Acceleration Mechanism of Chemical Reaction by Freezing:  The Reaction of Nitrous Acid with Dissolved Oxygen

Abstract

The oxidation of nitrite by dissolved oxygen to form nitrate is known to be accelerated ca. 105 times by the freezing of the aqueous solution. Here we report a detailed study on the acceleration mechanism of the above-mentioned oxidation. The reaction was studied at pH values between 3.0 and 5.6 at various freezing rates, by different freezing methods, and with and without additional salts. The effect of freezing which induced concentration (freeze concentration) of reactants into the unfrozen bulk solution was too small to explain the acceleration factor of ca. 105. Nitrate formations were completely prevented by addition of salts, such as NaCl and KCl, which make the freezing potential of ice negative, while the reaction was not affected by addition of salts, such as Na2SO4 and NH4Cl, which make the freezing potential of ice positive. When a sample solution was frozen in such a way as to form a single crystal of ice, most nitrite was exclusively liberated from the ice to the gas phase. This observation suggests the importance of ice in the polycrystalline form to retain nitrite during freezing. When freezing begins, grains of crystalline ice begin to grow. The solutes are rejected from the ice and concentrated in the interfacial water layer by assistance of the electrostatic force generated by the freezing potential. At a certain stage of freezing, the water layer is completely confined by the walls of some ice grains. Protons move from the ice phase to the unfrozen solution surrounded by the ice walls to neutralize the electric potential generated, and thus the pH of the unfrozen solution decreases. As a result, the reactant species, HNO2, increased more in the unfrozen solution. After this stage, the concentrations of the reactants in the unfrozen solution abruptly increase resulting in the acceleration of the rate of formation of nitrate. On the basis of the above mechanism, the concentration factor for nitrite was calculated as 2.4 × 103. The validity of this mechanism is further discussed.

1-D Air-snowpack modeling of atmospheric nitrous acid at South Pole during ANTCI 2003

“…How to correct for nonideality in the "QLL" is not clear, however, as the actual solute concentrations and the partitioning at low temperatures are not known. Other reactions can happen, for example, NO − 2 and HONO can be oxidized by molecular oxygen to nitrate ions at a much faster rate (Takenaka et al, 1992(Takenaka et al, , 1996. However, the importance of these reactions remains unexplored.…”

Using Singlet Molecular Oxygen to Probe the Solute and Temperature Dependence of Liquid-Like Regions in/on Ice

“…Takenaka and coworkers showed that as solutions are frozen, solutes partition out of the ice and form a concentrated brine in the unfrozen portion of the solution, where rates of nitrite oxidation are 10 5 (or more) times faster than in solution. 24,[28][29][30][31] In frozen solutions, Grannas and coworkers showed enhanced photoreaction rates of p-nitroanisole with pyridine and observed rates up to about 40 times 5 higher on ice than in the corresponding liquid. 32 34 which has a peak absorption band at 549 nm (Supporting Information, Figure S1).…”

Enhanced formation of formate by freezing in solutions of hydrated formaldehyde-metal-hydrogen peroxide

“…However, because freezing the solution containing HOOH and CH 2 (OH) 2 did not form any detectable formate, we concluded that the electron transfer was not the primary mechanism for the formation of formate. Takenaka et al (1992Takenaka et al ( , 1996 considered several mechanisms for the HNO 2 oxidation reaction to form nitrate in the freezing solution, and concluded that concentration effects around ice crystals could be the primary mechanism. Their study found that HNO 2 was 2400 times more concentrated in the solution phase surrounded by the ice crystals and HNO 2 was oxidized by dissolved O 2 (Takenaka et al, 1996).…”

“…Takenaka et al (1992Takenaka et al ( , 1996 considered several mechanisms for the HNO 2 oxidation reaction to form nitrate in the freezing solution, and concluded that concentration effects around ice crystals could be the primary mechanism. Their study found that HNO 2 was 2400 times more concentrated in the solution phase surrounded by the ice crystals and HNO 2 was oxidized by dissolved O 2 (Takenaka et al, 1996). Takenaka et al (1993) and Honda (2001) studied oxidation of H 2 SO 3 to H 2 SO 4 and concluded that O 2 had oxidized H 2 SO 3 .…”

“…Furthermore, winter fogs could be partially frozen. Water drops in high cloud and winter fogs may undergo not only wet chemical and photochemical reactions, but also reactions induced by freezing (e.g., nitrate formation from nitrous acid (Takenaka et al, 1992(Takenaka et al, , 1996). …”