Chemical actinometry

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 decomposition of the excited state, C02 — -COOX + 0(3P, 1D, and 3S) is generally assumed simple. The state 0(3P) has apparently not been seen, which is consistent with Wigner s (1927) rule. In any case, the reported quantum yield of unity for the production of CO is often used in far-UV chemical actinometry. 

All the 1-pyrenylbismuthonium salts photochemically decompose (2ex > 320 nm / > 150 mW cm 2) to generate their respective protic acids, accompanied by the formation of bismuth(III) compounds and pyrene (Scheme 25). The quantum yields of the photodecomposition (<7>dcc) in acetonitrile were determined by chemical actinometry to be 0.20-0.22. These values are comparable to the values reported for the triarylsulfonium and diaryliodonium salts (dec = 0.17-0.22) [98, 99]. 

There are several approaches to measuring actinic fluxes and photolysis rate constants. One approach is to measure the rate of decay of a species such as N02 directly, so-called chemical actinometry (e.g. see Madronich et al., 1983). Another approach is to measure the light intensity and convert this to an actinic flux. 

As described earlier, measurements of actinic fluxes are made using chemical actinometry, particularly the photolysis of N02 or 03, or using flat-plate or 277 radiometers. Intercomparisons of such measurements... 

Chemical Actinometry Using o-Nitrobenzaldehyde to Measure Light Intensity in Photochemical Experiments 251... 

First-Order Rate Constant for Quantification of Direct Photolysis Illustrative Example 15.3 Estimating the Photolysis Half-Life of a Weak Organic Acid in the Well-Mixed Epilimnion of a Lake Determination of Quantum Yields and Chemical Actinometry Advanced... 

Unfortunately, it is not possible in most cases to quantify a given photooxidant by a direct measurement. By analogy to chemical actinometry (Section 15.4), however, one may use a probe or reference compound (P, subscript ref) with known k p ox ref to determine [Ox]° in a given natural water. This involves adding the chemical at a known concentration to the water, illuminating, and measuring the compound s disappearance. Since the probe compound disappearance kinetics also obeys Eq. 16-7, [Ox]° can then be calculated from the slope of a correlation of In [P] versus time ... 

The reaction now affords investigators another valuable tool because the absolute intensity of the chemiluminescence has been carefully measured. It can thus be used as a standard light source against which other chemiluminescent reactions can be measured without the need of detector calibrations and geometry corrections. This convenience is due largely to the fine work of Fontijn, Meyer, and Schiff,145,147 who used chemical actinometry to measure the emission from a discharge-flow system. In their first report on this reaction, they determined the value of k12 as 1.0 x 104 M x sec-1 for emission in the... 

In practice both the incident light intensity I0 and the fraction of light absorbed /a vary with time, the former because of the instability of the output of the light source, and the latter because of the change in the absorbance of the sample at Ar in the course of irradiation. Although it is possible in principle to measure and integrate the light intensity by means of electronic detectors, the technique of chemical actinometry is still widely used in practice. 

Photolysis of oxalate complexes show that there is a strong tendency to undergo photoredox decompositions, resulting in oxides of carbon. Two useful applications of this are (i) the system based on the redox photolysis of aqueous [Fe(C204)3]3- is widely used for chemical actinometry 100 and (ii) UV irradiation of (phos)2M(C204) complexes (M = Pd, Pt) result in loss of two molecules of carbon dioxide and production of the synthetically useful, coordinatively unsaturated M° complex (phos)2M.19 One reaction which, if generally applicable to dicarboxylate complexes, may have considerable impact upon the validity of physical measurements upon these systems is the rather unusual, room temperature, solid-state reaction (2).102... 

Kuhn HJ, Braslawski SE, Schmidt R. Chemical actinometry. Pure Appl Chem. 1989 61 187-210. 

The output of a lamp may be monitored by chemical actinometry. The standard method remains the irradiation of potassium ferrioxalate which is useful in the range 254 to 480 nm. The method is described in detail by Calvert and Pitts (1966) and by Murov (1973). But perhaps, the most convenient way to check the lamp output is to keep a stock solution of a model compound or the reagent itself, if it is readily available, and determine its rate of photolysis periodically by irradiation in a spectrophotometer cuvet. In this way the output of the lamp in the region of interest can be rapidly checked. An alternative is to use one of many radiation measuring instruments that are commercially available, a thermopile and voltmeter, for example (see Appendix for a list of manufacturers). 

Heller and Langan38 reported that the quantum yield for photocoloration (c) of fulgide 35 to the 7,7a-DHBF (36) in toluene was 0.20, and the E->c value appeared to be wavelength independent over the range 313-366 nm. Temperature (10-40°C) had little effect on the quantum yield for photocoloration. Furthermore, the cycles of photochromism of 35-36-35 did not affect the (E - c) value. These results showed that fulgide 35 is well suited for chemical actinometry in the near-UV and visible spectral region. 

Disinfection of Water, Air, Surfaces Quantification of Radiation Actinometry Synthesis of Fine Chemicals AOPs AOTs... 

Kuhn HJ, Braslavsky SE, Schmidt R (1989) Chemical Actinometry, Pure Appl. Chem. 

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. 

Hans Jochen Kuhn and Silvia Braslavsky, headed the International Union of Pure and Applied Chemistry (lUPAC), Organic Chemistry Division Commission on Photochemistry Committee on Chemical Actinometry. The duty of this group was to provide a list of chemical systems that have been found suitable for the integration of incident radiation by chemical conversion. This list was published in 1989 in the lUPAC Journal, Pure and Applied Chemistry (113). 

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. 

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