Hydrogen Exchange Studies and Proton Transfer in β Iron(III) Oxyhydroxide

Abstract

The kinetics of hydrogen exchange between tritiated water and /3-FcOOH crystals have been found to be dependent on pH in the alkaline region but independent in the acid region. The exchange formally obeyed a diffusion controlled law, and yielded the same activation energy 97 joules mole-1 (23 kcal mole-1) in both the alkaline and acid regions. The difference in kinetic behaviour on increasing pH in the alkaline region was attributed to a decrease in Do, the pre-exponential factor in the self- diflfusion coefficient.

Pore size and surface area have been investigated in the characteristic cigarshaped crystals of /J-FeOOH by means of

nitrogen isotherms. The pore size distribution calculated from the multilayer region of the isotherm was found to be very narrow and gave a mean pore diameter of 28.4 A with a standard deviation of 7.0 A. This agrees with the value of 30 A found by Watson et al.1 using electron microscopy. Infra red data indicate weak hydrogen bonds of length 2.86 A.

The implication of the weak hydrogen bonds, the orthogonal array of the large parallel channels, and the small structural tunnels, are considered in relation to possible mechanisms for hydrogen exchange.