SPECTROPHOTOMETRIC DETERMINATION OF VANADIUM(III) AND VANADIUM(V) USING 2-[(E)-[3-[(E)-(2-HYDROXYPHENYL)METHYLENEAMINO] PHENYL]IMINOMETHYL]PHENOL

ABSTRACT

2-[(E)-[3-[(E)-(2-hydroxyphenyl)methyleneamino]phenyl] iminomethyl]pheno 1   was synthesized from the condensation reaction of 1,3-diaminobenzene and 2-hydroxyzaldehyde  m dimethylformaldehyde  (DMF).  Its  coordination  characteristic  with  vanadium(III)  and vanadium(V)    complexes    was   studied    via,   UV/Visible,    IR   and   NMR    spectroscopy; stoichiometric,  melting  point  and  conductivity  determinations.  The  analytical  data  of these complexes  and the mode of bonding  show that, the ligand acted as a tetradentate  ligand via coordination through the two azomethine nitrogen and the two hydroxylic oxygen. Mole ratio method indicated a 1: 1  ligand to metal ratio for the complexes. Vanadium(III) and vanadium(V) were  determined  spectrophotometrically  by  measuring  their  absorbance  at  400  and 405  nm respectively.   From   the   calibration   curve,   Beer’s   law  was   valid   for  vanadium(III)   and vanadium(V) between 0.488 -3.904 ppm. The calibration and analytical sensitivity of vanadium(III)   complex  is  0.074  and  0.32  while  that  of   vanadium(V)   is  0.024  and  24 respectively.  Optimum pH for the formation of the complex was determined to be 10 and 11  for vanadium(III)  and vanadium(V) respectively.   Very few elements were found to interfere with the method. The method was successfully applied in the determination of vanadium in steel.

CHAPTER ONE

INTRODUCTION

1.1   Spectrophotometry

In   Chemistry,   Spectrophotometry   is   the   quantitative    measurement    of  the   reflection   or transmission  properties  of a material  as a function of wavelength.  Spectrophotometry deals with visible  light, near-ultraviolet,  and near-infrared,  but  does not cover time-resolved  spectroscopic techniques’.

Spectrophotometry involves the use of a spectrophotometer.  A spectrophotometer is a device for measuring  light  intensity  which  is  a  function  of the  light  wavelength.  Important   features  of spectrophotometers  are spectral  bandwidth  and  linear range  of absorption  or reflectance measurement’.

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 on any of the listed light ranges that usually  cover around

200nm  –   2500nm  using  different   controls  and  calibrations   ‘.  Within  these  ranges  of  light, calibrations  are  needed  on  the  machine  using  standards  that  vary  in  type  depending  on  the wavelength  of the photometric  determination.

An example  of an experiment  in which spectrophotometry  is used  is in the determination  of the equilibrium  constant  of a  solution.  For  instance,  a  certain  chemical  reaction  may  occur  in  a forward  and  reverse  direction  where  reactants   form  products   and  products  break  down  into reactants.  At  some  point,  this  chemical   reaction   will  reach   a  point   of  balance   called   an equilibrium  point.  In order to determine  the respective  concentrations  of reactants  and products at this point, the light transmittance  of the solution  can be tested  using spectrophotometry. The amount  of light  that  passes  through  the  solution  is  indicative  of the  concentration   of certain chemicals that do not allow light to pass through  ‘.

The use of spectrophotometers  spans various scientific  fields,  such as physics,  materials  science, chemistry,  biochemistry,   and  molecular   biology.They  are  widely   used   in  many   industries including  semiconductors,  laser and  optical  manufacturing,  printing  and  forensic  examination, and as well in laboratories for the study of chemical substances.  Ultimately,  a spectrophotometer

is able to determine,  depending on the control or calibration,  what substances are present  in a target and exactly how much through calculations of observed wavelengths.

There  are  two  major  classes  of devices:  single  beam  and  double  beam.  A  double  beam spectrophotometer  compares the light intensity between two light paths, one path containing a reference sample and the other the test sample. A single beam spectrophotometer  measures the relative  light  intensity  of the  beam  before  and  after  a  test  sample  is  inserted.  Although comparison  measurements  from double  beam  instruments  are easier  and more  stable,  single beam instruments can have a larger dynamic range and are optically simpler and more compact. Additionally, some specialized instruments, such as spectrophotometer built onto microscopes or telescopes, are single beam instruments due to practicality.

