Beer-Lambert Law Calculator
Apply the Beer-Lambert law to relate the absorbance of a solution to its concentration. Absorbance A = ε·b·c, where ε is the molar extinction coefficient, b the path length and c the concentration. Choose what to solve for — absorbance, transmittance or concentration — and enter the known values.
Formula
A = epsilon * b * c, where A = absorbance, %T = 100 * 10^(-A)
- Absorbance A = ε × b × c (extinction coefficient × path length × concentration), and is dimensionless.
- Transmittance relates to absorbance by %T = 100 × 10^(−A); higher absorbance means lower transmittance.
- To find concentration, rearrange to c = A / (ε × b).
- ε (molar extinction coefficient) is specific to the substance and wavelength, in L/(mol·cm); b is the cuvette path length in cm.
- The law holds for dilute solutions; at high concentrations absorbance can deviate from linearity.
ε = 0.5 L/(mol·cm), b = 1 cm, c = 0.5 mol/L
Inputs
- Calculate: absorbance
- Molar extinction coefficient (epsilon): 0.5 L/(mol*cm)
- Path length (b): 1 cm
- Concentration (c): 0.5 mol/L
A = 0.5 × 1 × 0.5 = 0.25. Transmittance = 100 × 10^(−0.25) ≈ 56.2%, meaning about 56% of the light passes through.
Frequently asked questions
What is the Beer-Lambert law?
It states that the absorbance of a solution is proportional to the concentration of the absorbing species and the path length: A = εbc. It's the basis of quantitative spectrophotometry.
How do I find concentration from absorbance?
Rearrange the law to c = A / (ε × b). Measure the absorbance, then divide by the extinction coefficient times the path length.
What is the difference between absorbance and transmittance?
Transmittance is the fraction of light that passes through; absorbance is the log of how much is absorbed. They're linked by %T = 100 × 10^(−A).
When does the law break down?
At high concentrations, with scattering samples, or when the substance interacts chemically, absorbance stops being linear with concentration.