CHEMICAL REACTIONS IN POLYCRYSTALLINE ICE

The acceleration mechanism of chemical reactions by freezing has been investigated in the reaction of nitrite with dissolved oxygen to form nitrate. The reactants are concentrated into the unfrozen solution between ice crystals. In such a solution the reactants are further concentrated, and the pH of the solution decreases due to proton transfer from the ice to the solution to relax the freezing-potential that is generated. As a result, the reaction of nitrite with dissolved oxygen is accelerated considerably in freezing. To confirm this mechanism the reaction was investigated at various temperatures. At lower temperature than -3℃, nitrate formed rapidly until the sample was completely frozen, while the reaction did not proceed after the sample was completely frozen. On the other hand, at higher temperature than -3℃ nitrate formation was still observed even after the sample was completely frozen. The reactants were not very concentrated because the unfrozen solution was equilibrated at that temperature at low concentration of the reactants due to freezing point depression. The concentration factor of the reactants at the temperature of -3℃ was estimated to be several thousand

DECREASE OF SOLUTE IN THE AQUEOUS SOLUTION IN THE FREEZING PROCESS

Dilute volatile acidic aqueous solution was frozen by various freezing methods, and concentrations of solutes and pH of the sample were measured after the sample was completely thawed. Concentration of the volatile acid anion in the sample decreased, and pH of the sample increased after freezing. The decrease of solutes in the sample was almost equal to that detected in the gas phase. When dilute salt solution was frozen, solutes were excluded from ice and concentrated into the unfrozen solution. Concentrated anion combines with concentrated proton to form volatile acid such as acetic, formic or nitrous acid without reaction with any other chemicals. These volatile acids are saturated, and evaporation to the gas phase occurs. It is considered that the decrease in concentration of various solutes is mainly due to the freeze-concentration effect