Beer–Lambert law — In the equation A = ε b c, what does the symbol ε represent?

Introduction / Context:
Quantitative UV-Vis spectrophotometry relies on the Beer–Lambert relationship A = ε b c to determine analyte concentration from light absorption. Correctly identifying each term in the equation is fundamental to analytical chemistry.

Given Data / Assumptions:

• A is absorbance (unitless).
• ε is a proportionality constant characteristic of the analyte at a given wavelength.
• b is path length (commonly 1 cm), and c is concentration (typically mol/L).

Concept / Approach:
ε is the molar absorptivity (also called molar extinction coefficient), with units L mol^-1 cm^-1. It describes how strongly a species absorbs light at a particular wavelength per molar concentration and per path length. Using ε allows direct proportionality between absorbance and concentration for a fixed path length.

Step-by-Step Solution:

Verification / Alternative check:
Calibration curves plot A versus c at constant b; slope equals ε b, confirming ε as molar absorptivity.

Why Other Options Are Wrong:

• “Absorbtivity” (without “molar”) is ambiguous and not the standard term here.
• Path length is b, not ε.
• “None” is incorrect because ε has a precise meaning.

Common Pitfalls:
Confusing transmittance (%T) with absorbance; A depends linearly on c with ε, whereas %T does not.

Final Answer:
Molar absorbtivity