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Determination of absorbed intensity:

A photochemical reaction occurs by the absorption of photons of photons by the molecule.Therefore it is essential to determine the absorbed intensity of light to study the rate of reaction. Light beam from a suitable source(tungsten filament or mercury vapour lamp) is rendered parallel by the lens.The beam then passes through a filter or monochrometer which yields light of one wavelength only.The monochromatic light enters the reaction cell made of quartz.The part of the light that is not absorbed strikes the detector.Thus the intensity of the light is measured first with the empty cell and the cell filled with the reaction sample.The first reading gives the incident intensity and second gives the transmitted intensity. The difference between the two gives the absorbed intensity. There are diffrent types of detectors: •thermopile •photoelectric cell •chemical actinometer.

THERMOPILE: It is made of a series of thermocouples in which unlike metals such as bismuth and silver are joined ttogether.One end of the couple is blackened with lampblack and the other end is left as such.When the radiation strikes the black end ith absorbs ebergy and is heated up.The temperature difference between the two ends causes a current to flow in the circuit and is indicated by the galvanometer.The current is proportional to the intensity of radiation.

PHOTOELECTRIC CELL: In a photoelectric cell the light striking the active metal electrodes(cesium ,sodium or potassium)causes the emission of electrons.A current flows through the circuit which can be measured with an ammeter.The intensity of the light is proportional to the current.

CHEMICAL ACTINOMETER: It uses a chemical reaction whose rate can be determined easily.One such sime device is uranyloxalate actinometer.It contains 0.05M oxalic acid and 0.01M uranyl sulphate in water.When it is exposed to radiation oxalic acid is decomposed to carbon dioxide,carbon monoxide,and water. The concentration of oxalic acid that remains can be found by titration with standard KMnO4 solution.The used up concentration of oxalic acid is a measure of intensity of radiation.