# Your Guide to Colorimeter Principle

In the field of the appearance of products, the color of the product is the most important aspect of appearance. The color contributes a major portion of the appearance and hence it is essential to ensure that the color of the products is of the best quality and level. To ensure that, a colorimeter is used. A colorimeter is a light-sensitive device used for measuring the transmittance and absorbance of light passing through a liquid sample. The device measures the intensity or concentration of the color that develops upon introducing a specific reagent into a solution. Now that you know what a colorimeter is, let’s discuss the colorimeter principle. Read this new blog in Linquip to find out more.

## Colorimeter Principle

A colorimeter is based on the photometric technique which states that when a beam of incident light of intensity I0 passes through a solution,

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A part of the incident light is reflected (Ir),

A part is absorbed (Ia

and the rest of the light is transmitted (It)

Therefore,

I0 = Ir + Ia + It

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In the colorimeter, (Ir) is eliminated because of the measurement of (I0) and It is sufficient to determine the (Ia). For this purpose, the amount of light reflected (Ir) is kept constant by using cells that have identical properties. (I0) and (It) are then measured.

The mathematical relationship between the amount of light absorbed and the concentration of the substance can be shown by the two fundamental laws of photometry on which the colorimeter is based. These laws are explained below.

### Beer’s Law

This law states that the amount of light absorbed is directly proportional to the concentration of the solute in the solution.

Log10 I0/It =  asc

where,

as = Absorbency index

c = Concentration of Solution

### Lambert’s Law

Lambert’s law states that the amount of light absorbed is directly proportional to the length and thickness of the solution under analysis.

A = log10 I0/It = asb

Where,

A = Absorbance of test

as = Absorbance of standard

b = length / thickness of the solution

## Colorimeter Working

When a colorimeter is used, there is a ray of light with a certain wavelength is directed towards a solution. Before reaching the solution the ray of light passes through a series of different lenses. These lenses are used for navigation of the colored light in the colorimeter. The colorimeter analyzes the reflected light and compares it with a predetermined standard. Then a microprocessor installed in the device is used for the calculation of the absorbance of the light by the solution. If the absorption of the solution is higher then there will be more light absorbed by the solution and if the concentration of the solution is low then more lights will be transmitted through the solution.

The colorimeter is based on Beer-Lambert’s law, according to which the absorption of light transmitted through the medium is directly proportional to the medium concentration.

As mentioned earlier, the colorimeter is based on Beer-Lambert’s law, according to which the absorption of light transmitted through the medium is directly proportional to the medium concentration.

The mathematical representation of the combined form of Beer-Lambert’s law is as follows:

Log10 I0 / It = asbc

If b is kept constant by applying Cuvette or standard cell then,

Log10 I0/It = asc

The absorbency index  as is defined as

as = A/cl

Where,

c = concentration of the absorbing material (in gm/liter).

l = distance traveled by the light in solution (in cm).

In simplified form, the Beer-Lamberts law is commonly written as:

A= Ɛcl

Where A is the absorbance (no units, since A = log10 P0 / P ), Ɛ is the molar absorptivity (with units of L mol-1 cm-1), c is the concentration of the compound in solution (expressed in mol L-1) and l is the length that the light passes through (expressed in centimeters).

• Note that if there is a continual changing of the solution, then the percentage of transmittance against time is generally used.
• To measure concentrations, the amount of light absorbed is dependent upon the amount of solute. In the solution, a higher concentration of dissolved solute means that more light will be absorbed, and vice versa, hence, the concentration can be backed out from the absorption of specific wavelengths.
• To determine the concentration of an unknown sample, several sample solutions of a known concentration are first prepared and tested. The concentrations are then plotted on a graph against absorbance, thereby generating a calibration curve. The results of the unknown sample are compared to that of the known sample on the curve to measure the concentration.

## Colorimeter Applications

• The colorimeter is commonly used for the determination of the concentration of a colored compound by measuring the optical density or its absorbance.
• It can also be used for the determination of the course of the reaction by measuring the rate of formation and disappearance of the light-absorbing compound in the range of the visible spectrum of light.
• By colorimeter, a compound can be identified by determining the absorption spectrum in the visible region of the light spectrum.
• 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 by screening chemicals such as chlorine, cyanide, fluoride, dissolved oxygen, iron, zinc, hydrazine, and molybdenum.
• It is used to determine the concentration of haemoglobin in the blood.