Photochemical and spectroscopic studies of citric acid and selected alphahydroxydi- and tricarboxylic acid complexes of iron(III).

摘要

The overall objective of this work was: (1) to determine the chemistry of organic acids formic, acetic, succinic, malic, tartaric, isocitric, and citric acid in the photoinduced-reduction of iron(III) to iron(II), and (2) to determine some of the factors influencing the quantum yield for production of iron(II) from iron(III) in the presence of citric acid.; It was shown that the quantum yields for photoreduction of iron(III) to iron(II) in the presence of the selected organic acids acid depends on the structure and concentration of the carboxylate ligand as well as the pH of the solution. The quantum yields for the photoreduction of iron(III)-carboxylate complexes under our experimental conditions for (Ligand) / (Iron(III) = 167, (Carboxylate = 0.05 M, Iron(III)) = 3.0E-4 M and pH = 2.9) varied with the carboxylate ligand. The order from highest to lowest was oxalate {dollar}>{dollar} tartarate {dollar}>{dollar} malate {dollar}>{dollar} citrate {dollar}>{dollar} isocitrate {dollar}>{dollar} succinate {dollar}>{dollar} formate. Iron (III)-acetate had no photoactivity at all. The photoreduction of iron(III) to iron(II) was accompanied by the oxidative decarboxylation of the ligand.; Experimental results showed that the photoinduced-reduction of iron(III) to iron(II) in presence of the organic acids depends on two factors, the pH and the initial ligand to metal molar ratio. For the experimental condition where (Ligand) / (Iron(III)) = 5, (Carboxylate = 0.015 M, Iron(III) = 3.0E-4 M) the order from highest to lowest was oxalate {dollar}>{dollar} tartarate {dollar}>{dollar} citrate {dollar}>{dollar} malate {dollar}>{dollar} isocitrate when the pH of the reaction media was 2.7 as opposed to the order tartarate {dollar}>{dollar} citrate {dollar}>{dollar} isocitrate {dollar}>{dollar} oxalate {dollar}>{dollar} malate found for the pH of 4.0.; The NMR, UV-VIS and IR spectra of the organic acid complexes of iron(III) were studied for the structural examination of the metal complex species as a function of pH. The NMR and UV-VIS spectra were investigated with respect to the electronic interaction, and the IR spectra were studied with respect to the vibrational modes that constitute the complexations. The IR spectra provided information on the kind of molecular vibrations that changed as a result of the irradiation of the KBr salt of the iron(III)-carboxylate.; Work was then focused on the photochemical decomposition of the iron(III)-citrate. The intermediate product of the photochemical decomposition of iron(III)-citric acid was detected by HPLC and shown to be acetonedicarboxylic acid (ADA).; The rate of the decomposition of ADA in the absence of iron was fastest at pH 2.7. The kinetics of the decomposition of the ADA in the presence and absence of iron(II) and iron(III) were measured using various spectroscopic methods. At pH 2.0, the rate of the decomposition of ADA to acetone from fastest to slowest was iron(III)-ADA {dollar}>{dollar} ADA {dollar}>{dollar} iron(II)-ADA, while at values of pH above 4.0, the order was iron(II)-ADA {dollar}>{dollar} iron(III)-ADA {dollar}>{dollar} ADA.