Concentration from Absorbance Calculator

Concentration from Absorbance Calculator

(unitless)
M⁻¹cm⁻¹
cm

The Beer-Lambert Law

The Beer-Lambert Law (also known as Beer's Law) is a fundamental principle in optics and chemistry that relates the attenuation of light to the properties of the material through which the light is traveling. For chemical solutions, it provides a direct relationship between absorbance (A) and concentration (c).

The law is typically stated as:

A = ε ⋅ c ⋅ l

Where:
• A = Absorbance (unitless), measured by a spectrophotometer.
• ε = Molar Extinction Coefficient (or molar absorptivity), a constant specific to the substance and wavelength (units: M⁻¹cm⁻¹ or L·mol⁻¹cm⁻¹).
• c = Molar Concentration of the absorbing substance (mol/L or M).
• l = Path Length, the distance the light travels through the solution, usually the width of the cuvette (cm).

This calculator rearranges the formula to solve for concentration: c = A / (ε ⋅ l). By measuring the absorbance of a solution and knowing the substance's extinction coefficient and the path length, you can easily determine its molar concentration.

Frequently Asked Questions

What are the limitations of the Beer-Lambert Law?

The Beer-Lambert Law is highly useful but has limitations. It assumes:
• The solution is relatively dilute (typically A < 1.0). At high concentrations, interactions between solute molecules can affect absorbance.
• The incident light is monochromatic (single wavelength).
• The solute does not undergo chemical changes (like association or dissociation) that depend on concentration.
• There is no significant light scattering from suspended particles in the solution.
• The solvent itself does not significantly absorb light at the chosen wavelength.
Deviations from these assumptions can lead to non-linear relationships between absorbance and concentration.

What is a standard cuvette path length?

The most common path length (l) for cuvettes used in standard spectrophotometers is 1 centimeter (1 cm). This value is often assumed if not explicitly stated, but cuvettes with different path lengths (e.g., 0.1 cm, 0.5 cm, 10 cm) are also available for specific applications, especially for measuring highly concentrated or very dilute samples.

How do I find the Molar Extinction Coefficient (ε)?

The molar extinction coefficient (ε) is an intrinsic property of a substance at a specific wavelength and under specific conditions (like solvent and temperature). You typically need to:
• Look it up in scientific literature, databases (like the NIST Chemistry WebBook), or manufacturer's specifications for the substance.
• Determine it experimentally by measuring the absorbance of several solutions of known concentrations and plotting A vs. c (the slope of the line will be ε⋅l).
• Estimate it based on the substance's structure (as done for proteins and DNA in other calculators).