Actinometry, actinometer

Measurement of the light intensity under conditions identical to those used in the photolysis of the compound of interest is essential for the determination of a quantum yield. Although a number of instrumental methods for measuring light intensities are available, unless these are carefully calibrated, the most accurate means is to use a chemical actinometer. This can be any photochemical reaction for which the quantum yield at the wavelength of interest is accurately known. The following photochemical systems are most commonly used for solution actinometry. 

The fractional light absorption can be measured in a separate experiment. Knowing and estimating the extent of decomposition, f , the incident intensity can be calculated in the units of einstein cm-3s-1 falling on the reaction cell. To avoid geometrical errors due to differences in absorptivity of the actinometer solution and the sample, the same cell is used for actinometry and for the reaction, under conditions of equal optical densities. There are a number of photochemical reactions which have been found suitable as actinometers. They are useful within their specific wavelength ranges. 

Example of Application Large-Scale Actinometry. Neural network modelling was applied to large-scale actinometry in a continuous elliptical photochemical reactor with a concentric annular reaction chamber [2, 3,108, 148], Uranyl oxalate was used as an actinometer, which is based on the photosensitized decomposition of oxalate ions (Eq. 89) [2, 3] the experimental data were taken from the literature [108],... 

Gunter Gauglitz of the Eberhard-Karls University, Tubingen, Germany and his associates have done much research in the field of photochemistry and in particular chemical actinometry. His contributions include the development of new actinometers as well as practical guides to their usage and other articles. Four good... 

Much of his work, discussed in Chapter 8 of this book, is important because chemical actinometry is the only way of truly measuring the absorbed dosage of the incident radiation. A chemical actinometer corrects for container problems such as reflection, refraction, geometry, absorption, internal reflections, etc. 

In the paper are included solid phase, gas and liquid chemical systems, a listing of electronic actinometers and recommended actinometric procedures. Also included in this paper are "Recommended Actinometric Procedures," and "General Considerations on Chemical Actinometry." This latter topic contains items such as the pros and cons of using chemical actinometry, quality marks of a good actinometer, fields of application, as well as potential errors such a refractive index, temperature, absorption by photoproducts and the degree of absorption by the chemical actinometer itself. 

The total irradiance received by a sample can be determined by chemical actinometry, which uses a reaction of known photochemical efficiency. The ICH guideline proposes quinine actinometry as a standard based on the assumption that its increase in absorbance is proportional to the integrated UVA irradiance for a given lamp (22). The suitability of quinine as actinometer is, however, questioned in several different reports (10,23,24). The photolysis of quinine is sensitive to temperature, pH, and dissolved oxygen content. The physical characteristics of the quartz cell (e.g., cuvette dimension) are further shown to influence the result (24). 

The determination of photochemical quantum yields is not a simple task, and, in some cases, approximations are required. Nevertheless, according to the parameters chosen, various well-known photochemical reactions can be used to measure irradiance, which is an essential quantity in the field of photokinetics. Finally, some selected chemical actinometers will be discussed with respect to their pros and cons and their best areas of application. At the end, special applications of actinometry such as measurements of polychromatic light and high-intensity light sources (lasers) will be described. The overall aim of this chapter is to help the reader to choose the best actinometers out of the numerous examples in the literature and avoid technical mistakes. 

Demas JN, McBride RP, Harris EW. Laser intensity measurements by chemical actinometry. A photooxygenation actinometer. J Phys Chem 1976 80 2248-2253. 

Quantum yields are often useful quantitative measures of the efficiency of photochemical processes. Among the more traditional methods of quantum yield determination is ferrioxalate actinometry by UV-vis spectrophotometry. Since many organometallic complexes contain strong infrared chromophores, e.g. CEO, CENK, ve describe herein a method for determining quantum yields in Ca solution IK cells using a novel and simple actinometer the photochemical disappearance of tta lCO)in neat CCl to give Mn(CO)5CI (1). 

