Example:

A solution containing 36.5 mg of potassium dichromate per 500 cm³ was taken in a cuvette having a path length of 2 cm and its transmittance was measured at 455 nm. If the percentage transmittance is found to be 12, calculate the molar absorptivity of potassium dichromate.

Solution

The molarity of provided potassium dichromate solution can be calculated as

= 1000 × 0.0365/500 × 294 = 2.48 ×10 ^-4 mol dm ^-3

 (M_m of potassium dichromate = 294 g mol^-1)

12% transmittance means T = 0.12

∴ A = 1/0.12 = 0.921

∴ ε = A/cb = 0.921 / (2.48 ×10 ^-4 × 2) = 0.921/4.96 × 10 ^-4

∴ε = 1.86 × 10³ cm^-1 mol^-1 dm³

From the Lambert-Beer's law expression provided above, absorbance (A = log P₀ / P) is a direct measure of the concentration of the analyte if the thickness of the absorbing medium is kept constant. That proportionality is exploited in the determination of analyte concentration. Thus, a few factors challenge the validity of the Beer's law and need to be controlled to take up quantitative determinations.