Colorimeters and Beer-Lambert Law

Colorimeters are analytical devices commonly used in science labs to measure the amount of light of a specific wavelength that is absorbed by a sample. Absorbance is related to concentration (Beer-Lambert Law) so the greater the concentration (color intensity) of a sample, the greater the absorbance (A) value. Comparison of the absorbance measurements to a calibration curve provides the final concentration in the sample.

Our standard colorimeter consists of an red-green-blue (RGB) LED and a light sensor mounted into a black acrylic enclosure with a cuvette holder. (Other LED wavelength options are also available). The cuvette holder positions the sample between the LED and the sensor. The sensor board connects to a Arduino programmed with the colorimeter firmware via a simple shield. When the colorimeter is operating, the LED illuminates the sample in the cuvette with a specific wavelength of light. For example, with the standard colorimeter RGB LED:

  • Red Light - 632 nm peak wavelength, (625 nm domimant, 1077 mcd, 18 nm bandwidth)
  • Green Light - 523 nm peak wavelength, (528 nm domimant, 1570 mcd, 33 nm bandwidth)
  • Blue Light - 465 nm, peak wavelength, (470 nm domimant, 377 mcd, 25 nm bandwidth)

A small slit in the colorimeter allows light to pass through the sample to the light sensor. Absorbance (A) of the sample is determined by comparing the intensity of incident light (I0) to the intensity of light after it has passed through the sample (I) using the following equation: A = log10(I/I0).

Using the colorimeter

Colorimeters are extremely useful and flexible lab instruments for a wide range of science education labs. Some examples of how they are used include:

  • Color and absorbance - Investigate the relationship between the color of a substance and absorption of light at different wavelengths. See Lab 1: Introduction to Colorimetry
  • Beer’s Law - Investigate the relationship between concentration and absorbance using colored solutions (eg. food dye, copper sulfate). Calculate molar extinction coefficient. See Lab 2: Beer’s Law and Molar Extinction Coefficients
  • Nitrogen cycle - Quantify the breakdown (oxidation) of ammonia into nitrite and nitrate by nitrification bacteria. See Lab 3: Ammonia and nitrate measurements
  • Water quality - Measure several water parameters such as turbidity, chlorine content, pH, water hardness, phosphate content and more
  • Aquaponics - Measure levels of ammonia, nitrate, nitrite, phosphate and more in an aquarium
  • Population growth - Measure the turbidity of a microbial culture over time, which serves as an indicator of population growth
  • Enzyme kinetics - Measure the activity of an enzyme over time, even under different environmental conditions (temperature, pH, inhibitors, substrate)

The Educational Colorimeter uses eight (8) digital signals for normal operation. The provided Arduino shield connects 8 digital input-output (DIO) pins of the Arduino microcontroller board with the light sensor and RGB LED.

This project was launched on Kickstarter in 2012 and since then we have added more new features including expanded LED options, optional stir plate base and more assay testing.