ABSTRACT
Â
TABLE OF CONTENTS
Title page – – – – – – – – – – i
Approval – – – – – – – – – – ii
Declaration – – – – – – – – – – iii
Dedication – – – – – – – – – – iv
Acknowledgements – – – – – – – – – v
Abstract – – – – – – – – – – vi
Table of contents – – – – – – – – – vii
List of Tables – – – – – – – – – – xi
List of Figures- – – – – – – – – – xii
List of Schemes – – – – – – – – – xiii
CHAPTER ONE
1.0 INTRODUCTION – – – – – – – – 1
1.1 Spectrophotometry – – – – – – – – 1
1.1.1 Beer- lambert’s law – – – – – – – – 2
1.2 Schiff Base Ligands – – – – – – – – 4
1.2.1 Preparation of Schiff bases – – – – – – – 4
1.2.2 Uses of Schiff Bases – – – – – – – – 6
1.2.3 Biological Importance of Schiff Bases – – – – – 7
1.2.4 Schiff Base Metal Complexes- – – – – – – 8
1.3 Chromium – – – – – – – – – 9
1.3.1 Determination of Chromium – – – – – – – 9
1.3.2 Uses – – – – – – – – – – 10
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1.4 Statement of the Problem – – – – – – – 11
1.5 Aims and Objectives – – – – – – – 12
CHAPTER TWO
2.0 LITERATURE REVIEW- – – – – – – – 14
2.1 Catalytic Spectrophotometric Determination of Chromium – – – 14
2.2 Spectrophotometric Determination Of Trace Level Chromium Using Bis
(Salicylaldehyde) OrthophenyleneDiamine In Non-ionic Micellar Media – 14
2.3 Spectrophotometric Determination of Chromium(III) and chromium(VI)
in sea water.- – – – – – – – – – 15
2.4 Determination of Hexavalent Chromium in drinking water by ion chromatography
with post-column derivatization and UV-visible spectroscopic detection. – 15
2.5 Determination of Cr(VI) in environmental sample evaluating Cr(VI)
impact in a contaminated area. – – – – – – – 16
2.6 Indirect Extraction – Spectrophotometric Determination of chromium.- – 17
2.7 Sensitivity Determination of Hexavalent chromium in drinking water – – 18
2.8 Determination of Dissolved Hexavalent Chromium in Drinking Water, Ground Water
and Industrial Waste Water Effluents by Ion Chromatography- – – – 18
CHAPTER THREE
3.0 Experimental – – – – – – – – – 19
3.1 Apparatus – – – – – – – – – 19
3.2 Preparation of Stock Solution – – – – – – – 19
3.3 Preparation of Buffer Solutions – – – – – – – 20
3.4 Synthesis of the Ligand (HBAPP) – – – – – – 20
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3.5 Synthesis of Chromium (III) and Chromium (VI) Complexes of HBAPP – 21
3.5.1 Determination of the Stoichiometry of the Complexes by Slope-Ratio Method. 22
3.6 General Procedure for the Complexation Studies – – – – 23
3.6.1 Effect of Time on the Formation of the Complexes – – – – 23
3.6.2 Effect of Temperature on the Formation of the Complexes – – – 23
3.6.3 Effect of Concentration of Reagent on the Formation of the Complexes – 23
3.6.4 Effect of pH on the Formation of the Complexes – – – – 23
3.6.5 Effect of Interfering Ions on the Formation of the Complexes – – – 23
3.6.6 Calibration Curve-Beer’s Law – – – – – – – 24
3.7 Determination of Chromium in Alloy – – – – – – 24
3.7.1 Determination of Chromium in Alloy with Flame Atomic Absorption
Spectrophotometry – – – – – – – – 24
3.7.2 Determination of Chromium in Alloys with UV Spectrophotometry – – 24
CHAPTER FOUR
4.0 Results And Discussion – – – – – – – 26
4.1 Physical Characterization and Molar Conductivity Data of the Ligands and Its
Cr(III) and Cr(VI) Complexes – – – – – – – 26
4.2 Spectroscopic Characterization Of The Ligand And Its Cr(III) And Cr(VI)
Complexes. – – – – – – – – – 26
4.2.1 Electronic Spectral Data of the Ligand and Its Complexes – – 26
4.2.2 Infrared Spectra – – – – – – – – 27
4.2.3 1H and 13C NMR Spectra of the Ligand – – – – – 28
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4.2.4 13C NMR – – – – – – – – – 29
