Experimental evaluation of the isotopic exchange equilibrium 10B(OH)3+11B(OH)4−=11B(OH)3+10B(OH)4− in aqueous solution

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 (KB). The quotient of 11KB and 10KB, obtained using isotopically ⩾99% pure borate/boric acid buffers, provides an equilibrium constant for the reaction 10B(OH)3+11B(OH)4−⇔11B(OH)3+10B(OH)4− 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-pCO2 of the atmosphere, have yielded constants, 11–10KB=10KB/11KB, that have ranged between 1.0194 and approximately 1.033. The experimentally determined value 11–10KB=1.0285±0.0016 (mean±95% confidence interval) obtained at 25 °C and 0.63 molal (mol kg−1 H2O) ionic strength is in much better agreement with recent theoretical assessments of 11–10KB 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 11–10KB value obtained in this study will be applicable over a wide range of solution compositions and ionic strengths.