18-7: How do transmittance, absorbance, and molar absorptivity differ? Which one is proportional to concentration?

Spectrophotometry is a standard and inexpensive technique to measure light absorption or the amount of chemicals in a solution. It uses a light beam which passes through the sample, and each compound in the solution absorbs or transmits light over a certain wavelength.

Transmittance (T) is a dimensionless quantity equal to the ratio transmitted radiation flux to the total incident flux

$T=\frac{P}{P_0}$

Absorbance (A) is the logarithm of the intensity of the incident radiationand the transmitted radiation (P) through the sample.

$A=log\left(\frac{P_0}{P}\right)$

The absorptions of light through the solutions can be mathematically described as Beer- Lambert’ s law.

$A=ϵ\cdot c\cdot b$

Absorbance is proportional to Concentration, Molar absorptivity is the constant of proportionality between absorbance and concentration.