ACCELERATION MECHANISM OF CHEMICAL-REACTION BY FREEZING - THE REACTION OF NITROUS-ACID WITH DISSOLVED-OXYGEN

TAKENAKA N UEDA A DAIMON T BANDOW H DOHMARU T MAEDA Y

N. Takenaka et al., ACCELERATION MECHANISM OF CHEMICAL-REACTION BY FREEZING - THE REACTION OF NITROUS-ACID WITH DISSOLVED-OXYGEN, Journal of physical chemistry, 100(32), 1996, pp. 13874-13884

39

INGLESE

Article

Chemistry Physical

Journal of physical chemistry → ACNP

0022-3654

100

32

1996

13874 - 13884

ISI

0022-3654(1996)100:32<13874:AMOCBF>2.0.ZU;2-D

The oxidation of nitrite by dissolved oxygen to form nitrate is known to be accelerated ca. 10(5) times by the freezing of the aqueous solut ion.(1) Here we report a detailed study on the acceleration mechanism of the above-mentioned oxidation. The reaction was studied at pH value s between 3.0 and 5.6 at various freezing rates, by different freezing methods, and with and without additional sails. The effect of freezin g which induced concentration (freeze concentration) of reactants into the unfrozen bulk solution was too small to explain the acceleration factor of ca. 10(5). Nitrate formations were completely prevented by a ddition of salts, such as NaCl and KCl, which make the freezing potent ial of ice negative, while the reaction was not affected by addition o f salts, such as Na2SO4 and NH4Cl, which make the freezing potential o f 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 f rom 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 so lutes are rejected from the ice and concentrated in the interfacial wa ter layer by assistance of the electrostatic force generated by the fr eezing potential. At a certain stage of freezing, the water layer is c ompletely 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 un frozen solution decreases. As a result, the reactant species, HNO2, in creased more in the unfrozen solution. After this stage, the concentra tions of the reactants in the unfrozen solution abruptly increase resu lting in the acceleration of the rate of formation of nitrate. On the basis of the above mechanism, the concentration factor fur nitrite was calculated as 2.4 x 10(3). The validity of this mechanism is further discussed.