Constructing and Testing a Versatile Colorimeter

The construction and operation of a colorimeter are described. The instrument utilizes a standard cuvette of the type used in a Spectronic 20 spectrophotometer. The cuvette is held in a holder made of wood, which can be dissembled to change the light source (a light emitting diode, LED) and detector (a photocell or photoresistor). In the instrument, two inexpensive digital multimeters are employed, one to measure the voltage supplied to the LED and the other to measure the resistance produced in the detector. Power is supplied by three AAA batteries, and a potentiometer is incorporated to make it possible to control the voltage to the LED. Measurements were performed on potassium permanganate solutions using a green LED light source. The results obtained resulted in linear relationships between the absorbance measured by a spectrophotometer and the logarithm of resistance measured with the colorimeter. The colorimeter yielded results that are in good agreement with those obtained with a Spectronic 20 spectrophotometer. The total cost of the components for the colorimeter is approximately 20.