Question

The UV-visible spectrum of tetraphenylporphyrin exhibits four absorptions at 420 nm, 510 nm, 590 nm, and 645 nm. In practice, scientists typically use theabsorbance value at the wavelength maximum with the largest molar extinction coefficient to determine the concentration of the solution being analyzed. Which wavelength maximum is that for tetraphenylporphyrin?

	1.	420 nm
	2.	590 nm
	3.	510 nm
	4.	645 nm

Answer 1

Beer’s Law states that molar absorptivity is constant (and the absorbance is proportional to concentration) for a given substance dissolved in a given solute and measured at a given wavelength.

According to beer's law :

absorbance value

molar absorptivity coefficient or extinction coefficient ( unit -> lit / mol cm )

b => path length ( wave length )

c => analyte concentration ( mol / litre )

As from the above equation it is clear that absorbance is directly proportional to the wavelength, (if other parameters are constant ).

So, the maximum absorbance will give the maximum wavelength.

=> 645 nm is the maximum wavelegth (option 4 ).