Measurement of solvent isotope effects with the glass electrode. Part II. The dissociation constants of acetic and boric acid in D2O and D2O–H2O mixtures

Abstract

The dissociation constants of acetic and boric acid in D₂O and D₂O–H₂O mixtures have been determined by means of a cell with glass and silver–silver chloride electrodes and without liquid junctions. The fractionation factor Φ₁ for isotopic exchange between solvent and undissociated acetic acid has been determined from n.m.r. measurements and is close to unity. The isotope effects, ΔpK_n= pK_n– pK_H, are discussed in terms of the equilibrium theory of solvent isotope effects. For acetic acid ΔpK_n= 0·4630n+ 0·0377n²+ 0·0138n³ and for boric acid ΔpK_n= 0·399n+ 0·131n²– 0·014n³. The possibility of distinguishing, from these measurements, between the structures OB(OH)₂^– and B(OH)₄^– is considered.