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 light in a particular solution, that is, how much light is absorbed and how much light passes through in a liquid or solution by analysing colour intensity.
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 lightabsorbing substances.
It permits particular solutions to absorb a specific wavelength (colour) of light.
This instrument was invented by Louis J Duboscq in 1870.
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It is based on photometric technique. According to this, when an incident light 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 light 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 light absorbed.
According to this law, absorbance is directly proportional to the concentration. In other words, highly concentrated solutions absorb more light 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
According to this law, the amount or quantity of light 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 light 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 proportionality,$\mathrm{k^{'}}$ = k(2.303) = another constant
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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The working of colorimeter is explained in the following three stages -
All tools or instruments are needed to calibrate before starting any experiment. Here, the colorimeter is calibrated 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.
In this stage, a beam of light of a particular wavelength specific to the analysis is directed toward the sample solution. This light beam passes through various lenses and filters, before reaching the sample. This coloured light roams with the help of lenses and reaches a filter that allows the separation of the light beam into separate wavelengths and allows only the required wavelength to pass through and reach the cuvette.
Finally, the beam of light 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 light. Then, the photodetector converts the beam of light into electrical signals which are then calculated by galvanometer in digital form.
| S.no | Industry | Uses |
|---|---|---|
| 1 | Hospitals | Estimating biochemical samples, like plasma, serum, urine and CSF-cerebrospinal fluid. |
| 2 | Textiles | It helps in specifying the colour and ensuring the colours we see on screen are accurate. |
| 3 | Laboratories | Quantitative estimation of serum components as well as glucose, protein and other various biochemical compounds. |
| 4 | Water quality Department | It checks water quality for the chemicals like fluoride, zinc, iron, chlorine, cyanide, dissolved oxygen etc. |
| 5 | Paints | Check the consistency and quality of colours. |
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 qualitatively 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 light, making it difficult to take measurements.
It has low sensitivity.
It requires accurate wavelength bandwidth for more accurate analysis.
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 light passing through the sample solution while analysis. When the light beam of a particular wavelength is transferred to a sample, some light is absorbed and some are passed, only transmitted light waves are detected, which are used to measure colour density. It also helps specific solutions absorb a certain wavelength of light. 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.
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 light 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 ability to reflect light. Lighting can affect colour perception. Blurred light can wash away certain colours, while bright light can intensify others.
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