SPECTROPHOTOMETRIC DETERMINATION OF CHROMIUM(III) AND CHROMIUM(VI) USING 2-[E)-[{3-[(2-HYDROXYBENZYLIDENE) AMINO]PHENYL}IMINO)METHYL]PHENOL

ABSTRACT

The Schiff base ligand, 2-[(E)-[{3–[(2-hydroxybenzylidene)amino]phenyl}imino)methyl]phenol was synthesized by condensing 1,3-diaminobenzene and 2-hydroxybenzaldehyde in absolute ethanol.  Its  Cr(III)  and   Cr(VI)  complexes  were  equally  synthesized.  The  ligand   was characterized via UV, IR and NMR spectroscopy, whereas the complexes were characterized based on UV and IR spectroscopy and conductivity values. Stoichiometric studies indicated 1:1 metal to ligand ratio for both complexes. Cr(III) complex absorbed at 1042.56 cm-1  υ(C-O),

532.37 cm-1 υ(Cr-N) and 607.60 cm-1 υ(Cr-O) while the Cr(VI) complex absorbed at 1182 cm-1 υ(C-O), 749.37 cm-1 υ(Cr-O) and 457 cm-1 for υ(Cr-N).  Based on UV, IR and NMR studies, the ligand  coordinated to  the metals using the  nitrogen and  oxygen atoms. Spectrophotometric determination of the metals using the ligand was done at 368 nm for Cr(III)  and 465 nm  for Cr(VI).Optimum conditions for complexation and stability were studied and it was shown that optimum pH for Cr(III) and Cr(VI) were 13.0 and 2.0 respectively. Very few ions such as Co2+, Cu2+, Mn2+, Mg2+, Fe3+  and Zn2+  interfered with the determination. Beer’s law was obeyed between 0.02 to 0.14ppm for both metals. The method was successfully applied in the analysis of steel.

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.

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

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,  and the (number) density N’ of absorbers6.

For liquids:    =      =  10

Whereas in biology and physics, they are normally written

=      =                 =              

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

=  − log

Whereas, for gases, it is usually defined as

=  − ln

This implies that absorbance becomes linear with the concentration according to6

=         =

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

=      =                  =              

This implies that

=  − ln         =       =

And     =  − log             =              =       =

.

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  carbon- nitrogen 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

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.

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:

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.

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 non- nucleophilic, 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.

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

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

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)

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

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.

      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

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

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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