done to increase the amount extracted. However, it can be shown that for a given volume of extracting solvent, it is more effective to divide it into several small portions and use each portion successively rather than to make a single extraction with the whole volume³. This statement can be proved mathematically            as follows:

Suppose W^o_aq of a solute was originally present in V_aq(ml) of the aqueous phase. Let the solute be extracted with successive portion V_org(ml) of the organic phase. Let W_aq and W_org be the weight of the solutes in aqueous and organic phase respectively. The concentration values in aqueous and organic phases are C_aq and C_org respectively.

W^o_(aq)  = W_(aq) + W_(org)       …(20)

Where, W_(aq)  =  C_(aq)V_(aq)  and  W_(org)  =  C_(org)V_(org)

Fraction of Solute Remaining in Aqueous Phase After First Extraction

W_(aq) is weight of solute remaining in aqueous layer after the first extraction. Dividing through by C_(aq³₎,

 

That is to say:

Therefore;   21)

This last equation helps in determination of the amount of solute that will remain unextracted in the aqueous phase in a single equilibrium stage, provided the volume of the two liquid phases and the distribution ratio for the solute in the system are known²⁵. Also, the amount of solute that will be extracted into the organic phase in a single equilibrium stage can be calculated using:

22)

Although, if there is mutual solubility of the two liquids, the equilibrium volume of the two phases will not be the initial volume unless each liquid has been pre-saturated with the other before it is used.

Fraction of Solute after Second Extraction

Dividing through by ;

 

²⁵

If we substitute the value of W_(aq)1 into this equation , it will give;

 

And if we continued for nth extraction, we will have;

………(25)²⁵

Having known how to determine the amount of solute remaining in aqueous phase after nth extraction, the amount extracted into the organic phase after nth extraction will become;

ⁿ

In conclusion, if the value of the distribution ratio for a solute and the volume of the two phases are known, then the number of extraction required to get the desired degree of extraction will be obtained graphically

 

 

W_aq(n)/W^o_aq

N, no of extraction

Fig 1.7 The graph of W_aq(n) / W^o_aq against n, number of extraction

From the graph, it is observed that it is not possible to extract all the solute from the aqueous phase even at infinite number of extraction.

1.9.2                         Continuous Extraction

            This method solves the problem of successive batch extraction. It uses continuous flow of immiscible solvent through a solution, or a counter-current flow of both phases. The spent solvent is stripped and fresh solvent is added continuously from a reservoir or recycled by distillation²². Most device of continuous extraction operate on the same general principle which consists of distilling the extracting solvent from a boiler flash, condensing it and passing It continuously through the solution being extracted.

1.            b.

Figure 1.8 – Device for continuous extraction²⁶ (a) is used where the extraction solvent is more dense than the aqueous phase, and (b, where the extraction solvent is less dense.

1.11.3             Discontinuous Counter Current Extraction

This method separates more than one compound from preparation that was thought to be pure by all other techniques²⁷. It has fractionated and given clear-cut evidence of purity for the individual fractions from many substances, for which no clear-cut evidence has been obtained by any other techniques²⁷. The method was devised by Craig and it enables substances with similar distribution ratio to be separated. This extraction is very efficient because fresh extractant are brought in contact with solute-depleted aqueous phase and the solute-enriched extractants contacted with the fresh aqueous phase till the equilibrium state is attained by the system. It is used for fractionation purposes and it involves the use of a series of seperatory funnels or more elaborate contacting vessels to achieve many individual extractions rapidly and in sequence.

The apparatus are fully automatic and it comprises of many extraction tubes mounted on a shaking racks whose axis is attached to an automatic control unit. The control unit automatically affects the operations of shaking, settling and decantation. After each shaking operation, the rack is tilted through 90ᵒ and the upper phase decanted into the next extraction tube²². The cycle ts repeated up to 50 transfers. Discontinuous countercurrent method has been applied with great success to the fractionation of organic compounds, particularly where the distribution ratio, are of the same order of magnitude, but not in inorganic compounds where there are wide range in distribution ratio. Therefore batch and continuous extraction techniques are better employed.

