By Sunil Bhardwaj

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Photoelectric colorimeter is the instruments, with the help of which quantitative analysis of coloured solutions is possible. The photoelectric colorimeters are of two types. Single beam photoelectric colorimeters and Double beam photoelectric colorimeters.

TLC

Single beam photoelectric colorimeter is consisting of the following components:-

1. A source of visible light: An incandescent lamp (on heating glow) with a tungston filament .

2. Collimating convex lens: It collects the radiation from source and the transmitted radiation will be traveling parallel to the principle axis because the source is kept exactly at the focus of the lens.

3. Diaphragm: The function of diaphragm is to set 100% transmittance.

4. Filter: It is in the circular disc which is made up of no of colored glass patches. When the light from the source is allow to pass through this filter. The unnecessary radiations are absorbed and we get light of only one colour.

5. Sample Holder OR Cuvette: It is a rectangular transparent container made up of glass or quartz

6. Photo cell: It is the component which converts the radiations into current. It converts the transmitted beam emerging from sample into an electric current. This electrical pulse is due to the emission of electrons from the photocathode surface caused by the transmitted beam falling on the photo electrode.

7. Signal indicator Read-out meter or digital display: It is a meter calibrated in 0-100 transmittance or absorbance units. Some meter covers both absorbance and transmittance. The optical density or percentage transmittance can be read directly.

Working: Light from source such as incandescent lamp with tungsten filament passes through collimating convex lens and then through an adjustable diaphragm. By adjusting this diaphragm, the intensity of incident radiation can be altered to any required level. Light is then incident on a filter which allows only a narrow band of wave lengths to pass through it in the curette. The solvent or the sample solution is placed in the curette and transmitted light falls on the photo Cell producing small current. The current is then incident on Read out meter or dial, which reads transmittance or OD.

First we take blank i.e. solvent in the cuvette which has been properly cleaned and insert it in the optical path of the instrument. As it is blank therefore no absorption and light is fully transmitted and then incident on photocell and galvanometer shows 100% transmittance. If there is any absorption due to impurities, this can be nullified by adjusting the reading of O.D. to zero or transmittance to 100%.

Now remove the blank from cuvette and put the sample solution. When the light is incident on it some radiations are absorbed and the remaining is transmitted which is incident on photocell which will give O.D. or transmittance. If O.D. or transmittance is known then by using Beer-Lamberts law. $$ O.D. = A= \log { \frac { { I }_{ 0 } }{ { I }_{ t } } } =-\log { T } = \varepsilon cl $$

Single beam photo electric colorimeters have following disadvantages:-
1. As the source of radiations is AC. i.e. alternating Current i.e. Fluctuations in the voltage causes fluctuations in reading.
2. As the solvent contain the impurities, which also absorb the radiations. Therefore, it is not possible to get the correct amount of light absorbed.
3. As filter is used which do not give 100% monochromatic light. Therefore analysis is not accurate has error.

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