Quantum Yields of the Photochemical Reduction of Ceric Ions by Water and Evidence for the Dimerization of Ceric Ions

Abstract

Water solutions containing ceric perchlorate and perchloric acid evolve oxygen when they absorb ultraviolet light and the ceric is reduced to cerous perchlorate.2Baur2a and Weiss and Porret2b studied the photochemical reaction in the full light of a quartz mercury arc lamp. Weiss and Porret obtained maximum gross quantum yields of the order of one-tenth in solutions one-tenth molar in ceric perchlorate and one molar in perchloric acid. Their quantum yields decreased as the cerous perchlorate accumulated.We have measured the quantum yields of the reaction when the solutions are irradiated with monochromatic light of X 254 µ. The light intensity, I, and the concentrations of cerous, c3, and ceric, Ci, perchlorates were varied many fold. The perchloric acid concentration, c2, was held at 1.03 =*= 0.03 M and the ionic strength, µ, at 1.1 ± 0.1, all at 23 ± 3°.Solutions of ceric perchlorate in perchloric acid are thermally unstable at 25°. The equilibrium ratio Ci/Ci is about 10 _8 in molar perchloric acid in equilibrium with the atmosphere (po, = 0.2 atm.), but the rate of the thermal reduction of the ceric perchlorate by water is extremely slow. In one of our stock solutions which was 5.3 M in perchloric acid and was kept in the dark at 25 * 3°, Ci decreased from 1.36 to 1.20 M in eleven months while c3 increased from 0.32 M.Materials.-The chemical reagents were of analytical reagent grade or were prepared from material of this quality. The water was chloride-free distilled water.Stock solutions of ceric and cerous perchlorates were prepared from a sample of snow white granular ceric oxide, 98.5% pure Ce02, which was supplied by the Rohm and Haas Chemical Co., Philadelphia, Pa. The ceric oxide could not be converted directly into ceric perchlorate even when dispersed as a fine hydrous oxide in 72% perchloric acid at room temperature or at 100°f or periods of several months.The ceric perchlorate was finally prepared by reducing the ceric oxide to cerous ions by bromide in perchloric acid. The bromine and excess bromide were removed by boiling. The cerous ions were then oxidized electrolytically to the ceric state. The experimental details follow.Forty grams of the granular ceric oxide and 80 g. of sodium bromide were added to 275 ml. of 72% perchloric acid. The mixture was simmered for two hours under an appropriate hood. The hot solution was filtered by suction through a sintered glass filter. The filtrate was boiled(1) The part of this article concerned with the experimental study of the effect of cerous perchlorate upon the reaction is taken from the thesis submitted by Maynard E. Smith in September, 1946, to the Department of Chemistry of the Massachusetts Institute of Technology in partial fulfillment of the requirements for the degree of Master of Science.(2) (a) E. Baur, Z. physik. Chetn., 63, 683 (1908), was the first to identify oxygen as the gaseous product of the reaction. This was

Kinetic Study of the Oxidation of tert-Butanol by Cerium(IV) in Perchloric Acid Media

“…Several studies [20,21] have found that Ce IV species present in perchloric acid as solvent are Ce 4ϩ , Ce(OH) 3ϩ Concerning the reducer species, t-BuOH, it is well known that for tertiary alcohols the oxidation begins on the carbon atom linked to the hydroxyl group. On the other hand, the stoichiometric equation shows that both acetone and formaldehyde are produced.…”

Chlorophyll Sensitized Charge Separation in Phospholipid Vesicles

“…Efforts to use solar energy for the purpose of generating chemical energy through the photodecomposition of water can be traced back 30yr (Heidt and Smith, 1948; R i a and Weiss, 1952; Calvin, 1961). Schematically, the reaction can be represented as hv H20 [photosensitizer]' H2 + The energy which is required to transfer one electron from oxygen to hydrogen is 34.16 kcal per equivalent (Handbook, 1965).…”

Ionic Species in Ceric Perchlorate Solutions

“…Recently Heidt and Smith (2) postulated the existence of a stable dimei-of cerium in ceric perchlorate solution. This assumption enabled them to explain some unusual effects observed in the photochen~ical reduction of ceric ion in perchloric acid.…”