Which measurement method measures unknown concentrations in water by color intensity?

• A. Colorimetric
• B. Turbidimetric
• C. Gravimetric
• D. Potentiometric

Answer: (A)

Colorimetric analysis relies on how strong a color is to determine how much of a substance is present. When the analyte reacts with a reagent, a colored complex forms, and the darker the color, the higher the concentration (up to a practical limit). The instrument measures how much light at a specific wavelength is absorbed by the sample (or how much light passes through). Because absorbance is proportional to concentration for a fixed path length and wavelength, you create a calibration curve with standards and read off the unknown’s concentration from its color intensity.

In water testing this is common for substances like nitrate, chlorine residuals, or iron, where the reagent produces a measurable color change. Remember to use a blank to account for any background color and choose the appropriate wavelength to match the color produced.

The other methods don’t use color intensity to quantify concentration: turbidity measures cloudiness from scattered light, not color; gravimetric methods weigh a precipitate or residue, not an optical signal; potentiometric methods rely on electrical potential rather than color.