Oxygen isotope (18O/16O) fractionation between water and hydroxide ion
Authors: Bajnai David , Herwartz Daniel 
Published: March 2021

&lt;p&gt;In any reaction involving water, the educt oxygen is either derived from H&lt;sub&gt;2&lt;/sub&gt;O or OH&lt;sup&gt;-&lt;/sup&gt; (i.e., the hydroxide ion). For example, during carbonate precipitation the relative proportion of (de)hydration (CO&lt;sub&gt;2&lt;/sub&gt; + H&lt;sub&gt;2&lt;/sub&gt;O &amp;#8660;&amp;#160;H&lt;sup&gt;+&lt;/sup&gt; + HCO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) and (de)hydroxylation (CO&lt;sub&gt;2&lt;/sub&gt; + OH&lt;sup&gt;-&lt;/sup&gt; &amp;#8660; HCO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) 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 (1000ln&lt;em&gt;&amp;#945;&lt;/em&gt;&lt;sub&gt;H2O&amp;#8211;OH-&lt;/sub&gt;). Experimental studies from the 1960s determined 1000ln&amp;#945;&lt;sub&gt;H2O&amp;#8211;OH-&lt;/sub&gt; to be 39.22(&amp;#177;2.88)&amp;#8240; at 25&amp;#160;&amp;#176;C and estimated its temperature dependence at &amp;#8209;0.5&amp;#8240;&amp;#160;&amp;#176;C&lt;sup&gt;-1&lt;/sup&gt; (1-3). These empirical observations were recently questioned by a theoretical study that implied a much lower fractionation factor of 23.18&amp;#8211;18.91&amp;#8240; at 25&amp;#160;&amp;#176;C as well as a lower temperature dependence of &amp;#8209;0.05&amp;#8240;&amp;#160;&amp;#176;C&lt;sup&gt;&amp;#8209;1&lt;/sup&gt; (4).&lt;/p&gt;&lt;p&gt;To provide new experimental data to solve this controversy, we performed quantitative witherite (BaCO&lt;sub&gt;3&lt;/sub&gt;) precipitation experiments. Tank CO&lt;sub&gt;2&lt;/sub&gt; gas of known oxygen and carbon isotopic composition was injected into saturated Ba(OH)&lt;sub&gt;2&lt;/sub&gt; solution of known oxygen isotopic composition. Following the hydroxylation of the CO&lt;sub&gt;2&lt;/sub&gt;, BaCO&lt;sub&gt;3&lt;/sub&gt; instantly precipitated from the high pH (&gt;12) solution. Since the precipitate directly inherited 1/3 of its oxygen from the hydroxide ion and 2/3 from the tank CO&lt;sub&gt;2&lt;/sub&gt;, the &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;18&lt;/sup&gt;O value of the OH&lt;sup&gt;-&lt;/sup&gt; can be calculated via mass balance. Subsequently, the 1000ln&lt;em&gt;&amp;#945;&lt;/em&gt;&lt;sub&gt;H2O&amp;#8211;OH-&lt;/sub&gt; value can be derived. Altogether 18 experiments were performed at a range of temperatures (1&amp;#8211;80&amp;#160;&amp;#176;C) and using solutions of different oxygen isotopic composition (range of ca. 20&amp;#8240;). Minor variations between the &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C values (&lt;&amp;#160;1&amp;#8240;) of the BaCO&lt;sub&gt;3&lt;/sub&gt; and the tank CO&lt;sub&gt;2&lt;/sub&gt; attests the quantitative precipitation of the reference gas.&lt;/p&gt;&lt;p&gt;Our 1000ln&lt;em&gt;&amp;#945;&lt;/em&gt;&lt;sub&gt;H2O&amp;#8211;OH-&lt;/sub&gt; values show a similar temperature dependence as the recent theoretical study of Zeebe (4), but our fractionation factor at 25&amp;#160;&amp;#176;C is much closer to the values reported in the 1960s. Reasons for the discrepancies between our study and previous publications in terms of 1000ln&lt;em&gt;&amp;#945;&lt;/em&gt;&lt;sub&gt;H2O&amp;#8211;OH-&lt;/sub&gt; and its temperature dependency will be discussed. One of our hypotheses is that the H&lt;sub&gt;2&lt;/sub&gt;O &amp;#8658; OH&lt;sup&gt;-&lt;/sup&gt; + H&lt;sup&gt;+&lt;/sup&gt; reaction introduces a large kinetic effect as isotopically light H&lt;sub&gt;2&lt;/sub&gt;O 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.&lt;/p&gt;&lt;p&gt;(1) H. R. Hunt, H. Taube, The relative acidity of H&lt;sub&gt;2&lt;/sub&gt;O&lt;sup&gt;18&lt;/sup&gt; and H&lt;sub&gt;2&lt;/sub&gt;O&lt;sup&gt;16&lt;/sup&gt; co&amp;#246;rdinated to a tripositive ion. J. Phys. Chem. 63, 124-125 (1959).&lt;br&gt;(2) E. R. Thornton, Solvent isotope effects in H&lt;sub&gt;2&lt;/sub&gt;O&lt;sup&gt;16&lt;/sup&gt; and H&lt;sub&gt;2&lt;/sub&gt;O&lt;sup&gt;18&lt;/sup&gt;. J. Am. Chem. Soc. 84, 2474-2475 (1962).&lt;br&gt;(3) M. Green, H. Taube, Isotopic fractionation in the OH&lt;sup&gt;-&lt;/sup&gt;&amp;#8211;H&lt;sub&gt;2&lt;/sub&gt;O exchange reaction. J. Phys. Chem. 67, 1565-1566 (1963).&lt;br&gt;(4) R. E. Zeebe, Oxygen isotope fractionation between water and the aqueous hydroxide ion. Geochim. Cosmochim. Acta 289, 182-195 (2020).&lt;/p&gt;