Beer-Lambert Law Calculator

Beer-Lambert Law Calculator

Enter the absorbance of the solution (e.g., 0.85)
Enter the path length of the cuvette in centimeters (e.g., 1.0)
Enter the molar absorptivity (ε) in L·mol⁻¹·cm⁻¹ (e.g., 88000)
The concentration will be calculated automatically based on the inputs.

The Beer-Lambert Law Calculator is a powerful tool used to determine the concentration of a substance in a solution based on its light absorption properties.

This law, also known as Beer's Law or the Beer-Lambert-Bouguer Law, establishes a linear relationship between the concentration of a solution and its absorbance of light.

Beer-Lambert Law Chart

Concentration (mol/L)Path Length (cm)Molar Absorptivity (L/mol·cm)Absorbance
0.001150000.500
0.002150001.000
0.003150001.500
0.004150002.000
0.005150002.500

Beer-Lambert Law Equation

The Beer-Lambert Law is expressed by the equation:

A = εbc

Where:

  • A is the absorbance (dimensionless)
  • ε is the molar absorptivity (L/mol·cm)
  • b is the path length of the sample (cm)
  • c is the concentration of the solution (mol/L)

Let’s consider an example using this formula. Suppose you have a solution of methylene blue with a molar absorptivity of 95,000 L/mol·cm at 664 nm.

You measure its absorbance in a 1 cm cuvette and get a reading of 0.285. To find the concentration:

c = A / (εb)

c = 0.285 / (95,000 × 1)

c ≈ 3 × 10^(-6) mol/L or 3 μM

How to calculate molar absorptivity?

Molar absorptivity (ε) is a measure of how strongly a chemical species absorbs light at a given wavelength.

To calculate it, you need to rearrange the Beer-Lambert equation:

ε = A / (bc)

For example, let’s say you prepare a 0.002 M solution of potassium permanganate and measure its absorbance in a 1 cm cuvette at 525 nm.

If the absorbance reading is 0.786, you can calculate the molar absorptivity:

ε = 0.786 / (1 cm × 0.002 M)

ε = 393 L/mol·cm

This value indicates how effectively potassium permanganate absorbs light at 525 nm, which is useful for future concentration determinations or comparing the absorptive properties of different substances.

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