What This Calculator Does
This tool calculates the molar concentration of a substance in a solution from its measured absorbance, using the Beer-Lambert law. It is widely used in analytical chemistry, biochemistry, and spectrophotometry to convert UV-Vis readings into meaningful concentration values.
How to Use It
Enter three values: the absorbance (A), a dimensionless reading from your spectrophotometer; the molar absorptivity (ε), a constant for the analyte at the chosen wavelength expressed in L·mol⁻¹·cm⁻¹; and the path length (l), the width of the cuvette in centimetres (typically 1 cm). The calculator returns the concentration in mol/L and also converts it to micromolar (µM) for convenience.
The Formula Explained
The Beer-Lambert law states \(A = \varepsilon \cdot l \cdot c\). Rearranging for concentration gives $$c = \frac{A}{\varepsilon \cdot l}.$$ Absorbance increases linearly with concentration as long as the solution is not too concentrated. The molar absorptivity describes how strongly a chemical species absorbs light at a given wavelength, while the path length accounts for how far the light travels through the sample.
Worked Example
Suppose you measure an absorbance of \(A = 0.63\) for a protein at 280 nm with a molar absorptivity \(\varepsilon = 6300 \ \text{L}\cdot\text{mol}^{-1}\cdot\text{cm}^{-1}\) in a standard 1 cm cuvette. Then $$c = \frac{0.63}{6300 \times 1} = 0.0001 \ \text{mol/L} = 100 \ \mu\text{M}.$$
FAQ
What units should I use? Use L·mol⁻¹·cm⁻¹ for ε and cm for path length to get concentration in mol/L.
Why is my result negative or zero? A blank absorbance correction may be needed; also confirm ε and path length are non-zero positive numbers.
Does the law always hold? Beer-Lambert linearity breaks down at high concentrations (typically \(A > 1\)) due to scattering and analyte interactions, so dilute the sample if needed.
