Spectrophotometric Essay

The basic principle of the spectrophotometeric technique is the measurement of interaction between energy and electrons of the substance. Spectrophotometric technique is an analytical method used for estimating concentration of metal ion in liquid solution. One of the most magnificent effects of complex formation is the change of spectral properties. The reason for light absorption by complexes are as follows.
a) The excitation of electrons of both metal ion and ligand is influenced by their interactions.
b) The electrons of transition metal ions are easily excited and consequently absorbed in the visible region i.e. these ions give colored compounds.
c) The electron systems of non-transition metal ions and of the ligands are more stable,

If more than one complex is formed at different pH values, their existence can be decided by this type of study. The pH, at which the absorption due to a particular complex species is far greater than that of metal ion and or the ligand alone, is selected for the study of that species. As the complex formation is the function of pH; it should be kept constant for particular system. Similarly, ionic strength is maintained constant throughout by adding an appropriate volume of sodium perchlorate. pH can be remained constant by using suitable buffer, provided the buffer does not interfere with the complex formation at wavelength where complex species show maximum

The absorbance of these mixtures is measured at a suitable wavelength. If 'x' mole/litre are added to (1-x) mole/litre of M and if C1, C2 and C3 are equilibrium concentrations in mole/litre of M, L and MLn respectively then,
C_1= (1-x) - C_3............(1)
C_2= (X-nC_3) ............ (2)
C_3= C_1 C_2.............(3) where 'n' number of ligands attached to a metal ion which can be easily determined from the position of the maximum or minimum in the graph. If the metal ion and the ligand are colorless, but the complex species is colored, then there will be maximum at the composition of the complex in the plots of absorbance versus composition.
Job's method is ordinarily applicable to system in which only one complex is present. Robert Gould and Vosburgh [12] have applied above method to system where second equilibrium exists and showed that it also obey's Beer'slaw.
Calculation of Stability Constants:
The conditional stability constants of a complex can be determined from Job's curve by following two methods.
From Extinction Coefficient