A colorimeter is an analytical instrument designed to measure the intensity or concentration of a substance in a solution by analyzing how much light it absorbs. This device operates by passing a beam of light through a colored liquid sample and then quantifying the light that successfully passes through the material. The colorimeter converts the physical property of light absorption into a numerical value that relates directly to the chemical concentration of the dissolved substance.
Names and Terminology for the Device
The most common alternative name for a colorimeter is a Filter Photometer. This term highlights the device’s reliance on optical filters to select a specific color or wavelength of light for measurement. Sometimes, the instrument is simply referred to as a Photometer, a broader term for any device that measures light intensity.
Older or simpler models designed to measure color saturation might be called a Tintometer, especially when used in industrial color matching. A colorimeter is distinct from a densitometer. A densitometer primarily measures the optical density of materials like photographic film or printed ink, while a colorimeter measures the color value of a chemical solution.
How the Instrument Measures Color
A colorimeter operates on the principle that a colored substance absorbs light of one color more effectively than others. The device directs a beam of white light through a chosen colored filter. This filter is selected to transmit the color of light that is absorbed most strongly by the sample, which is typically the complementary color to the solution’s actual hue.
The resulting narrow band of light passes through the transparent container holding the sample solution. As the light travels through, photons are absorbed by the colored molecules, reducing the intensity of the light beam. A photodetector positioned on the opposite side of the sample measures the intensity of the light that was successfully transmitted.
A higher concentration of the colored substance means more light is absorbed, resulting in a lower intensity reaching the detector. This relationship between light absorption and concentration is defined by the Beer-Lambert Law, which forms the scientific basis for the instrument. The colorimeter calculates the absorbance value from the measured light transmittance, providing a quantitative measurement.
Key Differences from a Spectrophotometer
The main difference between a colorimeter and a spectrophotometer lies in the mechanism used to select the light wavelength. A colorimeter is a simpler instrument that uses a set of fixed, broad-band colored filters to isolate light within a wide range of wavelengths. These instruments are less expensive, more portable, and only capable of measuring at a few discrete wavelengths.
In contrast, a spectrophotometer uses a component called a monochromator (typically a prism or a diffraction grating). The monochromator separates light into its component wavelengths, allowing the user to select a very narrow, specific band of light. This ability to select any precise wavelength across the visible and ultraviolet spectrum makes the spectrophotometer far more accurate and versatile.
A colorimeter is limited to measuring substances specific to the few wavelengths provided by its filters. A spectrophotometer can generate a complete absorption spectrum (a graph showing the sample’s light absorption across all wavelengths). This spectral data is required for complex analytical tasks like identifying unknown compounds or determining optimal measurement conditions.