4.2.5 APT (Attached Proton Test) – – – – – – – 29
4.3 Stiochiomery of the Complexes – – – – – – 30
4.3.1 Metal-Ligand Mole Ratio of Cr(III) Complex – – – – 30
4.3.2 Metal-Ligand Mole Ratio of Cr(VI) Complex – – – – 31
4.3.3 Molecular Formulae and Structures of the Ligand and Its Complexes – 33
4.4 Complexation Studies – – – – – – – – 35
4.4.1 Effect of Time on the formation of the Complexes – – – – 35
4.4.2 Effect of the concentration of the reagent on the formation of the complexes- 36
4.4.3 Effect of temperature on the formation of the complexes – – – 38
4.4.4 Effect of pH on the absorbance of the complexes – – – – 41
4.4.5 Effect of interfering ions on the formation of Cr(III) and Cr(VI) complexes – 42
4.5 Calibration curve for determination of Cr(III) and Cr(VI) complexes – 44
4.5.1 Cr(III) complex – – – – – – – – – 44
4.5.2 Cr(VI) complex- – – – – – – – – 45
4.6 Application using steel solution- – – – – – – 46
4.6.1 Determination of Cr(III) in the steel solution – – – – – 47
4.6.2 Determination of Cr(VI) in steel solution – – – – – 47
4.7 Conclusion – – – – – – – – – 47
4.8 Recommendation – – – – – – – – – 48
References – – – – – – – – – – 49
Appendix A – – – – – – – – – – 55
Appendix B – – – – – – – – – – 58
CHAPTER ONE
INTRODUCTION
1.1 SPECTROPHOTOMETRY
Spectrophotometry is the quantitative measurement of the reflection or transmission
properties of a material as a function of wavelength1. It is more specific than the general term
electromagnetic spectroscopy in that spectrophotometry deals with visible light, near-ultraviolet,
and near-infrared, but does not cover time-resolved spectroscopic techniques. Spectrophotometry
is a very fast and convenient method of qualitative analysis, due to the fact that absorption occurs
in less than one second and can be measured very rapidly. Molecular absorption is valuable for
identifying functional groups in a molecule and for the quantitative determination of compounds
containing absorbing groups2,3. A spectrophotometer is commonly used for the measurement of
transmittance or reflectance of solutions, transparent or opaque solids, such as polished glass or
gases. However, they can also be designed to measure the diffusivity of any of the listed light
ranges that usually cover around 200 – 250 nm using different controls and calibrations1 .
The most common spectrophotometers are used in the UV and visible regions of the
spectrum and some of these instruments also operate into the near-infrared region as well.
Visible region (400 – 700 nm) spectrophotometry is used extensively in colorimetry science. Ink
manufacturers, printing companies, textile, vendors and many more, need the data provided
through colorimetry. They take readings in the region of every 5 – 20 nanometers along the
visible region and produce a spectral reflectance curve or a data stream for alternative
presentations.
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Spectrophotometeric method is undoubtedly the most accurate method for determining,
among other things, the concentration of substances in solution, but the instruments are of
necessity more expensive. A spectrophotometer may be regarded as a refined filter photoelectric
photometer which permits the use of continuously variable and more nearly monochromatic
bands of light. The essential parts of a spectrophotometer are (1) a source of radiant energy (2) a
monochromator i.e. a device for isolating monochromatic light or, more accurately, narrow
bands of radiant energy from the light source (3) glass or silica cells for the solvent and for the
solution under test and (4) a device to receive or measure the beams of radiant energy passing
through the solvent4.
