molar absorptivity
/ MOH-lar ab-sor-TIV-ih-tee /
Some dyes are so intense that a single drop colours a whole bathtub, while others barely tint a glass. Molar absorptivity is the number that captures this difference: it tells you how greedily a particular substance grabs light of a particular colour.
It is the constant ε in Beer-Lambert law (A = ε c l): the absorbance produced by a one-molar solution in a one-centimetre path, with units of L mol⁻¹ cm⁻¹. A large ε means the substance is a strong absorber, so even tiny amounts can be measured; a small ε means you need a lot before any colour shows.
Because molar absorptivity is fixed for a given substance at a given wavelength, it sets how sensitive a spectrophotometric method can be. Analysts deliberately pick the wavelength where ε is largest — the absorption peak — to detect the smallest possible concentrations.
The intensely coloured permanganate ion has a molar absorptivity of about 2,400 L mol⁻¹ cm⁻¹ at 525 nm, so even a faintly purple, very dilute solution gives a readable absorbance.
A large ε means high sensitivity — trace amounts still register.
Molar absorptivity is wavelength-specific: the same compound has a different ε at each colour. Always quote the wavelength when reporting an ε value.