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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.


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 nonreactive
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
1. A high distribution ratio for the solute and a low distribution ratio for undesirable
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
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) , 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



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