Colorimeter

What is a Colorimeter?

A colorimeter is a tool that is used in Colorimetry. It is a type of device which helps the solution to absorb a specific wavelength of light. It is generally used to measure the concentration of a known solute in a solution using the beer lamberts law. The colorimeter was invented by Louis J Duboscq in the year 1870.

Colorimetry means the measurement of color. But, in terms of chemical analysis, it is used for the measurement of the concentration of a particular solute in a colored solution that is solvent.

During experimental work, we are usually required to measure the amounts of a particular solute in a mixture or the concentration of the solution. It is used to identify the difference in colors of several mixtures and determine their absolute values. It is a more informative and scientifically very useful technique than simply having subjective judgments such as solutions being light or dark in color.

These are off Two Types;
• First is, Color densitometers, which are used to measure the density of primary colors.
• Second is Color photometers, which are used to measure the transmission and color reflection.

Principle of Colorimeter

It is a photometric technique that asserts that when a beam of incident light of intensity Io passes through a test solution, then;
• A part of it is reflected which is denoted as Ir
• A part of it is absorbed which is denoted as Ia
• The rest of the light is transmitted and is denoted as It
Therefore,
Io = Ir + Ia + It

Beer-Lambert Law

The Beer-Lambert Law states that the concentration of a substance in solution is directly proportional to the ‘absorbance ‘, A, of the solution.

The law is only true for monochromatic light, that is light of a single wavelength or narrow band of wavelengths and provided the physical or chemical state of the substance does not change with concentration.

When monochromatic radiation passes through a homogeneous solution in a cell, the intensity of the emitted radiation depends upon the thickness (l) and the concentration (C) of the solution.

The Components of a Colorimeter

This instrument is used to determine the concentration of a solution by measuring the quantity of absorbed light of a particular wavelength.

One of the earliest and modern designs, the Duboscq Colorimeter, was invented by Jules Doboscq, in the year 1870.

A colorimeter has the following components;
• Usually, a blue, green, and red LED light source is used to illuminate the solution.
• Filters are used for red, blue, and green wavelengths of light.
• A slit is used to focus the beam of light.
• A condenser lens is used to focus the beam of light into parallel rays.
• A cuvette is used to hold the solution. It is made up of glass, quartz, or plastic.
• A photoelectric cell, which is made up of selenium which is a light-sensitive material. It is a vacuum-filled cell and is used to identify the transmitted light and convert it into electrical output.
• An analog or digital meter to display output as transmittance or absorbance.

Working of Colorimeter

• The first step includes the calibration of the colorimeter instrument. It can be done with the help of the standard solutions of the known solute concentration that has to be determined. Then, fill the standard solutions in the cuvettes and place them in the cuvette holder of the Colorimeter.
• The second step includes the light ray of a particular wavelength is in the direction of the solution.
• The light travels through a series of filters and lenses. The colored light deviates by taking the help of lenses.
• The filter provides the split of a beam of light into different wavelengths which required a specific wavelength to pass through and reach the standard test cuvette.
• When the beam of light reaches the cuvette, then, it is transmitted, reflected, and absorbed by the solution.
• The transmitted ray reached the photodetector system, where the intensity of transmitted light is measured.
• Now, the photodetector system converts the beam into electrical signals and sends it to the galvanometer.
• The electrical signals that are measured by the galvanometer are shown in digital form.

Formula to ascertain the substance concentration in the test solution;
A = ∈cl
Where, ∈ and l are constant for the test solutions;
AT = CT —- (i)
AS = CS —- (ii)
From these two equations, we get the colorimeter formula;
AT × CS = AS × CT
CT = (AT/AS) × CS
Where;
AT – is the optical density
AS – is the absorbance 
CT – is the concentration of the test solution
CS – is the concentration.

Uses of Colorimeter

There are various uses of colorimeter such as;
• It is used in laboratories and hospitals to determine biochemical samples such as urine, plasma, serum, etc.
• It is used in the production of paints.
• This device is used to identify the strength and durability of colors which is used in paints and fabrics to ensure similar quality.
• It is used in the textile and food industry.
• It is used in the quantitative analysis of proteins, glucose, and other biochemical compounds.
• It is used to test water quality by screening the chemicals such as Cl, Fe, Zn, dissolved oxygen.
• It is used to determine the concentration of hemoglobin in the blood.
• It is used in the identification of drugs.
• Colorimeters are used to observe the bacterial growth or yeast culture.
• Some colorimeters can even measure the colors used in fax machines, copy machines, and printers.
• It is used to determine the plant nutrient concentrations such as nitrate, ammonia, and phosphorus in soil.

Advantages of Colorimeter

• It is an inexpensive method.
• It is widely used in the quantitative analysis of colored samples.
• It is easy to carry and transport.
• This technique is highly accurate and gives reliable results.

Disadvantages of Colorimeter

• It is not possible to the analysis of colorless compounds.
• It does not work in IR and UV regions.