Experimental Evaluation of the Isotopic Exchange Equilibrium sup10/supB(OH)sub3/sub+sup11/supB(OH)sub4/subsup−/sup=sup11/supB(OH)sub3/sub+sup10/supB(OH)sub4/subsup−/sup in Aqueous Solution

Robert H. Byrne, Wensheng Yao, Kateryna Klochko, John A. Tossell, Alan J. Kaufman

Research output: Contribution to journalArticlepeer-review

Abstract

<p> The precision of spectrophotometric measurements of indicator absorbance ratios is sufficient to allow evaluation of small isotopically induced differences in the dissociation constant of boric acid ( <em> K </em> <sub> B </sub> ). The quotient of <sup> 11 </sup> <em> K </em> <sub> B </sub> and <sup> 10 </sup> <em> K </em> <sub> B </sub> , obtained using isotopically &ges;99% pure borate/boric acid buffers, provides an equilibrium constant for the reaction <sup> 10 </sup> B(OH) <sub> 3 </sub> + <sup> 11 </sup> B(OH) <sub> 4 </sub> <sup> &minus; </sup> &hArr; <sup> 11 </sup> B(OH) <sub> 3 </sub> + <sup> 10 </sup> B(OH) <sub> 4 </sub> <sup> &minus; </sup> which heretofore had not been experimentally determined. Previous theoretical and semi-empirical evaluations of this equilibrium, which is important for assessments of the paleo-pH of seawater and the paleo- <em> p </em> CO <sub> 2 </sub> of the atmosphere, have yielded constants, <sup> 11&ndash;10 </sup> <em> K </em> <sub> B </sub> = <sup> 10 </sup> <em> K </em> <sub> B </sub> / <sup> 11 </sup> <em> K </em> <sub> B </sub> , that have ranged between 1.0194 and approximately 1.033. The experimentally determined value <sup> 11&ndash;10 </sup> <em> K </em> <sub> B </sub> =1.028 <sub> 5 </sub> &plusmn;0.001 <sub> 6 </sub> (mean&plusmn;95% confidence interval) obtained at 25 &deg;C and 0.63 molal (mol kg <sup> &minus;1 </sup> H <sub> 2 </sub> O) ionic strength is in much better agreement with recent theoretical assessments of <sup> 11&ndash;10 </sup> <em> K </em> <sub> B </sub> that have ranged between 1.026 and 1.033, than the much-cited original estimate (1.0194) of Kakihana et al. (1977) [Fundamental studies on the ion-exchange separation of boron isotopes. Bulletin of Chemical Society of Japan 50, 158&ndash;163]. Since the activity quotient for the fractionation reaction is almost equal to unity, it is expected that the <sup> 11&ndash;10 </sup> <em> K </em> <sub> B </sub> value obtained in this study will be applicable over a wide range of solution compositions and ionic strengths.</p>
Original languageAmerican English
JournalDeep Sea Research Part I: Oceanographic Research Papers
Volume53
DOIs
StatePublished - Jan 1 2006

Keywords

  • Boron isotopes
  • Boron isotopic exchange equilibrium
  • Paleo-pH of seawater
  • Paleo-pCO2 of the atmosphere
  • Spectrophotometric pH measurement

Disciplines

  • Life Sciences

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