Infrared (IR)5 light is electromagnetic radiation with longer wavelengths than those of
visible light, extending from the nominal red edge of the visible spectrum at 700 nm to 1mm.
Infrared spectroscopy is very useful for obtaining qualitative information about molecules. For
absorption in infrared region to occur, there must be a change in the dipole moment (polarity) of
the molecule. Absorbing groups in the infrared region absorb within a certain wavelength region,
and the exact wavelength will be influenced by neighbouring groups. Their absorption peaks are
much sharper than the ultraviolet or visible regions and easier to identify. The most important
use of infrared spectroscopy is in identification and structure analysis; it is useful for qualitative
analysis of complex mixtures of similar compounds because some absorption peaks for each
compound will occur at a definite and selective wavelength, with intensities proportional to the
concentration of absorbing species.
Nuclear magnetic resonance spectroscopy5 is a research technique that exploits the
magnetic properties of certain atomic nuclei. It measures the absorption of electromagnetic
radiation in the radiofrequency region of roughly 4 MHz to 750 MHz, nuclei of atoms rather than
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outer electrons are involved in the absorption process. It determines the physical and chemical
properties of atoms or the molecules in which they are contained. It relies on the phenomenon of
NMR and can provide detailed information about the structure, dynamics, reaction state and
chemical environment of molecules. NMR is used to investigate the environment of molecules.
NMR is used to investigate the properties of organic molecules, although it is applicable to any
kind of sample that contains nuclei possessing spin.
1.1.1 Beer- Lambert’s Law
In optics, the Beer-Lambert law, also known as Beer’s law or the Lambert- Beer’s law (named
after August Beer, Johann Heinrich Lambert and Pierre Bouguer) relates the absorption of light
to the properties of the material through which the light is travelling6.
The law states that there is a logarithmic dependence between the transmission
(transmissivity), T, of light through a substance and the product of the absorption coefficient of
the substance, the light travels through the material (the path length), l. The absorption
coefficient can, in turn, be written as a product of either a molar absorptivity (extinction
coefficient) of the absorber, £ and the molar concentration, c of absorbing species in the material,
or an absorption cross section, s and the (number) density N’ of absorbers6.
For liquids: ı = ı
ıı
= 10ıııı
Whereas in biology and physics, they are normally written
ı = ı
ıı
= ıııı = ııııı
Where IO and I are the intensities (power per unit area) of the incident light and the transmitted
light respectively. α is cross section of light absorption by a single particle and n is the density of
absorbing particles. The transmission (transmissivity) is expressed in terms of an absorbance
which for liquids, is defined as6
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ı = − logıııı
ıı
ı
Whereas, for gases, it is usually defined as
ıı = − lnıı
ıı
ı
This implies that absorbance becomes linear with the concentration according to6
ı = ııı= ıı
Historically, the Lambert law states that absorption is proportional to the light path length,
whereas the Beer law states that absorption is proportional to the concentration of absorbing
species in the material6.
The modern derivation of the Beer-Lambert law combines the two laws and correlates the
absorbance to both, the concentration as well as the thickness (path length) of the sample6
ı = ıı
ıı
= ııııı = ıııı
This implies that
ı = − lnııı
ıı
ı= ıı= ııı
And ı = − logııııı
ıı
ı= ıı
ı.ııı = ıı= ııı
The linearity of the Beer-Lambert law is limited by chemical and instrumental factors.
1.2 Schiff Base Ligands
Schiff base (imine or azomethine)7, named after Hugo Schiff 8, contains a carbonnitrogen
double bond,C=N, with the nitrogen9 connected to an aryl or alkyl but not hydrogen10.
Schiff bases are of general formula R1R2C=NR3, where R is an organic side chain. R3 is a
phenyl or alkyl group that makes the Schiff bases a stable imine. Some restrict the term to the
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secondary aldimines (azomethines where the carbon is connected to a hydrogen atom, thus with
the general formula RHC=NR1 11
Schiff base compounds were reported for the first time by Hugo Schiff in 18648. These
bases are very efficient as ligands. Many Schiff bases have a second functional group, generally
an OH, near the imine function. This proximity of the functional group permits the formation of
five or six member chelate rings when coordinated with metal ions. Schiff bases have a
diversified structure with nitrogen and oxygen donor systems being the most numerous.
