Some photo-chemical properties of ferric iron solutions

Solutions of 1.3 x 10-3M ferric chloride of pH between 2.0 and 3.05, and in the absence of added substrate are found to be photoactive. The photoactivity is to be attributed to the 'dimer' Fe3+(OH)- and possibily also to the species Fe+3OH-. Experimental proof of OH Radical formation is shown by the hydroxylation of substrates benzene and benzoic acid, which are oxidised to phenol and salicylic acid respectively. Experimental evidence is adduced to show that the photoactivity is not due to oxidisable organic impurities in the distilled water. Possible processes are oxidation of chloride to chloride and/or oxidation of water to oxygen. No chloride was found with a sufficiently sensitive test. In fact, irradiation of Fe+3/Cl- solutions leads to the disappearance of chlorine. Using the 'freezing and thawing' technique of Dain and Kachan for measurement of small quantities of gas, a non-condensible gas was found to be photoproduced in these iron solutions. The amount of gas was shown to accord with the stoichiometry of the equation:- [equation]. Further the gas was identified as pure oxygen by (a) the phosphor method of Kautsky and Hirsch, (b) sparking with hydrogen, resulting in a 2/1 combination. The photo-oxidation of water by ferric ion is thus established. The following reaction scheme is proposed. [equation] iron catalysis. The quantum yield of Fe+2 formation [equation]. Powdered silica was found to provide an active surface for increase in photo-reduction of Fe+3. In the analogues ceric cerous system Dain and Kachan suggest that the increased photo-activity is due to a recombination of OH radicals on the surface of the silica, leading to a higher oxygen yield. It was shown in the present work that a 'fresh' surface of colloidal ferric hydroxide increases the photoreduction of the iron. It is suggested that this is due to some 'dark' heterogeneous reaction such as the recombination of OH radicals or catalysis of some electron-transfer reaction, which would in time increase the oxygen yield. The effect of light intensity (365mmu) was measured. The initial [equation]. However, the maximum yield i.e. [Fe+2] stationary is a linear function of √I. Relatively high intensities are therefore necessary for easily measurable oxygen evolution, a suggestion already made by Uri. Light was found to initiate and accelerate the process of secondary hydrolysis of the iron to Fe(OH)?, as compared with the dark hydrolysis. Experimental results point to the conclusion that the reaction, Fe+2 + H2O2 is basically responsible for the phenomenon, and provide further, independent evidence that H2O2 is present in the system Fe+3/U.V