1.10           Classification of Inorganic Extraction Systems

Metal extraction system can be classified based on the nature of extractible species. Also inorganic extraction is possible only if the metal-aquo complexes are converted into uncharged species, metal chelate, or ion-association complexes. Usually the complex are formed in the aqueous phase prior to extraction, many a times, the complex is dissolved in the organic phase, so that comlpexation and extraction take place simultaneously when the two phases are shaken together. Therefore, inorganic extraction systems are classified into three; metal chelates, ion association complexes and additive complexes.

1.10.1         Metal Chelate

For a reagent to form an uncharged chelate with a metal, the reagent must behave as a weak acid whose anion is to participate in the complex formation. In addition, it must contain hydrophobic groups, which reduce the aqueous solubility of metal chelate formed. A metal ion Mⁿ⁺ that is equilibrated with an organic and aqueous phase is first solvated in the aqueous phase by water molecule to form metal-aquo complex. The organic anion from the dissociated organic reagent HL, displaces the water molecule in the aquo-water complex forming neutral metal chelate ML_n²⁵. The metal chelate distributes itself between the aqueous and organic phase according to Nernst distribution law. For the fact that charge neutrality reduces electrostatic interaction between the solute and water; and the presence of hydrophobic groups in the metal chelate reduces its aqueous solubility, the overall solubility of the metal chelate in the organic phase is enhanced. As in the equation below:

M(H₂O)_xⁿ⁺  +  nL^–   ⇌    ML_n  +  xH₂O …………….(27)

Some examples of metal-chelate solvent extraction systems are the inner complexes formed by acetyl acetone, dithizone, 8-hydroxyquinoline and dimetyl glyoxime.

Organic reagents with one anionic group like: -OH, -SH and one neutral basic group like = N and = O are suitable chelating ligand³⁸. They can easily replace coordinated water molecule from metal ions and provide more than one point of coordination to the metal ion forming chelate compounds which are essentially neutral and covalently bonded.

1.10.1.1 Extraction Equilibrium in Chelate Extraction System

            The metal chelate extraction process consists of four equilibrium steps. Using an equilibrium that develops when an aqueous solution of Zn(II), is extracted with an organic solution containing excess of 8-hydroxyquinoline, as an example gives:

1. Distribution of the 8-hydroxyquinoline HQ, between the organic and aqueous layers.
2. Acid dissociation of HQ, to give H⁺ and Q^– ions in the aqueous layer
3. Equilibrium is the complex-formation reaction giving MQ₂.
4. Distribution of the metal chelate between the two phases. The equilibrium ensures that MQ₂ is not precipitated out of aqueous solution²⁹, as stated in these four steps of reaction given as:

M (H₂O)_2(aq) + 2HQ_(org)                       MQ_2(aq) + 2H₂O + 2H⁺_(aq) ………….(28)

This will give extraction constant of    …….(29)

Also similar chelating agent are described using several extractive separation with 8-hydroxyqiunoline²⁷. Generally, there are four equilibrium steps in the extraction of metal complex, ML that can dissociate in aqueous phase, using a chelating agent, HL can be formulated based on the four equilibrium steps below.

1. The chelating agent HL distributes between the aqueous and the organic phase²²

(HL)_org            ⇌                  (HL)_aq ——————–(30)

  ——————–  (31)

Where K_D,HL is the distribution constant of the ligand, HL.

2. The chelating agent dissociates in the aqueous phases;

(HL)_aq                 ⇌              ……………………..(32)

………………….(33)

K_a is the dissociation constant of the ligand. The metal-aquo complex reacts with the reagent anion to form an uncharged molecule; were K_F is the formation constant of the metal chelate.

………..(34)

……………………………….(35)

Then the metal chelate is distributed between the organic and aqueous phases.

