SOLVENT EXTRACTION STUDIES ON Zn(II) AND Cd(II) COMPLEXES OF 1,5-DIMETHYL-2-PHENYL-4[(E)-(2,3,4- TRIHYDROXYPHENYL)]DIAZENYL-1,2-DIHYDROXYL-3H- PYRAZOL-3-ONE.

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.

CHAPTER ONE

1.0  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) state5,6. 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 processes1,6. 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 nA moles of the one are mixed with nB 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, ΔM HAB, 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:

ΔMGAB = ΔM HAB -TΔM SAB = RT [xA ln xA + xB ln xB ] ……… (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

. µ ′′  =   µ∞  +         ln     ′′  =   µB

=   µ∞B′ +  RT ln XB′

… . (2)

(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

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