However, nitrogen and sulfur donor systems and only nitrogen systems have been studied. The
presence of lone pair of electrons in sp2 hybridized orbital of nitrogen atom of the azomethine
group is of considerable chemical importance and impart excellent chelating ability especially
when used in combination with one or more donor atoms close to the azomethine group. This
chelating ability of the Schiff bases combined with the ease of preparation and flexibility in
varying the chemical environment about the C=N group makes it an interesting ligand in
coordination chemistry12.
1.2.1 Preparation of Schiff bases 13
A Schiff is the nitrogen analog of an aldehyde or ketone in which the C=O is replaced by
a C=N-R group. It is usually formed by condensation of an aldehyde or ketone with a primary
amine. Schiff base that contain aryl substituents are more stable and more readily synthesized,
while those which contain alkyl substituents are relatively unstable. Schiff bases of aliphatic
aldehydes are relatively unstable and readily polymerizable, while those of aromatic aldehyde
having effective conjugation are more stable 14-19.
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The formation of Schiff bases from aldehydes or ketones is a reversible reaction and
generally takes place under acid or base catalysis, or upon heating:
R NH2 + R1 R
O
R1 R
OH
NHR
R1 R
NH
+ H2O
Primary amine Aldehyde or ketone
carbinolamine
The formation is driven to completion by separation of the product or removal of water or
both. Many Schiff bases can be hydrolyzed back to their aldehydes or ketones and amines by
aqueous acid or base.
The mechanism of Schiff base formation is another variation on the theme of nucleophilic
addition to the carbonyl group. In this case, the nucleophile is the amine. In the first part of the
mechanism, the amine reacts with the aldehyde or ketone to give an unstable addition compound
called a carbinolamine.
The carbinolamine loses water by either acid or base-catalysed pathways. Since the
carbinolamine is an alcohol, it undergoes acid catalysed dehydration.
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R C N R
H
OH2
O
R C N R
OH
O O O
C
R
N H O
R
R
H
C
R
N H O
R
R
H
2 2
+ 2
+ 3
(acid-catalyzed dyhydration)
Typically the dehydration of the carbinolamine is the rate-determining step of Schiff base
formation and this is why the reaction is catalysed by acids20. Yet the acid concentration cannot
be too high because amines are basic compounds. If the amine is protonated and becomes nonnucleophilic,
equilibrium is pulled to the left and carbinolamine formation occurs. Therefore,
many Schiff bases syntheses are carried out at mildly acidic pH.
1.2.2 Uses of Schiff Bases
Schiff bases have wide application in food, dye, analytical chemistry, catalysis and agrochemical
industries21.
Schiff bases are widely used as pigments and dyes, catalysts, intermediates in organic
synthesis, and as polymer stabilizers22. They are also used in optical and electrochemical sensors,
as well as in various chromatographic methods, to enable detection of enhanced selectivity and
sensitivity23,24. Schiff bases possess excellent characteristics, structural similarities with natural
biological substances, relatively simple preparation procedures and the synthetic flexibility that
enables design of suitable structural properties25,26.
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Schiff bases are widely used in analytical determination, using reaction of condensation of
primary amines and carbonyl compounds in which the azomethine bond is formed
(determination of compounds with amino or carbonyl group). Schiff bases play important roles
in coordination chemistry as they easily form stable complexes with transition metal ions27,28. In
organic synthesis; Schiff base reactions are useful in making carbon-nitrogen bonds.