(ML_n)_aq        ⇌         (ML_n)_org      …………………..(36)

          ………………………….(37)

Where K_D,MLn is the distribution constant of the metal chelate. The analytical concentration of this metal in the aqueous phase is the sum of the equilibrium

[M]_aq = [ML_n]_aq + [Mⁿ⁺]_aq      …………………(38)

If we assume that the chelate is essentially undissociated in the non-polar

                               ……………………..(39)

Furthermore, assuming that the metal chelate distributes largely into the organic phase,

[Mⁿ⁺]_aq » [ML_n]

Equation (39) becomes;           …………………….(40)

From equation (37), [ML_n]_org = K_D,MLn [ML_n]_aq     ………………(41)

From equation (8),                   ………………….(42)

From equation, (11)                ……………………(43)

From equation (31),           …………………….(44)

Substituting for [ML_n]_org and [Mⁿ⁺]_aq in equation (13);

…………….(45)

i.e.,          ………………….(46)

Substituting for [L^–] in equation (89),      ……….(47)

Substituting for [HL]_aq in equation (90),   ………….(48)

Since,               ……………………..(48i)

                  …………………………..(48ii)

The logarithm of the equation (48ii) gives.

Hence, the distribution ratio depends on the extraction constant together with the concentration of the chelating agents, HL, and pH of the solution. The distribution ratio is an experimental parameter and its value does not necessarily imply that distribution equilibrium between the phases has been achieved.

Furthermore, the experimental value of the distribution ratio can be altered by varying the solution conditions. For example, consider the extraction of benzoic acid, HB, from water (acidified with HCl to suppress dissociation of the benzoic acid) into an organic solvent such as ether. The equilibrium is given by;

HB_aq   ⇌     HB_org      ……………(49)

……………..(50)

But if the aqueous phase is not acidified, the benzoic acid dissociates in the aqueous phase;

HB_aq       ⇌              …………(51)

 

Also, if benzene is used as the organic solvent in place of the ether, the benzoic acid is partially dimerized in benzene.

2(HB)_org   ⇌    (HB,HB)_org ………(53)

The ratio of the total concentration of benzoic acid in each phase regardless of its form is given by:

From equation (52),   ……………..(56)

From equation (54), [HB,HB]_org  =  K_d[HB]²_org  …………..(57)

Substituting equation (56) and (57) into equation (55), we obtain

………(58)

D =     ……………………(59)

Therefore, at low pH, benzoic acid will be found more in organic layer as D will be large. But at high pH, when D will be small, benzoic acid will be found in aqueous layer almost entirely as benzoate ion.

1.10.2                       Ion Association Complexes

            Ion-association complexes are uncharged species formed by the association of ions due to purely electrostatic forces of attraction. The extent of such association increases sharply as the dielectric constant of the solvent decreases below 40 to 50 ³⁰. This condition not only exists in all of the commonly used organic solvents but also in highly concentrated aqueous solutions of strong electrolytes ³¹. Generally, the complexes are formed when an anionic complex of metal ion attracts with cationic species formed by the association of ligand in the aqueous phase. The three equilibrium steps, that are involved for the formation of these complexes includes:

1. The information of anionic complex by replacement of the aquomolecules attached to the metal ion by major anions in the aqueous phase.
2. The transfer of the organic reagent from the organic phase to the acidic aqueous phase where it is protonated and becomes cationic.
3. The formation of the ion-association of the anionic metal complex with the cationic ligand. The ion-association complexes are easily attracted into the organic phase because of the weak solvation in the organic phase and the electrostatic interaction between the cations and anions³². High dielectric solvents like dichloromethane and nitrobenzene favour their formation

Additive complexes are either ion associated or chelated but in addition may have other molecules coordinated to them as ligand. The coordinated molecule may be a reagent molecule, organic solvent molecule or hydroxyl molecule.

1. Additive Complex with Organic Reagent: The metal may be incorporated into a large cation or anion or ion containing bulky organic group. This in association with an ion of opposite charge, constitutes an ion pair, which may be readily extracted by an organic solvent. For instance: copper(1) reacts with 2,9-dimethylphenanthroline to form a large univalent cation which associates with a nitrate or perchlorate anion to form a compound extractable into chloroform³³. Zinc as associates with two tribenzylammonium ions [(C₆H₅CH₂)₃NH⁺] to give an uncharged species soluble in xylene. Tetraphenylarsonium perrhenate (C₆H₅)AS⁺.ReO^–₄ and tetraphenylarsonium permanganate, (C₆H₅)AS⁺.MnO^–₄ are examples of such additive complexes.In order to illustrate the possible equilibrium in additive complexes of this type, the association of perrhenate and tetraphenylarsonium chloride are used as follows³⁴:

 

 

Where R  =  C₆H₅. However, not all additive complexes with organic reagent system behave fully in accordance with this simple expression because it usually involves many other complicated equillibria.

