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 ⩾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> − </sup> ⇔ <sup> 11 </sup> B(OH) <sub> 3 </sub> + <sup> 10 </sup> B(OH) <sub> 4 </sub> <sup> − </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> 11–10 </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> 11–10 </sup> <em> K </em> <sub> B </sub> =1.028 <sub> 5 </sub> ±0.001 <sub> 6 </sub> (mean±95% confidence interval) obtained at 25 °C and 0.63 molal (mol kg <sup> −1 </sup> H <sub> 2 </sub> O) ionic strength is in much better agreement with recent theoretical assessments of <sup> 11–10 </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, 158–163]. Since the activity quotient for the fractionation reaction is almost equal to unity, it is expected that the <sup> 11–10 </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 language | American English |
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Journal | Deep Sea Research Part I: Oceanographic Research Papers |
Volume | 53 |
DOIs | |
State | Published - 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