Analytical chemistry practice — Why is absorbance (A) generally preferred over percent transmittance (%T) as the measure of absorption in quantitative spectrophotometry?

Introduction / Context:In UV-Vis spectroscopy, one can report data as percent transmittance (%T) or as absorbance (A). For quantitative analysis via Beer–Lambert law, absorbance is the preferred metric because it relates linearly to concentration.

Given Data / Assumptions:

• Beer–Lambert law: A = ε b c.
• %T = 100 × (I / I0), and A = −log10(T) where T = I / I0.
• Calibration methods require linear relationships for straightforward regression.

Concept / Approach:Absorbance is directly proportional to concentration at constant path length and wavelength because ε is constant for a given analyte. %T is logarithmically related to concentration; equal concentration changes do not produce equal %T changes, complicating calibration and error analysis. Therefore, using A simplifies quantitation and statistical treatment.

Step-by-Step Solution:

Verification / Alternative check:Typical calibration plots of A vs. c yield straight lines with slopes ε b; plots of %T vs. c are curved.

Why Other Options Are Wrong:

• %T can be measured accurately; accuracy is not the core issue.
• Dependence on incident power cancels when T = I / I0 is used properly; modern instruments compensate.
• “None” is incorrect because there is a clear quantitative reason to prefer A.

Common Pitfalls:Confusing precision with linearity; the critical advantage of absorbance is proportionality to concentration.

Final Answer:Because absorbance is proportional to analyte concentration whereas %T is not