1.2.3 Biological Importance of Schiff Bases
Schiff bases appear to be important intermediates in a number of enzymatic reactions
involving interaction of the amino group of an enzyme, usually that of a lysine residue, with a
carbonyl group of a substrate29. Stereochemical investigation30 carried out with the aid of
molecular models showed that Schiff bases formed between methylglyoxal and the amino group
of the lysine side chains of proteins can bend back in such a way toward the N atom of peptide
group that a charge transfer can occur between these groups and the oxygen atoms of the Schiff
bases. Complexes of Co(II), Cu(II), Ni(II), Mn(II) and Cr(III) with Schiff bases derived from
2,6-diacetyl pyridine and 2-pyridine carboxaldehyde with 4-amino-2,3-dimethyl-1-phenyl-3-
pyrozolin-5-one show antibacterial and antifungal activities against Escherichia coli,
Staphylocccus bacteaureus, Klebsiella pneumonia, Mycobacterium snegmatis, Pseudomonas
aeruginosa, Enterococcus cloacae, Bacillus megaterium and Micrococcus leteus. The results
showed that the ligand had a greater effect against E. Coil than other bacteria while it has no
activity against S.aureus. Metal complexes had greater effect than the ligand against almost all
bacteria. Schiff bases derived from pyridoxal (the active form of vitamin B6) and amino acids
are considered as very important ligands from biological point of view. Schiff bases are involved
as intermediates in the processes of non-enzymatic glycosylations. These processes are normal
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during aging but they are remarkably accelerated in pathogeneses caused by stress, excess of
metal ions or diseases such as diabetes, Alzheimer’s disease and atherosclerosis. Non-enzymatic
glycosylation begins with an attack of sugar carbonyls or lipid peroxidation fragments on amino
groups of proteins, aminophospholipids and nucleic acid, causing tissue damages by numerous
oxidative rearrangements. One of the consequences is cataract of lens proteins31. Many
biologically important Schiff bases have been reported in the literature. These possess
antimicrobial, antibacterial, antifungal, anti-inflammatory, anticonvulsant, antitumor and anti
HIV activities 32-37. Another important role of Schiff base structure is in transamination38.
Transamination reactions are catalysed by a class of enzymes called transaminases.
Transaminases are found in mitochondria and cytosal of eukaryotic cells.
1.2.4 Schiff Base Metal Complexes
Transition metals are known to form Schiff base complexes. Schiff bases have often been
used as chelating ligands in the field of coordination chemistry. Their metal complexes have
been of great interest for many years. It is well known that N and S atoms play a key role in the
coordination of metals at the active sites of numerous metallobiomolecules39. Schiff base metal
complexes have been widely studied because they have industrial, antifungal, antibacterial,
anticancer, antiviral and herbicidal applications. They serve as models for biologically important
species and find applications in biomimetic catalytic reactions. Chelating ligands containing N, S
and O donor atoms show broad biological activity and are of special interest because of the
variety of ways in which they are bonded to metal ions. It is known that the existence of metal
ions bonded to biologically active compounds may enhance their activities40-41. Schiff base
metal complexes have occupied a central place in the development of coordination chemistry
xxiii
after the work of Jergensen and Werner42. Pfeiffer and his co-workers43 reported a series of
complexes derived from Schiff bases of salicylaldehyde and its substituted analogues. The
configuration of the chelate group in the four coordinate complexes may be square-planar,
tetrahedral, distorted tetrahedral or distorted trigonal pyramidal with the metal atom at the apex.
The advantages of the salicyaldiimines ligand systems is the considerable flexibility of the
synthetic procedures, which have resulted in the preparations of a wide variety of complexes
with a given metal whose properties are often dependent on the ligand structure. A number of
structural studies on the effect of the number of CH2 groups between the two azomethine
moieties in VO2+, Co2+, Ni2+, Cu2+, Zn2+ complexes of tetradentate Schiff bases derived from
salicyladehyde and a variety of diamine (1:2 ratio) have been reported44-45. It has been shown
that an increase in the methylene chain length allows adequate flexibility for the complexes to
change their structure from planar towards a distorted or pseudotetrahedral coordination
depending on the magnitude. In addition, the longer chains cause the ligand field strength to
decrease46-48. Metal complexes of this type have been prepared for the series n=2 to 10 for the
bivalent cobalt, nickel, copper, zinc and manganese. For n=2 most divalent first-row transition
metals are expected to form square-planar complexes. The v stretching frequencies fall in the
range 861-994cm-1 and the effective magnetic moments at room temperature of the complexes
are between 1.64 and 1.81 BM. The complexes with [(n=2, R1=R2=H), (R1= H, R2=CH3),
(R1=R2=CH3)] are green and their spectroscopic and magnetic properties suggest that they have
tetragonal pyramidal structures. A corresponding complex (R1 = R2 = H, n = 3) is orange-yellow
and its x-ray structure shows that it is polymeric, having a distorted octahedral geometry.