2. Additive Complexes with Organic Solvents: In these additives complexes, the organic solvents plays role in the extraction processes. These complexes are usually formed when hydrophobic reagents are employed as organic reagents (solvent), and or the maximum coordination number of the metal and the geometry of the ligand are favourable. For example: uranyl nitrate, UO₂(NO₃)₂, is extracted from nitric acid solution by diethyl ether the hydrated species, UO₂(NO₃)₂.6H₂O, in aqueous becomes, UO₂(NO₃)₂.2[(C₂H₅)₂O].2H₂O in the organic phase³⁴. As can be observed above, the solvent ether, replaces four water molecules, attached to the metal chelate to form a less hydrophilic additives complex, which is readily extracted by non-polar solvent. Other solvents beside diethyl ether have been used, including derivatives of phosphoric acid such as tri-n-butyl phosphate (C₄H₉O)₃P = O. Some conditions that favour these reactions are;

1. Where the maximum coordination number of the metal and the geometry of the ligands are favourable.
2. Where the solvent easily displaces the coordinated water molecule from the neutral chelate.

• Where the organic reagent is less hydrophilic and less nucleophilic than the chelating ligand.

The formation of additive complexes of this type is established by measuring the distribution ratio D₁ and D₂ of the metal using two different organic solvents at the same reagent concentration. Variation of the ratio D₁/D₂ with pH confirms that these additive complexes were formed, otherwise, it will remain constant.

3. Additive Complexes with Hydroxyl Group: The general formular of these complexes is ML_n(OH)_p. The extraction of such complexes requires high alkakine media and organic solvent with high dielectric constant³⁵. The conditions above increase the partition constant- thus enhancing the extraction processes

1.11   Factors that Influence Stability and Extractability of Metal Chelate Complexes

The quantitative description of extraction given by Kolthoff and Sandell³⁶ and the theoretical treatment of solvent extraction of metal chelates developed by some workers^37,38 are reviewed by briefly discussing some important points below:

1.11.1. Effect of Acidity (pH): The attainment of selectivity in metal chelate extractions is greatly dependent upon proper pH control. The distribution of chelates in a given system is a function of pH alone, provided the reagent concentration is maintained constant. Increased selectivity can be achieved in the extraction of acidic or basic organic substances by the addition of buffer salts to the aqueous phase to control the pH ³⁹. The hydrogen ion concentration of aqueous phase governs the degree of dissociation of the chelating agent and hence the amount of the anion present in the solution to chelate with metal ion (see equation 55). The value of D increases with decrease in hydrogen ion concentration (as the pH increases). This increment depends on the oxidation state of the metal ion being extracted. Consequently, a high pH will favour dissociation, chelate formation, and extraction. Also for a change of one pH unit, for example the concentration of organic reagent being constant, the value of D is increased to 100 times if a bivalent metal ion is extracted, or 1000 times for the extraction of a tetravalent metal ion ⁴⁰.

1.11.2. Effect of Organic Chelating Agent: As clearly indicated in equation (55) above, the distribution ratio and the degree of extraction increase as the concentration of the chelating agent increases at constant pH. But the concentration of chelating agent in organic phase is limited by its solubility (8-hydroquinoline is soluble to the limit of 2.6M in chloroform). Also, there is a shift to lower pH value with increasing concentration of the chelating agent and this enables the extraction to be carried out at a pH low enough to prevent hydrolysis³³. Sometimes, the chelon forms an adduct with metal chelate, ML_n(HL)_r, and an increase in the concentration of the chelating agent will have a greater effect than that predicted in equation (55), since the dependency upon concentration of the chelating agent is now in (n + r) instead of just n. Cobalt oxinate and oxime form such an adduct.