The various actinometer systems applicable to all types of photoprocesses have been summarized by Kuhn et al. (1989) for the International Union of Pure and Applied Chemistry Commission on Photochemistry. The fundamental problem is that actinometry is a technically demanding task, and it is then a complicated matter to relate the photon absorption rate to a reaction rate applicable to a particular experimental setting. In the following sections, the procedure will be briefly described for chemical actinometry and quantum yield determination then it will be explained that the rate of a photochemical reaction can be predicted from knowledge of the quantum yield and the incident photon intensity. Finally, treatment of experimental rate data for a photostability study will be covered in detail. 

The principles of actinometry are well established, but there is no well-established actinometric system sensitive only to UV radiation (Kuhn et al., 1989 Piechocki and Wolters, 1993). Quinine was originally proposed by the Japanese National Institute of Health Sciences and the Japanese Pharmaceutical Manufacturer s Association (Yoshioka et al., 1994). The results of the work of the FDA and U.S. laboratories have confirmed its suitability (Drew et al., 1998). At the recommended concentration (2%), the quinine solution has an absorbance of >2 from 320 to 367 nm, so the UV irradiance over this range will be determined using this system. Quinine usually contains a number of alkaloid impurities, principally dihydroquinine. However, the level of the latter does not affect its performance as a UV actinometer. 

Accordingly, we determined the quantum yield of the photohydrate of 1,3-dimethyluracil (DMU) for initial DMU concentrations of 1 X 10-3M, and 1 X 10-4M in unbuffered triple-distilled water. The measurements were made according to the conventional double cell technique (12), using uranyl oxalate as the actinometer. In this method, the total number of incident and transmitted quanta are measured by chemical actinometry. The quantum yield for DMU disappearance was found to be 3.93 X 10-3 at 1 X 10-4M DMU and 3.79 X 10-3 at 1 X 10-3M DMU (single determinations), and is thus independent of concentration in this range. 

It is clear from these equations that a standard method of measuring light is needed for any quantitative assessment of a photochemical process. For this purpose, the intensity of light falling on a sample must be precisely defined in photons per second, so that it can be related to the quantum yield. This is done by using a reference sample, commonly termed the actinometer, of known quantum yield. The method itself is known as chemical actinometry (see Borrell, 1973, for details). 

Actinometry is an indirect spectroscopic monitor of the reactive species densities. A low known quantity of an actinometer (more often argon) is introduced in the flow and the evolution of the intensities ratio Io/Iat of an oxygen line to an argon line is measured along the discharge zone. 

An alternative approach is to use chemical actinometry. A chemical actinome-ter makes use of a photochemical reaction that has an accurately known and easily reproducible quantum yield. One such chemical actinometer uses a solution of potassium ferrioxalate in 0.1 M sulfuric acid. Irradiation of this oxygen-ffee... 

An apparatus that allows actinometry and photochemistry to be carried out simultaneously is called a merry-go-round. In such an apparatus the tubes containing the actinometer and the chemical reactions are held equidistant from a single light source, thereby ensuring that each solution receives the same number of incident quanta for a given time interval. Such an apparatus also compensates for fluctuations in both light intensity and temperature that may occur during the elapsed time of the measurement. 

Chemical actinometry has the advantage over physical methods in that the actinometer can be irradiated under conditions similar to those of the photoreaction to be studied [1762]. This eliminates the need to make corrections due to reflectance and nonuniformity of the incident light beam. The liquid-liquid phase chemical actinometer traditionally used is the potassium ferrioxalate system, but it requires the careful preparation of standard solutions and of a calibration graph before measurements can be made [299, 920]. 

There is a direct relation between the rate of a photochemical reaction and rate of light-quantas absorbed, and their ratio is known as quantum efficiency. To determine quantum deld of a particular reaction, it is necessary to know incident light flux /q. There are a number of techniques available for the measurement of Iq. The equipment used for this measurement is known as actinometer and phenomenon Actinometry. Actinometry provides determination, measurement and standardisation of the light source. For perfect calibration, a standard lamp (light-source) of known colour-temperature is used to standardise the detector which may be ... 

Several actinometers have been proposed in the literature, but none of these meets all the given criteria. A general report on chemical actinometry was prepared by the lUPAC Commission on Photochemistry [6]. Surveys on the most relevant used actinometers can be found in [1, 2, 4, 7]. 

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