The colorimetric method used in early laboratory analysis is based on which principle?

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Colorimetric analysis hinges on how the color of a solution relates to how much of the absorbing substance is present. This is governed by Beer's Law, which states that, for a given wavelength and a fixed light path, absorbance is proportional to the concentration of the absorbing species (A = εlc). In other words, more colored molecules mean more light is absorbed and less is transmitted, so the darker the solution, the higher the concentration. Early labs used this principle by forming a colored product with the analyte and comparing color intensity to standards, either by eye or with a simple colorimeter. With a standard path length (usually 1 cm) and a known molar absorptivity, you can create a calibration curve that lets you determine unknown concentrations from measured absorbance. This method works best in dilute solutions where Beer's Law holds true and where interference from other colors or scattering is minimized. The other listed principles describe gas behavior or electrical relations, not light absorption, so they don’t underpin colorimetric analysis.