Friday, August 23, 2019

How we determined Fe +3 ion colourimetrically ?


How we determined Fe +3 ion colourimetrically?

In colourimetric method Beer’s law is followed . According to this law intensity of colour is the absorbance or optical density of a coloured solution is directly proportional to its molar concentration.

 In colourimetric estimation the metal ion must produce a coloured complex or compound in solution.
Therefore a specific ligand is necessary for a given metal ion to produce such a coloured complex. 


Fe +3 ion in aqueous solution is not very strongly coloured , but when thiocyanate ion is added to the Fe +3 solution an intense blood red colour is produced due to formation of iron-thiocyanate complex.

Thiocyanate ( NCS -1) ion is very specific towards Fe +3 ion but it does not produce any coloured complex in Fe +2 . Thus it can be said that Fe +3 ion can be determined by colourumetric estimation .


What is the increasing order of energy of the CT bands of halides of a metal ?

Charge transfer spectra ( C .T spectra ) is obtained when electron transition takes place between two different atomic centres. 

In case of metal halides  charge transfer takes place when the electron on the halide ion is taken up by the metal ion and then electron transfer occurs from metal to halogen atom. 
This is shown below for a given metal ion of oxidation number +n’ .

The electronic configuration of halide ions can be represented in the general form ns2 np6. Where n is the principal quantum number of the valence shell. 

The energy of the outer most electron of the halides gradually increases as the value of ‘n’ increases. Since during charge transfer the electron from the halide ion is to be removed completely . 

The necessary energy for this, gradually decreases from F to I -.  Again for a given metal ion the energy for charge transfer bands is independent of the metal ion but mainly depends on the halides ions. 

So the sequence of the energy of C.T bands in of halide ions in increasing order is ,


Prcatice problem :
How we determined Fe +3 ion colourimetrically  ?
What is the increasing order of energy of the CT bands of halides of a metal ?


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