Oxygen-18 Isotope Effect in the Reaction of Oxygen with Copper

Abstract

The fractionation of oxygen‐18 in the reaction of copper with oxygen of natural isotopic composition has been measured over the temperature range 68—256°C. The oxide films (estimated thicknesses varying from 150—2500 A) were removed for O¹⁸ assay by treatment with hydrogen at 350°C. Using the CO₂ equilibration technique, the O¹⁸/O¹⁶ ratios for the resulting water samples were determined mass spectrometrically and compared with the ratio for a reference sample of water prepared from the original oxygen gas. The direction of the fractionation indicated that O₂¹⁶ reacts preferentially compared to O¹⁶O¹⁸. The isotope effect appeared to be independent of the oxygen pressure over the limited range studied (2—25 cm Hg). The magnitude of the fractionation factor was 2.0% at 150°C, with a small negative temperature coefficient. From this it was possible to estimate a value of ΔE_act=17 cal/mole, which may be compared with the calculated isotopic zero‐point energy difference of 64 cal/mole. The theoretical implications of the experimental results are discussed.