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

Abstract

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 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- 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 °C 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.