Historically,  spectrophotometers   use  a  monochromator   containing  a  diffraction  grating  to produce the analytical spectrum. The grating can either be movable or fixed. If a single detector, such as a photomultiplier  tube or photodiode is used, the grating can be scanned stepwise so that the detector can measure the light intensity at each wavelength  (which will correspond to each “step”).  Arrays of detectors,  such as charge coupled devices (CCD) or photodiode arrays (PDA) can also be used.  In such systems,  the grating is fixed and the intensity of each wavelength of light is measured by a different  detector in the array. Additionally, most modem mid-infrared spectrophotometers  use a Fourier transform technique  to acquire the spectral information. The technique is called Fourier Transform Infrared.

When making transmission  measurements, the spectrophotometer  quantitatively  compares  the fraction of light that passes through  a reference  solution  and a test  solution.  For reflectance measurements,  the spectrophotometer  quantitatively compares the fraction of light that reflects from  the  reference   and  test  samples.  Light   from  the  source  lamp   is  passed  through   a monochromator,  which diffracts  the light into a “rainbow”  of wavelengths  and outputs narrow bandwidths  of this  diffracted spectrum.  Discrete  frequencies  are transmitted  through  the test sample.  Then the photon  flux density (watts per meter squared usually)  of the transmitted  or reflected light is measured with a photodiode, charge coupled device or other light sensor. The transmittance or reflectance value for each wavelength of the test sample is then compared with

the transmission (or reflectance) values from the reference sample3.

In short, the sequence of events in a modem  spectrophotometer is as follows: The light source is imaged upon the sample

A fraction of the light is transmitted  or reflected  from the sample

The light from the sample is imaged upon the entrance slit of the monochromator

The monochromator separates the wavelengths  of light and focuses each of them onto the photo detector sequentially.

Many  older  spectrophotometers  must  be  calibrated  by  a procedure  known  as  “zeroing.”  The absorbancy  of a reference  substance  is set  as a baseline  value,  so the  absorbance  of all  other substances  are recorded relative  to the  initial  “zeroed”  substance.  The  spectrophotometer then displays percent  absorbance  (the amount  of light absorbed  relative  to the initial substance).The concentration  of the  solution  can  be  determined  by  measuring  the  amount  of  light  it  absorbs

which requires a quantitative relationship.  This is provided by Beer’s-Lambert’s Law 4

1.2  Beer-Lambert’s Law

Lambert  concluded  that the power  P of the transmitted  light varies exponentially  with the path length,  band directly with the power of Po of the incidence  light. If P (or I) represents  the power (or  intensity)  of transmitted   light  and  P,  (or  I)  represents  the  power  incident  light,  then  the change in P is proportional  to the power of incident  light multiplied  by the change in thickness  b of the material through which the light passes.

Mathematically, dP = KPdb

K  is proportionality   constant,  and the  negative  sign  indicates  that  P becomes  smaller  when  b becomes  larger. Rearranging  and integrating the above equation 5  .

dP  = –Kdb p

And

or

p

In       =Kb

P,4

or

p      K

Log        =b

P,            2.303

Beer modified the law to apply to solution. He found that doubling the concentration  of light absorbing  molecules  in  a solution  produced  the  same effect  as doubling  the  thickness.  The modified form of the above law is     logP/P,=  cbc

In this expression;

C is the concentration of the solution and is expressed in moles per liter   .

&   is molar absorptivity (molar extinction coefficient) and b is the cell width expressed in centimeters,

thus log(P/P,,) is directly proportional to concentration of solution.

If log (P/P,) is plotted against concentration for a solution which obeys the Beer’s -Lambert law, a straight line results whose slope is – cb.

P/P is called the transmittance of the solution.

Beer-Lambert  law is a combination  of two absorption laws and tells us quantitatively  how the amount of transmitted power depends on the concentration of the absorbing molecules and the path length over which absorption occurs  .

Beer-Lambert  law is well obeyed with dilute solution, where there is a linear relationship. In the plot of absorption or transmittance  (i.e A or log T) at the wavelength of maximum absorption, max,  versus concentration  for a series of standard solution.  The concentration range in which the  Beer-Lambert   law  is  obeyed  is known  as  linear  dynamic  range,  and  only  quantitative determination done within it can be accurate and reliable.

