Beer-Lambert Law Calculator — Absorbance & Concentration

Beer-Lambert Law Guide

What do I need to know about The Beer-Lambert Law?

A = ε × c × l. Where A = absorbance (no units — dimensionless), ε = molar extinction coefficient (L/mol·cm), c = concentration (mol/L), l = path length of cuvette (cm, usually 1 cm). Transmittance: T = I / I₀ = 10^(−A). Percent transmittance: %T = T × 100. Relationship: A = −log₁₀(T) = log₁₀(I₀/I). A = 0: 100% transmittance — no absorption. A = 1: 10% transmittance — 90% absorbed. A = 2: 1% transmittance. Each unit increase in absorbance reduces transmitted light by 90%.

What should I know about Calibration Curves?

In practice, ε may not be known. A calibration curve is produced: make a series of solutions of known concentration. Measure absorbance of each at the wavelength of maximum absorbance (λ_max). Plot A vs concentration — should be linear (Beer-Lambert law). Fit a line of best fit: A = gradient × c. Gradient = ε × l. For an unknown concentration: measure A, read off concentration from the calibration curve. Limitations: Beer-Lambert law is only linear at low concentrations — typically below 0.01 mol/dm³ for many coloured compounds — at higher concentrations the relationship becomes non-linear due to intermolecular interactions and changes in the refractive index of the solution, so dilute an unknown sample if its absorbance falls outside the linear region of the calibration curve rather than extrapolating beyond it.

What do I need to know about Colorimetry in A-level Chemistry?

Colorimetry is used to determine concentrations of coloured solutions. Principle: a coloured solution absorbs complementary colours of light. Red solution absorbs green/blue light — use a green filter to maximise absorbance. Purple solution absorbs yellow light — use yellow filter. The colorimeter measures the light transmitted through the sample — more colour means less transmission means higher absorbance. Applications: monitoring reaction rate (absorbance vs time), enzyme kinetics, determining the concentration of coloured ions (e.g. Cu²⁺ solutions, which are blue, absorb strongly in the red/orange region) and following the rate of reactions that produce or consume a coloured species, such as the iodine clock reaction or the acid-catalysed reaction between propanone and iodine.

What's the difference between Spectrophotometry and Colorimetry?

Colorimeter: simple, uses coloured filters, measures visible light only, adequate for school and routine lab work. Spectrophotometer: uses a monochromator (diffraction grating) to select precise wavelengths, covers UV and visible range, higher precision. UV-visible spectrophotometry: proteins absorb at 280nm (aromatic amino acids). Nucleic acids (DNA/RNA) absorb at 260nm. A260/A280 ratio used to assess DNA purity: ~1.8 = pure DNA, <1.8 = protein contamination. NADH absorbs at 340nm — used to monitor enzyme-catalysed redox reactions in biochemistry, since NADH absorbs strongly at 340nm while its oxidised form NAD⁺ does not — following the change in absorbance at 340nm over time gives a direct measure of reaction rate in many metabolic assays.