1.11.3. Effect of Masking Agent: In the extraction procedures for metal pairs that are difficult to separate, masking or sequestering agents are introduced to improve the separation factor. The masking agent forms water-soluble complexes with the metals in competition with the extracting agent. Masking agents form sufficiently strong complexes with interfering metals to prevent their reactions with the extracting agents, either altogether or at least until the pH is much higher than the value needed for quantitative extraction of the metal of interest. When a masking agent (sequestering agent) such as ammonia, cyanide, tartarate, citrate, nitrilotriacetic acid or ethylenediaminetetraacetic acid is present in the aqueous phase, it complexes with the metal ion thereby competing with the chelating agent and the value of D is lowered. In solvent extraction, a masking agent is a compound that can form a non-extractable complex with metal ion in the aqueous phase. Consequently, at constant concentration of extracting agent and pH, the presence of masking agent decreases the percentage of metal extracted. Furthermore, there is a shift of the %E-pH curve to high pH curve to higher pH value in the presence of masking agent. The magnitude of this pH shift depends on the strength of metal-masking bond and on the concentration of the masking agent.

The application of masking agent in extraction separation of metals is of extreme important. By a selective utilization of the masking agent, it is possible to extract a particular metal of interest from a mixture of metals, which form non-extractable ionic complexes and remain masked in the aqueous phase.

1.11.4. Effect of Variation of the Oxidation State of Metal: A useful method of increasing the selectivity of metal extractions involves modification of the oxidation states of the interfering ions present in solution, in order to prevent the formation of their extractable metal complexes. For example, the extraction of iron from chloride solutions can be prevented by reduction to iron-II (using hydroxylamine hydrochloride), which is not extractble. Similarly, antimony-V may be reduced to the tervalent state to suppress its extraction. Conversely, it is important in the preparation of a solution for extraction to adjust the proper valence state of metal ion required for formation of the complex in order to insure complete extraction of that element. Selectivity can also be achieved by variation of the oxidation state of the co-extracted interfering ions during the stripping operation.

1.11.5. Effect of Salting-Out Agent: The addition of high concentrations of inorganic salts to the aqueous phase greatly increases the distribution ratio of many metal complexes to the organic phase. ⁴¹This salting-out effect may be explained in part by the pronounced effect of the added salt on the activity of the distributing species, the common ion effect, as well as the strong ability of these ions to bind water around them, thereby depleting the aqueous phase of water In solvent extraction, salting-out agents are ionic salts which when dissolved in the aqueous phase increases the ionic strength of the medium and distribution ratio of a particular solute, hence causing a decrease in solubility of non-ionic metal chelate in the aqueous phase. Consequently, salting-out agent increases D of the metal during extraction. The mechanism of salting-out is as follows³⁸;

1. The ionic activities of extractable metal chelate is decreased
2. The highly hydrated ions of the salt deplete the effective concentration of free H₂O molecules, thereby making the metal chelate formation easy.
3. The dielectric constant of the aqueous phase increased, resulting in decrease in solubility of the metal chelate in aqueous phase, but increase increase in the organic phase.

1.11.6. Effect of the Stability of the Metal Chelate: Equation (77) shows that the stability constant of the metal chelate is directly proportional to the distribution ratio²⁵. Thus, the larger the value of the stability constant the greater the percent of the metal extracted at a constant pH, other factors remaining unchanged. Conversely, the more stable the complex the lower the pH of extraction.

1.11.7. Influence of Organic Solvent: In solvent extraction, choice of organic solvent highly depends on the solubility of the organic reagent and the degree of immiscibility with the aqueous phase²³. In addition, the organic solvent should possess the following properties:

1. It should have high density difference with the aqueous phase in order not to for, emulsion.
2. It should have very low viscosity to permit good contact between the two phases while shaking.
3. The metal chelate should be readily soluble.
4. The organic solvent should have low toxicity and inflammability.
5. It should have such a property that will make for easy stripping-off the metal.
6. It sould have high boiling point for easy recovery after extraction.
   • The Features of Ligand which Affects Chelate Formation
7. The basic strength of the chelating group: the stability of the chelate complexes formed by a given metal ion generally, increases with increasing basic strength of the ligand, as measured by the pKa values²⁴.

