Molar Absorptivity

Molar Absorptivity

The property that measures the strength by which a chemical species absorbs light at a particular wavelength is known as molar absorptivity. Read this article to know more about this property and its uses in chemistry.
Molar absorptivity is a term used in chemistry to measure how a particular chemical absorbs light at a particular wavelength. It is also known as molar extinction coefficient denoted by 'ε'. This property can be calculated by using the Beer - Lambert Law. The intrinsic property of the chemical known as absorbance A is measured using the path length l and concentration c of the species. This is illustrated by the following equation,

A = εcl

Using this formula, the molar absorptivity equation becomes,

ε=A/cl

Beer-Lambert's Law
In simple words, the law states that the absorbance (A) of an absorbing chemical species is directly proportional to the path length and concentration of the chemical. This length is the distance source of light by which it travels. The SI units for ε 'epsilon' are m2/mol, but very commonly, molar absorptivity units are expressed as: M-1cm-1, and also as L mol-1 cm-1. This property is confused with extinction co-efficients used in physics. It is important to remember that this property is almost exclusive to chemistry. Sometimes, it so happens that there are more than one absorbing species in the chemical. In such a case, absorbance is a summation of all the individual absorbances of each species. It is given by the equation,

A= l1c12c2+.....)

Here, the concentration and the molar absorptivity for each species changes, whereas the path length remains the same. Certain biological components like proteins are known to show maximum absorption at 280 nm, which is only due to the aromatic amino acids that are present in the proteins. This explains the presence of a number of absorbing species that affect the total absorbance.

How to Calculate Molar Absorptivity
Using the Equation in an Experiment
  • A simple way to calculate it is using the formula given above.
  • Define all the variables with a value. Absorbance (A), is a measurement without any units, obtained from a spectrophotometer at a particular wavelength of light. Path length is usually considered to be 1, when calculating this value experimentally, unless stated otherwise. Concentration of the substance c should also be known.
  • Substitute these values in the equation mentioned for epsilon.
Using a Graph
  • In a graph, several values of 'A' are plotted on Y-axis against a number of concentrations on X-axis.
  • The slope of the line will be εl, and the l path length will be 1. Thus, the slope will give you the molar absorptivity. Calculators are the easiest way to get these values.
Calculating Concentrations Using Known Molar Absorptivity
When this value is known, it is used to determine unknown concentrations of chemical components. For example, known molar absorptivity of iron complexes is often used to determine the iron content in different branches of biology. Reactions between iron and phenanthroline gives a compound whose molar absorptivity is 11,100 at the wavelength of 508 nm. This method was used to estimate iron in blood, and is reliable and sensitive, as the complex of iron is very stable once you add a reducing agent that keeps iron in the ferric state. There are various other complexes of known values that are being used to determine concentrations of biologically important chemicals.

Thus, molar absorptivity can most easily be calculated using a graph, when you have varied known concentrations of the same chemical species. Its values are constant at a particular wavelength and concentration for a given species. There are also other ways to determine this value using Avogadro's constant and absorption cross section (σ). This is illustrated by,

σ = 1000ln10ε/NA=3.82 x 10-21ε

This formula or equation can be used only when you know the absorption cross section.