 # Colorimeter

## What is Colorimeter?

A colorimeter is a device that is used in Colorimetry. It refers to a device which helps specific solutions to absorb a particular wavelength of light. The colorimeter is usually used to measure the concentration of a known solute in a given solution with the help of the Beer-Lambert law. The colorimeter was invented in the year 1870 by Louis J Duboscq.

Check out the derivation of Beer-Lambert law here.

## Principle of Colorimeter

It is a photometric technique which states that when a beam of incident light of intensity Io passes through a solution, the following occur:

• A part of it is reflected which is denoted as Ir
• A part of it is absorbed which is denoted as Ia
• Rest of the light is transmitted and is denoted as It

Therefore, Io = Ir + Ia + It

To determine Ia the measurement of Io and It is sufficient therefore, Ir is eliminated. The amount of light reflected is kept constant to measure Io and It.

Colorimeter is based on two fundamental laws of photometry. We have discussed them below:

### Beer’s law:

According to this law the amount of light absorbed is proportional to the solute concentration present in solution.

Log10 Io/It = asc

where,

as is absorbency index

c is the concentration of solution

### Lambert’s law:

According to this law the amount of light absorbed is proportional to the length as well as thickness of the solution taken for analysis.

A = log10 Io/It = asb

Where,

A is the test absorbance of test

as is the standard absorbance

b is the length / thickness of the solution

## Diagram of Colorimeter ## Working of Colorimeter

Step 1: Before starting the experiment it is important to calibrate the colorimeter. It is done by using the standard solutions of the known solute concentration that has to be determined. Fill the standard solutions in the cuvettes and place it in the cuvette holder of colorimeter.

Step 2: A light ray of a certain wavelength, which is specific for the assay is in the direction of the solution. The light passes through a series of different lenses and filters. The coloured light navigates with the help of lenses, and the filter helps to split a beam of light into different wavelengths allowing only the required wavelength to pass through it and reach the cuvette of the standard test solution.

Step 3: When the beam of light reaches’ cuvette, it is transmitted, reflected, and absorbed by the solution. The transmitted ray falls on the photodetector system where it measures the intensity of transmitted light. It converts it into the electrical signals and sends it to the galvanometer.

Step 4: The electrical signals measured by the galvanometer are displayed in the digital form.

Step 5: Formula to determine substance concentration in test solution.

A = ∈cl

For standard and test solutions

∈ and l are constant

AT = CT ….. (i)

AS = CS ….. (ii)

From the above two equations,

AT × CS = AS × CT

CT = (AT/AS) × CS

Where,

CT is the test solution concentration

AT is the absorbance/optical density of test solution

CS is the standard concentration

AS is the absorbance / optical density of standard solution

## Uses of Colorimeter

• It is used in laboratories and hospitals to estimate biochemical samples such as urine, cerebrospinal fluid, plasma, serum, etc.
• It is used in the manufacturing of paints.
• It is used in 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.
• It is used to determine the concentration of haemoglobin in the blood.