• The nature of the donor atoms in the chelating agent: ligand which contain donor atoms of the soft base type, form their most stable complexes with the relatively small group of class B metal ion (soft acids) and are thus more elective reagents. This can be demonstrated by dithizone used for solvent extraction of metal ions like Cu²⁺, Zn²⁺, Pb²⁺, e. t. c.
• Ring size: Five- or six-membered conjugated chelate rings are most stable, since these have minimum strain. The functional groups of the ligand must be so situated that they permit the formation of stable ring.
• Resonance and steric effects: the stability of chelate structures is enhanced by contributions of resonance structures of chelate ring-thus copper acetylacetonate has greater stability than copper chelate of salicyladoxime. A good example of steric hinderance is given by 2,9-dimethylphenathroline, which does not form a complex with iron (II) as does the unsubstituted pheanthroline; this hinderance is minimum in the tetrahedral grouping of the reagent molecule, about a univalent tetracoordinated ion such as that of copper (I). AA nearly specific reagent for copper is thus available.

 

1.13     Applications of Solvent Extraction

Solvent extraction of heavy metals is widely applied in many fields ranging from the environmental to the biomedical discipline. In the environmental field, some of the more prominent applications include: removal and recovery of heavy metals and dyes from wastewater; ranging from aqueous solutions in hydrometallurgical treatment to environmental applications. It is also considered a useful technique to increase the initial concentration of the solute, commonly used in the separation processes of analytical applications³⁵.

In the biomedical field, liquid extraction has been used in the determination of heavy metals in human waste (e.g urine), also supported liquid membrane methodology was used for trace analytes determination by facilitating chromatogram differentiation between samples, water and blood plasma⁴². It is also used to enrich human wastes (e.g. urine) with heavy metals prior to concentration determination using atomic absorption spectroscopy (AAS)^42,43,44 .

It can be a useful tool for purifying DNA from a sample while simultaneously protecting it from nucleases. It is used by the food industry to isolate or eliminate particular flavours. This is an important parameter in drug discovery and development. It is crucial concept in understanding how cells are able to pass some molecules through their membranes while and yet reject others.

1.14    Drawbacks on Solvent Extraction

1. It is laborious.
2. It is difficult to automate and connect on-line to analytical instruments
3. Large amounts of organic solvents is avoided for environmental and health reasons.

1.15    Scope of Study

The scope of this work entails:

1. Synthesis and characterization of 1,5-dimetyl-2-phenyl-4-[(E)-2,3,4-trihydroxylphenyl) diazenyl]-1,2-dihydro-3H-pyrazol-3-one,(H₃L), and its Zn(II) and Cd(II) complexes , based on UV spectra, IR-spectra, conductometric test, and mole ratio.
2. Using the ligand to extract both Zn(II) and Cd(II) complexes in aqueous solution at varying extraction conditions, such as pH, acidity effect, salting-out effect and masking agent effect
3. Using the ligand to separate the two metals in aqueous solution at a particular extraction condition.
4. Using the ligand to extract the two metals from a real industrial material.

1.16           Aims and Objectives

The research aims and objectives entails- synthesis of 1,5-dimetyl-2-phenyl-4-[(E)-2,3,4-trihydroxylphenyl) diazenyl]-1,2-dihydro-3H-pyrazol-3-one,(H₃L) and its zinc(II) and cadmium(II) complexes; characterization of both the ligand and the two complexes based on UV spectra, IR spectra, solubility test, conductivity test, melting point, texture and mole ratio. The ligand should be used to extract zinc(II) and cadmium(II) complexes from aqueous solution at varying extraction conditions like pH, acidity effect, salting-out effect and masking effect; determining the optimum extraction conditions for their extraction from aqueous phase solution with the ligand and using results obtained from pH to determine the extraction constant of the two metal ions. The ligand should be to used to separate the two metal ions in aqueous solution at a particular extraction conditions; obtaining the optimum extraction condition for their separation from a particular condition. The ligand should   be used to extract the two metal ions from a real synthetic material and the optimum extraction of the metal complexes from the real material should be determined by applying the conditions that favour their maximum yield.

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