How scientists use light to weigh the invisible contents of a solution
In short: Colorimetry is more than just observing hues; it is a precise mathematical method for measuring the concentration of substances. By analyzing how light is absorbed or transmitted, researchers can identify chemical elements even when they exist in concentrations as low as one part in several hundred million.
At its core, colorimetry relies on the relationship between light and matter. The practice is governed by two fundamental principles: Pierre Bouguer’s law (often called Lambert’s law), which relates light absorption to the distance light travels through a medium, and Beer’s law, which connects absorption to the concentration of the substance. When combined, these laws allow scientists to use the equation log I0/I = kcd to calculate unknown concentrations. In this formula, I0 represents the incident light intensity, I is the transmitted intensity, c is the concentration, d is the path length, and k is a constant specific to the substance and wavelength.
The process often involves comparing the intensity of light passing through a solution of unknown concentration to one of a known concentration. By balancing transmitted light levels through varying solution depths, one can calculate the unidentified concentration using the ratio of both path lengths. While early instruments like the 1870 Duboscq colorimeter relied on the human eye to compare intensities, modern photoelectric colorimeters use a photoelectric cell. This advancement allows for much higher precision in detecting substances.
While colorimetry typically uses white light to observe the complementary color of what is absorbed, the field has largely been superseded by spectrophotometry. Unlike a standard colorimeter, a spectrophotometer uses monochromatic light or narrow bands of radiation, allowing measurements to extend into the ultraviolet and infrared regions. Advanced tools like the spectrocolorimeter can even estimate tristimulus values through numerical integration, providing a highly accurate spectral reflectance curve without the manufacturing variances found in traditional optical filters.
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Sources
- Summary by Intellect · source: Colorimetry | Light Measurement, Photometry & Spectrophotometry | Britannica
- Also: Colorimetry
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