EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Oxygen isotope (18O/16O) fractionation between water and hydroxide ion

David Bajnai and Daniel Herwartz
David Bajnai and Daniel Herwartz
  • University of Cologne, Institute of Geology and Mineralogy, Cologne, Germany (

In any reaction involving water, the educt oxygen is either derived from H2O or OH- (i.e., the hydroxide ion). For example, during carbonate precipitation the relative proportion of (de)hydration (CO2 + H2O ⇔ H+ + HCO3-) and (de)hydroxylation (CO2 + OH- ⇔ HCO3-) reactions is pH-dependent. When modelling this system, the oxygen isotopic composition of water can be measured directly, but the oxygen isotopic composition of hydroxide must be calculated from the respective fractionation factor (1000lnαH2O–OH-). Experimental studies from the 1960s determined 1000lnαH2O–OH- to be 39.22(±2.88)‰ at 25 °C and estimated its temperature dependence at ‑0.5‰ °C-1 (1-3). These empirical observations were recently questioned by a theoretical study that implied a much lower fractionation factor of 23.18–18.91‰ at 25 °C as well as a lower temperature dependence of ‑0.05‰ °C‑1 (4).

To provide new experimental data to solve this controversy, we performed quantitative witherite (BaCO3) precipitation experiments. Tank CO2 gas of known oxygen and carbon isotopic composition was injected into saturated Ba(OH)2 solution of known oxygen isotopic composition. Following the hydroxylation of the CO2, BaCO3 instantly precipitated from the high pH (>12) solution. Since the precipitate directly inherited 1/3 of its oxygen from the hydroxide ion and 2/3 from the tank CO2, the δ18O value of the OH- can be calculated via mass balance. Subsequently, the 1000lnαH2O–OH- value can be derived. Altogether 18 experiments were performed at a range of temperatures (1–80 °C) and using solutions of different oxygen isotopic composition (range of ca. 20‰). Minor variations between the δ13C values (< 1‰) of the BaCO3 and the tank CO2 attests the quantitative precipitation of the reference gas.

Our 1000lnαH2O–OH- values show a similar temperature dependence as the recent theoretical study of Zeebe (4), but our fractionation factor at 25 °C is much closer to the values reported in the 1960s. Reasons for the discrepancies between our study and previous publications in terms of 1000lnαH2O–OH- and its temperature dependency will be discussed. One of our hypotheses is that the H2O ⇒ OH- + H+ reaction introduces a large kinetic effect as isotopically light H2O is pyrolysed more frequently. In contrast, the back reaction proceeds rapidly without an isotopic preference. Hence, the self-ionisation of water cannot be described as a classic equilibrium. Alternative explanations such as unidentified kinetic isotope effects in our precipitation experiments (i.e., on the crystal surface) cannot be ruled out.

(1) H. R. Hunt, H. Taube, The relative acidity of H2O18 and H2O16 coördinated to a tripositive ion. J. Phys. Chem. 63, 124-125 (1959).
(2) E. R. Thornton, Solvent isotope effects in H2O16 and H2O18. J. Am. Chem. Soc. 84, 2474-2475 (1962).
(3) M. Green, H. Taube, Isotopic fractionation in the OH-–H2O exchange reaction. J. Phys. Chem. 67, 1565-1566 (1963).
(4) R. E. Zeebe, Oxygen isotope fractionation between water and the aqueous hydroxide ion. Geochim. Cosmochim. Acta 289, 182-195 (2020).

How to cite: Bajnai, D. and Herwartz, D.: Oxygen isotope (18O/16O) fractionation between water and hydroxide ion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4909,, 2021.