Experimental evaluation of the isotopic exchange equilibrium ~(10)B(OH)_3 + ~(11)B(OH)_4~- = ~(11)B(OH)_3 + ~(10)B(OH)_4~- in aqueous solution

摘要

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 (K_B). The quotient of ~(11)K_B and ~(10)K_B, obtained using isotopically ≥ 99% pure borate/boric acid buffers, provides an equilibrium constant for the reaction ~(10)B(OH)_3+~(11)B(OH)_4~- < = > ~(11)B(OH)_3 + ~(10)B(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-pCO_2 of the atmosphere, have yielded constants, ~(11-10)K_B = ~(10)K_B/~(11)K_B, that have ranged between 1.0194 and approximately 1.033. The experimentally determined value ~(11-10)K_B = 1.028_5 ± 0.001_6 (mean ± 95% confidence interval) obtained at 25? and 0.63 molal (mol kg~(-1) H_2O) ionic strength is in much better agreement with recent theoretical assessments of ~(11-10)K_B 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 ~(11-10)K_B value obtained in this study will be applicable over a wide range of solution compositions and ionic strengths.