Quantum Yields and Actinometry

2 Quantum yields and actinometry (V.Balzani, V.Carrassit/. Photochemistry of Co-ordination Compounds, Academic Press, 1970, London, pp8-12) 

The rate of a photochemical reaction depends on the wavelength of radiation and the rate of light absorption and it is generally little affected by temperature. The rate is expressed as quantum yield, I , defined as  

Ordinary or instrumental analytical methods are used to determine the rate of disappearance of and/or rate of formation of species involved in the reaction. The number of quanta absorbed depends on the nature and concentration of the absorbing species as well as the intensity of radiation absorbed i.e. the output of the lamp. Although physical methods can be used for this purpose (Sec.2.4.1), a more convenient and simpler method is to use a chemical actinometer. The term refers to a chemical system whose 0 is accurately known at various wavelengths. 

A mixture of a uranyl salt e.g. nitrate and an excess of oxalic acid (giving mainly uranyl oxalate) is one of the earliest and better studied systems for actinometry. The rate of disappearance of oxalic acid on irradiation is easily determined by titrating an aliquot of the solution and repeating the titration after exposing an identical aliquot to the same light source used for the photo-reaction and under identical conditions, to those used in the reaction under study. 

The % of light absorbed by the layer of the actinometric solution irradiated should be determined spectrophotometrically (Sec.2.4.1) at the wavelength of radiation used. A similar determination for the solution investigated is also required. The situation is complicated when polychromatic radiation is used. 

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