Which instrument measures the proportions of light at different wavelengths absorbed and transmitted by a pigment solution?

• A. Colorimeter
• B. Spectrophotometer
• C. Turbidimeter
• D. pH Meter

Answer: (B)

Assessing how a pigment solution absorbs light across different wavelengths requires a spectrophotometer. This instrument passes light through the sample and uses a monochromator or filter system to select a specific wavelength, then a detector measures the amount of light that makes it through. By repeating this across many wavelengths, you obtain the absorbance or transmittance at each color, creating an absorption spectrum. This is essential because different pigments have unique patterns of light absorption, and the data can be related to concentration via the Beer-Lambert law (A = log10(I0/I)).

A colorimeter, by contrast, uses fixed-wavelength filters to estimate color intensity, not the full spectrum. A turbidimeter measures how much light is scattered to assess turbidity, not how much is absorbed. A pH meter determines hydrogen ion concentration with an electrode, not optical absorption.