Page 41 - Practical book PC406-MNU-2025

MANSOURA NATIONAL UNIVERSIY
PHARM D- CLINICAL PHARMACY LEVEL II INSTRUNENTAL ANALYSIS (PC 406)

Lambert’s Law:
The absorbance is directly proportional to the path length of the absorbing medium.
A α b

Beer-Lambert Law:

 There's a linear relationship between absorbance and concentration of an
absorbing substance.
 The absorbance is directly proportional to the concentration of the absorbing

substance and the path length of the absorbing medium.

A α b c or A = const × b c

Beer-Lambert law can be expressed in one of three forms as follows:

A = a b c …………… when c is in g/L

-1
-1
“a” is absorptivity (L . g . cm )

A = ε b c …………… when c is in mol/L

-1
“ε” is molar absorptivity (L . mol . cm ) {ε is pronounced Epsilon}
-1

A = 1% b c ………… when c is in g% {g% means grams per 100 mL}
 cm 1

-1
-1
“ 1% ” is specific absorbance (dL . g . cm ) {dL means deciliter or 100 mL}
 cm 1

In all cases, A = Absorbance (No Unit) and b = Path length (cm)

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