Colorimeters are photometric measurement instruments that can be used to measure the concentration of organic and inorganic compounds in solution by determining the absorbance of
specific wavelengths of light. Integral to water quality analysis, colorimeters go beyond simple testers by allowing users to develop a chemical profile of the sample rather than just
simple detection of the presence of a chemical compound.
Colorimeters have many laboratory and practical applications. Besides being valuable for basic research in chemistry and biology laboratories, they are valuable tools for measuring water
quality by determining the concentration of a wide range of substances and contaminants such as heavy metals, radionuclides, nitrates, phosphates, fluorides, pesticides, and many others.
They can also be used to measure the concentrations of plant nutrient such as phosphorus, nitrate and ammonia in the soil.
Colorimeter Technology
When testing for water quality, most organic and inorganic compounds are colorless and undetectable to the human eye. Testing with a colorimeter introduces chemical reagents and light providing
a way to “see” these invisible compounds.
Reagents are chemicals formulated to react with specific ions causing them to change color. For the purpose of water quality analysis, test samples are put in a cuvette (sample container)
to which reagents are added. If the compound specific to the reagent is present, the reagent will turn it a color. The greater the concentration of ions, the deeper the coloration. In order
to convert this color into a numerical value, light is required.
White light consists of numerous wavelengths of light which correspond to different colors. If that light is passed through a test solution which has been colored by a reagent, one color, or
band of the wavelength of white light, is absorbed. The wavelength of light absorbed is a function of the ion present. Different chemical substances absorb different wavelengths of light on
the visible spectrum.
Colorimeters use colored filters to isolate light of a specific wavelength which is passed through the test sample. Since colorimeters generally test for a single ion, the wavelength of
light that is passed through the test solution is selected to identify that particular ion. The test sample, colored by the reaction between the reagent and the ion, absorbs some of the light.
Any light that passes through the solution is measured by a photodetector and compared to a colorless reference sample. According to the Beer-Lambert law, the quantity of
light absorbed by a substance dissolved in solution is directly proportional to the concentration of the substance. Less light passing through the test solution means more light has been
absorbed, indicating a high concentration of the ion being tested for.
Colorimeters generally measure transmittance (a linear scale from 0-100%) as well as absorbance (a logarithmic scale from zero to infinity). The displayed
value, however, is usually either mg/L or ppm which are calculated from measured values.
When setting up the colorimeter, it’s important to select the correct wavelength of light for testing. Generally, the wavelength used is complementary to the color of the test sample. For
example the nitrate reagent creates a pink color. The greater the concentration of nitrate, the deeper the pink color. When testing a wavelength of light in the green region should be selected
since a pink solution absorbs green light.
Colorimeters have three main components:
- A light source, usually a tungsten or xenon lamp
- A combination of filters to isolate the wavelength of light absorbed by the solute
- A photoelectric detector to measure the transmitted light and convert that value into electrical output
Things to Consider When Selecting a Colorimeter:
- What chemical ions are being tested for?
- Is a single parameter tester suitable for your needs or are multiple parameters needed?
- What consumables (reagents and cuvettes) are required?
- What level accuracy is needed?
- Are any advanced features such as data logging or communications needed?
If you have any questions regarding colorimeters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
