Chapter 13 – UV-VIS AND NEAR IR ABSORPTION.pptVIP

Chapter 13 – UV-VIS AND NEAR IR ABSORPTION SPECTROSCOPIES First part of this chapter applicable to other absorption spectroscopies e.g. IR and AA even though it is covered in this section. Beer’s Law – Ideal Behavior Decrease in power is proportional to power going through cell and distance traveled: Rearrange and integrate over the length of the cell: where e = molar absorptivity and is the collection of other terms. %T = P/Pox100 Absorbance Another term called absorbance is generally used in place of either of these and is defined as A = log(Po/P); Beers law equation becomes: A = ebC. Beer’s Law predicts linear behavior between concentration and absorbance but not between amount power coming out of sample. Losses During Absorption Real sample cells have losses due to reflection and scattering; Minimized by using a reference cell. with the same spectral characteristics. Measured absorbance is A = log Psolvent/Psolution and is assumed to be approximately equal to the correct absorbance in the absence of these effects i.e. log Po/P. Real Limitations Linearity is observed in the low concentration ranges(0.01), but may not be at higher concentrations. This deviation at higher concentrations is due to intermolecular interactions. As the concentration increases, the strength of interaction increases and causes deviations from linearity. The absorptivity not really constant and independent of concentration but e is related to the refractive index (h ) of the solution by the expression: At low concentrations the refractive index is essentially constant-so e constant and linearity is observed. CHEMICAL DEVIATIONS Apparent deviations in Beers law sometimes occur from various chemical effects, such as dissociation, association, complex formation, polymerization or other equilibrium. E.g. K2Cr2O7 solutions exist as a dichromate, chromate equilibrium: At lmax of 350 (and 450) nm and 372 nm respectively. There is a strong dependence of position of this equi