Kinetic Fractionation Factor Research Articles

The kinetic isotope effect in the reaction of H2 with OH

Atmospheric hydrogen, H2, is removed to a large extent by reaction with OH. That reaction is expected to show a large kinetic isotope effect, enriching the deuterated hydrogen molecule, HD, in the atmospheric H2. We measured the kinetic fractionation factor using a static system with a UV‐irradiated mixture of O3, H2O and O2 to produce OH, H2 with a natural D content, and standard isotope mass spectrometry to measure the D/H ratio in the hydrogen. At room temperature the fractionation factor was found to be α = 1.65 ± 0.05 (±1σ), where a is the ratio between the rate constants of the H2 and HD reactions with OH. A calculation based on BEBOVIB‐IV gave exactly the same result and was extended to include the corresponding kinetic fractionation factors of other isotopic H2 molecules and their temperature dependence. The possible applications of the obtained fractionation factors to the tropospheric HD and HT balance and the resulting information for the H2 cycle are briefly illustrated.

Carbon isotopic fractionation during the CO 2 exchange process between air and sea water under equilibrium and kinetic conditions

Carbon isotopic fractionation during the air/sea exchange process is not fully understood at present. Information on the equilibrium and kinetic fractionation factors is an essential requirement, together with the value of the CO 2 partial pressure, for understanding the carbon cycle in the atmosphere and marine environments. Using a specially designed countercurrent equilibrator system, the fractionation factors between gaseous CO 2 and dissolved inorganic carbon in sea water were determined under both kinetic and equilibrium conditions. The following results were obtained: kinetic fractionation factor for air to sea ( α as ) is 0.998 at 288.2 K; kinetic fractionation factor for sea to air ( α sa ) is 0.990; equilibrium fractionation factor ( α eq ) is 0.991 at pH = 8.3 and 288.2 K. From these results, the carbon isotopic ratio of CO 2 passed through the air/ sea interface is estimated to be about −10 %. for air to sea and −8 %. for sea to air when CO 2 exchange takes place between air ( δ 13 C = −8 %.) and surface sea water ( δ 13 C = 2 %.) at 288.2 K.

Kinetische Isotopentrennung bei der Verdampfung von Wasser

Water evaporating into air not saturated with water vapour shows a stronger enrichment of D and O-18 than could be ascribed to the simple equilibrium isotopic fractionation. Kinetic effects are responsible for the increase in isotopic fractionation between liquid and gaseous phase. The so-called kinetic fractionation factor is calculated by means of a simple model. Comparison with the experimentally obtained fractionation factors proves the assumption that the kinetic fractionation is produced by the different diffusion velocities of H2O-16, H2O-18 and HDO-16 in the surrounding turbulent air. The consequences of the kinetic separation for the evaporation of the oceans are briefly discussed. Those for the evaporation of a limited water reservoir in isotopic exchange with the water vapour of the air are more extensively investigated. In the latter case calculation and experiments show that during evaporation the D - and O-18-contents approach stationary values as the water mass decreases. DOI: 10.1111/j.2153-3490.1965.tb01431.x