Colorimeter

Colorimeter refers to a device used in colorimetry that aids in the absorption of a particular wavelength of
light by a specific sample solution.
Louis J. Duboscq invented it in the year 1870. It is employed to measure how much light transmits and
absorbs as it passes through a liquid. These are used to identify the color and establish the concentration of a
solution. By comparing a solute’s color intensity in a solution to that of a reference solution with a known
solute concentration, one can estimate the concentration of colored solute in that solution.

Principle of Colorimeter
When a beam of incident light of intensity I0 passes through a solution, following events occur:
● A part of incident light is reflected. It is denoted by Ir
● A part of incident light is absorbed. It is denoted by Ia
● Remaining incident light is transmitted. It is denoted by It
As Ir is kept constant by using cells with identical properties, The light that is not absorbed is transmitted
through the solution and gives the solution its color. Note that color of the incident light should be
complementary to that of color of the solution.
The ratio of the intensity of transmitted light (It) to the intensity of incident light (I0) is called transmittance
(T). Photometric instruments measure transmittance. In mathematical terms,
T = It/I0
The absorbance (A) of the solution (at a given wavelength) is defined as equal to the logarithm (base 10) of
1/T. That is,
A = log (1/T)

These measurements are dependent on two important laws:

1. Beer’s law:
When monochromatic light passes through a colored solution, the amount of light absorbed is directly
proportional to the concentration (C) of solute in the solution.
2. Lambert’s law:
When monochromatic light passes through a colored solution, the amount of light absorbed is directly
proportional to the length (L) or thickness of the solution.
When combining Beer-Lambert’s law,
Absorbance (A) α CL
or, A= KCL
where K is a constant known as absorption coefficient.
As the path length is same (as same cuvette is used), Concentration of an unknown solution can be
determined by using equation:

Figure: Schematic diagram of Colorimeter

A series of lenses in a colorimeter guide a beam of light with a particular wavelength through a solution as it
makes its way to the measuring apparatus. This compares the color to a current standard to examine it. The
absorbance or % transmittance is then calculated using a microprocessor. By measuring the difference
between the amount of light at its source and that after passing the solution, it is possible to determine the
concentration of the solution and how much light will be absorbed. Several sample solutions with known
concentrations are first prepared and evaluated to ascertain the concentration of an unknown sample.
Plotting the concentrations versus absorbance on a graph yields the calibration curve. To determine the
concentration, the results of the unknown sample are contrasted with those of the known sample on the
curve.
Components of Colorimeter
• Light Source: The source of light should produce energy with enough intensity to cover the entire visible
spectrum (380-780 nm). Commonly, Tungsten lamps are used as a light source for measurement in the
visible spectrum and near-infrared ranges. Halogen deuterium is suitable for measurement in the UV range
(200-900 nm).
• Slit: It reduces unwanted or stray light by allowing a light beam to pass through.
• Condensing lens: Parallel beam of light emerges from condensing lens after the light passes through slit
incidents on it.
• Monochromator: It filters the monochromatic light from polychromatic light, which absorbs unwanted light
wavelengths and permits only monochromatic light. These are of three types: prism, grating, and glass.
• Prism: It facilitates the refraction of light when it passes from one medium to another.
• Glass: It selectively transmits light in certain ranges of wavelengths.
• Gratings: These are made of graphite, which separates light in different wavelengths.
• Cuvette (Sample cell): The monochromatic light from the filter passes through the colored sample solution
placed in the cuvette. Their sizes range from square, and rectangle to round and have a fixed diameter of
1cm. These are of three types based on the substances these are made of: Glass, Quartz, and Plastic cuvette.
• Glass cuvettes are cheap and absorb light of 340 nm wavelength.
• Quartz cuvettes facilitate entry of both lights of UV and visible ranges.
• Plastic cuvettes are cheaper, easily scratched, and have shorter lifespans.
• Photocell (Photodetector): These photosensitive devices measure light intensity by converting light energy
into electrical energy.
• Galvanometer: The electrical signal generated in a photocell is detected and measured by a galvanometer. It
displays optical density (OD) and percentage transmission.

Fig: A basic Colorimeter’s user interface

Advantages of Colorimeter
• A quick and affordable means of evaluating the quality of a liquid sample is the colorimeter.
• The Colorimeter makes it simple to perform a quantitative examination of colored chemicals.
• Results are available in under seconds.
• Just Four AA batteries are enough to run a portable colorimeter for 100 and 300 times.

Disadvantages of Colorimeter
• The procedure of determining the concentration of colorless substances becomes laborious.
• Colorimeter does not function in the ultraviolet or infrared spectrum since it only measures wavelength
absorbance in the visible range of light (400nm to 700nm).
• A spectrum range must be set rather than a specific wavelength to measure the absorbance.
• Measurements might be challenging on surfaces that can reflect light.

Colorimeter Operating Procedure (General):

1. Switch the device on by rotating the Power Switch knob in a clockwise direction (toward the right). 15
minutes of warming up time for the colorimeter is required to stabilize the light source and the detector.
2. After the warm-up period, turn the Wavelength Control knob to the appropriate wavelength.
3. Press the MODE control key until the light next to “Transmittance” turns on to switch the display mode to
transmittance.
4. Use the Zero Control knob to set the display’s T-factor to 0.0%. Make this adjustment while ensuring the
sample chamber is empty and the cover is securely closed.
5. Place the blank solution in a cuvette until it reaches the top of the triangle on the side of the cuvette. To get
rid of any fluids or fingerprints on the cuvette’s exterior, wipe it with a Kimwipe. Both will obstruct the light’s
ability to travel and result in inaccurate readings.
6. Place the tube gently but completely into the cuvette chamber, with the vertical guide line facing in the
direction of your right. The guideline on the cuvette should now be lined up with the guideline on the sample
chamber by rotating the cuvette 90 degrees in a clockwise orientation. This method protects the cuvette against
scratches in the light transmitting portions. Erroneous measurements can result from scratches on the cuvette.
Close the compartment’s cover.
7. Set the display to 100.0% using the Transmittance/Absorbance control knob.
8. Press the MODE control key and switch the Status Indicator light to read Absorbance. The display should
indicate 0.0 if the Transmittance calibration was done correctly. No further adjusting is necessary. Use the
Transmittance / Absorbance control knob to set the display to 0.0 if it does not already show that value. Switch
the display back to Transmittance using the MODE key.
9. Reverse the previous process to remove the cuvette from the compartment by rotating it 90 degrees
counter-clockwise before doing so. You should put the solution whose absorbance you want to test in another
cuvette. Similar to before, place it inside the chamber.
a. Directly from the digital display, read the %T value.
b. Select Absorbance using the MODE key, then take the A value directly from the digital display. Select
Transmittance once more.
10. Reverse the process you used to insert the cuvette to remove it from the sample compartment. Close the
compartment’s cover.