Page 16 - Instrumental Analysis - Pharm D Clinical- 07-PA403

UV-visible Spectrophotometry

The Beer-Lambert Law

A=kb A = k’ c

• By combining the 2 laws together, we get the Beer-Lambert

law: A = a b c

• “a" is a constant known as absorptivity which is the
absorbance at specific λ when pathlength is unity (i.e. 1 cm)

and concentration is unity.

• If concentration is measured in molarity (M), “a” will be known
as molar absorptivity or epsilon (ε).

• ε is defined as the absorbance of 1 M solution and the
pathlength is 1 cm. Unit of ε is L mol–1 cm–1.

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• If concentration is measured in g% or g/100 mL, “a” will be known

as specific absorbance or A(1%, 1 cm).

• A(1%, 1 cm) is defined as the absorbance of 1% solution and the

pathlength is 1 cm.

A=εbc A = A(1%, 1 cm) b c

• Both ε and A(1%, 1 cm) are characteristic values for each
absorbing substance. These constants are affected by λ of the

monochromatic radiation and the solvent used for preparation of

sample solution.

• What is the unit of A(1%, 1 cm)?

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