A, colorimetric test is a chemical assay in which the concentration of a substance in a solution is estimated by the colour it produces when it reacts with a specific reagent. The intensity of the colour correlates with the amount of analyte, and the result can be observed visually or measured with a colorimeter or spectrophotometer.
Explanation
Colorimetric assays rely on the formation of coloured complexes. When a reagent is added to a sample, it reacts with the target molecule to produce a coloured product. The Beer–Lambert law relates absorbance to concentration, so the amount of light absorbed at a given wavelength indicates how much of the analyte is present. The test is convenient for routine analysis in clinical, environmental and industrial laboratories because it does not require expensive equipment and can be performed with simple cuvettes and photometers. Different tests are used to detect specific classes of biomolecules. The Biuret test uses copper ions to form a violet complex with peptide bonds, indicating the presence of protein. The Bradford assay uses Coomassie Brilliant Blue dye, which binds to basic and aromatic amino acids and turns blue when proteins are present. The ninhydrin reaction produces a purple product with amino acids. For carbohydrates, Benedict’s and Fehling’s reagents detect reducing sugars by forming a red precipitate. Enzymatic assays for glucose and cholesterol use coupled reactions that produce coloured compounds whose absorbance is proportional to analyte concentration.
Applications and examples
Colorimetric tests are widely used to evaluate water quality. Chlorine levels in drinking water are checked using N,N‑diethyl‑p‑phenylenediamine (DPD) tablets that turn magenta in the presence of residual chlorine. In microbiology, colour changes in pH indicators such as phenol red signal acid production during fermentation tests. The Lowry and bicinchoninic acid (BCA) assays quantify protein in cell lysates. Medical laboratories use colorimetric kits to measure blood urea, creatinine and enzyme activities. Colour charts provided with kits allow semi‑quantitative interpretation without instrumentation. These tests are simple and accessible but may be affected by interfering substances that absorb at similar wavelengths or by turbidity. Careful control of reaction conditions and calibration with standards help ensure reliable results. Related Terms: colorimeter, spectrophotometer, Beer–Lambert law, biochemical assay, reagent