Der heterogene Isotopenaustausch. I. Der Austausch an Fe 3 O 4 mit 59 Fe

The isotopic ion exchange between various forms of 59 Fe labelled synthetic Fe 3 O 4 and FeC1 2 ‐solution was investigated from 9 to 95°C, by measuring the activity of the solution after various reaction times. The observed exchange rate was compared with the theoretically calculated rate for a diffusion controlled reaction. The mathematics of the heterogeneous isotopic ion exchange is discussed. For short times a deviation from the y |√ t relationship was observed and attributed to an initial process, superimposed on the diffusion reaction. This initial process probably consists of a rapid exchange of the solution with the first atomic layers of the solid. This problem will be treated mathematically in a subsequent paper. Diffusion coefficients (D 8 ) can be estimated from the slope of the straight line in the y |√ t plot. From the temperature dependence of the D 8 ‐values the activation energy was calculated to be about 7 kcal/mole and the self diffusion coefficient (D 0 ) of Fe in Fe 3 O 4 was estimated to be 5 ± 3 · 10 −6 cm 2 s −1 .