ABSORPTION STUDY OF Cr(+6) AND HENCE TO FIND OUT A METHOD OF ESTIMATION

Principle:

Absorption of wavelength in UV and visible range are characteristics of outermost electronic configuration in atoms/ions. Based on this principle an approach was made to estimate Cr (+6) in presence of Cr(+6),Cr(+3),Al(+3),Fe(+3) without addition of any reagent. Following are the electronic configuration of Cr(+6), Cr(+3), Al(+3), Fe(+3) respectively.

Cr(+6):


Cr(+3):


Al(+3):

 

 

 

Fe(+3):

It appears from the electronic configuration that in case of Cr(+6), 3s and 3p electron will be promoted to next higher permissible energy level ,i.e. 4 thshell absorbing energy which are in the UV range,most prominent of those absorbed wavelength are 257 nm and 351 nm.

Absorption study of Cr(+6) 60 ppm

                                                                          

 

 

 

 

 

 

In case of Cr(+3) the outer electrons are 3dt2g orbital electrons which will be promoted to next higher permissible energy level by absorbing less energy that is corresponds to wavelength in visible range.

Absorption study of Cr(+3) 60 ppm

 

 

 

 

 

 

 

In case of Fe(+3) the electron that will be promoted are 3d electrons and also 3s,3p electrons as the 3d levels are half filled, it is quite stable and before that 3s and 3p will be promoted to a permissible level.so it appears that Fe(+3) may have additive interference with Cr(+6) absorption spectra

Absorption study of Fe(+3) 60 ppm

 

 

 

 

 

 

 

In case of Al(+3), the outermost electrons are 2s and 2p electron,which will be promoted to next higher permissible 3rd shell by absorbing wavelength which is quite different from Cr(+6) wavelength absorbing.

Absorption study of Al(+3)

 

 

 

 

 

 

 

 

Depending on this theory, we have developed a method for estimation of Cr(+6) in solution by UV absorption spectrometry.

Instrument:

Carry 60 UV absorption spectrophotometer.

Method:

For estimation of Cr(+6) we have prepared Cr(+6) standard 1000 ppm from CRM k2cr2o7 solid. This 1000 ppm standard diluted to 20,60,80,100 ppm

Calibration Curve  of Cr(+6)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

It follows a straight line.so any unknown solution having Cr(+6) in it,can definitely be identified and analysed on the basis of this calibration curve.

 Recovery Study:

50 ppm—–50.9 ppm——101.8% recovery

50 ppm—–50 ppm——–100 % recovery

Study of interference:

Cr(+3):

Cr(+3) 1000 ppm was made from Cr(NO3)3. 9H2O solution and a scan of Cr(+3) 100 ppm was taken and UV visible absorption spectrum studies.It has found no interference on the two wavelength for which Cr(+6) have maximum absorption.

Cr(+6)/Cr(+3) mix 60 ppm absorption study

 

 

 

 

 

 

 

Al(+3):

Al(+3) was made from CRM metallic Al and it shows it has no interference on the two wavelength for which Cr(+6) has maximum absorption

Cr(+6)/Al(+3) 60 ppm absorption study

 

 

 

 

 

 

 

Fe(+3):

Fe(+3) has additive interference on the Cr(+6) on those two lines.so in case of any solution contain Fe(+3) knowing  that amount of Fe(+3) correction factor applied in case of Cr(+6) or Fe(+3) can be removed by addition of NH4OH/ NH4Cl buffer when Fe will be precipitated as Fe(OH)3.

Cr(+6)/Fe(+3) 30 ppm absorption study

 

 

 

 

 

 

 

 

Cr(+6)/Fe(+3) 15 ppm absorption study

 

 

 

 

 

 

 

 

Estimation of unknown solution:

Unknown solution is analysed from Cr(+6) content from this method.

 

Conclusion:

This method does not require any reagent for estimation and almost accurate and less time consuming but we have studied up to 20 ppm. Further study is required for a concentration below 20 ppm and this can be achieved by vacuum evaporation of this solution.

 

Contributed by: PayelDey under Guidance of Prof.Barun Gupta

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