The Autoxidation of I⁻ in Ice

Abstract

The rate of the autoxidation of 1-is largely enhanced in ice u compared to the liquid state. A peculiar effect of acidity on the rate hu been observed. Remarkable inhibition is caused by doping the ice with some foreign substances. The rate in H 20-ice is 2-3 times larger than in~v-ice. The frozen system displays a high photochemical sensitivity.The remarkable chemical and radiative properties of frozen solutions, which were first pointed out by Szent-Gyorgyi [1 J, have been the subject of several works. In particular it has been observed that the rates of some reactions, both thermal and photochemical, are largely enhanced when occurring in ice as compared to the liquid phase. The reactions studied were of two types: (a) hydrolysis and dehydration, where water is directly involved [2-4J: (b) dimerisations [5,6J. The rates of both types probably owe their enhancement to a concentration effect, but in case (a) the high proton mobility in ice [7J should also be considered, since these reactions are generally acid-base catalyzed.We wish to report the observation of a similar ice-effect on a simple inorganic redox system and comment on the origin of the enhanced reactivity. It was observed that aerated acidic solutions of iodide salts, when frozen, liberate iodine and turn yellow much faster than at room temperature. Moreover, the frozen system exhibits a high photochemical sensitivity. No such effect was observed in frozen methanol, or evacuated aqueous solutions. These results led us to a systematic study of the various factors which affect the reaction. ExperimentalAll experiments were carried out by freezing 7 ml of solution in pyrex testtubes (10 mm diameter), revolving rapidly in a water-alcohol cooling• Correspondence mould be directed to this author.

Freeze-Induced Reactions: Formation of Iodine−Bromine Interhalogen Species from Aqueous Halide Ion Solutions

“…The reaction between iodide ions and nitrous acid, under both freezing conditions and in nonfrozen solutions, produces greater quantities of product when dissolved oxygen is present. , Such behavior is similar to that observed in many other freeze-induced reactions. In fact, both iodide and nitrite ions undergo greatly enhanced oxidation by dissolved oxygen when they are frozen separately in dilute solutions. ,, In order to determine the role of dissolved oxygen for the current reactions under study, two further experiments were carried out. In the first, mixture 1 ([Br − ]/[I − ] = 10) was frozen under very acidic conditions (pH 2.7) in the absence of dissolved oxygen.…”

“…As observed in the case where X = Cl − , it was shown that the freezing process itself can lead to unexpected chemical behavior . Previous reports in the literature have shown that a variety of reactions are in fact accelerated as a result of the freezing process. − In particular, a number of atmospherically relevant autoxidation reactions, such as those involving nitrite, iodide, sulfide, or sulfite ions, are known to be promoted when dilute solutions are frozen. − ,− Indeed, an acceleration of reaction rates up to 10 5 times above those at room temperature have been reported in the literature . When a solution of dilute electrolytes is frozen, it is known that some species are incorporated more readily into the ice than others, the so-called Workman−Reynolds effect. , The concentrations of electrolytes in the liquid phase may increase dramatically as liquid water is removed to the growing solid phase and, as the freezing front advances, the remaining liquid becomes trapped between grain boundaries, leading to the formation of unfrozen, liquid-phase “micropockets”.…”

“…In fact, both iodide and nitrite ions undergo greatly enhanced oxidation by dissolved oxygen when they are frozen separately in dilute solutions. 19,24,25 In order to determine the role of dissolved oxygen for the current reactions under study, two further experiments were carried out. In the first, mixture 1 ([Br -]/[I -] ) 10) was frozen under very acidic conditions (pH 2.7) in the absence of dissolved oxygen.…”

“…[15][16][17][18][19][20][21][22][23] In particular, a number of atmospherically relevant autoxidation reactions, such as those involving nitrite, iodide, sulfide, or sulfite ions, are known to be promoted when dilute solutions are frozen. [19][20][21][24][25][26] Indeed, an acceleration of reaction rates up to 10 5 times above those at room temperature have been reported in the literature. 20 When a solution of dilute electrolytes is frozen, it is known that some species are incorporated more readily into the ice than others, the so-called Workman-Reynolds effect.…”

An overview of snow photochemistry: evidence, mechanisms and impacts

“…For example, it has been shown that both sulfur dioxide and sulfide ions incorporated into ice are oxidized to sulfate ions (Valdez et al, 1989;Finnegan et al, 1991, Betterton and. Similarly iodide and bromide ions become oxidized to higher valence species when frozen (Eyal et al, 1964). It was later discovered that the N(III) species, NO − 2 and HONO, can be oxidized by molecular oxygen to nitrate ions upon freezing in aqueous solution at a rate about 10 5 faster than that found at room temperature (Takenaka et al, 1992).…”