By Sunil Bhardwaj

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Double beam photoelectric colorimeters are the instrument with the help of which we can have quantitatively analysis of coloured solutions.

TLC

It is consist of the following components

Source of radiations: Incandescent lamp with tungsten filament i.e. ordinary electric bulb can be used.

Collimating convex lens: It is convex lens. The source is kept at the focus of the convex lens; the transmitted radiations are traveling parallel to principle axis.

Diaphragm: it is used to set 100% transmittance.

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.

Plane mirror: to change the direction of the beam of light.

Two cuvettes (sample holders): one for sample solution and the other for Blank.

Two photo cells: one is connected to that cuvette which contains sample solution and the other is connected to that photocell which contains Blank.

Two Jockey: Two Jockeys are connected on two resistances AB & CD to adjust null point.

Galvanometer: Sensitive galvanometer is used to get the null point.

Working: Light from source such as incandescent lamp with tungsten filament passes through a collimating convex lens and then through diaphragm to set the 100% transmittance and then through colored glass filter to obtain monochromatic light. A mirror placed at an angle in the path of the light beam, emerging from the filter splits the beam into two. One part of beam is made to pass through the sample solution, placed in a cuvette and the photo cell. The other part is made to pass through the solvent or Blank placed in an identical matched cuvette and then to another photo cell (reference cell). When the transmitted lights from two cuvettes falls on photo cells produce electric currents. These currents are passed through resistance AB and CD. AB is calibrated in 0-100 transmittance. A sensitive galvanometer is connected across AB and CD which serves as null indicator (detector).

The sample solution is then placed in the same cuvette. The radiant power falling on the photocell opposite the sample cell will decrease and the galvanometer will not be balanced. Now we will slide the J1 Jockey toward the lower value till no deflection is obtained this will be the transmittance of the solution.

Then by using Beer Lamberts law, concentration of the coloured solution can be determined. $$ O.D. = A= \log { \frac { { I }_{ 0 } }{ { I }_{ t } } } =-\log { T } = \varepsilon cl $$

Double beam photoelectric colorimeters have following advantages:
1. As the two beams of radiations are passed simultaneously from sample solution and Blank Therefore any fluctuation in voltage can be compensated (cancelled).
2. As the two beams are passed from Blank and solution any impurities present in the solvent will not affect the absorbance.
3. Accuracy is more than single beam photoelectric colorimeter.
4. The main disadvantage is the instrument is costly.

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