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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