Beer's Law Lab

Absorbance, transmittance, and spectrophotometry

Beer-Lambert Law states that absorbance is directly proportional to both the concentration of the absorbing species and the path length of light through the solution. A = εbc, where ε is the molar absorptivity (a constant for each substance at a given wavelength), b is the path length in cm, and c is the molar concentration. Transmittance T = I/I₀ is the fraction of light that passes through. A = -log₁₀(T). A calibration curve (A vs c) at fixed wavelength and path length gives a straight line through the origin. The slope is εb. To find an unknown concentration, measure its absorbance and read from the calibration curve. Deviations from Beer's Law occur at high concentrations (>0.01 M for most species) due to intermolecular interactions.