Ferrioxalates, actinometry

Ferrioxalates, actinometry, 1225 Ferrocene biosensors diacyl peroxides, 701... 

Wilkinson s catalyst. Irradiation at 366 nm of 0.001 M RhCPPh NO and 1 M cyclohexene in o-dichlorobenzene was carried out under 1 atm H2 at room temperature. The hydrogen uptake was monitored using a mercury manometer attached to the reaction flask. Hydrogen was added periodically in order to maintain 1 atm pressure in the system. The solvent and olefin were distilled twice and degassed by three freeze-pump-thaw cycles before use. A 1000 watt Hg lamp filtered with a glass filter to isolate the 366 nm Hg line was used for all photolysis experiments. The light intensity, measured by ferrioxalate actinometry, was 1.0 x 10 6 einsteins/min. 

While Hatchard and Parker [32] concluded that complexes of Fe(III) with 1-3 oxalate ligands have equal quantum yields at 366 nm, Vincze and Papp [50] determined quantum yields at 254 nm of 0, 1.18 and 1.60 for Fe(C204)+, Fe O - and Fe O 3", respectively. In view of these markedly different quantum yields, and the changes in Fe(III) oxalate spe-ciation expected with change in solution composition (Fig. 4), it is to be expected that the quantum yields of Fe(II) production from Fe(III)-oxalate complexes will be significantly affected by the solution composition and pH. While the quantum yields reported by Hatchard and Parker [32] for ferrioxalate actinometry have been confirmed by several other authors (references cited in [26]), values of c Pedi) under other conditions have not been well studied. 

The reader may have recognized Dobereiner s system as the basis of ferrioxalate actinometry [22,23]. Indeed, this system was utilized throughout the nineteenth century in numerous efforts to determine the energy of solar radiation [24-32], giving rise on more than one occasion to disagreements and arguments about its merits and its most appropriate application. 

Light intensities were measured by potassium ferrioxalate actinometry (51). 

Y. Quan, S. Pehkonen, and M.B. Ray, Evaluation of three different lamp emission models using novel application of potassium ferrioxalate actinometry, Ind. Eng. Chem. Res. 43, 948-955 (2004). 

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). 

Here Eg is the semiconductor band gap [eV] and Eq (+4.44 eV) is the energy of a free electron on the Hi redox scale. ° " Activities of photochemical water-splitting catalysts are usually assessed with the rates of evolved gases [moFh] per catalyst amount [g] under the specified irradiation conditions. From the measured evolution rate [Hi], the apparent QE = 2[Hi]// of the catalyst can be calculated using the known photon flux I [mol/s] incident on the reaction mixture (as determined by, e.g., ferrioxalate actinometry ). If available, this information is included with the experimental conditions in Table 1. The structures of selected semiconductors are shown in Figures 2-4. 

The exciting light was furnished by an irradiator composed of a 500-W xenon or 450-W high pressure mercury lamp as the light source and filters. The irradiation for the measurement of the quantum yields was performed with a JASCO CRM-FM spectroirradiator composed of a 2000-W xenon lamp as the light source and a grating monochromator. The light intensity was measured by potassium ferrioxalate actinometry [20], and the actinometric estimates were checked with an Eppley thermopile. 

The experiments were carried out in a glass-jacketed reactor vessel filled with 60 cm (or, in some cases 180 cm ) of reaction mixture and wrapped with aluminium foil. A 125-W immersion-type medium-pressure mercury lamp was applied as a radiation source. The light intensity was determined by ferrioxalate actinometry [20] (7a 1.5x10 mol photons dm" s" at 366 nm). A GBC UV-vis 911A spectrophotometer was used for the analysis. 

Absorption spectra were recorded using a GBC 911/A UV-vis spectrophotometer. For most of the continuous-wave experiments an AMKO LTl photolysis apparatus (containing a high-pressure 200-W Xe-Hg lamp and a monochromator) was applied. For irradiation of bromomercurate(II) complexes (2ir=253.7 nm), a low-pressure 16-W mercury-arc lamp was used. The light intensity was measured by ferrioxalate actinometry [15] and frequently checked by a thermopile. The experimental results were processed and evaluated by Quattro and Microsoft Excel programs on personal computers. The quantum yields for the formation of BrJ were determined from the initial rates of the photoinduced reactions, following the spectral changes. 

Bowman WD, Demas JN (1976) Ferrioxalate actinometry a warning on its correct use. J Phys Chem 80 2434-2435... 

At first glance, the results appear quite scattered. The values obtained under conditions of chemical ferrioxalate actinometry represent the upper boundary of the reported values, which mostly agree with each other. Between 250 and 350 nm the quantum yields are fairly constant around 1.25. Ferrioxalate actinometry is performed under standardized conditions using millimolar concentrations of ferrioxalate (and above millimolar at A > 436 nm) and an acidic pH (0.05 M H2SO4) of about 1.2 [206]. Other measurements have been carried out at lower initial Fe(III) concentrations as well as different Fe(III) to oxalate ratios and different pH values these mostly result in lower Fe(II)-quantum yields. Some investigations discriminating between individual complexes of Fe(III) and oxalate have been performed, while others did not provide an analysis of the individual complexes and are thus valid only for their respective complex-mixtures. However, all measurements with initial Fe(HI) concentrations below millimolar result in lower quantum yields. It is therefore desirable to characterize systemically any possible effects of initial Fe(III) complex concentration, speciation, and other experimental conditions on the ferrioxalate quantum yield to be able to interpret reported differences. 

It had been observed that the UV spectrum of a methanolic humulone solution, exposed to light, changes over a period of 28 days to match that of the isohumulones (49). In a more detailed study a 0.2% methanolic solution of humulone was irradiated with a high-pressure mercury lamp during 36 h (50). A crystalline substance was obtained after removal of the solvent and recrystallization of the residue from iso-octane. This so-called photo-isohumulone was identical to trans isohumulone (66). The photoreaction also proceeds upon irradiation with UV light of 365 nm or 254 nm. The presence of oxygen and the nature of the alcoholic solvent are irrelevant (51). The quantum yield, measured by ferrioxalate actinometry at 350 nm, is 0.022 (52). Since no... 

In order to provide a quantitative description of the product formation from the photolysis of PMPP, the quantum yield of biphenyl formation in methylene chloride was determined. Photolysis of PMPP in methylene chloride was therefore conducted with a medium-pressure mercury lamp using 254 nm band-pass filter. The light intensity was measured with ferrioxalate actinometry and the product yield was determined by gas chromatographic analysis of the samples photolyzed in methylene chloride. The quantum yield for biphenyl formation is ca. 5 x 10 . No change in the quantum yield for biphenyl was noticed in the presence and in absence of molecular oxygen. Since only a trace amounts of benzene and phenol were detected by gas chromatography, their formation quantum yields could not be determined accurately. 

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