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