Measurement of solvent isotope effects with the glass electrode. Part II. The dissociation constants of acetic and boric acid in D 2 O and D 2 O–H 2 O mixtures

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, ΔpKn= pKn– pKH, are discussed in terms of the equilibrium theory of solvent isotope effects. For acetic acid ΔpKn= 0·4630n+ 0·0377n2+ 0·0138n3 and for boric acid ΔpKn= 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.