How to Calculate Concentration from Absorbance.

In this article we will discuss how to calculate concentration from absorbance but first lets begin by defining absorption.

Absorption means that a substance captures and transforms energy while concentration refers to the amount of a substance in a defined space.

If we increase the concentration of the solution then there are more molecules for the light to hit when it passes through.

Scientists use the Beer-Lambert Law also known as Beer’s Law in order to find concentration from absorbance.

The relationship between absorbance and concentration (c) is proportional. Usually, the more concentrated a substance, the more light will be absorbed. Thus the absorbance (A) of the material is related to the initial intensity of the light, I₀, and the transmitted intensity of the light.

Formula to Calculate Concentration from Absorbance.

We can find concentration by plotting a graph of Absorbance against Concentration of known solutions. So if you subtract your y-intercept from the absorbance and divide by the slope, you are finding the concentration of your sample.

Example:

Suppose the molar absorptivity of Na Cl is 193L mol^-1 cm^-1 and the length of its light path is 5 cm, calculate the concentration if the absorbance is 200.

Therefore, the concentration is 0.21M.