Experimental evaluation of the isotopic exchange equilibrium 10B(OH)3+11B(OH)4- = 11B(OH)3+10B(OH)-4 in acqueous 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 (K-B). The quotient of K-11(B) and K-10(B), obtained using isotopically >= 99% pure borate/boric acid buffers, provides an equilibrium constant for the reaction B-10(OH)(3)+B-11(OH)(4)(-)double left right arrow B-10(OH)(3)+B-10(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, K-11-10(B) = K-10(B)/K-11(B), that have ranged between 1.0194 and approximately 1.033. The experimentally determined value K-11-10(B) = 1.028(5)+/- 0.001(6) (mean +/- 95% confidence interval) obtained at 25 degrees 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. (c) 2006 Elsevier Ltd. All rights reserved.