In colorimetric quantification of analytes using reagent, why we measure the absorbance of the solution (at a specific wavelength) rather than measuring the transmittance of the solution (at that specific wavlength)?
Dear Sibasish Dutta ,
In short, Absorbance A= log Io/I =εbC, which means that A α C, but T=I/Io. This means that A gives you a straight line with C at a specific wave length while T is not proportional linearly with C . But for detail of all those I mentioned, pleas see any text book for Analytical Instrumentation, such as: INSTRUMENTAL METHODS OF ANALYSIS, 7th. Ed, by, Willard, Merritt, Dean and Settle. Thank you and
Good Luck
Dear Sibasish Dutta ,
If my previous answer was not enough, here I shall add some more information about absorbance A and transmittance T. If you see the scale of A and T on the instrument, or you just make a horizontal line the upper part for %T and the lower for A. at the starting point on the left you will have 100% on the T scale for 0 zero value on A scale. At the extreme end of the line at the right side where T= 0% and at 1% T we have A=2. If you calibrate T scale linearly from 100 on the left to zero on the right with equal divisions, you will see at 10% T, the opposite will be A=1 according to the equation A=-log T. this means A=log10/100 =1. Now you can see that bwtween T=100 to t=10, which is 90% of the scale equqls only 1 absorbance unit between 0 to 1. the 10% remaining of T% is also = 1 absorbance unit. Surely you know that is because of the log scale of A and linear scale of T...This means it is also possible to measure T instead of A, but for making the plot of linear relationship between A and C, you have to convert the T values you measure into A values. In this way, it will be possible to read T at high values of A where the reading divisions are so close together makes reading A not accurate. But on the other hand reading T divisions will be more accurate. For you information also, usually A readings between 0.03 to less than A=1,will have better accuracy than at high concentration where A readings will not be accurate apart from other unfavorable factors. Ihope you also read the reference I gave you before. Thank Yoy and Good luck.
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