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

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.

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

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.

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.

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.

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.

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.