Introduction

While performing experiments, scientists often need to measure the values of a particular compound in the concentrated solution or coloured solutions. Only eyes are not good enough to decide the concentration in coloured solutions. To carry out such experiments, a device is used known as a colorimeter. Therefore, it helps detect colour and determine the colour and determine a solution’s concentration. It works on the principle of absorbance of a specific wavelength of pght in a particular solution, that is, how much pght is absorbed and how much pght passes through in a pquid or solution by analysing colour intensity.

What is a Colorimeter?

It is a device or tool that identifies colour samples or solutions to provide an objective measure of colour characteristics.

It helps in determining the concentration of a particular compound that is a solute in a colour solution or solvent.

It is widely used to identify and determine the concentration of pghtabsorbing substances.

It permits particular solutions to absorb a specific wavelength (colour) of pght.

This instrument was invented by Louis J Duboscq in 1870.

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Principle of Colorimeter

It is based on photometric technique. According to this, when an incident pght of intensity, say $mathrm{I_{0}}$ passes through a solution, then,

some part of the incident ray is reflected at $mathrm{I_{r}}$

some part of the ray is absorbed in $mathrm{I_{a}}$ and

some part of the incident ray is transmitted at $mathrm{I_{t}}$.

$$mathrm{I_{0}=I_{r}+I_{a}+I_{t}}$$

Therefore, the value of $mathrm{I_{r}}$ can be ignored as $mathrm{I_{0}}$ and $mathrm{I_{t}}$ values are enough to calculate the value of $mathrm{I_{a}}$. Hence, values for the amount of pght transmitted and absorbed are measured by keeping $mathrm{I_{r}}$ constant.

It is based on two essential laws of photometry that develop the relationship between the concentration of the substance and the amount of pght absorbed.

Beer’s law

According to this law, absorbance is directly proportional to the concentration. In other words, highly concentrated solutions absorb more pght than highly diluted solutions.

Mathematically it can be expressed as

$$mathrm{A=varepsilon cl}$$

Where,A = absorbance,$mathrm{varepsilon}$ = molor absorption coefficient,c = molor concentration and l = optical path length

Lambert’s law

According to this law, the amount or quantity of pght absorbed directly depends on the length and intensity of the solution taken for analysis. In other words, equal parts in the same absorbing area absorb the equal particles of pght that enters them.

Mathematically it can be expressed as

$$mathrm{log_{10}frac{I_{0}}{I_{t}}=frac{k}{2.303}b=k^{ }b}$$

Where,b = thickness,k = constant of proportionapty,$mathrm{k^{ }}$ = k(2.303) = another constant

Diagram of Colorimeter

Here is the diagram of the colorimeter showing different components of the colorimeter, and where the result is obtained from the galvanometer as output.

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Working of Colorimeter

The working of colorimeter is explained in the following three stages -

Stage 1: Capbrate the colorimeter

All tools or instruments are needed to capbrate before starting any experiment. Here, the colorimeter is capbrated by using a standard solution for known solute concentration which is required to find out. Then fill the standard solution in the tube or cuvettes, and put this tube in the colorimeter stand.

Stage 2: Passes through lens and filters

In this stage, a beam of pght of a particular wavelength specific to the analysis is directed toward the sample solution. This pght beam passes through various lenses and filters, before reaching the sample. This coloured pght roams with the help of lenses and reaches a filter that allows the separation of the pght beam into separate wavelengths and allows only the required wavelength to pass through and reach the cuvette.

Stage 3: Transmitted pght converted into an electrical signal

Finally, the beam of pght reaches the tube or cuvette, where it is transmitted, reflected, and absorbed by the solution. Only transmitted rays will be allowed to fall into the photodetector and help measure the intensity of the transmitted pght. Then, the photodetector converts the beam of pght into electrical signals which are then calculated by galvanometer in digital form.

Uses of Colorimeter

Advantages and disadvantages of Colorimeter

Some of the advantages are:

It is very fast, results in even less than 1 second.

Easily variable and transportable.

It is very simple to operate.

It can be quaptatively or semi-quantitatively identified by the naked eye.

A pocket-sized colorimeter can make 100-300 measurements of four AA batteries.

Some of the disadvantages are

It doesn t work in UV and IR regions.

It requires a large number of samples to be analyzed.

It may create errors sometime when the same colours of interfering material are used.

Some surfaces reflect pght, making it difficult to take measurements.

It has low sensitivity.

It requires accurate wavelength bandwidth for more accurate analysis.

Conclusion

A colorimeter is an instrument or tool that helps in measuring the concentration of a particular compound (known as solute) in a colour solution (known as solvent). This tool contains a photocell that can detect the amount of pght passing through the sample solution while analysis. When the pght beam of a particular wavelength is transferred to a sample, some pght is absorbed and some are passed, only transmitted pght waves are detected, which are used to measure colour density. It also helps specific solutions absorb a certain wavelength of pght. This is how it measures the concentration of a known solute in a solution provided with the help of the Beer-Lambert law, which mainly depends on the absorbance and the concentration of the solution.

FAQs

Q1. Does absorbance depend on colour?

Ans. According to Lambert s law, the absorbance directly depends on the thickness and path length of the absorbing substance. More thickness means more intensity, hence more the colouration of the substance. Hence absorbance depends on the colour.

Q2. What is the purpose of colorimetry?

Ans. A colorimeter tool is used to find the concentration of coloured compounds dissolved in solutions. Hence it is useful in finding the concentration of solute.

Q3. How do you maintain a colorimeter?

Ans. It is essential to clean the colorimeter and sample cells, all the time. For that purpose, use a soft and clean cloth, and distilled water to clean the sample cell.

Q4. Why does absorbance increase with concentration?

Ans. A more concentrated solution means more particles of solute are present in the solution, hence pght gets absorbed by a large number of particles in the solution, increasing absorption.

Q5. What s the easiest colour to see?

Ans. Strong and vibrant colours, such as orange, red, and yellow are often more visible. As bright colours are usually easily visible due to their abipty to reflect pght. Lighting can affect colour perception. Blurred pght can wash away certain colours, while bright pght can intensify others.