Colorimetry — Measuring Concentration by Color
8 experiments
Colorimetry uses the Beer-Lambert law to determine the concentration of colored solutions by measuring how much light they absorb. The deeper the color (higher concentration), the more light is absorbed. This technique is widely used in clinical chemistry (blood tests), environmental monitoring (water quality), and food science (additive quantification).
Learning Objectives
Understand the Beer-Lambert law (A = epsilon * l * c), construct calibration curves, determine unknown concentrations, and appreciate the limitations of the linear range. Learn wavelength selection — measure at the absorption maximum (lambda_max) for maximum sensitivity.
Key Techniques
Prepare a series of standard solutions of known concentration. Measure absorbance at the wavelength of maximum absorption. Plot absorbance vs. concentration to create a calibration curve. Measure the unknown sample's absorbance and read its concentration from the curve. Use a blank (solvent only) to zero the instrument.
Equipment Overview
Required: colorimeter or spectrophotometer, matched cuvettes, volumetric flasks for standard preparation, micropipettes. Simple colorimeters use LEDs and photodiodes; UV-Vis spectrophotometers provide full wavelength scanning capability.
Beer-Lambert Law Verification
Demonstrating the linear relationship between absorbance and concentration
Verify the Beer-Lambert law by measuring the absorbance of CuSO₄ solutions at various concentrations and constructing a calibration curve.
Colorimetric Determination of Glucose
Measuring blood glucose analog using the DNS method
Determine glucose concentration using the dinitrosalicylic acid (DNS) colorimetric method and understand reducing sugar chemistry.
Colorimetric Determination of Phosphate
Measuring phosphate levels in water using the molybdenum blue method
Determine the phosphate concentration in a water sample using the ammonium molybdate colorimetric method.
Colorimetric Protein Assay (Bradford Method)
Quantifying protein concentration using Coomassie Brilliant Blue dye binding
Determine the protein concentration of an unknown sample using the Bradford assay and a BSA calibration curve.
Determination of Iron in Water
Measuring dissolved iron concentration using thiocyanate colorimetry
Determine the concentration of iron(III) in a water sample using the thiocyanate colorimetric method.
Food Dye Analysis by Spectrophotometry
Identifying artificial food colors by their absorption spectra
Determine the identity and concentration of artificial food dyes in commercial beverages by measuring their absorption spectra.
Spectrophotometric Analysis of a Mixture
Resolving a two-component mixture using simultaneous equations
Determine the individual concentrations of two dyes in a mixed solution using absorbance measurements at two wavelengths and simultaneous equations.
Spectrophotometric Determination of Aspirin
Measuring aspirin content in commercial tablets by UV absorption
Determine the amount of acetylsalicylic acid (aspirin) in commercial tablets using UV spectrophotometry after hydrolysis to salicylate.