Beer-Lambert Law Calculator — Absorbance & Concentration

Beer-Lambert Law Guide

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%.

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 mo

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, determinin

Spectrophotometry vs 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 mon