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
This material content is developed to serve as a GUIDE for students to conduct academic research
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.>
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