Beer-Lambert  law  as  expressed  in  the  equation  above  can be used  in  several  ways.  Molar absorptivities of species can be calculated,  if the concentration is known.  The measured value of absorbance can be used to obtain concentration if absorption and path length are known. The law also applies to solution containing more than one kind of absorbing  substance.  Provided,  that there  is no interaction among the various  species,  the total absorbance  substance.   The total absorbance  for  multi-component   system  at  a  single  wavelength   is  the  sum  of individual absorbancies.

1.3    Ultra Violet (UV) Spectrophotometry

The most common spectrophotometers  are used in the UV and Visible regions of the spectrum. Light of wavelength between 400nm and 750 nm is visible and the instrument used to measure it is ultraviolet spectrometer and it absorbs light in the visible and near ultraviolet region, that is in the 200-750 nm range. This light is of higher frequency with respect to the nearby protons.

In Ultra Violet spectrophotometer, the samples  are usually prepared  in cuvettes; and fill up to mark, the chosen wavelength  is  set and the maximum absorbance  is taken once the cuvette is placed inside the UV machine 6 .

1.4  Infrared (IR) Spectrophotometry

Spectrophotometers  designed  for the  main  infrared region  are quite  different  because  of the technical  requirements   of measurement   in  that  region.  One  major   factor  is  the  type  of photosensors  that are available for different  spectral regions, but infrared measurement  is also challenging because virtually everything emits IR light as thermal radiation 7   .

A molecule is constantly vibrating,  that is,  its bonds stretch and bend with respect to each other,

changes in vibrations of a molecule are caused by absorption infra red light.

The infrared spectrum helps to reveal the structure of a new compound by telling us what groups are present in or absent from the molecule   .   IR is a highly characteristic property of an organic compound/  element because a particular  group of atoms give rise to characteristic  absorption bands.

1.5  Nuclear Magnetic Resonance (NMR)

NMR is a research technique that exploits the magnetic properties  of certain atomic nuclei to determine  physical  and  chemical  properties  of atoms  or  the  molecules  in  which  they  are contained. It relies on the phenomenon ofNMR and can provide detailed information about the structure, dynamics reaction state and chemical environment of molecules ‘.

‘H,  ‘N, ‘C  and   P are highly abundant isotopes whilst ‘N and   ‘C are present at only low level <1 %.  In simply terms,  when the sample is placed in the magnet the nuclei of the atoms align with the magnetic field.  Typically the magnets used in NMR spectroscopy are very strong

with pulses of energy in the radio frequency (RF) range,  typically 40-800 MH, to the sample. The pluses cause Nuclei to rotate away from their equilibrium position and they start to precise

(rotate) around the axis of the magnetic field.   The exact frequency at which the nuclei precise is related to both the chemical and physical  environment  of the atom in the molecule.  This results in a spectrum showing many absorption   peaks, whose relative position, reflect different environments  of  portions  which  can  give unbelievable  detailed    information about  molecular structure.

The various aspects of the NMR spectrum are 7

The number of signals which tell us how many different kinds ofprotons that are in a molecule. The position  of the signals which tells us about the electronic  environments  of each kind  of proton.

The intensities of the signals which tells us how many protons of each kind.

The splitting of a signal into several peaks, which tells us about the environment of the protons. NMR  can also be used to  look  at dynamic processes.  These  include  internal  motions within regions oflarger molecules such as loops in a protein or the base pair in DNA or RNA.

1.6 Schiff Base

Schiff  base  is  a  term  used  to  describe  the  product   formed  when  an  amme  undergoes  a condensation reaction with a carbonyl compound  or it  is said to be the nitrogen analog of an aldehyde or ketone in which the C = 0 group is replaced by a C = N-R group “.  A German Chemist named Hugo Schiff discovered these bases. He discovered the Schiff bases and other imines,  and was responsible for research into aldehydes,  the field of amino acids and the Biuret reagent. He also had the Schiff test named after him’.  Schiff bases are synonymous with imines, and even Azomethines ” An  imine  is a functional group  or chemical compound  containing  a carbon- nitrogen  double bond”. Imines can be classified further as Aldimines and Ketimines and this mainly depends on the type of carbonyl  compound  involved  in the reaction.  Imines  derived  from aldehydes  are called aldimines [Scheme l] while those from ketones arc called ketimines

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