In general, Co(II) complexes have a higher tendency to assume a tetrahedral
configuration than the corresponding Ni(II) complexes. The complexes of Cr(III), Fe(III), Co(III)
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and Ni(II) ions with a Schiff base derived from 4-dimethylaminobenzaldehyde and primary
amines have been prepared and investigated using different physio-chemical techniques, such as
elemental analysis molar conductance measurements, and infrared spectra. The analytical data
showed formation of the complexes and a square planar geometry was suggested for Co(II) and
Ni(II) complexes and an octahedral structure for Cr(III) and Fe(III) complexes. Nair, et al
synthesized two Schiff bases from 5-ethyl-2,4-dihydroxyacetophenone49. Their copper, nickel,
iron and zinc complexes were screened for antibacterial activity against some clinically
important bacteria, such as Pseudomonas aeruginosa, Proteus vulgaris, Proteus mirabilis,
Klebsiella pneumoniae and Staphylococcus aureus.
The metal complexes showed differential effects on the bacterial strains investigated and
the solvent used, suggesting that the antibacterial activity is dependent on the molecular structure
of the compound, the solvent used and the bacterial strain under consideration.
1.3 Chromium
1.3.1 Determination of Chromium
Chromium is found throughout the environment in 3 major oxidation states: chromium(0),
chromium(III) and chromium(VI)50. The most stable form, chromium(III), occurs naturally in the
environment, while chromium(VI) and chromium(0) are generally produced by industrial
processes51. The trivalent and hexavalent states of chromium are the most biologically
significant. Chromium in biologic tissues is almost always trivalent and helps to maintain the
normal metabolism of glucose, protein and fat51,52. However, trivalent chromium may be harmful
if ingested in large amounts. Chromium(VI) is a strong oxidizing agent and highly toxic to
humans and animals due to its carcinogenic and mutagenic properties51. Hence, the
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determination of chromium in environmental and biologic samples is of great interest. There are
many sensitive techniques for chromium determination, such as ICP –MS53-55, ICP-AES56,57,
NAA58-60,UV-visible56,61 and AAS56,62,63.
On the other hand, the application of kinetic catalytic methods for trace analysis allows
one to achieve detection limits and sensitivity comparable with the above mentioned instrument
techniques and offers simple and low-cost equipment64. Moreover, catalytic methods for
chromium determination take a very small place among the many sensitive methods reported for
the determination of chromium. Most catalytic spectrophotometric methods for chromium
determination reported so far are based on its catalytic effect on a given redox reaction61,65. The
oxidants most frequently used are hydrogen peroxide, chlorate, bromate or ceric ions and most of
the substrates used are organic compounds: aromatic amines, phenols and their derivatives61,65-69
.
1.3.2 Uses
· Metallurgy70 : The strengthening effect of forming stable metal carbides at the grain
boundaries and the strong increase in corrosion resistance made chromium an important
alloying material for Stainless steel is formed when chromium is added to iron in sufficient
concentrations71. The relative high hardness and corrosion resistance of unalloyed chromium
makes it a good surface coating with unparalleled combined durability72.
· Dye and pigment70: lead chromate, PbCrO4 was used as a yellow pigment shortly after its
discovery. Chromium oxides are also used as a green colour in glass making and as a glaze in
ceramics73. It is also the main ingredient in IR reflecting paints, used by the armed forces to
paint vehicle, to give them the same IR reflectance as green leaves.
xxvi
· Synthetic ruby and the first laser70: Natural rubies are aluminium oxide crystals that are
colored red due to chromium(III) ions. A red-colored artificial ruby may also be achieved by
dropping chromium(III) into artificial aluminium oxide crystals, thus making chromium a
requirement for making synthetic rubies74.
· Wood preservative70: chromium(IV) salts are used for the preservation of wood. Chromate
copper arsenate is used in timber treatment to protect wood from decay fungi, wood attacking
insects, including termites and marine bores75.
· Tanning70: Chromium(III) salts, especially chrome alum and chromium(III) sulfate are used
in the tanning of leather. The chromium(III) stabilizes the leather by cross linking the collages
fibres76.
· Refractory material70: the high heat resistivity and high melting point makes chromite and
chromium(III) oxide a material for high temperature refractory applications, like blast
furnaces cement kilns, molds for the firing of bricks and as foundry sands for the casting of
metals77.
· Catalysts70: Several chromium compounds are used as catalysts for processing hydrocarbons.
For example the Philips catalysts for the production of polyethylene are mixtures of
chromium and silicon dioxide or mixtures of chromium and titanium and aluminum oxide78.
· Chromium(IV) oxide is used to manufacture magnetic tape used in high performance audio
tape and standard audio cassettes79. Chromic acid is a powerful oxidizing agent and is a useful
compound for cleaning laboratory glassware of any trace of organic compounds.
1.4 Statement Of the Problem
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Chromium(VI) is very toxic and have accumulative effects. The determination of Cr(VI)
in environmental samples plays an important role in the monitoring of environmental pollution
and the associated health hazards to both terrestrial and aquatic lives.
Different classical and instrumental techniques for the determination of Chromium are
very expensive, readily unavailable and require high cost of maintenance. Instrumental methods
like atomic absorption spectrophotometry is very sensitive and highly selective in metal
determination, but cannot give information about Cr(III) and Cr(VI) as found in their various
compounds69,70. Atomic absorption spectrophotometry does not take cognizance of complexation
studies of ions present in complexes as do the ultraviolet/visible spectrophotometry. Only UV
spectrophotometry can give information about the ions present in metals already determined by
AAS. Cr(III) and Cr(VI) can be determined spectrophotometrically by forming light absorbing
coloured– complexes with organic reagents. This method is cost-effective, rapid and its
sensitivity and selectivity can be enhanced by masking other ions present in the sample of a
given analyte. This research work is inspired by a serious need to search for more reagents and
also establish the optimum and fundamental conditions of complex formation needed for
application in the determination of metal ions.
1.5 Aims And Objectives
Spectrophotometric determination of chromium(III) and chromium(VI) ions requires the
formation of stable chelates with a light absorbing reagent that can be absorbed in the UV
/visible region of the electromagnetic spectrum. Therefore, the main aim of the present work
were to ascertain the possibility of direct determination of Cr(III) and Cr(VI) in steel with the
xxviii
Schiff base ligand derived from 1, 3 – diamino benzene and salicyaldehyde. The specific
objectives were to:
(a) synthesize a Schiff base derived from 1,3–diamino benzene and salicyaldehyde.
(b) synthesize Cr(III) and Cr(VI) complexes of the ligand
(c) characterize the ligand and the metal complexes on the basis of melting point, electronic
spectra, infrared spectra, nuclear magnetic resonance (1H and 13C) spectra.
(d) conductivity test of the ligand and the complexes
(e) propose structures for the synthesized ligand and complexes on the basis of their spectral
data, as precursors for further structural studies.
(f) determine Cr(III) and Cr(VI) by looking at the following parameters below:
i. the composition of the complexes
ii. the effect of time on the formation of the complexes
iii. the effect of the concentration of the reagent on the formation of the complexes
iv. the effect of temperature on the formation of the complexes
v. the effect of pH on the formation of the complexes
vi. the effect of some interfering ions on the formation of the complexes
vii. Calibration curve
(g) application/direct determination of Cr(III) and Cr(VI) in standard steel to ascertain the
possibility of the determination of the ions.
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